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close this bookThe Biogas/Biofertilizer Business Handbook (Peace Corps, 1982, 186 p.)
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View the documentNew ideas
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View the documentBioinsecticides
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View the documentFacts & Figures
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View the documentFeasibility Studies
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New ideas

This section is a brief look at a few new ideas that have the potential for increasing the uses and profits of biogas systems.

Carbon Dioxide

Biogas is primarily two gases: methane and carbon dioxide. Some people say that the carbon dioxide should be removed from the biogas and some people say that removing the carbon dioxide would cost more than the costs involved in not removing it.

There are costs in not removing the carbon dioxide because carbon dioxide, which is 30 to 40 percent biogas, does not burn. Carbon dioxide takes up expensive gas storage space and dilutes the heat value of biogas in the same way water dilutes the food value of soup.

What if there were uses for the carbon dioxide, just as there are uses for the methane? That would mean there would be a third biogas system product, an additional way to make biogas systems profitable to the system owners and useful to the community. Two possible uses for carbon dioxide are in greenhouse operations and in the manufacture of dry ice. First, an outline on removing carbon dioxide from biogas and then a few notes on the uses of carbon dioxide.

The heat available from a unit volume of biogas can be increased if the carbon dioxide is removed. In a sample of biogas which is 55 percent methane and 45 percent carbon dioxide, completely removing the carbon dioxide will increase the heat available from a unit volume of that biogas by a factor of 1.8, more than one and a half times the original heat value.

Where biogas is produced for several days but only used intensively at the end of that period or where pressurized bottled gas storage is used, scrubbing the carbon dioxide out will significantly reduce the cost of gas storage. A simple carbon dioxide scrubber consists of a tank with stirrers to mix lime or ash in water (see Diagram 17).

The scrubber is placed on the gas pipe line between the digester and the gas storage tank. When the biogas from the digester bubbles through the lime water, the carbon dioxide is absorbed by the lime water mixture. The lime residue is removed from time to time from the bottom of the scrubber and can be used as potash fertilizer.

For many uses and most biogas systems, carbon dioxide scrubbing is usually not necessary. A simple method that will get rid of some of the carbon dioxide is to add a few cups of lime to the water in the gas storage tank. This will absorb some, but not all, of the carbon dioxide. Every once in a while, by siphoning, a gate valve, or some other method it will be necessary to replace the used lime water with fresh lime water. This must be done without losing the water seal which traps the biogas in the gas tank or when the gas tank is not in use.



-NOTE: Place scrubber between digester and gas storage tank. The handle could be turned by mechanical power generated by a biogas fueled engine.

-From: Methane Digesters

L. John Fry had two 80 cubic meter digesters, used much of his biogas to fuel engines, and as far as I can tell, never scrubbed out either the carbon dioxide or the hydrogen sulfide. For those interested in removing the hydrogen sulfide (less than one percent of most biogas), there is in the book, The Compleat Biogas Handbook for Farm and Home, instructions on how to make and use a hydrogen sulfide scrubber. VITA also has design information for carbon dioxide and hydrogen sulfide scrubbers.

The Indian Agricultural Research Institute has reported the following method for removing carbon dioxide from biogas (as reported in Nepal's Biogas Newsletter). Biogas is bubbled through a solution of ten percent monoethanolamine (MEA) in water. The carbon dioxide content of the biogas is reduced to 0.5 to 1.0 percent by volume from an initial content of about 40 percent by single bubbling through a plain column 6.0 cm (2½ inches) in height. The scrubbing column is made of an inexpensive plastic bubbler (5.0 cm in diameter and 15 cm in height), having only one opening. Maximum removal of carbon dioxide is observed when the bubbling coming from the opening comes out slowly enough so that each bubble is separate from the others and the stream of bubbles is never in danger of merging into a continuous "tube."

The ideal rate of flow is 100 milliliters per minute at the regulator. This rate yields 60 milliliters of pure methane per minute. A decrease in this rate of flow does not result in any increase in scrubbing effect. Initial pressure of the biogas introduced into the bubbler is 10.0 cm (4.0 inches) of water column, and the drop in pressure head is about 5.0 cm (2.0 inches) of water.

Both caustic potash and MEA are equally effective. But MEA solution can be completely stripped of the carbon dioxide by boiling it for five minutes, after which it can be used again. MEA is also less caustic (able to cause chemical burns) than other solutions that can be used. It will not cause any damage to the skin in case of accidental contact.

Once the carbon dioxide is separated from the biogas, it can be used. The two uses outlined below are adding the carbon dioxide to the air in greenhouses and freezing the carbon dioxide to make dry ice.


Greenhouses have glass or plastic walls and roof and are used for growing plants. They trap the solar energy of the sun that green plants need to make their food. Greenhouses also protect plants from cold weather, wind, heavy rain, dry weather, and insects.

Solar greenhouses are greenhouses designed to be heated primarily by the sun, even during cold weather. Traditional greenhouses require large amounts of fuel for heating. Solar greenhouses can also be designed to heat attached houses, acting as solar collectors.

Solar greenhouses require double glazing (two layers of glass) and/or night insulation, a storage mass, the proper glazing tilt, and an insulated upper roof and northern wall. For a solar greenhouse attached to a house, the northern wall of the greenhouse is replaced by the house so that it can receive heat from the greenhouse during the day.

Solar greenhouses also work well in hot climates. They can be designed for cooling by venting hot air up a chimney and drawing cool air in the bottom. In all climates, hot and cold, greenhouses save water when compared with crops grown outside. Some reports have shown water usage for greenhouse crops to be 1/10 to 1/30 of field crops.

The Philippines is the first tropical country in which greenhouse food production has been done on a commercial basis. The main reason for using greenhouses was to protect the crops from the force of heavy seasonal rains. The greenhouses also provided other benefits of water control, such as reduced leaching of fertilizer, less washing-off of insecticides, fewer cases of crop diseases, and easier weed control.

When plants are grown in greenhouses, the crop yields can be increased by increasing the percentage of carbon dioxide in the air (normally only 0.04 percent) during the daylight hours. Plants absorb carbon dioxide through their leaves and use it to manufacture plant food.

Greenhouse experiments with carbon dioxide were done at Michigan State University in the United States during the winter of 1961-62. Plastic tubing was used to distribute the carbon dioxide through the greenhouse.

In the first year increases of 30 percent in lettuce crop yield were obtained, and in the second year increases of 70 percent were obtained--in part because very cold weather made it necessary to close the ventilators more than usual. Increases in tomato yields ranged from 25 to 70 percent, depending on variety. The percentage of the air which is carbon dioxide gas is the main factor controlling plant growth and development under all levels of light intensity except the very lowest.

In countries with cold weather, a greenhouse could be built as part of a biogas system. The greenhouse could even be built around and over the digester in order to reduce the amount of heat needed to maintain the digester temperature at 35 degrees centigrade. The excess engine heat could be used to help heat the greenhouse as well as the digester. In such a system it may be economical to increase the carbon dioxide content of the greenhouse air by separating the carbon dioxide from the methane in biogas. The carbon dioxide gas would then be used as a plant growth factor to increase yields in greenhouse crops.

There are two ways to separate carbon dioxide from biogas so that it can be used by greenhouse plants. The carbon dioxide could be chemically removed from the biogas (the MEA method described in Diagram 19) or it could be released by burning the biogas.

One way to do that would be to heat a water boiler inside the greenhouse. The boiler would produce steam which would power a Rankine-cycle engine (more on that later), and the carbon dioxide (and some heat) would be released into the greenhouse air where the plants could use it.

Detailed information on solar greenhouse design and construction methods are available from the same sources of information that were used to write this introduction to the subject. They were: Chris Ahrens' Greenhouse from VITA; The Appropriate Technology Sourcebook, Volumes I and II; and Energy for Rural Development, the original book and the supplement. VITA might also be able to find more information on the process of carbon dioxide enrichment of greenhouse air.



Dry Ice

Leo Pyle, Chairman of the Methane and Fertilizer Working Group of the Intermediate Technology Development Group, makes a very interesting statement in the book, Biogas Technology in the Third World: A Multidisciplinary Review. He said, "The possible reuse of carbon dioxide merits serious consideration. Carbon dioxide can be regenerated easily from lime water and could be used as dry ice for local health services, refrigerators, etc..."

It is true that carbon dioxide can be easily separated (scrubbed) from the methane of biogas and then separated from the liquid that was used to do the separating. But scrubbing costs money and time; it should only be done if the increased value of the methane and the value of the separated carbon dioxide is greater than the costs involved.

One way to get a value out of carbon dioxide is to freeze it; make it into what is called dry ice.

· Dry ice has a 70 percent greater cooling value than the same amount of water ice.

· When water ice melts, it becomes a puddle of water; when dry ice melts, it becomes a part of the air.

· Dry ice is much more useful than water ice for shipping and storing foods and medicines that would rot or otherwise go bad if not kept cold.

· When compared with water ice, the process of making dry ice is much more expensive and requires much more complicated machines and methods.

At normal, everyday air pressure, there is a 1.512 kilograms of carbon dioxide in a cubic meter of carbon dioxide. In other words, a cubic meter of carbon dioxide gas weighs 1.512 kilograms.

· If that carbon dioxide is in a biogas system's gas storage tank at 7.5 column inches of water pressure, there will be 1.789 kilograms of carbon dioxide in a cubic meter of carbon dioxide.

· In an ordinary biogas system that produces 100 cubic meters of biogas per day (60 percent methane, 40 percent carbon dioxide), there will be 40 cubic meters of carbon dioxide produced per day.

· Because the carbon dioxide scrubbing systems and the dry ice manufacturing systems will not be 100 percent efficient, estimate that only approximately one kilogram of carbon dioxide in a cubic meter of biogas will end up as dry ice.

· So a biogas system that produces 100 cubic meters of biogas will be able to produce 30 to 50 kilograms of dry ice per day.

This is a very approximate estimate because very little work has been done or information been made available on the production of dry ice from small-business biogas systems.



Diagram 20 shows how dry ice is made. A combination of very cold temperatures and very high pressures turns the carbon dioxide gas into a liquid; then a "snow," and then into a solid (dry ice). If a dry ice business already exists, it might want to start a biogas system and use the methane for fuel, sell the sludge for fertilizer, and use the carbon dioxide to reduce the amount of carbon dioxide it has to buy--but only if the biogas system would reduce, not increase, costs.

A biogas business thinking of starting a dry ice operation would have to make sure that there would be a market for the dry ice at a price that would cover costs and allow for a profit. A biogas business thinking about going into the dry ice business should visit dry ice businesses in their own country to find out such things as local dry ice business expenses, profits, markets, potentials, and problems. Then if dry ice still looks like a profitable business, VITA and/or a Peace Corps Volunteer might be able to help find ways to reduce the costs and risks of operating a biogas system that included a dry ice business.

Solar Water Heaters

How does a solar water heater work? Any surface that is exposed to the sun will get hot as it absorbs radiant solar heat. When the surface becomes hotter than its surroundings, it will begin to lose heat in three ways:

1) By conduction--through any object with which it is in contact;

2) By convection--through contact with air or water;

3) By re-radiation to the sky or to other nearby cooler objects.

By minimizing heat losses and diverting the heat to a storage tank, a "solar collector" has been created.



-from; Technology for Solar Energy Utilization

Several conditions must be met before a useful amount of heat can be collected. The collector must be facing toward the sun and be tilted to the sun's rays in order to collect the maximum amount of solar radiation. The conduction of heat from the absorber surface to a storage tank must be fast and efficient, and the heat loss from the absorber surface must be kept at a minimum.

VITA has a very good small book on how to build and operate solar hot water heaters for less than US$ 100. The hot water can be used to heat biogas digesters or preheat the slurry when an engine is not part of the biogas system. A solar hot water heater would also be useful for a biogas system where the engine was not run all the time. The hot water could fuel Rankine-cycle engines which could be used to produce mechanical and/or electrical power. And last but not least the hot water could be used for bathing, cleaning, and heating purposes.

There are many books and magazines on the different kinds and prices of solar systems (the Appropriate Technology Sourcebook, Volumes I and II, have very complete lists). The following introduction to the subject is from the VITA book, Solar Water Heater. The design is simple, inexpensive, can be made just about anywhere, and can be easily used in biogas systems.

"Heat from the sun's rays is easily captured. Black-painted surfaces exposed to the sun will get hotter than those of any other color. A metal surface painted flat black and placed in contact with water will heat the water. The black metal plate is called an absorber.

"Once the water is heated, it is kept hot with insulation. The heated water behind the absorber can be insulated with a variety of substances such as fiberglass, straw, sawdust, hair, or polyurethane foam. In some designs a sheet (or two) of glass is placed between the absorber plate and the sun (Diagram 21). Glass transmits the high radiation from the sun that heats the water but stops the low-energy infrared radiation that is reradiated from the absorber. It also keeps air from passing over the absorber and causing heat loss. The reduction of the two forms of heat loss makes glass an ideal insulator. Clear plastics can be used but their life expectancy is limited.

"(The VITA) system will heat 70 liters (18.5 gallons) of water to 60 degrees centigrade (140° F) between sunrise and noon on a clear day with an average outside temperature of 32 degrees centigrade (90° F). (If one or two glass plates are placed between the absorber and the sun, as in Diagram 21, more water can be heated to higher temperatures in the same or less amount of time.) Obviously, water does not have to be this hot for many purposes: very hot water can be mixed with cool water to provide water warm enough for bathing and washing clothes and dishes. This factor should be taken into account when estimating the amount of water needed each day.

"There are two main parts to the solar water heater (Diagram 22): (1) a heat-absorbing collector that is rather like an envelope made of (two) metal sheets; and (2) a storage tank that holds the water for the system. The collector can be made either of flat or corrugated galvanized metal sheets that have been painted with several coats of black rustproof paint. (If the cost is not too high, the sheets can be made of copper, which will not rust. Copper will also capture more heat than any other metal.)

"It is important to remember that the storage tank must be located 46 cm (18 inches) or higher above the collector to enable the thermosyphon to work (Diagram 23).

"The Thermosyphon Principle: The tank, filled with water, is connected to the collector. The collector is positioned below the bottom of the tank. Water runs through a hose at the bottom of the tank to the collector. Hotter water flows toward the top of the collector. Hot water is forced out of the hose at the top of the tank and cooler water flows to the collector. The flow established continues until the water is no longer being heated by the sun (Diagram 23). For example, at night the flow becomes stable and the hot water remains until it is used or it cools.





"(It would cost more money, but if the hot and cold water pipes were connected inside the water tank by a long coil of copper pipe, a heat exchanger, the water would get heated without it being necessary to keep the water level in the tank above the hot water connection in order to maintain the thermosyphon circulation. A closed circulation heat exchanger design would have to be used if a solar water heater is used to heat biogas slurry and digesters.)

"Site conditions are important. Collectors should face directly south. Turning a collector southeast or southwest can affect its performance by 20 percent or more. If hot water is needed by noon, face the collector to the southeast; if hot water is more important in late afternoon, face the collector to the southwest. (Building a collector platform that can be easily turned is a simple but effective way to follow the sun and increase hot water production.)

"The site should be free from shade. Collectors should be placed so that they can be tilted from the horizon to an angle equal to the latitude of the location. In more temperate climates (cold winters and hot summers), the angle should equal the latitude plus ten degrees..."

Rankine-Cycle Engines

Rankine-cycle engines are simple, low-cost steam engines that can be powered by solar collector heated water. Rankine engines involve the evaporation and condensation of a fluid under differing pressure and temperature conditions.

When the fluid is evaporated at high temperature and pressure and condensed at low temperature and pressure, useful work can be extracted from the vapor during its expansion from the higher pressure to the lower pressure. Only a small part of this work need be re-invested in pumping the liquid back to the high-pressure state because of the small liquid volume. This cycle of pumping/evaporation/expansion/condensation is the basis of all steam engines, whether they employ pistons or turbines to extract work from vapor.

The Rankine engines described here get their energy from heat exchangers that are heated by hot water coming from solar collectors. If biogas is used to heat the heat exchanger, in addition to the heat coming from the solar collectors, the engine can run day and night, in all kinds of weather. If the biogas digester and the Rankine engine, or at least the heat exchanger, are in a greenhouse, the carbon dioxide that is released when the biogas is burned to heat the heat exchanger will increase the percentage of carbon dioxide in the greenhouse air. The extra carbon dioxide will act like a fertilizer and increase the crop yield of the greenhouse plants.

A note of caution. Just as in the case of a building with a biogas powered stationary engine in it, the roof of the greenhouse must be vented in case something goes wrong and unburned gas escapes.

When the engine part of a biogas system is a Rankine engine, the preheating of slurry and the heating of the digester might be more efficiently achieved than it is with gasoline or diesel engines.

No books or magazine articles were found that describe combining solar collector/Rankine-cycle engines with biogas systems. What follows are quotes from a book and an article on Rankine engines. Anyone interested in Rankine engines, powered either by solar collectors, biogas digesters, or both, should write to the authors or to VITA for more information.

This first report is from the book, Energy for Rural Development. "The traditional example of a Rankine engine is the familiar steam engine....This discussion, however, will be restricted to engines that are more useful on a smaller scale and that operate at temperatures in the range that can be achieved with solar collectors. For these lower temperatures, Rankine-cycle engines have been designed that use organic materials (such as freon) instead of water as the working fluid. The analysis that follows illustrates the type of system that would be associated with a solar-powered Rankine-engine-operated electric power plant (see Diagram 24). It is based on current manufacturing capabilities for the individual components.



"The engine consists of four major components: expander, boiler, feedpump, and condenser. During operation, a heat-transfer fluid (typically pressurized water) flows through the collector array and is heated to a temperature in the 200-400° F (100-200° C) range depending on solar-collector configuration (design), solar flux (changes), and engine operating conditions. This would entail a system capable of operating at about 235 psi (16.5 kg/sq cm gauge).

"The hot fluid (water) is then used to vaporize the working fluid (freon) of the engine in a heat exchanger; a number of common refrigeration fluids are appropriate for use in the engine loop. The hot, high-pressure, working fluid is then used to drive the expander of the Rankine-cycle engine. For higher-power-output applications (greater than 100 kw), the expander will be a turbine. Lower-power systems can use positive displacement configurations (designs) such as reciprocating (piston) or vane-type expanders. After leaving the expander, the working-fluid vapor is condensed and the liquid is then pumped back into the solar collector heat exchanger, completing the cycle.

"Engine performance calculated on the basis of an expander efficiency of 80 percent and a feedpump efficiency of 80 percent indicates that it is possible to achieve about 65 percent of the ideal Carnot efficiency when the engine is operating in its appropriate temperature range.

''Both focusing and non-focusing collectors can be used as the heat source for organic Rankine-cycle engines. Flat-plate (non-focusing) collectors are used here to illustrate the effect of thermal collector characteristics on overall system performance for the following reasons: Flat plate collectors have the advantage of being capable of effective operation mounted in a fixed position. The collectors and their mounting structure are, therefore, structurally simple--an advantage for use in remote areas. The technology of flat-plate collectors is advancing...(and) organic Rankine-cycle engines can effectively use the low to moderate temperatures achievable with flat-plate collectors."

The second report on Rankine engines is from an article entitled, "Conversion of Solar into Mechanical or Electrical Energy: Indian Experience." The development and production of solar pumps in the range 1.5 - 4.0 kw (two to five hp.) have received highest priority (in India). The pumps will preferably be in a modular form so that when pumping is not required, the same system can be used to produce equivalent mechanical or electrical energy for minor industrial operations or lighting.

"(Diagram 25) shows the Abhimanyu solar water pump developed by the National Physical Laboratory (NPL, New Delhi, India). Its primary components are a flat-plate collector array and a closed-cycle Rankine-cycle engine. During operation, a heat-transfer fluid (water) flows through the collector array and is heated to a temperature of 80 to 90 degrees centigrade, depending upon the collector efficiency and configuration and the solar flux. This hot water is used to vaporize an organic fluid with a low boiling point (such as freon) in a reverse-flow heat-exchanger (a boiler). The hot, high-pressure organic vapor is then used to drive the expander of the Rankine-cycle engine. After leaving the expander, the vapor is condensed in a condenser, where the water being pumped is used as the heat sink. The condensed organic liquid is pressurized and then pumped back into the boiler with the help of a reinjection feed pump and driven by the shaft of the expander. For some organic fluids, there may also be a regenerator that utilizes exhaust vapor superheat to preheat the fluid coming into the boiler.


Abhimanyu solar water pump

"The organic Rankine-cycle engine is particularly suitable for solar pumping, power generation, and for waste-heat utilization for several reasons:

(a) High thermal efficiency (60-70 percent) even when operating with the low to moderate temperatures (80-200 degrees centigrade) achievable with flat-plate collectors;

(b) Low-cost components owing to the use of commonly available construction materials and simple mechanical components;

(c) High reliability because of its sealed construction, which protects it from harmful effects of environment such as sand, dust, and moisture;

(d) No problem with freezing, since the working fluids have very low freezing points;

(e) Finally, they are adaptable for use over a wide power range from one kw pumping systems to multimegawatt power stations.

"(While) the engine efficiency increases as the collector temperature increases, the collector efficiency decreases on increasing the collector temperature. It is therefore necessary to determine the temperature range for each collector-engine combination that produces the maximum system efficiency. This optimum system efficiency depends upon several factors such as the insulation, the condenser temperature, and the characteristics of the solar collector array, expander, reinjection feed pump, and working fluid. Nevertheless, it is obvious that the higher the efficiency of the collector array and the higher the temperature it can produce, the higher the system efficiency. It is all the more necessary to optimize the collector assembly in a large-scale power plant because a substantial fraction of the total cost is accounted for by the solar collectors.

"Flat-plate collectors were used in the Abhimanyu pump. The absorber plate is made of aluminium alloy with channels built into it by the pressure-bonding technique (Diagram 21). The complete mechanical design of the collector was optimized.

"Since it is advisable to operate the expander at a constant input temperature, a reservoir is needed to store hot water; the reservoir is connected to the array by means of a thermostatically controlled bypass valve. Under operating conditions nearly one-half of the heat is required to preheat the organic liquid and the other half is used during the process of boiling. Some saving on the collector area requirement can be effected by having one collector array for low temperatures and one for high temperatures. The former is used to provide preheating whereas the latter serves to boil and super-heat the organic vapor."

A few additional points:

(a) If the solar collector is built on a base that can be turned during the day so that the collector always faces the sun, more water will be heated to higher temperatures. The collector could rotate on wheels set along the outer circumference of the base.

(b) Mechanical power uses such as a water pump, thresher, small lathe, or a one kw generator for electrical power can be obtained from one ten-square meter solar collector. A 400-square meter collector array can provide enough power to generate ten kw of electrical power.

(c) The response of turbines to variable loads is poor. Screw and spiral expanders work better with Rankine engines that get their power from flat-plate solar collectors.

(d) The costs of solar collector/Rankine engine combinations compare favorably with diesel engines, especially where the maintenance and repair of diesel engines is a problem.

What follows is a list of people, organizations, and businesses that are working in the field of solar collectors and Rankine-cycle engines. The sources quoted above are listed in the Sources and Resources section of the Appendix.

1. Professor Redfield Allen, Dept. of Mechanical Engineering, College of Engineering, University of Maryland, College Park, Maryland 20742, U.S.A. He has done research on organic Rankine-cycle engines.

2. All-India Solar Energy Working Group, c/o Dr. K. S. Rao, Central Salt and Marine Chemical Research Institute; Bhavnagar, Gujarat, India. Publishes proceedings devoted to the promotion, development, and spread of knowledge in several areas, including solar energy and biogas systems.

3. Rolf T. Skrinde, Olympic Associates Company, 1214 John Street, Seattle, Washington 98109, U.S.A.

4. Jerry P. Davis, Thermo Electron Corporation, P.O. Box 459, 101 First Ave., Waltham, Massachusetts 02254, U.S.A. Organic Rankine-cycle engines from low-temperature heat sources.

5. Ormat Turbines Ltd., New Industrial Area, Szydlowski Road, P.O. Box 68, Yavne, Israel. The company was formed in 1965 to develop, manufacture, and market Rankine-cycle turbine-driven systems for the generation of electrical power. Examples of products include solar-powered turbogenerators and water pumps.

Plant Shredder

If a biogas digester is going to produce biogas instead of a scum layer when it tries to digest plants, the plants have to be properly prepared; and that can mean time and money. How can plants be used without the costs being higher than the profits?

The simplest way is to feed the plants to animals and then feed the animals' manure to the digester. The animals best suited to do this are the ones with double stomachs, the cud chewers (ruminants): cattle, cows, sheep, goats, water buffalo (carabao), and horses. Cud-chewers have digestive systems especially designed for breaking down plants.

If there are not enough cud-chewers or other animals available, the plants have to be partly composted before they can go into the digester. For the partial composting to be effective, as much plant surface area as possible must be exposed; that is where shredding, grinding, and pulping of plants becomes important.

Still the question remains; how can plants (and if possible the plant fibers in manure) be shredded, ground, or pulped in a way that does not cost more than it is worth? The method or methods must be low-cost in time and money, and they must be easy to build and operate. One good choice is the use of simple machines powered by bicycles (pedal power). Another possible source of power is mechanical power from the biogas system's stationary engine.

Both VITA and the Appropriate Technology Sourcebook, Volumes I and II, have lots of information on using bicycles to power simple machines such as lathes, threshers, washing machines, saws, pumps, and generators. But no designs were found that could easily be used to prepare plant leaves and stalks for biogas digesters. It can be done; it may have already been done, but reports on the work have not yet found their way into the mainstream of appropriate technology literature.

People interested in finding low-cost, easy ways to prepare plants for biogas digesters should get a good pedal-power manual and adapt the information to preparing plants for digesters. After the plants are finely ground, they will still need several days of partial composting in order to begin the rotting process that breaks down the walls of plant cells. Then when the plants are in a digester, they will, like manure, produce lots of biogas and very little scum,

Solids Separator

It would be very useful if large plant fibers could be easily separated from the liquids before slurry went into digesters. Solids in slurry made from plants are not the only problem. There is also a lot of undigested plant matter in most manure. If separated, the solids could be ground up so that they could not form a scum layer in the digester. The biogas bacteria could digest a higher percentage of the organic matter with an end result of higher biogas production rates.

It would also be very useful is the solids could be easily separated from the liquids in the sludge taken out of the digester. It would then be easier to dry (and bag if so desired) the solid sludge for use as fertilizer and to use the liquid sludge as fertilizer for fish ponds and crops or as the liquid portion of new slurry.

Both these things can be done now, and this book has many suggestions on how to do them. But none of the methods for separating solids from liquids is at the same time: low-cost, efficient, easy to build, simple to operate, and not very time consuming.

The principle of the centrifuge (Diagram 26) offers the possibility of an ideal way to separate the solids from the liquids in slurry and sludge. A centrifuge is a machine that spins fluids at high speeds to separate matter of different densities. Powered by a bicycle or the mechanical power of a biogas-fueled stationary engine, a simple centrifuge could quickly separate solids from liquids.

The big question is what does a simple, practical centrifuge design look like? Then there are the other important questions: How much would it cost? Would it need a pump? What is the smallest size digester that a centrifuge would be practical to use with? And so on.

VITA has a US$ 1.00, seven page booklet entitled, Centrifugal Honey Extractor. It might have enough information about centrifuges to make designing one for separating the solids from the liquids in biogas slurry and sludge possible. The VITA description of the booklet says, "Made of wood, this is an easy-to-build, simple-to-operate method of extracting honey from a comb."




Partial composting (seven to ten days) can be used to prepare ground-up plants for biogas digesters. Complete composting (three to six weeks) can be used to make a solid organic fertilizer. With composting there is no methane gas, and compost does not have as much nitrogen as biogas sludge made from the same organic waste would. With compost the fertilizer is a solid, while 90 percent of the fertilizer from biogas digesters is a liquid. What follows is a method of composting adapted from the Filipino handbook, The Samka Guide to Homesite Farming.

When organic waste is composted, it becomes a biofertilizer that was made by organic waste rotting in air. When organic waste is digested in a biogas digester, it becomes a biofertilizer that was made by rotting where there was no air. Both types of fertilizer are very useful. The method that is chosen depends on local resources and needs.

The old method of making compost fertilizer is to pile plant wastes on the ground until the pile is 30 centimeters (1.0 foot) high, then to add animal manure. Ashes and lime are sprinkled on top of the manure. Another layer of plant waste is added, then more manure and lime, and in this way alternating layers are built up until the pile is about 1.5 meters (5.0 feet) high, wide, and long.

The compost pile is kept moist, but not wet or flooded with water. Most farmers cover their compost with a grass or banana leaf roof in the rainy season so it will not get too wet. In about three months the compost pile shrinks to about one tenth of its original size and has become a rich organic fertilizer which can be spaded into garden planting beds or plowed into field.

Now there is a better way to change organic wastes into rich organic fertilizer. Instead of waiting three months, compost fertilizer can be made in three weeks. And it will be a better fertilizer! The secrets of this quick process are as follows:

The first secret is to use plenty of organic waste that is high in nitrogen to add to the compost mixture, such as urine, fish and meat scraps, algae, and manure (see Facts and Figures section of Appendix). Most plant and crop wastes are high in fiber which means they are also high in carbon and will need lots of nitrogen to help them rot. Scientists call this the carbon/nitrogen ratio. It has been found that the best compost mixture is about 30 parts carbon to every one part nitrogen (C/N 30).

When a good mixture of organic matter has been collected, chop the plants into small pieces, 3.0 to 5.0 cm (1.0 to 2.0 inches) long, to speed up the rotting (decay) process. The rotting process is faster when there is more plant surface area exposed to moisture and air. The more pieces a plant is cut into, the greater the exposed surface area will be. If the plants can be ground, shredded, or pulped, the quality of the compost will be even higher. On a small farm there is no way to be sure of getting the right mix of carbon and nitrogen except by trying different combinations and observing the results. It is better to have too much nitrogen than too little.



-From the Samaka Guide to Homesite Farming


-Build your compost pile with alternating layers of fresh materials, dry matter, and manure.

All plant waste should be composted for four to seven days before being put in a biogas digester. If the plants are to be completely rotted, they must be composted for three to five weeks. The shorter times are for hot weather, and the longer times are for cold weather.

If there is not enough plant or animal waste with high nitrogen content, add chemical fertilizers containing nitrogen to the compost. This has three advantages.

· It will help the compost materials change quickly to organic fertilizer.

· It will make the chemical fertilizer into a better fertilizer which will be easier for plants to take up and digest.

· It will help prevent the nitrogen in the chemical fertilizer from evaporating or being washed away by the rain.

The second secret is to turn the compost pile every few days.

· If the right compost mixture is used, the pile will get very hot in the center.

· If it stays too hot too long, it will begin to smell bad and the pile will become dry.

· The solution is to push a bamboo stick into the center of the pile and pull it out after three minutes.

· If the stick is very hot, turn the pile. e If the stick smells bad, turn the pile.

· If the stick is dry, turn the pile.

The third secret is to keep the compost pile moist, but not wet, something like a wet rag out of which the water has been squeezed.

· The hot temperature in the compost pile causes much of the water to evaporate as steam.

· So every time the pile is turned, sprinkle water or sludge from a biogas digester on the compost to keep it moist. e If the pile is turned often and kept moist, it will smell sweet.

· If it smells bad, it is because the pile was not turned often enough. The bad smell is the smell of valuable nitrogen fertilizer escaping into the air.

To turn a compost pile, first gather the material from the outside of the old pile and place it in the center of the new pile. Being dry, it will need more water. Then take the material from the inside of the old pile and place it on the top and sides of the new pile. In this way, every time the pile is turned, the mixture alternates its position from inside to outside and from outside to inside.

A successful compost pile needs three things: a good mixture of organic materials broken up into very small pieces with enough high nitrogen content waste, the right amount of water or biogas sludge, and plenty of air.

If farm animals are kept in pens, the floors of the pens should be made of concrete so that channels and holes can be made to guide the manure and urine to holding areas where the wastes can be easily added to compost piles. The holding areas can be filled with shredded straw and leaves to absorb the urine and begin the rotting process.

For thousands of years people have used animal manure, ash, and plant wastes to restore to the soil the elements that plants need to grow strong and healthy. Plants take certain substances from the soil in order to grow. These substances are food for the plants. They are just as necessary to plants as rice, corn, meat, fish, and vegetables are to people.

When a family's rice sack is empty, the rice must be replaced or else they will starve. The same is true for plants; the farmer must return to the soil the substances that are taken from it when a crop is harvested or the next crop will starve. About 100 years ago scientists began to study what was in the soil that gave food to the plants through their roots. Today three elements are believed essential to all plant food. These three elements are: nitrogen (N), phosphorus (P), and potassium (K). There are secondary elements in addition to these three which are needed in small amounts, but which must be present in the soil to make healthy plants. They are: calcium, magnesium, sulfur, boron, iron, manganese, copper, zinc, molybdenum, and cobalt.

In fertile soil all of these elements are present in the correct amounts so crops grow well and harvests are big. Poor soil does not have enough of some of these plant foods so the plants are not healthy and harvests are small. Plant food can be made in factories and put into the soil to supply what is lacking or to replace what has been taken from the soil by harvested crops. Today, farmers all over the world are using manufactured chemical fertilizers to increase the yields of their crops. It has been found that fertilizers can increase the value of crops by as much as ten times the cost of the fertilizer. It is often said that "fertilizers pay; they do not cost."

The kind of fertilizer that should be used depends on two things: what kind of crop is to be planted and what the soil lacks that is needed in order to make that crop grow well.

When there is not enough nitrogen in the soil:

1) The plants have light green or yellowish leaves.

2) The plants do not grow big and they grow slowly.

3) The plants have stalks which are thin and stiff.

4) The plants have leaves which form a small angle with the stem.

When there is not enough phosphorus in the soil:

1) The plants are thin and short.

2) Beans have dark green or bluish color, like the leaves of plants during a long dry season.

3) The stems and leaves of corn and some vegetables become purple in color.

4) Grains are thin and light in weight.

When there is not enough potassium in the soil:

1) The leaves die along the edges. In bean leaves, gray spotted half circles form on the edge of the leaves.

2) The stems are weak and brittle.

3) Growth is slow.

4) Grains are thin and light in weight.

Leafy vegetables such as lettuce, cabbage, and mustard need more nitrogen the phosphorus or potassium. A complete fertilizer containing the three major elements should be used before planting. The fertilizer should be mixed very well with the soil. After two weeks if the leaves of the plants are not dark green in color, add fertilizer containing only nitrogen to the soil.

Plants which have fruits and seeds such as corn, mango, tomato, peanuts, string beans, potatoes, cucumbers, papayas, bananas, pineapples, peppers, and squash need as much phosphorus and potassium as they need nitrogen.

The appearance of the plants will tell when there is a serious lack of a fertilizer element. When in doubt as to what fertilizer element is lacking, the best thing to do is to use a complete fertilizer.

It is not only fertilizer which plants need. Plant food must be "seasoned." It must be prepared correctly, and it must be mixed with water. When people put salt and sugar on food, it makes the juices of the mouth and stomach come out in large quantities. Without these juices, the food we eat will not nourish us. If soil is too sour (too acid), plant food cannot be completely digested by the plants. In such cases, add seasoning by mixing fine powdered lime into the soil at least two weeks before planting. This will help the plants in absorbing and digesting the fertilizer. Finally, water is needed to dissolve the fertilizer and lime, so that the roots of the plants can take up the nutritional elements.

Always remember that chemical fertilizers work best when used only as a supplement to organic fertilizers. The more organic material that is mixed with chemical fertilizers, the better it will be for the plants and for the continued fertility of the soil. A large percentage of expensive chemical fertilizers can be washed out of fields by rain and evaporated into the air. If chemical fertilizers are mixed with organic fertilizers, the soil will be able to hold more of the nutrients from being washed away or evaporated.

Organic fertilizers, which add humus (decayed plant matter) to the soil, are necessary to profitable farming and to successful home gardening. Chemical fertilizers can be a valuable supplement if there is not enough organic fertilizer. When chemical fertilizers are used: mix them with compost first and make it into an enriched organic fertilizer--this will benefit the crops, the soil, and it will reduce costs to the farmer.

What follows is another way to profit from plant wastes. It will not directly help biogas systems, but it should be of help to farmers who have plant wastes. This method is especially good for crop wastes, such as straw from grain crops.

Crop waste can be:

1) Composted for a week until the fibers are broken down enough to be used in a biogas digester.

2) Composted for a month and used as compost fertilizer.

3) Composted by the following method which is adapted from an article by Art Bell in the magazine, "Peace Corps/Philippines" of December, 1979.

In order to clear fields, crop wastes are often burned. Every time plants are burned, nitrogen and other fertilizer compounds are wasted that the crops need.

Nitrogen is energy--plant energy; to burn it is to lose that energy. It is like burning fertilizer instead of spreading it on the fields. Using all of a field's crop wastes as fertilizer for future crops, returns approximately 25 percent of the nitrogen, 40 percent of the phosphorus, and 75 percent of the potassium, along with the humus which holds the soil together, and stops erosion.

In order to understand how to make use of plant energy, the situation of the rice farmer can be used as an example. The rice is harvested, threshed, the grain is sacked, and now the fields are covered with six-inch strands of rice stubble, and there are large piles of rice straw. The rice straw is often burned, a process that burns up thousands of pesos of plant nutrients.

If this stubble and straw is plowed under, what would happen? How could this waste be made to rot, to decompose fast enough and completely enough so that another crop could be plants in two to three weeks? In nature there is what is called a carbon/nitrogen ratio which must be kept in balance for normal plant growth to occur. This is just a scientific way of saying that energy for breaking down plant and animal matter, which is usually carbon, comes from nitrogen. This decay process is carried out by bacteria working on the carbon materials, and the size of the bacteria populations increase in proportion to the amount of nitrogen that is present in their surroundings.

Plowing under a large amount of plant waste, such as rice stubble and straw, calls upon the soil to supply a large amount of nitrogen for the bacterial decay of all this plant material. This process is the first priority. Plants growing in the soil will be starved for nitrogen as long as there are high levels of carbon plant matter left to be decayed by the bacteria.

To offset this nitrogen starvation and speed up the decomposition process, a nitrogen fertilizer such as sulphate of ammonia or urea is added. What this means is that the farmer can subtract this from the basic fertilizer application for the next crop. In practice, many farmers still use their normal fertilizers, and the added nitrogen plus the increased organic matter produces crops with much higher yields.

Normally, broadcasting (scattering) 30 kilograms of nitrogen per hectare (three bags sulphate of ammonia or one and a half bags of urea per hectare), followed by plowing under the crop wastes; and if the soil is not very moist, a light irrigation is all that is needed to break down the plants.

It should be pointed out that this nitrogen cannot be lost by leaching from the soil because it is captured and held within the bacterial cells that are decaying the plants. When the last of the plant matter is broken down into basic organic plant nutrients, the bacteria die and the nitrogen inside their cell bodies becomes available for new plants as nutritional nitrogen and other plant nutrients.

The time required for this decomposition process is only two to three weeks. It should be noted that this process is the same for all crops, whether they are cereal grains such as rice and wheat, or crops such as corn, cotton, or vegetables. For very woody-stemmed crops like cotton, pull the plants up and chop or grind them into six inch or smaller pieces. Spray the chopped-up plants with a water solution of sulphate of ammonia or urea, which will supply the 30 kilograms of nitrogen per hectare. Then immediately plow the plants under and if the soil is not very moist, follow with a light irrigation. Again, all this crop waste will be decomposed and part of the soil within the same two to three weeks as with the lighter, less woody crop wastes.

This is a simple process which takes a little work and a little time. Who knows, maybe some enterprising person can put together a well-paying business, making fertilizer out of all the plant wastes that are being dumped or burned.


Bioinsecticides do not come out of biogas digesters, but there is a strong connection between the products of biogas digesters and bioinsecticides.

Bioinsecticides, like biogas and biofertilizers, can be made and used by the people who need them, at a low cost, using local resources. Two of the most important products that farmers use to increase crop production are fertilizers to help plants grow and insecticides to protect the plants from insects.

Bioinsecticides are one weapon in a battle plan of pest control called integrated pest management (IPM). The IPM battle plan has fewer risks and lower costs than total dependency on chemical poison pest control. Groups such as the Peace Corps, Volunteers in Asia, and VITA can provide information on integrated pest management battle strategies and tactics.

A low-cost, safe, and easy-to-use insecticide is what this section is all about, just as low-cost, high-quality fertilizers are one of the main subjects of this book. This bioinsecticide information is adapted from four articles written by Jeff Cox (October, 1976; May, 1977; April, 1978; July, 1979) and one article written by Michael Lafavore (August, 1978) for the "Organic Gardening and Farming" magazine.

From the October, 1976, Article

Frank Batey grows peanuts and soy beans in bug-ridden Archer, Florida. For years he and his father used the chemical insecticides guthion, toxaphene, sevin, methyl parathion, and other insecticides to kill the fierce semitropical insects that would otherwise have destroyed their crops. Today, Batey's fields are crawling with insects, but his crops suffer only minor, unimportant damage. So far he has saved US$ 5,000 in insecticide costs. Best of all, he says, "I do not have to breathe that peanut poison anymore."

Mike Sipe, a pest-control specialist, suggested a simple method of controlling insects to Mr. Batey.

· Go into the fields and collect a cup or two of the insects that are damaging the crops.

· Grind up the insects in a blender with some rain water, strain out the solids, and add more water.

· Put the solution in an ordinary sprayer, and spray the crops that the insects were found on.

So effective has this method been for over two years that in 1976, the third year, Mr. Batey did not even have to spray the bug juice insecticide on his fields.

"We had cabbage looper, stink bug, army worm, velvet bean caterpillar, granular cutworm, southern corn borer, and other pests on our crops," Batey says. "I collected samples of all of them, except for the southern corn borer, which makes a webbed tunnel underground and attacks the pegs of peanuts. It is Just too difficult to find enough of them to make a spray. But on the other pests, the method did a great Job. I am not against using chemicals to help grow food crops, but I hate insecticides and always have. We should not be eating food with these poisons on them. It is important to have good, clean food."

Batey told of a farmer who lives near him. "Last year he used so much insecticide that his peanuts almost died. He came over and looked at our crop. There were ants, earwigs, and beetles crawling around on the ground and on the plants, but none of the major pests. He did not understand what was happening." Batey says his neighbors do not like to experiment with new methods, and so he has not told them about the bug juice method yet. "They would think it is crazy. But I can prove it works. I know it works."

It seems most likely that the bioinsecticides kill insects by spreading diseases that can kill insects. Batey farms 74 acres (29 hectares) of his own land and he sharecrops neighbor Bill Matthew's 39 acres (16 hectares) allotment. "I told Bill that we were going to use the bug Juice to combat insects on his land, and he thought I was crazy. When I sprayed the bug juice on the crops, he thought it was very funny. But in three or four days most of the worms were hanging dead from the leaves or lay dead in the rows. Bill had the worst insect problem I had ever seen, but the bug Juice method took care of it." Batey also farms 100 acres (40 hectares) of soy beans and uses the bioinsecticide spray effectively on that crop, too.

A handful of insects should be enough to spray a large garden. But Mike Sipe has a few important warnings: "In most cases where insects are only doing a little damage, it is best to do nothing. There must be pests for pest-eating insects, lizards, frogs, and birds to eat. When there are too many pests and a crop is threatened, use the bug juice method. It should reduce the pest populations to a safe level but allow enough pests to survive to support a population of the enemies of the pests. Also, make sure that you are collecting only those pests that are threatening to destroy the crops. If you gather harmless insects or insects that eat other insects, you will be upsetting the natural insect controls."

To make sure you have found the real pests, study the plants very closely and watch for the insects that feed on leaves or fruit. If any insect is parasitized (has tiny cottony cocoons of insect-eating insects on its back), do not take it; or you will be making a spray that could kill insects that eat insects.

The results from Alachua County in Florida, a hot weather area where crops are grown all year, are encouraging. Batey's peanut production was 5,251 to 5,351 pounds per acre (6,000 kilograms per hectare) in 1974--a very large harvest that won him the local County Peanut Production Award. Minor insect pest damage had a part in making that record. The average production in Alachua County is between 2,000 and 3,000 pounds per acre (2,200 to 3,300 kilograms per hectare). Batey's chemical insecticide costs were about US$ 2,000 per year, so over the three years that he has been using the bug juice bioinsecticides, he has saved more than US$ 5,000.

From the May, 1977, Article

Safe insect control has always been a goal of farmers, but what is safe for people, is often safe for bugs; and what is dangerous for insects, is often dangerous for people. Is there then, any danger to people from the bug juice bioinsecticide method?

The following comments are from quotes by several plant scientists:

· "The method is definitely safe, but I would still use caution in applying the bug juice. I would rather see people use as little as possible to control insect problems. If all of a pest insect population is killed in an area, the insect's natural enemies might die out too..."

· "I have been working in this area for eight years, and I do not think there is any danger. The only danger I can see is that if the bug juice method does not work, the farmer may never try biological control methods again..."

· "Most people who have worked with insect pathogens (organisms that cause disease) feel that they are safe..."

· "Californians have been using a modified bug juice method for 25 years, collecting sick insects and grinding them up to make a bug juice spray...these insect pathogens are very effective, but this is the first time that using healthy insects has been recommended..."

· "He applied it to his okra (a vegetable), which was infested with five different kinds of caterpillars and army worms. It killed all the caterpillars and worms. I did not believe it at the time and told him I thought he had forgotten to clean the chemical insecticide out of the spray equipment he has used. But he insisted he had cleaned the sprayer..."

· "The use of insect disease successfully and completely controlled the bug problem in a Malaysian rubber plantation."

It seems that the use of bug juice bioinsecticide is safe, according to the scientists we talked to. One said, "If these insect diseases were transmissible, farm animals and farmers everywhere would have been made sick by them many, many years ago." U.S. Department of Agriculture studies have shown that insect viruses (diseases) of common pest insects reproduce themselves only in specific insects.

(According to World Health Organization figures, chemical pesticides poison 500,000 people annually, killing 5,000 people; and most of the victims are in the Third World. Companies regularly sell pesticides in the Third World that are banned in North America and Western Europe because the chemical poisons are too dangerous.)

The following are adapted excerpts from some of the hundreds of letters "Organic Gardening and Farming" magazine received after the October, 1976, report on the bug juice method.

"I had scale infection on my plants, so I scraped, with difficulty, about a teaspoon of scale into my blender, added water; and after blending, I diluted the mixture with more water and sprayed the solution on the plants. For three days I did not mention my experiment to anyone. When I told my husband and we went to inspect, I was very happy. A shake of a branch or a push on a pile of scale caused the scale to fall off. Scale, I have read, does not fall off. I guess the remains on the plants are as dead as the scale I shook off. The insect-eating praying mantis bug that lived on one plant was not harmed by the bug juice spray, which would have been true with a chemical poison..."

"I noticed my Chinese celery cabbage beginning to wilt. I pulled one up and found most of the roots badly eaten by some kind of white maggot (wormlike larva of a fly) and by wireworms. There were also slugs on the leaves. I collected some of the slugs, maggots, and worms, blended them with a cup of water and poured (not sprayed) a diluted mixture on the plants very slowly, so that the bug juice ran off the leaves and down around the roots. There was no more wilting, but after heavy rains a week later, the maggots started to come back. Maybe the rains reduced the effectiveness of the bug juice over time. But I believe the method worked very well."

Do not use the bug juice method against ticks, fleas, mosquitoes, or other insects that attack people, because these insects can carry human disease. The diseases of plant-eating insects are everywhere in the world. They are all over the leafy vegetables that we eat, and we have been constantly exposed to them for millions of years on the foods we eat and in the gardens and fields we work in. If plant insect diseases could make us sick, we would have them by now.

The most important part of making bug juice is the collection of enough pest insects to increase the chances in favor of spreading an inactive insect disease. The amount collected in weight is not as important as the number collected. For example: if 300 insects are collected and only one tenth of the blender concentrate is used to make a dilute solution, success will be ten times more likely than if only 30 insects are collected and all of the concentrate is used to make the dilute solution.

The reason for this is: even if there is only one sick insect in a batch, its disease will be spread throughout the dilute solution. That one sick insect can be the source of 1,000,000,000 (one billion) virus spores (a "disease seed"). This means that when those virus spores are spread evenly over the leaves of a plant, almost all of the insects will eat some spores. Many of these insects will be killed by the virus. Each pest killed in this way will provide many more virus spores in the weeks to come.

If any pest insects look weak or are moving slowly, they may be sick and should definitely be used to make a bug spray. Three or four days after a solution has been sprayed on the plants, look for dying pests. These insects should be collected and stored in a jar in a freezer or dried slowly at about 38 degrees centigrade (100 F) for four to five hours and then stored in an airtight container until needed. These stored insect pests can be saved until the next crop or shared with a friend who is having trouble with the same pest.

Do not expect to control one kind of insect by grinding up and using another kind of insect in a spray. Each insect carries diseases that harm no other kind of insect. Only a few kinds of insects can be killed by diseases that affect other, even closely related, insects. Do not expect to control aphids with ground up corn earworms.

If ground up bugs are saved for use at another time, keep some record of what insects went into the bug juice solution. A simple way to do this is to place two or three of each kind of insect in a jar of 70 percent isopropyl alcohol (rubbing alcohol). Label the jar as to when the insects were collected, off of what kind of plants they were collected, and how many of each kind of insect was used in making the bug juice concentrate.

If the pest shows up again, the insects on the crops can be compared with the ones in the bottle of rubbing alcohol. The insects in the alcohol cannot be used in a bug juice spray; they are for identification purposes only. Only the dried or frozen concentrate can be used to make a dilute solution for spraying. Pest insects in jars of rubbing alcohol would be a good way for stores that sell bug juice bioinsecticides to advertise which insects they have frozen bug juice concentrate for.

The following precautions are based on the idea that too much safety is better than too little safety. Wash your hands after handling the insects. Use drinking quality water to dilute the concentrate. Sterilize a blender after using it to grind insects by placing the non-electric blender parts in boiling water for 15 minutes after they have been cleaned. Do not breathe the spray. Wash fruits and vegetables before eating them if the spray has been used recently. These precautions should be followed much more carefully when chemical insecticides are used.

From the April, 1978, Article

The bug juice method of insect control proved its worth last summer, according to reports from "Organic Gardening and Farming" readers and newspaper accounts from around the country. The bioinsecticide method was successfully used on an 80 hectare (200 acre) peanut and soy bean farm in Florida, when first reported in this magazine's October, 1976, issue. Now there are reports from many areas of the country saying that it works.

Two letters were received saying that it did not work. One man tried it on grasshoppers, and a woman tried it on Japanese beetles, both without success. The bug juice also failed to kill Japanese beetles on grapevines when "Organic Gardening and Farming" staff tried it, but the bioinsecticide did clear up infestations of aphids on corn. (Other scientifically controlled experiments have been very successful in killing Japanese beetle grubs with a bug juice insecticide. The scientists made sure that the grubs they used were sick with milky spore disease.)

Warning: There are some plant diseases, such as the rice plant virus called tungro in the Philippines, which are carried from plant to plant by insects, in the case of tungro by leafhoppers. Do not use the bug juice method if the crop is sick with a plant disease. Using the bug juice could spread the plant disease. Use the following plant juice method when plants are being eaten by insects and may also have a plant disease.

Sprays that spread insect diseases may not be the only effective biological blender spray. The magazine "Soil News" from September, 1977, reports that blending weeds that insects will not eat and then spraying a diluted solution on crops that insects love to eat is an effective way of protecting crops. Plants that insects refuse to eat contain distasteful substances that when sprayed on tasty crops, makes the tasty crops just as impossible for the bugs to eat as are the never-eaten weeds.

One letter writer said he collected the leaves from plants that insects never seem to eat, ground them up, made a tea with water from the solution, and sprayed it on his garden. "When I checked the garden a few days later, I swear I could not find a moth, bug, or worm of any kind under the leaves, on top of the leaves, or on the ground. The bugs had said ugh and silently crawled away."

Find weeds and tree leaves that insects never eat, as indicated by a lack of holes in the leaves.

· Grind them up in a blender with a little water, add some more room temperature water, not hot water, and let the solution sit for 24 hours.

· Then strain, dilute, and use as a spray on vegetables and other crops.

· Do not cook the solution, let the ground-up leaves just soak in water like a tea for a day before straining, diluting, and spraying.

· Do not dilute the solution more than five times the original volume.

· Smooth leaves are more likely to have chemicals that taste bad to insects than "hairy" leaves because hairy leaves repel insects mechanically.

· Choose plants like pines, poplars, and herbs that are not poisonous, yet contain volatile oils.

It is very possible that whatever insects do not like about one plant might make them hate another, different type of plant, if by using a spray made from the first plant, the second plant can be made to taste like the first plant.

There have also been reports that using a plant juice solution made from weeds might stop the growth of more weeds of the same type. There is very little information on this idea of killing a weed by spraying it with a "tea" made from its own kind. It may not work, but it is worth experimenting with.

One plant spray that has met with success is the hot pepper spray made from grinding up hot peppers and diluting the concentrate with water. There are reports of hot pepper spray stopping rats, mice, and some kinds of insects from eating plants. From Papua, New Guinea, comes the suggestion to add a little liquid or powdered soap to the solution (make sure it dissolves). The soap will help the pepper spray stick to the leaves.

From the July, 1979, Article

The bug juice method works successfully on over 20 pest insects, including: cabbage loopers, grape skeletonizers, stink bugs, armyworms, velvet bean caterpillars, granular cutworms, ants, slugs, fungus gnats, sawfly worms, aphids, wireworms, striped potato beetles, and several types of caterpillars.

The Center for Tropical Agronomy in Turrialba, Costa Rica, reported that insect populations in bean plots sprayed with bug juice were very low (one or two insects per 12 foot row), while the untreated plots had normal insect populations (four or five insects per plant). There was a need for once a week reapplication until pest levels decreased permanently.

The bug juice method is not totally understood yet. Sometimes it does not work as well as was hoped. Sometimes it does not work at all. One scientifically controlled study showed that cutworm bug juice, which has worked successfully many times, actually attracted cutworms. (Maybe the insects were collected during their mating season and were releasing chemicals meant to attract others of their kind. When the bug juice spray was made, these chemicals naturally enough ended up in the solution.)

The usual difference between success and failure with bug juice is whether or not any sick insects are used in making the solution, It is not always easy to see if an insect is sick, so all pest insects of the kinds wanted should be collected, in hopes that some of them will be sick.

More information on the "plant" juice spray: Certain chemicals in plants may serve as barriers to plant-eating insects. If those chemicals are put into a solution and sprayed on crops, the bugs will look elsewhere for food.

The neem tree of India is reported to have a taste that insects hate to eat, and some pine tree needles contain chemicals that can kill houseflies, codling moths, and apple moths.

Healthy plants in a healthy soil will attract few insect pests. More insecticides are needed to control insects when a crop has been given too little or too much fertilizer. A balance of plant nutrients in the soil grows plants that are resistant to insects, just as people who eat well do not get sick as often as people who do not eat well. Rotate crops, use biofertilizers and chemical fertilizers wisely, and less insecticide of any kind will be needed (August, 1980).

From the August, 1981, Article

The history of organic pest control is one of trial and error, success and failure. Gardeners and farmers have planted aromatic herbs and flowers to ward off bugs. They have planted several different vegetables in the same plot to confuse insects. They have tried to disguise their vegetables with sprays made from plants which bugs seem to avoid. They try to kill insects with sprays made from grinding up the insects themselves. Sometimes these measure work, and sometimes they do not work.

Do not let the fact that all attempts have not met with success stop you from trying. Try several small-scale experiments of the methods described here. Ideas that do not work can be easily dropped; ideas that do not work as well as one would hope can be improved on; and in the end large-scale safe, low-cost biological control of insect pests can be done successfully with confidence.

One man who successfully uses bug juice bioinsecticides does not separate the bugs from the leaves. He collects several leaves that are covered with aphids; then he grinds up the leaves and the bugs together in a blender and sprays a diluted solution back onto the vegetables. In one day he reports many of the aphids are gone; in three days they are all gone.

Pill bugs and slugs are not insects, but the bug juice method has been used successfully against them. However, more than one spraying is often necessary.

Bug juice has been used to treat soil planted to squash and beans.

· An ounce of ground up pill bugs (root-eating soil pests) was mixed with two ounces of water to make a paste and then diluted, one ounce to a gallon of water (three milliliters to 3.78 liters of water).

· One section of land was soaked with the solution; the other section was soaked with plain water,

· Two days after sowing, the seedlings came up. The roots of the plants in the untreated section were eaten by pill bugs on the first night, but the roots of the plants in the section watered with bug juice were not eaten.

A few warnings:

1) Bug juice bioinsecticides can quickly lose their effectiveness by becoming spoiled with bacteria; in other words, bug juice can rot. Use all of it right away or freeze the remainder. The only possible danger with the bug juice method is to let it sit out and become contaminated with salmonella bacteria as it decays.

There is no danger to people if the bug juice is used within an hour or two of being made or if the unused portion is frozen. Since the concentrate can be diluted up to 25,000 times, it is a good idea to freeze some for reuse after a rain.

2) Some people develop allergies when working with insect parts. If this starts to happen, wear a mask, long-sleeved shirt, pants, and gloves when spraying with bug juice.

3) Do not use the bug juice method against fleas, ticks, mosquitoes, and other insects that attack people and may carry human disease.

4) If a grinder is used to grind up the bugs, use that grinder only for making bug juice. If a blender has to be used to prepare food for people, use a mortar and pestle to grind up the insects.

Directions for making and using bug juice bioinsecticides:

1) Check for damage. Do not use the method if the crop is not being seriously attacked. If there are only a few pest insects, then there is a balance between insects that eat plants and insects that eat insects. Do not upset this balance.

2) Identify the pest insect. If the crop is being attacked by a pest, identify which insects are doing the damage. It is important not to kill insects that are not major pests. They provide food for insect-eating insects, birds, and animals.

3) Collect the pest insects. Collect as many insects as can be found in 15 minutes or at least 100 insects of each type for every five hectares (12½ acres). Protect the insects from the sun; they will not make a good bug spray if they die before being ground up.

4) Put the insects in a blender. Cover the insects with a cup of two of rain water or boiled water that has been allowed to cool. The volume ratio should be about one third insects to two thirds water. Run the blender at high speed until the solution is all liquid. This should take about one minute.

5) Strain the solution through cheese cloth, a paper filter, a clean rag, or towel. This will prevent the sprayer from becoming clogged with small insect parts.

6) Dilute the concentrate solution. For gardens, use a small hand sprayer, the degree of dilution is not critical. Dilute one quarter cup of concentrate with one or more cups of rain water or boiled water that has been cooled. Freeze, do not just refrigerate, the rest of the concentrate in small batches in case repeat sprayings are necessary.

For farms, a dilution as weak as one ounce (30 milliliters) of concentrate in about 100 gallons (378 liters) of rain water has worked. (Liquid soap, corn syrup, or other sugars that dissolve easily in water can be added to the solution to help the spray stick to plants.)

7) Spray the crops with the diluted concentrate. Use a standard insecticide sprayer to spray bioinsecticides. Spray both sides of the leaves and spray the stems, too. If a rain occurs soon after spraying, repeat with the solution that was frozen or make a new batch. Even if it does not rain, it is not unusual for repeat sprayings to be necessary.

8) Check the plants before spraying, in a few hours after spraying, one day, three days, and one week later to see what is happening to the pest insects. Some effects may not show up until a few days or weeks after spraying, depending on the type or types of insect disease or parasite that is in the bug juice; so keep watching.


This ferrocement concrete information can be used to make concrete biogas digesters and gas storage tanks. This information can also be used for the construction of water tanks, grain storage tanks, or for almost any construction project that needs strong concrete walls.

This section is adapted from the book, Ferrocement Water Tanks and Their Construction. The book describes methods of constructing water storage tanks from wire-reinforced concrete. These tanks are widely used in many parts of the world to collect and store water for housing, irrigation, and industrial purposes. The tanks are made by hand troweling a cement-rich concrete onto a mesh of wire reinforcement to form cylindrical (round) tanks with thin walls which vary in thickness from 3.0 to 10.0 centimeters (1.0 to 4.0 inches) depending on the size of the tank.

Part One

Although the word ferrocement is used here, this is not strictly true. These tanks should really be called wire-reinforced concrete tanks. The main difference is that in ferrocement there is a very dense layer of mesh reinforcing wire that has to have a minimum value of wire volume for each unit volume of concrete. The quantities of straight wire reinforcement used here fall far below this minimum value, but they provide enough strength for the tank sizes described here.

Still, wire-reinforced concrete is closer to ferrocement concrete than to ordinary steel-bar reinforced concrete. The wire distributes the load throughout the concrete, preventing the force of the load from concentrating in planes of weakness which would lead to early failure (Diagram 29, Figure 3). Straight wire is used because it is many times cheaper than the same weight of mesh wire.

This method of water tank construction is useful in rural areas for several reasons.

· The basic raw materials are available and are already being used for many familiar purposes.

· Except for the cement, which must be kept dry, the materials are not easily damaged.

· The skills needed to use the materials are often already known, and people can learn to make good tanks after only a few days of instruction.

· The users of the tank can contribute their labor to pay for part of the cost.

· The construction techniques are not complicated and do not demand the use of expensive and complicated machines or electricity.

· Trained supervision can be kept to a minimum.

· Leaks resulting from bad workmanship or damage are easy to repair, and only minimal maintenance is needed.

· The framework can be carried from site to site and used to build many separate tanks.

Wire-reinforced concrete is used for a wide variety of purposes, but its main advantage for water tanks (and biogas systems) is its ability to resist rusting and its low cost in comparison with other building materials. Galvanized iron (GI) tanks have been widely used in the past for water storage, but they are expensive and galvanized iron will rust and leak in five to ten years, even if they are carefully maintained. The life of reinforced concrete water tanks is expected to be more than 50 years.

The thin walls of ferrocement tanks are able to bend under load; this helps prevent the concentration of stresses that cause cracking. The wire reinforcement distributes the stresses throughout the concrete, increasing its ability to carry pulling and bending loads (tensile loads) without cracking. The thick walls of steel-bar reinforced tanks are not able to bend very much under load; they are more likely to crack than to bend.

Wire-reinforced concrete tanks have been proven in use over many years in all extremes of weather conditions and can be built confidently with capacities up to 125 cubic meters (4,400 cubic feet or 33,000 gallons). But the tanks should not be built off the ground unless expert advice is available.

Cost of construction: While the final price of the tanks will vary according to local conditions, it will depend on the following.

· Materials: the cost of sand, cement, straight wire, mesh wire, and formwork. The formwork can be made either for one usage with temporary local materials or of more permanent construction from steel sheeting and angle iron. Well-made forms will last for many years and the formwork cost for each tank will become increasingly smaller.

· Wages: the cost of wages for plasterers, laborers, and supervisors if the tank is not built totally by self-help.

· Transportation: the cost of transporting people and materials.

The quality of the water stored in the tank will depend initially on the quality of the water put into the tank. Diseases that are related to water and water use are causes of sickness and death to people who do not have protected or purified water supplies or efficient sanitation. Any measures that reduce the risk of these diseases will be of great value to water users.

The bacteria that cause some of the most serious illnesses, especially cholera, typhoid, and diarrhea in children, cannot live long outside of the human body. In stored water, most will die within one week, although cholera and typhoid germs can remain dangerous in stored water for up to four weeks.

Water in a tank must be protected from contamination. As shown in Diagram 28, a cover is usually necessary. The tank must not be allowed to become a breeding ground for malarial mosquitoes. Overflow pipes should be covered with window screen and so should the inlet. These precautions will prevent trash, insects, rats, and mice from falling into the tank.

To further improve the quality of the water, certain chemicals can be added. Both chlorine and iodine kill disease-causing bacteria and viruses in water. But unlike chlorine, iodine has no taste or smell, and iodine is medicinal in that it can prevent and cure goiter.

Chlorine, even though it is used all over the world to make water safe to drink, can increase the risk of some types of cancer; it is made inactive by organic matter in water, and its strength is reduced if the water is alkaline (base).

Too much iodine, like too much chlorine, can be poisonous. Follow the directions for using iodine very carefully. The result will be a method of purifying water that is an improvement on the chlorine method. Iomech Ltd., a Canadian company, sells systems that add measured amounts of iodine to water systems in many developing countries, including Egypt and New Guinea.

Designing the tank: The great advantage of wire-reinforced concrete over standard reinforced concrete is its ability to resist shrinkage during the curing (drying) process, its resistance to severe cracking under tensile load, and the need for only one set of forms for construction. Pouring a thin shell of concrete between two closely spaced shutters, the usual method of ferrocement construction, is a highly skilled and difficult job.

The weakness in tension and the brittle type of failure occur because no matter how carefully the concrete is mixed and placed, there will always be planes of weakness between the edges of the different lumps that make up the concrete. In compression (top-down pressure), these planes of weakness are held together by the load; but under tensile loading (sideways pressure), they can open up beyond their elastic limit. The separate cracks then join together, causing the concrete to fail (Diagram 29, Figure 3).

In standard reinforced concrete, the reinforcing bars work to limit and control the tendency of the concrete to crack under tensile load according to the amount and distribution of the steel bars and the degree of loading. In wire-reinforced concrete under moderate tensile loads, the concrete contributes greatly to the tensile strength of the whole structure. This is because the wire, distributed throughout the concrete, will allow the load to be absorbed throughout the concrete and will prevent the buildup of too much stress in weak spots.


Figure 1: Catching and using the rain water from a roof

Secondary storage, fish pond, or irrigation

-Strainer (mesh sieve) to prevent rubbish from flowing into the tank with the water and to stop insects, flies, and rodents from flowing into the tank.

Figure 2: Protecting the tank from further contamination

-The concrete diagrams in this section are from the book, Ferrocement Water Tanks and Their Construction.

Foundation: The foundation of the tanks carry the weight of the tank and water down to the ground. The floor in smaller tanks is usually continuous with the walls. The floor slab carries the weight of the walls and the water directly onto the foundation (Diagram 29, Figure 5).

Walls: The vertical tensile stresses set up across a horizontal plane at the joint of the wall and the floor are nearly double the hoop tension stresses produced by the walls stretching outward under load. The tanks should be designed with vertical reinforcement both within the walls and between the walls and the floor to prevent cracking (Diagram 32, Figures 21 and 22).

The tanks described here have one layer of straight wire reinforcement and one layer of the more expensive woven mesh (chicken) wire. The mesh wire, which starts in the floor and goes into the walls, will provide the reinforcement that limits cracking between floor and walls.

The joint between walls and floor will not remain completely rigid. It is likely that very small cracks will open and that the joint will bend outwards and throw a greater part of the load onto the wire reinforcement (Diagram 30, Figure 9). These small cracks are unlikely to be serious. If the cracks become wide enough to allow water to reach the wire because there is not enough wire or the wires are too widely spaced--the wire will rust and eventual tank failure will occur. In most of the tanks a thick concrete layer is built up around the junction of the floor and the walls to strengthen the joint.

The chicken wire layer that is wrapped around the formwork (before the straight wire is added) will play an important part in preventing cracks from occurring as a result of shrinkage and loading. Although the mesh wire is expensive, it will contribute greatly to the strength of a water tank.

Floor: The concrete floor of a tank is usually built first to give the walls a solid foundation. There should be mesh wire running from the floor up into the walls to provide resistance against cracking. A bitumen (asphalt or tar) movement joint between the floor and the walls is usually provided to take expansion and settlement.

The concrete floor of large tanks (over 10.0 meters in diameter) must be cast in sections with bitumen-sealed expansion joints between the sections. This will allow the floor to expand and contract without cracking. The floors of the smaller tanks can be built as one continuous piece.

Roof: A roof provides a good cover against the loss of water from evaporation, which in hot, dry parts of the world, can exceed two meters per year. It prevents trash, insects, rats, and mice from getting in and it keeps the water cool. The extra stresses set up by the weight of a concrete roof are not great if the junction between the roof and the walls is curved. Sharp angles concentrate stresses and start cracks.

The greatest stresses set up in a roof of this kind are caused by its expansion in hot weather from the heat of the sun. These stresses can be over 20 times greater than the static load stresses due to the weight of the tank. For this reason, the roof and walls should be painted white to reflect the heat of the sun or be protected with a grass roof (Diagram 30, Figure 11) over the concrete roof. The roof may also be built from lightweight materials, such as galvanized iron, fastened to a conventional structure built over the tank (Diagram 30, Figure 12).



Figure 4: Curved walls are stronger than flat walls

Figure 5A: Foundation of small tank

Figure 5B: Walls and base joined

Figure 6A: Foundation of Large open tank -

Figure 6B: Walls and floor separate



Tank fittings: Most tanks will have pipes of some sort to take water in and out. The outlet pipe for smaller tanks often goes through the bottom of the walls, the overflow pipe through the top of the walls, and the water enters through the roof. With larger tanks, the inlet and outlet pipes should both go through the floor. This reduces the risk of cracking and leakage around the pipes which could happen if the pipes were cut into the walls. The overflow pipe should still go through the top of the walls. In all cases where the outlet pipe passes through the floor, a wire screen of some sort is recommended to prevent the outlet pipe from being blocked by trash.

The opening of the outlet pipe is usually placed 10.0 centimeters (4.0 inches) or so above the level of the tank floor to prevent dirt that has settled to the bottom of the tank from being used. The hole for the overflow pipe is cut out while the concrete is still green (half dry). The overflow pipe is then set in the hole. The open end of the pipe should be covered with a layer of window screen to stop mosquitoes and other insects from entering and breeding in the tank.

On large tanks used for watering animals (Diagram 30, Figure 13), the outlet is often provided with an adjustable pipe section fitted inside the tank. If the water trough is damaged by the animals or if the ball valve in the trough sticks open, only a little water will be lost. The outlet to the tank should be fitted with a sand filter to screen the water before it is used in order to remove most organic matter and bacteria. This sand filter must be kept permanently wet if it is to work correctly. The sand should be cleaned or replaced from time to time (Diagram 30, Figure 14).

Reinforcing wire: There are many different types of steel reinforcing mesh that can be used to make reinforced concrete. These generally consist of thin wires, either woven or welded into a mesh, but the main requirement is that the wire must be easy to use and, if necessary, flexible enough to be bent around sharp corners. The wire is tied and held firmly in place so that the concrete can be troweled over it.

Chicken wire can be expensive to buy. It can cost over ten times as much as an equal weight of straight wire. For these reasons, straight wire is used for the second wire layer. Because reinforcement is needed in the vertical direction, a single layer of mesh wire is wrapped onto the formwork before the straight wire. To reduce the cost of buying ready-made mesh wire, handlooms can be adapted to weave mesh wire from straight wire.

The straight and mesh wire can be galvanized iron or steel, but under no circumstance should aluminum painted wire be used. Aluminum can react with cement to give a very poor bond between the wire and the concrete.

Cement, sand, and water: The cement that is used to make the concrete can be an ordinary Type One or Type Two Portland cement (to BS-12 or similar specification). Lower strength cement has been used in the past with some success, but it cannot be recommended. Sometimes lime is mixed with the cement in the ratio of one bag of lime to five bags of cement to improve the workability of the concrete and to reduce shrinkage cracks. The cement should always be kept dry until it is used.

The main requirement for the sand is that it be free from organic and chemical impurities that would weaken the concrete. Most clean sands are usable. If the quality of the sand is in doubt (such as salt contamination), it should be washed with clean water. A silica sand is probably best, but sands consisting of other hard minerals can be used. Moderately coarse sand, although it makes the concrete more difficult to work, will resist shrinkage and cracking better than a fine grain sand. If the sand has a high dirt content, the concrete will be weak. There should be a mixture of all grain sizes without too many of any one particle size.


Figure 9: How the loads are taken

Figure 11: Roofs for tanks less than 5m diameter

Figure 12: Roof for larger tank

Figure 13: Ball valves for cattle watering

Figure 14: Slow sand filter in tank over outlet

The water must be fresh and free of chemicals in solution or suspended dirt and organic material. Clean water is a must for a strong, long-lasting concrete. Salt water should never be used.

The concrete mix: Making a strong concrete mix from cement, sand, and water is one of the most important stages in building with concrete. The concrete must be prepared with the correct proportions of materials. It must be well mixed and workable enough to be troweled by hand onto the formwork and between the wires to form a dense, compact layer. It must also be dried slowly in order for the concrete to achieve its full potential strength and durability.

Ratio of cement and sand by volume: Increasing the proportion of cement in the concrete will increase the concrete's strength and make it more workable. Extra cement will also increase the danger of shrinkage and cracking which may cancel out the value of the increase in strength.

The concrete mixture preferred here uses a cement to sand volume ratio of 1:3 (one part by volume cement to three parts by volume sand). Measuring boxes or buckets should be part of the construction equipment and should be used to achieve the correct proportions. Measuring the materials on a shovel does not give reliable results.

Ratio of water and cement plus sand by weight: A dry, low water concrete mix will be stronger than a wet, high water mix made with the same proportions of cement and sand. Dry mixes are difficult to work with on formwork. They are more likely to contain air pockets and be poorly bonded to the reinforcing wire. Wet mixes are easy to trowel by hand, but the finished concrete will not be as watertight or airtight as a carefully made and used dry mix nor will the wet mix be as strong or as long lasting as a dry mix can be.

A compromise between wet and dry mixes must be made. Experience has shown that a 1:3 cement to sand by volume ratio and a 0.5:1 water to cement plus sand by weight ratio will work. If, in order to make the mix more workable, extra water has to be added to give a water to cement plus sand weight ratio of 0.6:1, then a better grade of sand or a larger proportion of cement should be used. (Other makers of ferrocement structures use a 1:2 cement to sand by volume and a 0.4:1 water to cement plus sand by weight ratio.)

Under most conditions the workability is usually controlled by eye during mixing. (If someone is skilled in using a funnel to make what is called a slump test, they should use it to determine the workability of the mix.) If the water to cement plus sand by weight ratio is not to exceed 0.5:1, the mixing must be carefully supervised.

The amount of water to be added to the dry mix of cement and sand is affected by the presence of moisture in the sand, which can vary greatly between the top and bottom layers of the sand pile. The wetter the sand is to begin with, the less additional water is needed. One or two small-scale trial mixes using a sample of completely dry sand and a measured water to cement plus sand weight ratio will allow the people who will be mixing the concrete to see and learn the feel of a good mix.


Figure 15: Mixing the concrete by hand

Figure 16: A tool for straightening and tightening wires that are bent and slack

Figure 17: Trowelling the concrete onto the formwork - 1. concrete onto hawk

Figure 17: Trowelling the concrete onto the formwork - 2. concrete onto float

Figure 17: Trowelling the concrete onto the formwork - 3. concrete onto formwork

Formwork: Formwork is needed to support the walls while the concrete that has been troweled on hardens and sets. Well-made formwork is expensive but will, with care, last for years. Its initial cost will be spread out over many construction projects. Experience shows that well-made formwork makes construction work almost foolproof. The chicken wire and the straight wire are wound easily and quickly around the cylindrical forms and the concrete is then troweled over the wire layers. The formwork must be rigid enough to hold the weight of the concrete as it is being applied and dried without moving. If the formwork moves during the setting period, the concrete is likely to crack and be very weak.

The formwork can be made from circular corrugated iron (GI) sheets. The "hills and valleys" of the corrugations provide a mark for the tank builders to help them wind the reinforcing wire on at the correct spacing, although the straight wire will tend to bundle together in the valleys of the corrugations.

The main advantage of the corrugated sheets, besides durability, low cost, and light weight, is that they allow for an accurate measurement of the final wall thickness when the corrugations on both the inside and the outside faces of the tanks are filled with concrete. In this way the total wall thickness is easily achieved.

Tools: The necessary tools should be collected in advance to make a standard kit for tank construction. It is possible to use local tools and equipment if they are available, but it is always safer to make sure that all necessary tools are on hand by bringing the standard tools. The concrete can be mixed either by hand or with a concrete mixer. Hand mixing is hard work and if incomplete will result in a lumpy concrete that is difficult to trowel onto a tank wall. Hand mixing can manage drier mixes better than concrete mixers, because the drums of concrete mixers tend to do a poor job at mixing low-water mixes. Treading the concrete under foot on the mixing slab is one of the best ways of working a dry mix. Steel (plasterer's) floats are much easier to use than floats made of wood. Because overworking wet concrete, especially with a steel float, can cause the concrete layer to drop off, the concrete should be applied quickly and carefully. The surface of the first layer (there are two per side) must be roughed or brushed when it has hardened a little, to give the second layer something to hold on to.

List of tools and equipment: Four plasterer's steel hand floats, four hand hawks 30 x 30 cm with 4.0 cm diameter handles, four troweling boards 75 x 75 cm, two wire brushes for cleaning formwork, two oiling brushes, one brush or scratching tool for concrete, one hacksaw and spare blades, one woodsaw, one set of wrenches, one crowbar--1.0 meter long one wire snip for mesh wire one bolt cutter for straight wire, two cold chisels for cutting green concrete, one two-kilogram sledgehammer, one gauging box for sand and cement 50 x 50 x 40 cm or other container to hold 100 liters of sand, one sieve 5.0 mm maximum openings for sand, four flat-ended shovels for mixing, one spirit level, one 50 meter long cloth or plastic tape measure, one string line, one axe for trimming wood, two pickaxes for digging, two mattocks for ground leveling, one wire tension tool, one set of formwork sections, one wheelbarrow for carrying concrete, water containers, and plastic sheeting to cover the drying concrete.

Part Two

Summary of methods: This section contains a detailed description of standard tank construction practices that have been used successfully for over 25 years in many countries all over the world. What is outlined here is a design for a small ten cubic meter capacity water tank. The book, Ferrocement Water Tanks and Their Construction, from which this information was taken, has information for building large 150 cubic meter capacity open tanks and many other different water tank designs, sizes, and adaptations of the basic design described here. VITA is another good source of information on ferrocement construction.

Clearing the site and preparing the foundation: The site chosen for the tank should be cleared of plants, loose soil, and any rocks that could cut through the floor of the tank. For small tanks it is often only necessary to clear the site and put down a 15 cm (6.0 inch) layer of sand and gravel. Larger tanks need a ring of concrete, separate from the tank floor, to support the walls. The ring is prepared by digging a trench under the line of the walls and backfilling it with concrete (Diagram 29, Figure 6).

Collecting the materials: The sand, gravel, and water should be collected before construction starts. If this is done by the people who will be using the tank, they will be able to contribute a large proportion of the total cost of the construction. With experience, a trained builder with five helpers can build a tank in about three days.

The sand and gravel should be stored in piles next to the concrete mixing slab. The slab is built by troweling a layer of concrete onto a layer of gravel 2.0 meters by 2.0 meters square. The slab is finished with a small wall to prevent the spillage of concrete slurry during mixing (Diagram 30, Figure 15). For smaller tanks, it may be cheaper to include a wooden mixing platform as part of the basic equipment. Mixing the concrete on the bare ground will contaminate and weaken the concrete with dirt.

Make sure that enough sand is collected for each tank before construction begins. The cement, mesh wire, and straight wire should be collected and stored under cover. The formwork and any other materials or tools should be collected and checked. All underground pipes should be installed before the concrete is laid. Although approximate quantities of materials needed for each tank are known, it is always a good idea to have extra materials on hand to allow for wastage.

Design: The tanks have been designed for construction by relatively unskilled workers. The tanks have a diameter of 2.5 meters, a height of 2.0 meters, and a capacity of approximately 10.0 cubic meters. The final wall thickness will be about 4.0 centimeters (1.5 inches). The tanks are built on site and should not be moved. (Portable tank construction is described in the book, Ferrocement Water Tanks and Their Construction.)

Casting the foundation: The foundations of small tanks are below and separate from the tank floor. They are made by casting a 7.5 cm (3.0 inch) thick slab of concrete 2.8 meters in diameter onto a 10.0 cm (4.0 inch) thick layer of sand and gravel. The concrete is prepared from a by volume mix of 1:2:4 of cement to sand to gravel. It should be kept wet in such a way that drying takes seven days. The ring trench of large tanks is filled with concrete after the floor slab has been cast.

Into the concrete foundation is cast a 1.0 meter length of 20 mm diameter steel water pipe with a tap on the outside end. The pipe is curved (with 45 percent joints) so that it projects 10.0 cm (4.0 inches) above the floor of the tank. A piece of wire is threaded through the pipe to act as a "pull-through" after the tank is completed (Diagram 32, Figure 20).

Mixing the concrete for the floors and walls: The cement and sand are gauged either onto the mixing slab or into a cement mixer in the by volume ratio of 1:3. It is difficult to judge these volumes accurately by shoveling. A gauging box should be made to measure the sand. If the box is 50 x 50 x 40 cm, then one box of sand mixed with a 50 kilogram bag of cement will give a concrete volume mix of 1:3 (cement to sand).

The dry cement and sand on the mixing slab are turned by hand (if a cement mixer is not used) several times into piles from one side of the slab to the other until they are completely mixed together. A hole is then made in the center of the pile and water is added into the mix until the desired consistency is achieved (0.5:1 water to cement plus sand by weight. Add only a small amount of water at a time. It is very difficult to make a wet mix drier by adding more cement and sand. The workability of a dry mix can be increased by treading it with one's feet to break up any dry lumps. (Warning! If lime is used in the mix, it can burn!)

It is usually easier to build the floor of a tank before the walls are built. The floor is made of two 1.0 cm (0.5 inch) thick layers of concrete with one layer of chicken wire between the two concrete layers. This mesh (chicken) wire extends 1.5 meters from the floor into the walls if the floor is built first. The mesh wire extends 1.5 meters from the walls into the floor if the walls are built first. The inside corners between the floor and the walls should be built up with extra concrete. This is called "coving."

Take care that the concrete does not block the outlet pipe. Before the concrete on the floor has stiffened, make a shallow depression in the middle; this will allow the tank to be easily cleaned in the future. The dirt can be brushed into the valley and cupped out (Diagram 32, Figure 22). After the concrete has dried, the inside of the tank is painted with a thick adhesive cement or asphalt layer to seal the tank.

Formwork: The 2.0 meter high formwork is made from standard galvanized iron (GI) roofing, 0.6 mm thick with 7.5 cm corrugations. The formwork is rolled into a cylinder with a radius of 1.25 meters. Steel angle iron (40 x 40 x 5.0 mm) is bolted vertically on the inside face at the ends of each set of four sheets. This allows the metal sheets to form a circle when they are bolted together. Between the ends of each section is placed a wedge which is pulled out to allow the formwork to be dismantled (Diagram 32, Figures 18 and 19).

The formwork is erected after the concrete floor has hardened. The bolts passing through the angle iron and wood wedges are tightened to provide a rigid cylindrical form. The formwork is cleaned free from concrete and dirt. It is then oiled to prevent the new concrete walls from sticking to it. Mesh wire is then wrapped tightly around the formwork to a single thickness and tied with short pieces of straight wire.

The 1.5 meters of mesh wire coming out of the concrete floor are overlapped with and tied tightly with straight wire to the wall's mesh wire. The mesh wire has a 50 mm mesh and is made from 1.0 mm wire (Diagram 32, Figure 21). Other makers of ferrocement structures use chicken wire with mesh openings as small as 15 mm. The smaller the mesh opening, the stronger and more expensive the tank will be.

To complete the reinforcement, the straight wire, 2.5 mm in diameter, is then wound tightly around the tank from the base up, at the following spacings:

· two wires in each corrugation for the first eight,

· one wire in each corrugation to the top, and

· two wires in the top corrugation.


Figure 18: Standard formwork for 10 cubic meter tank - top view of format

Figure 18: Standard formwork for 10 cubic meter tank - cross section of form

Figure 19: Tank foundation

Figure 20: Assembling the formwork

Figure 21: Erecting formwork and winding on reinforcing wire

Figure 22: The completed tank

About 200 meters of wire will be needed (weight 78.0 kg). The chicken wire must be wound tight enough to hold the straight wire out of the valleys in the corrugations. If the straight wire is bent, it can be straightened by making and using a special tool (Diagram 31, Figure 16). A wrench or a pair of pliers may be able to do the same job.

When building ferrocement biogas digesters, use two wires in all corrugations and two layers of mesh wire in the end pieces. Because digesters lie on their sides, the stress on the concrete may be greater.

The concrete is applied by hand to the wire covered walls with a plasterer's steel float in layers not greater than 1.0 cm (½ inch) thick because thicker layers tend to fall off. The concrete which has already been prepared at the mixing slab is carried to the side of the tank and emptied onto a square board (A). This board prevents dirt from becoming mixed with the concrete and catches any concrete that falls off of the wall during troweling. The concrete is scooped off this board using a plasterer's float (B) and a builder's hawk (C). The concrete is then transferred back onto the face side of the float and pressed into the formwork (Diagram 31, Figure 17).

The concrete is troweled onto the formwork from the base of the tank upwards to fill the corrugations and just barely cover the reinforcing wire. The walls are built up in this way in horizontal sections around the tank. The square board is kept under the section being worked on. Some experts say that vibrating the float as the concrete is pushed into the formwork will help the concrete penetrate the wire mesh so that there will be no air pockets.

When the first layer of concrete has hardened sufficiently, the surface is brushed rough or scratched so that it will be easier for the second layer to stick to the first layer. The 1.0 cm (3/8 inch) thick second layer, which provides the outside surface, is finished with a smooth surface. It must be well bonded to the first layer, which should be only a day or so old when the second layer is applied.

Concrete must be applied quickly. If it is more than half an hour old, it should be used to make a floor slab or concrete path. The concrete quickly becomes unworkable without adding an excessive amount of water that would weaken the final product. In hot climates the concrete pile should be covered with wet sacks or plastic to prevent rapid drying.

In order to make tanks more watertight and airtight, an asphalt seal can be used between the two layers of concrete. The seal is not necessary; it is extra protection. The asphalt seal (bitumen emulsion) is applied in two layers. The first layer is mixed with water--one part asphalt, three parts water. The second layer contains no water. After the asphalt seal has dried, the second layer of concrete can be applied.

After the second layer of concrete has had a day or two to dry, the formwork is removed by removing the holding bolts and pulling out the wooden wedges which will leave the formwork free to be pulled away from the inside of the concrete tank. The sections are then lifted clear of the tank, cleaned of all concrete, and stored for later use.


Figure 31: Constructing the roof

Figure 32A: Making the access hatch

Figure 32B: Making the access hatch

The exposed corrugated concrete on the inside of the tank is brushed rough, and two 1.0 cm (3/8 inch) layers of concrete are troweled on a day or two apart to fill up the corrugations and cover up any exposed reinforcing wires. The methods for applying the concrete on the inside are the same as those used on the outside of the tank.

If the tank has to be left overnight with a layer incomplete, the top edge of the concrete should be at the same height all around the tank. On the next day, the joint should be brushed rough and coated with a cement and water mix to make a strong bond before applying fresh concrete. It is best to complete any one layer in the same day. If that cannot be done, finish a complete band around the tank. Concrete joints will then only occur on a horizontal line. Vertical joints are more likely to crack when the concrete dries.

Roof: The tank can have a galvanized iron sheet roof supported by angle-iron, or a reinforced concrete roof can be made. A ferrocement roof is built onto a tank by laying concrete onto shaped formwork which is supported from below (Diagram 32, Figures 31 and 32). The roof is reinforced with two layers of mesh wire which are tied onto mesh wire coming out of the top of the walls. A prefabricated angle-iron frame is set into the wire mesh to provide formwork for an access hatch into the tank.

This is how both ends should be built when making digesters, though of course only one access hatch will be needed.

Concrete is troweled onto the roof in 1.0 cm (3/8 inch) layers with two layers of mesh wire in the middle and allowed to slowly dry over a seven day period. When the concrete is strong enough, the roof and access hatch formwork is removed and two 1.0 cm (3/8 inch) layers of concrete are troweled onto the bottom of the tank roof and the access hatch. Do not add a concrete roof to a concrete tank until the walls are strong enough to hold it; that will be at least several days after the walls have been finished.

Applying concrete to a tank may look difficult at first, but most people can learn within a few hours. The secret is to apply well-mixed concrete quickly and firmly with the float and work it smoothly over the wall. The surface finish is not of greatest importance on water tanks. What is important is that the layers be of uniform thickness throughout, with no gaps or weak spots.

Drying the tank: After the concrete has been applied, the tank should be covered with black plastic or wet sacks. If the drying concrete is exposed to hot sunlight or wind, it will dry too fast and the final strength will be greatly reduced. Loss of moisture at too fast a rate enlarges the shrinkage cracks that form in the drying concrete and can lead to tank failure.

In very hot weather, tanks must be covered anytime work is stopped. In milder weather, the tanks can be left uncovered except at night. When finished, keep the tanks covered for seven days or more to allow the concrete to slowly dry. The concrete should be sprayed with water as often as three times a day for the seven days. The concrete will take at least one month to reach its full strength, and for the first few days it will be soft enough to allow holes to be cut in it for pipes.

Slow drying is absolutely necessary for strong, waterproof tanks. It is one of the most important construction steps. It is also one of the most difficult things to guarantee in the field.

Filling the tank with water: Concrete shrinks as it dries. If the tank is filled rapidly with water, the concrete will not have enough time to expand again as it reabsorbs water. There is a great risk of severe cracking or even tank failure. An empty tank (or biogas digester), especially a new one, should always be filled over a several day period. It should be left for a week or so with only a shallow depth of water before complete filling is begun.

Materials needed for a ten cubic meter water tank (roof not included): 600 kg of cement, 200 meters of 2.5 mm diameter straight wire, 20 meters of 50 mm mesh and 1.0 mm wide chicken wire, water pipes, water tap, gate valves, overflow pipe, one cubic meter of sand, and 0.5 cubic meter of gravel.

The ferrocement information described above can be used to make water tanks, concrete biogas digesters, the water tanks for concrete biogas storage tanks, or just about anything that can be made out of concrete, including boats.

Remember that whatever is built should be built on the ground where it is to be used and that it will be hard to construct if it is too small to be worked on from the inside.

In this ferrocement design, gravel is not used except in foundations which are not really part of the ferrocement structure. The foundations are beds upon which the floor of the ferrocement tanks are built. Thin steel reinforcing rods can be added to the mesh and straight wire reinforcement, but they are not necessary in small tank (ten cubic meter capacity or less) construction. Ferrocement walls are thin, usually only 2.0 cm (3/4 inch) to 10.0 cm (4.0 inches) thick, but they are very strong.

If these directions are adapted to design a concrete biogas digester, two layers of mesh wire should be used at both ends of the digester. Digester walls can use the straight wire and mesh wire combination for digesters of ten cubic meters capacity or less. Digesters with capacities between ten and 100 cubic meters should probably use two layers of mesh wire. Building digesters with capacities larger than 100 cubic meters will require a more complicated level of ferrocement techniques than those described here. VITA should be able to help anyone interested in more information on different ferrocement techniques.

If it becomes too complicated to make totally round ferrocement biogas digesters, try making the digesters with flat floors and round walls and roofs. The sides of the floor should tilt down a few degrees towards the center line of the digesters. This tilt, in addition to the few degrees tilt down from inlet to outlet (necessary to the biogas digestion process), will make the digesters easier to clean than completely level floors would be.

Keeping the walls and roof curved will make for stronger digesters. The walls and floors should be connected with overlapping mesh wire. Concrete coving should be used on the insides of the tanks where floor meets wall.

A few ideas on making concrete digesters watertight and airtight:

1) Make a cement paste, replacing five to ten percent of the water with poly vinyl acetate latex paint. The combination can also be used as a paint to coat the inside or outside of a digester or gas tank.

2) Sodium silicate can be mixed with cement for patching gas, digester slurry, or water leaks. The sodium silicate and cement solution will harden in the mixing bucket within two to three minutes. To make a large area airtight, extra water can be used when making the mixture.

3) Paint the concrete with a chlorinated rubber-based coal tar or bitumen paint in order to make sure the concrete is completely airtight and watertight.

4) Wet new concrete three times a day for seven days. Cover the concrete with moist sacks and make sure the cover is kept wet. It is very important not to let the sun dry the concrete too quickly. Slow drying, called curing, gives concrete much of its durability, strength, and helps make it watertight and airtight.

There is a Chinese method of sealing concrete digesters against leaks that does not cost very much, does not kill biogas producing bacteria, and does not harm the fertilizer value of the sludge. It is the paraffin/wax method. The following description of the method is adapted from the book Chinese Biogas Digester by Charles H. Nakagawa of the U.S. Peace Corps and Q. L. Honquilada of The Philippine Rural Reconstruction Movement.

The tools needed to apply the paraffin/wax are: a blow torch, a large paint brush, an old pot or tin can to melt the wax in, a small bowl or can with a wide mouth to hold the melted wax while it is being applied, and a steel brush.

The sealing of the inside surface of a concrete digester should be done at least two weeks after the concrete work has been completed.

The steps involved in paraffin/wax application are as follows:

1) Inspect the inside of the digester carefully.

1.1) Check for cracks or weak sections that make a "hollow" sound when tapped. If there are problems, make repairs.

1.2) Check for dampness by spreading cement powder on the concrete to see if it "wets" the cement. If a wetting or leak rate is fast, concrete repair is necessary.

1.3) Check for water on the floor of the digester. If there is any, it should be dried up and the floor should be checked again on the next day. If the water returns, there is a leak that has to be repaired.

Use the dry cement powder method to determine where the leak is coming from.

2) If there are no damaged areas in the digester, clean the walls and floor of loose concrete and sand particles with a steel brush. Wax sticks to rough surfaces much better than it sticks to smooth surfaces.

3) Use colored wax if possible. Colored wax makes it easy to see which sections have been painted with wax.

4) Melt the wax and carefully add kerosene to the melted wax in a volume ratio of 3.0 parts kerosene to every 10.0 parts melted wax. This will lengthen the hardening time of the wax and make it easier to work with. Be careful when adding the kerosene to the melted wax over open fires--it may burst into flame!

5) Place the melted wax and kerosene mixture in a smaller, wide-mouth container and allow it to cool off a little. Very hot wax does not stick to concrete very well. The hot wax drips off the paint brush, wasting much of the wax and may also ruin the brush hairs. A good sign of a workable temperature for wax is when it starts to develop a thin skin or film on the surface.

6) Apply the melted wax with a paint brush.

6.1) Place the bowl of melted wax below the work area to collect any drippings or fill the bottom of the digester with water. Any dripping wax will float in the water and can be easily recovered.

6.2) Apply with up-strokes and by "splashing" the wax on the surface using the brush.

6.3) Paint from the bottom up, working towards the top.

6.4) When finishing a small area, use left-right-left and diagonal strokes. This will guarantee that most of the concrete is covered with a layer of wax.

6.5) Place extra wax around joints and corners.

7) When the inside of the digester has been completely covered with wax, smooth it and force it into the concrete with heat. Use a blow torch operating on a medium-high flame. Using a flow torch will change the wax finish into a smooth, continuous layer of wax. This wax layer should effectively seal the digester against gas and water leaks.

8) When using the blow torch, do not let the flame remain on one section too long. This will burn away all of the wax and the high heat may crack the concrete, Constant left-right-left and circular motions work best.

9) After blow torching, there will be many very small holes visible. These areas will need "touch-up" patching with another layer of wax and blow torch application. Inspect again and repeat if necessary.

After a concrete digester has been in operation producing gas for a few weeks, it would be a good idea to check the walls and roof for leaks. Leaks below the slurry level should be easy to spot. For biogas leaks above that level, pour very soapy water on the concrete. If gas is leaking, fine bubbles will appear in the soapy water. A coat of adhesive cement should be applied to any area that fails the soap test. After the adhesive cement has dried, the area should be tested again for leaks.

Using concrete culverts to make digesters or water tanks for gas storage tanks can cause more problems than they are worth. Concrete culverts often sweat water under normal conditions. They are made strong enough to hold water underground, not aboveground. They are not made airtight or watertight. They often crack if used aboveground. If concrete culverts are used, at least one layer of cement or asphalt sealant will be needed. The least expensive method for making concrete digesters and gas storage tanks is to use the ferrocement method.

Facts & Figures

Conversion Tables

Units of Length:

1 centimeter (cm) = 10 millimeters (mm) = 0.39 inches
1 meter = 100 centimeters = 39.37 inches = 3.28 feet
1 kilometer (km) = 1,000 meters = 0.62 miles
1 inch (in) = 2.54 centimeters
1 foot (ft) = 12 inches = 0.30 meters
1 yard (yd) = 3 feet
1 mile = 1,760 yards = 1.61 kilometers

Units of Area:

1 square centimeter (sq cm) = 0.15 square inches
1 square meter (sq m) = 10.76 square feet
1 hectare = 10,000 square meters - 2.47 acres
1 square inch (sq in) = 6.45 square centimeters
1 square foot (sq ft) = 0.69 square meters
1 acre = 4,840 square yards = 0.40 hectares

Units of Volumes and Capacity:

1 milliliter = 1 cubic centimeter = 0.33 fluid ounces
1 liter = 1,000 milliliters = 0.26 US gallons = 0.22 Imp gallons
1 liter = 0.001 cubic meters = 0.035 cubic feet
1 cubic meter (cu m) = 1,000 liters = 264.2 US gallons
1 cubic meter = 35.31 cubic feet = 993 kg of water = 2,184 lb of water
1 fluid ounce = 30 milliliters
1 US gallon = 64 fluid ounces = 0.13 cubic feet = 3.78 liters = 8 pints
1 cubic foot (cu ft) = 1,728 cubic inches = 7.48 US gallons
1 cubic foot = 28.3 liters = 0.028 cubic meters = 62.4 lb of water

Units of Weight:

1 gram = 1,000 milligrams = 0.035 ounces
1 kilogram (kg) = 1,000 grams = 1 liter water = 2.2 pounds
1 ounce = 28.35 grams
1 pound (lb) = 16 ounces = 0.45 kilograms

Units of Pressure:

1.0 foot of water = 0.433 pounds per square inch (psi)
1.0 kilogram per square centimeter = 14.22 pounds per square inch
1.0 pound per square inch = 0.07 kilograms per square centimeter
1.0 pound per square inch = 27.7 inches of water

Units of Power:

1 horsepower (English) = 764 watt = 0.746 kilowatt
1 horsepower (English) = 1.0139 metric horsepower
1 metric horsepower = 736 watt = 0.736 kilowatt = 0.98 HP/English
1 kilowatt (kw) = 1,000 watt = 1.34 horsepower HP/English

Digester Dimensions

Dimensions of the volumes of the insides of round digesters: volume of digester in cubic meters = radius x radius x 3.14 x length.

digester volume



ratio of length/diameter

in meters·

1.3 cubic meters

3.5 m

0.7 m


2.2 cubic meters

4.3 m

0.8 m


3.2 cubic meters

5.0 m

0.9 m


5.3 cubic meters

5.6 m

1.1 m


10.6 cubic meters

6.9 m

1.4 m


20.6 cubic meters

9.1 m

1.7 m


30.9 cubic meters

10.9 m

1.9 m


41.0 cubic meters

10.8 m

2.2 m


51.1 cubic meters

12.3 m

2.3 m


61.3 cubic meters

12.5 m

2.5 m


Dimensions of the volumes of the insides of rectangular digesters: volume of the digester in cubic meters = length x width x height.

digester volume




ratio of length/ width x height

in meters

1.2 cubic meters

2.5 m

0.8 m

0.6 m


2.2 cubic meters

3.5 m

0.9 m

0.7 m


3.2 cubic meters

4.0 m

1.0 m

0.8 m


5.2 cubic meters

5.3 m

1.1 m

0.9 m


10.4 cubic meters

7.3 m

1.3 m

1.1 m


20.5 cubic meters

10.5 m

1.5 m

1.3 m


30.7 cubic meters

12.8 m

1.6 m

1.5 m


41.1 cubic meters

15.1 m

1.7 m

1.6 m


51.1 cubic meters

16.7 m

1.8 m

1.7 m


61.2 cubic meters

17.9 m

1.9 m

1.8 m


When the cubic measurement of a container is referred to, it is usually in cubic feet or cubic meters. It is the volume, the capacity of the container that is being talked about. When people talk about the size of a biogas digester, they are usually talking about the digester's capacity in cubic feet or cubic meters. In this book it will always be in cubic meters.

The actual volume of the insides of the digesters that are listed above are all a little bit bigger than they should be (such as 5.3 cubic meters instead of 5.0 cubic meters). A little extra space has been added to each digester so that the chances of overfeeding a digester are reduced.

One extra step has to be added before checking the 5:1 ratio on rectangular digesters if the measurements are in feet--divide the result of multiplying the "width x height" by 3.28 or change the feet into meters.

The digester sizes listed above are just examples. If there is a need for a different size, the charts and the equations can be used as guides to figure out the dimension.

The ratios between the length and the diameter of the round digesters and between the length and the surface area of a cross section of the rectangular digesters are also listed in the chart. Any ratio between 3:1 and 9:1 is ok. It is just that a 5:1 ratio is one of the variable factors like a slurry temperature of 35 degrees centigrade (96° F), an acid/base balance of pH 7.0 to 7.5, and a carbon/nitrogen ratio of 30. When the variable factors are all at their best, the digester's biogas production rate and the sanitary quality of the biofertilizer will be at their best.

A 1.0 cubic meter digester is a demonstration model. A 1.0 cubic meter digester has almost the same capacity as five 55-gallon oil drums. The 2.0 and the 3.0 cubic meter digesters are small family-size digesters. Depending on family size, they could be used to meet a family's cooking needs. Remember, digesters will produce less gas during cold weather and rainy seasons, so a digester that is big enough most of the year, may not be big enough all of the year.

A 5.0 cubic meter digester is either a large family digester or a small business digester. The larger digesters are definitely business and cooperative-size digesters. A 10.0 or a 20.0 cubic meter digester may be large enough to economically fuel a stationary engine. The engine could run pumps, small machines and electric generators, or anything that can be powered by stationary engines. The excess engine heat could be used to heat the digester.

I believe that digesters smaller than 5.0 cubic meters will cost more in time and money than they are worth. The smallest digester I would want to build would have a capacity of ten cubic meters. But I would prefer to risk my time and money on larger systems operated as business or cooperative enterprises. The costs of business systems are bigger than the costs of family systems but so are the chances of making money instead of losing money.

Digester Building Materials

When using metal to make digesters or gas storage tanks, use rolled sheet metal (often called mild steel) or galvanized iron (GI), if it is available in large enough sheets to make its use economical. In either case, the sides should be made from 22 gauge metal and the end pieces from thicker 20 gauge metal. The metal sections should be welded together or first riveted, then soldered. (Warning: The hydrogen sulfide in biogas will dissolve lead solder.)

The following two paragraphs are adapted from the Guidebook on Biogas Development. In most countries mild steel is the material which costs the least. Its disadvantage is that it rusts. Usually it is only possible to wirebrush or sandpaper the steel to remove rust before the metal is painted. Ideally, the steel should be sand- or grit-blasted to remove all rust and millseals (the dark blue or black color on new steel) prior to painting. If this is done properly, the life of the paint will be about three times longer than the life of the same paint applied on wirebrushed steel.

The best paints for rustproofing seem to be:

· Low cost--red oxide primer (one coat) followed by enamel paint (two coats).

· Medium cost--anti-saline primer (one coat) followed by high-build black bitumen (two coats).

· High cost--epoxy primer (one coat) followed by epoxy paint (two coats). Epoxy paint should only be used on sand- or grit-blasted steel.

The life of mild steel and galvanized iron could be extended greatly if it is painted on a regular basis or whenever rust appears. Galvanized iron is a low-cost metal, but the joints should be soldered as mastic glue dries, allowing gas and water to escape. Paint does not easily stick to new galvanized iron unless the metal is pretreated with a special preparation sold by large paint companies. If not painted, galvanized iron has a life expectancy of about five years. If properly painted and maintained, galvanized iron, like mild steel, should give many years of good service,

Metal digesters and gas storage tanks can be made with the same types of metal that water tanks are made of and by the same welding shops that make water tanks. e All surfaces must be painted with asphalt or other rustproofing paint, and the outside surfaces should be painted with black enamel paint in addition to the rustproofing paint. The black color of the paint will help the digester absorb more heat.

· Painting the inside of metal digesters and tanks before welding and soldering work is done, will not work. The paint will crack and peel away from the hot metal.

· After the welding or soldering work is finished and the metal has cooled, pour an epoxy primer paint or a plastic paint such as Nylon-60 into the digester. To completely cover the inside of the digester with paint, slowly turn the digester over several times, then pour out the excess paint.

· The inside surface of galvanized iron digesters must be painted because the zinc galvanizing can kill the biogas-producing bacteria.

· If a metal digester is big enough for a person to crawl inside to do the necessary painting, remember that most paint fumes are dangerous, especially when breathed in an enclosed space. Paint fumes can kill. Asphalt paints are the only relatively safe paints.

If it is available from a local hardware store, put what is called pipe joint compound on the threads of all metal pipe to metal pipe connections. Use a pipe joint compound that does not contain lead because biogas destroys lead. The compound will improve the gas-tight quality of the joint, and if the piping system has to be changed in the future, the compound will make it easier to unscrew the pipes from each other because the compound prevents the rust caused by the moisture in biogas from forming on the metal threads.

Red Mud Plastic Bag Biogas Digesters

There is a method of making biogas digesters out of plastic bags. According to the manufacturer. more than 1,000 plastic bag digesters are now in use in Taiwan and more than 60 are in use in the Philippines, Tahiti, Hong Kong, Egypt, Thailand, Singapore, Australia, Brazil, and Paraguay. Red mud plastic is also made into a rigid, hard type of plastic, but so far biogas digesters are just being made from plastic sheets. The digesters look like very large, very long sealed bags with an inlet, an outlet, and a gas pipe coming out of the top near the inlet (Diagram 34).


outlet tank & digester & inlet tank

transverse (crosswise) section of trench

Red mud plastic is a strong plastic which, according to the manufacturer, is ideal for many agricultural purposes because it does not get brittle and crack as fast as most plastics will when exposed to years of direct sunlight. It is flame resistant (but not fireproof) and can be made into either a flexible film or a rigid plastic,

Red mud plastic is easy to repair; a simple patch of the same material can be heat welded onto a hole or cut. Red mud plastic should last more than ten years and maybe as long as 30 years. The plastic gets its name from the waste product called "red mud" that is left over after bauxite ore is used to make aluminum metal. For every ton of aluminum produced, a half ton of red mud is also produced. The plastic is being made by mixing the red mud with (new or used) poly vinyl chloride (PVC) plastic and used engine lubricating oil.

Any country that makes aluminum or plastics should look into the possibility of buying the rights to manufacture red mud plastic products. Where local aluminum industries do not exist, blocks of red mud plastic could be imported and fabricated into products. In these ways, long-lasting agricultural plastic products could be designed for local markets, reflecting local needs and conditions.

The makers of red mud bag digesters claim that the digesters are easy to manufacture, easy to install, easy to transport, easy to maintain, and easy to clean. They say that red mud plastic makes better digesters than those made from other plastics, metal, or concrete. They do add a warning. Even though the plastic bags are strong, it would be very dangerous to walk on the digesters or drop heavy or sharp objects on them. The bags should be protected from these dangers. Red mud digesters, like all digesters, should be heated. Plastic hot water pipes running through a layer of concrete under the digester would heat the digester and the concrete would protect the plastic digester from the colder ground and from attack by insects, rats, and mice.

In Volume 6, Number 2, 1981 issue of the Mexican magazine, "Tropical Animal Production," B. Pond and others wrote an article on the use of a red mud digester in the Caribbean island nation of the Dominican Republic. Diagram 34 shows how the digester was set up.

There was no gas storage tank, so approximately 5.0 cubic meters of the 15 cubic meter digester was used for gas storage. The concrete inlet tank was built big enough to hold one day's slurry load, and the concrete outlet tank was built to hold five day's worth of sludge.

The digester was placed in an unlined trench to support the weight of the slurry. Because the digester was not heated, the average digester temperature was 27 degrees centigrade. Using manure from 15 head of cattle that were kept in a building with a concrete floor, a slurry was made which was only six percent solids (a manure to solids ratio of approximately 1:2). A detention time of 40 days was used and the daily slurry put in and sludge removed volume was 250 liters. The digester was on a slight downhill tilt and the sludge flowed by gravity to fertilize vegetable gardens.

The rate of biogas production was only 0.5 cubic meters of gas per cubic meter of slurry volume. This very low rate of gas production is probably due to the cool operating temperature of the digester. More gas could be produced if the trench was lined with concrete in which hot water pipes were buried in order to heat the digester. It is also possible that a method could be devised to run the heating pipes through the digester itself. Building a greenhouse over the digester would also raise the slurry temperature.

A higher gas production rate could also be produced by reducing the manure to water ration from 1:2 to 1:1.2 or 1:1.5 so that the solids percentage of the slurry would go up from six percent to approximately ten percent. A separate gas storage tank would also help increase gas production. When the biogas is not used fast enough on a continuous basis in a combination digester and gas storage tank, undigested waste is forced out of the digester. A clue that the Dominican Republic digester could produce more gas is the fact that biogas continued to bubble out of the sludge in the outlet tank.

The cost of the 15 cubic meter red mud digester, including shipping to the Dominican Republic (no import tax) and the installation, was US$ 890 in 1980. When compared with other digester designs, the red mud digester was considered by those that used it to be inexpensive, easy to install, and with adequate management, easy to operate. The payback period on the digester was estimated at two and one-half years, using bottled gas (propane) as the only basis for comparison. The value of the fertilizer was not used in the payback comparison.

The June, 1982, export prices (shipping not included) for red mud digesters from Taiwan include: US$ 930 for a 15 cubic meter digester; US$ 1,050 for a 20 cubic meter digester; US$ 1,240 for a 30 cubic meter digester; and US$ 1,820 for a 50 cubic meter digester. The red mud digesters have length to diameter ratios ranging from 2.4:1 to 8:1 in the 15 to 50 cubic meter capacity size range.

As of 1982, red mud plastic was not being used to make the small diameter, flexible pipe which is used for piping biogas for digester to storage tank to use. It would be worth writing the manufacturer to encourage them to produce both flexible and rigid pipe in the ¼ inch to 2.0 inch diameter size range. Red mud plastic pipe would have the advantage of lasting longer than uncovered ordinary plastic pipes, although it would have the disadvantage of not being transparent. It would be impossible to see condensed water blocking the pipes, but if the pipes are laid out correctly, all water should drain into condensation traps.

For current information and prices on red mud agricultural, fish pond, and biogas digester products, write the Taiwan manufacturer. The person to write to is: Daniel F. Lee, President; Lupton Engineering Corp.; 12F-1, Chung Nan Building E; 7 Tung-Feng St.; Taipei, Taiwan, R.O.C. The telex and cable addresses are--TLX: 20246 Daniel Taipei and CABLE: Lenco, Taipei.

Important Digester Loading Rates Information

The organic waste (human, animal, or plant) going into a digester is mixed with water. The liquid portion of the used sludge can be used instead of water, but only after it has been strained of its ten percent solids portion. The solids can be used as fertilizer after being dried in the sun. If the liquid portion is not used to dilute fresh waste, it must be aged in a pond before it can be safely used as a fertilizer.

The organic waste that feeds a biogas digester is only supposed to stay in the digester for a certain number of days; after that time, very little additional biogas can be produced.

All digesters have a limit as to the amount of waste that can be put into them in any one day. The waste will be pushed through the digester too fast and less gas will be produced if the digester is "fed" too much.

The following information shows how much slurry (waste plus liquid) to put into different size digesters.

1) Plant and animal waste is already part liquid; in fact the normal wet weight of pig manure is about three times what it would be if all of the water was cooked out of it: the dry weight.

Moisture content is different for different wastes. Cattle manure contains more water than pig manure, and chicken manure contains less water than pig manure. Digester slurry should be eight to ten percent total solids (the dry weight percentage). The total solids portion is what is left after the liquid portion has been "cooked" out.

2) To make sure the sludge that is taken out of the digester is not carrying any disease organisms, the slurry should stay in the digester for 40 days; this is called the detention time. In other words, 40 days is the total time that any one part of the slurry is in the digester. The daily slurry loading rate is 1/40 of the digester capacity.

3) The loading rate for wet pig manure is approximately ten kilograms per day per cubic meter of digester space. The ratio of pig manure to added liquid is 1.0 to 1.5. If the slurry is weighed in a container, such as a bucket, remember to subtract the weight of the bucket.

4) Take out of the digester a volume of sludge equal to the volume of slurry that is put in. Digesters should be fed at least once a day. Biogas production can be increased by diving the daily slurry volume into two or more feedings, the more the better; but do not increase the total amount added per day.

Digester volume


detention time


daily slurry volume.

digester size in cubic meters and liters


detention time in days


quantity of slurry added daily and sludge removed daily for pig manure, divided into waste and added liquid



40 days


10 kilo manure


15 kilo liquid/25 liters



40 days


20 kilo manure


30 kilo liquid/50 liters



40 days


30 kilo manure


45 kilo liquid/75 liters



40 days


50 kilo manure


75 kilo liquid/125 liters



40 days


100 kilo manure


150 kilo liquid/250 liters



40 days


200 kilo manure


300 kilo liquid/500 liters



40 days


300 kilo manure


450 kilo liquid/750 liters



40 days


400 kilo manure


600 kilo liquid/1,000 liters



40 days


500 kilo manure


750 kilo liquid/1,250 liters



40 days


600 kilo manure


900 kilo liquid/1,500 liters

This slurry ratio formula is for pig manure. If plant waste is used, more liquid will be needed to achieve a ten percent solid content in the slurry. The waste to water ratio may be 1.0:2.0 or even higher. The higher the water content of the original waste, the less additional liquid is needed. The lower the liquid content of the original waste, the more additional liquid is needed. This table is only a guide.

The slurry, when it goes into the digester, should look like thin, watery mud (manure mud, plant mud, manure plus plant mud). The total number of liters going in and coming out of the digester remains the same; it is the ratio between waste and added liquid that changes--depending on the water content of the waste.

An example: Only cattle (water buffalo, carabao or milk cow) manure is used and there are 25 cattle, each producing nine kilograms of manure per day. Cattle manure has a higher liquid content than pig manure, so to maintain an eight to ten percent solids content in the slurry, a ratio of cattle manure to additional water of 1:1 is used, not 1:1.5, which is the ratio for pig manure and water. The daily slurry volume is then 225 (25 x 9) kilograms of manure plus 225 liters of water (one liter of water weighs one kilogram) for an approximate total of 450 kilograms/liters of slurry.

To find the best size for a digester, multiply the daily slurry volume times 40 (the number of days it should take before one day's load is taken out of the digester). The total volume is 18,000 (450 x 40) and since there are 1,000 liters in a cubic meter, the volume of the digester should be 18 cubic meters (18,000 divided by 1,000), or better yet, 20 cubic meters to allow space for extra manure and/or plant waste.

Approximate total solids content of several digester fuel sources:

water lily (hyacinth)


plant waste average








chicken manure (fresh)


chicken manure (day old)


pig manure


human feces






Slurry in glass jar

-Hydrometer floating in slurry; make sure there are no lumps and that the slurry is stirred in order to make an accurate reading.

Relationship of percentage of solids to hydrometer readings.

A hydrometer is an instrument that can be used to measure the percentage of solids in a liquid such as digester slurry. There are several different types of hydrometers. A common design is used to measure the strength of car batteries. Do not use that type. Use the type shown in Diagram 35.

Make sure the hydrometer has a scale designed for the range 1.00 to 1.40 specific gravity (1.00 = water) or 0 to 40 Baume (0 = water).

If the hydrometer reading is between 1.10 and 1.34 specific gravity (12.5 and 36.5 Baume), then the percentage of solids in the slurry will be between eight and ten percent, which is what is wanted.

The best way to read a hydrometer is to look straight across at the scale. It is easy to get a false high reading when looking at an angle, because liquid often "rides" up the outside surface of the scale a few millimeters.

The smaller the pieces of organic matter in a slurry are, the more accurate a hydrometer reading will be. Make hydrometer readings on three separate batches of slurry, add the readings together, and divide by three to get an average (and more accurate) reading. Make sure the slurry samples contain the same percentage of solids as the slurry as a whole does, or the readings will not reflect what the solids percentage of the slurry is.

pig weight

manure only





















chicken manure per day

0.1 kilogram of manure from a 2.0 kilogram chicken

human (adult) waste per day

urine 1 liter (1 kilo)
feces 0.2 kilogram

cattle (full grown)

10 to 15 kilograms of manure per day

Biogas Facts

BTU stands for a common measurement of heat: British Thermal Unit. One BTU is the amount of heat needed to raise the temperature of one pound of water one degree Fahrenheit. It takes 3,413 BTUs to equal one kilowatt hour of electrical power. Biogas has a BTU per cubic foot of 600-700. After the carbon dioxide has been scrubbed out (removed) and the water vapor is removed, biogas is almost pure methane. Methane has a heat value of approximately 1,050 BTUs per cubic foot. The BTU per cubic foot of butane is 2,900-3,400. The BTU per cubic foot of propane is 2,200-2,600.

Can biogas, scrubbed or unscrubbed, be used for welding purposes? There are reports that unscrubbed biogas can be used for welding. A magazine article on biogas described a farmer's one cubic meter of biogas per day as being "enough to run his welding machinery."

The United Nations Guidebook on Biogas Development says that the temperature of an oxygen-methane flame is 3,000 degrees centigrade, about 250 degrees lower than an oxygen-acetylene flame. The temperature of an oxygen-biogas flame will be lower than the flame of the oxygen-methane combination. The actual temperature depends on the percentage of methane in the biogas. The lower temperature of the oxygen-biogas combination means that it can be used for brazing (hard soldering) purposes but that it should not be used for welding metals containing iron.

Percentage and density (in kilograms per cubic meter) of basic gases in air and in biogas:


nitrogen: 78% of air = 1.25
oxygen: 21% of air = 1.43
carbon dioxide: 0.04% = 1.98


methane: 65% of biogas = 0.72
carbon dioxide: 32% of biogas = 1.98
hydrogen sulfide: 1% of biogas = 1.54

Rates of use for methane and biogas from several sources:

gasoline engine (standard 25 percent efficiency):


0.30 cu.m. per brake horsepower per hour


0.43 to 0.56 cu.m. per brake horsepower per hour


0.42 cubic meters per kilowatt hour


0.60 cubic meters per kilowatt hour

as gasoline alternative:


0.95 to 1.20 cubic meters per liter of gasoline


1.30 to 1.80 cubic meters per liter of gasoline

as diesel fuel oil alternative:


1.10 to 1.40 cubic meters per liter of diesel fuel


1.50 to 2.10 cubic meters per liter of diesel fuel

as liquid (bottled) butane alternative:


1.16 cubic meters per liter of bottled butane

One cubic meter of biogas can:

1) keep a 1-horsepower engine working for two hours;

2) do the work of 0.6 liters of gasoline;

3) generate 1.25 to 1.70 kilowatt hours of electricity;

4) fuel a gas refrigerator for almost 24 hours; and

5) keep one gas lamp lit to a brightness equal to a 60-watt electric light bulb for six hours, or seven gas lamps lit for one hour, or twenty-five 60-watt electric light bulbs lit for one hour.

More examples of biogas rates of use:

1) a 1-hp engine used 12 cubic meters of biogas per 24 hours;

2) a 7-hp engine used 85.3 cubic meters of biogas per 24 hours;

3) a 13-hp engine used 170 cubic meters of biogas per 24 hours;

4) a 24-hp engine used 300 cubic meters of biogas per 24 hours;

5) a 1-kw generator used 0.62 to 0.81 cubic meters of biogas per kw hour;

6) a 12-kw generator used 1.5 cubic meters of biogas per kw hour.

The American consulting firm of Shaeffer and Roland, Inc., found the following to be true about a small biogas system it developed.

A 22.7 cubic meter digester produced: 22.4 cubic meters of biogas per day, 38 kilowatt hours of electricity per day (using a 2-kw engine-generator with an induction motor as the generator). 75.5 million BTUs of heat per year were produced of which 63.6 million BTUs were used to heat the digester and 12.2 million BTUs were used for other heating purposes.

The manure of 25 beef cattle was found to be equal to the manure of 17 dairy cows, or 183 pigs, or 23 horses, or 43,750 laying chickens.

This 22.7 cubic meter biogas system in the United States cost US$ 6,800 in 1980, produced US$ 1,800 per year in fuel and fertilizer, had a payback period of 3.8 years. The economic value of the fertilizer was considered to be greater than that of the fuel.

Biofertilizer Facts

The results of five different studies of the chemicals in five different organic fertilizers:

1) Manure from 25 beef cattle (weighing approximately 455 kilograms each) can produce a biogas digester sludge with an annual value of: 1,273 kilograms of nitrogen; 352 kilograms of phosphorus; and 811 kilograms of potassium. That is a N:P:K ratio of 3.6:1.0:2.3. (Shaeffer and Roland, Inc.)

2) Digester sludge is 0.5 to 2.2 percent nitrogen.

3) Compost is 0.75 to 1.0 percent nitrogen.

4) Digester sludge, liquid and solid, compared:

nitrogen %

phosphorus %

potassium %


0.03 - 0.08

0.02 - 0.06

0.10 - 0.50


0.08 - 1.50

0.40 0.60

0.60 - 1.20

5) Digester sludge in India and Brazil studies compared:




nitrogen (N)



phosphorus (P)



potassium (K)


One study found that the sludge from biogas digesters is a fertilizer which can increase crop yields 10 to 20 percent. A 20 cubic meter digester produced enough sludge to fertilize approximately one hectare (2.5 acres) of land at a rate of one cubic meter (1,000 liters) of sludge per 100 square meters of land.

The Guidebook on Biogas Development says that biogas fertilizer increases crop yields for potatoes, tomatoes, melons, guavas, mangoes, onions, sugar cane, rice, and jute; but that cotton and coconut yields did not increase very much when biogas fertilizer was used.

According to another study, when compared with compost, biogas fertilizer resulted in the following increased crop yields: rice 6.5 percent, corn 8.9 percent, wheat 15.2 percent, and cotton 15.7 percent. Biogas fertilizer is also often mentioned as a good fertilizer for coffee crops.

Biogas and Engines

Reading Industrial Engines of Reading, England, adapts engines to run on biogas. What follows are some of the things they have learned about biogas and engines.

When using biogas to fuel spark-ignition (gasoline) engines, use non-rotating valves with stellite-faced and induction hardened valve seats to prevent valve and seat recession.

A Ford six-cylinder water-cooled diesel engine with a 6.2 liter capacity and a 9:1 compression ratio was operated at 2,400 rpm and reduced through a gearbox to 1,500 rpm to drive an AC generator at an electrical output of 56 to 62 kilowatts. The engine had a biogas consumption rate of 24 to 30 cubic meters per hour. When the same engine was run at 1,500 rpm with an electrical output reduced to 25 to 30 kilowatts, biogas consumption dropped to 18 to 24 cubic meters per hour.

Using a Ford four-cylinder 1.6 liter water-cooled engine, operated at an engine speed of 3,000 rpm to drive a 20-kilowatt, 50-cycle, 3-phase AC generator, 16 to 20 kilowatts of electricity were generated with a biogas consumption rate of 9 to 12 cubic meters per hour.

The heat from engine coolant and exhaust systems were used in both cases to heat biogas digesters. Heat exchangers were used and in both cases this increased efficiency approximately 80 percent.

Important Slurry Carbon/Nitrogen Information

The carbon to nitrogen ratio (C/N) represents the proportions of two very important elements for successful digestion of organic wastes in a biogas digester. An organic material with 15 times more carbon than nitrogen has a C/N of 15 to 1. This ratio is often written as C/N = 15/1, 15:1, or simply 15.

A C/N of 30 will permit digestion to proceed at the highest possible rate. But: the nitrogen and carbon content of plant and animal waste changes greatly, depending on the type, age, and growing conditions of the plant, and the kind, age, diet, and degree of confinement of the animal. The list of ratios for different plant and animal wastes are averages.

The composting of plants is another factor that changes the C/N ratios; it lowers them. This can be very helpful if the digester is to use a plant waste with a high C/N ratio such as cornstalks and leaves. One week of Composting can bring a plant waste with a C/N of 100 down to a C/N of 20, which is a good C/N ratio for biogas digestion.

It is very important that all plant waste be composted before being used in a digester. Four things should be remembered when plants are used:

· compost for seven to ten days (longer in cold weather below 21 degrees centigrade/70° F);

· always shred, grind, or pulp the plants before Composting

· let nothing get into the digester than can float; and

· plants put into a digester should not look like plants. They should look like a thin soup, a watery mud.

Animal Waste Carbon/Nitrogen Ratios:





dairy cow


water buffalo












human feces




fish and meat (no fat)


Plant and Crop Waste Carbon/Nitrogen Ratios:

rice straw


wheat straw


oat straw


cornstalks & leaves


peanut stalks & leaves


soy bean stalks


nonleguminous vegetables


sugar cane waste (bagasse)


green plants


water lily (hyacinth)


fallen leaves


grass and weeds












Important Slurry Recipe Information

1) It is not possible to completely predict how any one organic material will respond to biogas digestion just because we know something about the material's chemical composition; in this case, how much more carbon than nitrogen is in the material? The biogas process is a biological process, and biological processes tend to have many variables that make them hard to predict.

2) As a guide to tell if the digester is getting what it wants to "eat," prepared and served the way it likes its "food," remember this basic rule--if the digester is producing close to one cubic meter of biogas per 24-hour day (in hot weather: 35° C, 96° F), for every cubic meter of digester space, all is well. This is true for unheated digesters that have a capacity of at least five cubic meters. Larger, heated digesters can produce up to two cubic meters of biogas per cubic meter of digester space per 24-hour day.

3) To prepare plant waste for biogas digester, the Guidebook on Biogas Development suggests the following:

(a) Cut, grind, or crush the plants into very small pieces.

(b) Break down the plant fibers so that they will not float by composting for one week with lime or a suitable enzyme such as cellulose. If local agricultural supply stores or septic tank dealers do not sell such enzymes, ask them to see if their suppliers can get them. (Warning: Too much lime in the slurry can make the sludge too alkaline to be a good fertilizer for crops or fish ponds. Too much enzyme in the slurry can make the sludge too dangerous to be a good fertilizer for crops or fish ponds.)

(c) Mix the dissolved plant matter with enough water to make a slurry which is ten percent solids, 90 percent liquids.

4) When plant wastes are used in 8 digester, high levels of organic acids will be released during digestion and the all-important pH level will fall. Alkaline (base) chemicals such as lime solutions or grass ash mixed in water should be added on a regular basis to slurry that has a high plant content in order to maintain a neutral balance of acids and bases.

Add lime. Some people say yes; some say no. Should it be added occasionally to digesters that are not primarily plant waste digesters? As far as D. House (1978) is concerned, the answer is yes. But all biogas system operators should make their own experiments and decide for themselves. Andrew Barnett (1978) says that sodium bicarbonate, which is commonly called baking soda, is a better buffer than lime.

Lime is one of those words that is not very well defined. Generally speaking, it refers to a group of calcium compounds. Limestone, the common rock, is largely calcium oxide (calcium + oxygen). Here, lime water will refer to calcium hydroxide which is also known as slacked lime (calcium + oxygen + hydrogen). Lime will be used to describe calcium carbonate (calcium + carbon + oxygen). All of these calcium compounds are, in other publications, sometimes referred to as lime. Lime water and lime can be of help to biogas digestion, but limestone is no help at all.

Responses to lime vary depending on the organic waste used in the digester. Lime also has the interesting characteristic of combining, above pH 5.0 with carbon dioxide, removing it from the slurry, and therefore increasing the percentage of methane in biogas.

The addition of relatively large amounts of lime to the slurry can cause carbon dioxide to be removed from the biogas atmosphere above the slurry. This can create a partial vacuum above the slurry. The danger in this situation is that if there are any leaks in the biogas system, air will be pulled into the digester, killing the biogas-producing bacteria and creating the possibility of an explosion.

If lime is used, not much is needed. 0.2 to 0.3 grams of lime dissolved in water should be mixed with every liter of slurry, or the lime can be added slowly while the pH is checked until the pH rises to between pH 7.0 and 8.0.

If lime is added too fast after the digester becomes stuck (acid pH, no digestion, no biogas production), it may cause foaming to occur. If a digester has a very high acid pH, add a little lime; then wait a day or two before adding more lime. Repeat the treatment until gas production starts; then wait and see if the pH level can correct itself. If it does not, continue small, cautious additions of lime until all is well again.

Lime has a problem. It does not dissolve easily, so it often does not mix evenly throughout the digesting slurry. A measurement of pH, therefore, may not give an accurate picture of the true pH average inside the digester.

Baking soda or ammonia can be used as a substitute for lime. Ammonia (nitrogen + hydrogen) is toxic to the biogas process if used in too great a concentration. Too much will kill the biogas bacteria, but a little prepared in the right way has been found to help correct acid pH conditions. That right way is to mix 1/10 (0.1) of a liter of ammonia in four liters of slurry for every one cubic meter of digester space.

Ammonia can be bought in most drug stores, but it is usually diluted with water. If possible, buy a bottle which lists the concentration of ammonia so that a 1:10,000 ammonia volume to digester volume ratio can be maintained. Do not use ammonia which contains anything else besides ammonia and water.

Usually pH is measured with litmus paper, which can be bought in some drug stores and from chemical supply companies. Litmus paper changes color in response to the pH level. When litmus paper is dipped into a solution, the color it changes to can be compared with a color chart that is sold with the litmus paper. Each color on the chart represents a different pH level.

pH measurements:




biogas composition


gas production is low, gas will not burn, sludge poorly digested

add buffer: lime, baking soda, or ammonia

high in carbon dioxide and hydrogen sulfide, low in methane


all is well


high in methane


rare situation, no common indicators other than litmus test

wait, raise C/N ratio

low in methane, other gases vary

5) Toxins are poisonous substances produced by living organisms. In many biological processes, the by-products of the life process (in animals: manure and urine) are toxins. Most creatures can be poisoned by their own toxic wastes--if they cannot get rid of them.

Under normal conditions of biogas production, the ecosystem (the world) of the digester prevents the buildup of toxic wastes. An ecosystem is a living system which supports not one, but many different kinds of life, each in some way dependent on the others. In a biogas digester this means that one bacteria's waste is another bacteria's food. The acid forming bacteria, which are the first bacteria to attack fresh slurry, come in many different types, with many different "food" preferences. They break down very complex molecules into simpler molecules.

The by-products of acid forming bacteria are almost all used by the methane forming bacteria. Some of the carbon dioxide passes off with the methane, some stays in the slurry, and some is used as a source of carbon by the methane bacteria when they make methane.

Ammonia, which is not produced in large quantities in a digester which has a balanced carbon/nitrogen ratio, apparently acts as a toxic waste rather than a food when, because of a low C/N ratio, it is produced in large quantities. In a similar way, sugar is an excellent food for bacteria. But honey, which has a very high sugar content, will not support bacterial life.

One biogas expert wrote that the waste from citrus fruits such as oranges, lemons, and grapefruits stops the production of biogas so completely that the digesters in which the citrus fruit slurry was used had to be cleaned out before they could be started again. This toxic reaction may be caused by a substance which is a chemical inhibitor to biogas bacteria called d-limonene, which is found in the peels of citrus fruits.

If soap of any type gets into the digester, it will slow down--maybe even stop--the production of gas and fertilizer. The chemicals in soap can kill biogas bacteria. This is something to remember when human and/or kitchen waste is used to make a digester slurry.

The antibiotics in human medicine, veterinary medicine, and in medicated animal feed can kill biogas bacteria. Antibiotics have a definite life. In other words, a certain number of days after being used beyond which they can no longer kill bacteria. Antibiotics will try to kill all bacteria, harmful disease-producing bacteria, and helpful biogas-producing bacteria.

Use manure from medicated animals only after the medicine is supposed to be ineffective. If a digester's bacteria are killed by the antibiotics in manure from animals that were fed medicated feed, the digester can usually be restarted only by completely cleaning out the digester and starting over.

Other toxins: Chemical herbicides, chemical pesticides, and heavy metals which are often the result of industrial pollution can kill biogas bacteria. The most dangerous metals are chromium (Cr), copper (Cu), nickel (Ni), zinc (Zn), and mercury (Hg). Of these, copper is often used in pesticides to kill fungus, and zinc is used in galvanized iron (GI) sheets, pipes, water tanks, and buckets.

Zinc will not usually be found in a digester unless a galvanized surface is directly exposed to the slurry, as with digesters made from galvanized iron. If galvanized iron is used to make a digester, it must be painted on the inside to protect the biogas bacteria from being killed by the zinc.

One source believes that red lead paint, which is a possible choice for painting metal digesters, is itself harmful to biogas bacteria. They suggest that tar or plastic paints be used to paint the insides of digesters. Lead is a very real danger to people if plants picked up lead from lead paint contaminated fertilizer.

There are three things that can be done to reduce or eliminate the deadly effects of toxins. For biogas bacteria, the major toxins include antibiotics, soaps, copper, zinc, and too much ammonia.

· The toxins can be diluted.

· They can be chemically changed.

· They can be stopped from getting into biogas digesters to begin with.

If worst comes to worst, do not surrender; just clean out the digester and start over.

6) Chicken feathers and hair rot very slowly. Do not let them get inside biogas digesters. Fats and grease will always float and become part of the scum layer. If ground up meat scraps are used, make sure all fat has been removed.

7) There is a belief that storing manure for a few days will increase its biogas-producing capability. Experiments using manure stored for one and three days show that fresh manure is best for biogas production.

8) How can chicken manure be collected for use in biogas digesters? The Compleat Biogas Handbook has two ideas.

· Let the manure fall through wire bottom cages onto glass plates. Fog the glass with a fine spray of water and use car windshield wipers to scrape the wet manure into a collection gutter.

· Another wire cage method is to let the manure fall on concrete and collect it daily by shovel or broom. With this second method, some ammonia nitrogen, a valuable fertilizer, is lost when it is absorbed into the concrete. Both methods must include a way to remove the chicken feathers.

9) The more plant waste is shredded, ground up, and pulped into a soft, moist mud; the more manure is mixed to break up the lumps; the easier and more successful will be the biogas digestion process. Plant waste will digest rapidly and completely if it is first cut into tiny little pieces and partly composted. What goes into the digester should not look like plants; it should be a watery, dirt-free mud, with nothing floating in it.

10) Water buffalo (carabao): If the manure is collected with the urine, mixed with cut-up and partly composted plants, and digested with a little lime and enough water, an excellent digester food will be the result.

11) Cornstalks: When cornstalks and leaves are shredded and put directly into a digester, a lot of scum will be produced. The scum soon stops biogas production. By soaking the cornstalks and leaves in water for four days after they have been shredded and then bringing the pH level back to neutral before the slurry goes into the digester, there will be a very large increase in biogas production without major scum problems.

12) Kelp or any kind of brown seaweed: Not only was washed kelp used successfully in fresh water digesters, unwashed kelp was used in sea water digesters. The digesters were slowly changed from fresh water to increasing concentrations of salt water. A stable and successful ecosystem was established in one of the digesters that was converted from fresh water to sea water.

13) Leaves: Biogas has been successfully produced from leaves, using urine as the nitrogen source to balance the high carbon content of the leaves. Leaves that will not do well in aerobic compost will probably not do well in anaerobic biogas digesters.

14) Rice straw: When using rice straw, an almost 30 percent increase in biogas production was achieved because the nitrogen content of the slurry was increased. The normal C/N ratio of rice straw is 50 to 70.

15) Water lilies (water hyacinths): Using water weeds in digesters can reduce transportation problems caused by large numbers of the weeds blocking rivers and lakes. Soaking chopped up water lilies in lime water for four days is one way to prepare the weeds for biogas digestion.

Another method is to grind the water lilies into a pulp, put them in a pile, keep them moist, and let them get moldy--in other words--compost them for a week. The mold fungi will accomplish much the same thing naturally as the lime water can accomplish chemically. Because molds, like biogas bacteria, may need to be cultured, keep a portion of the moldy plants so that fresh batches of the same plant can be infected with the molds best suited for breaking down the fibers of the plant.

16) Algae does not always digest well in digesters because the environment inside biogas digesters does not always kill the algae. One solution is to first dry the algae, so that it is dead when added to the slurry. Algae does not have much plant fiber, so it does not need to be partly composted before it is used in digesters. Another solution is to let animals drink water that is full of algae.

17) Wood: Few studies are available on the digestion of wood, sawdust, and rice hulls. In nature wood is almost never attacked by bacteria. Wood is attacked first by fungi, and only later by bacteria. Fungi are a large group of simple plants that cannot use sunlight, as most plants do, to produce energy. Fungi include molds, mildews, and mushrooms. Some parts of wood such as tannins and turpentines are toxic to bacteria. Wood is designed to resist breakdown because it lives so long. Complete Composting of hard to rot plant wastes that are high in fiber and lignin, such as wood, sawdust, and rice hulls, is probably the best way to use them. Never let wood, sawdust, or rice hulls get inside a biogas digester.

18) Lignin is a common material (a type of carbon) in plants that causes most of the floating scum problems in biogas digesters. Lignin makes plants rigid and stiff. Lignin gives plants the ability to stand up and the ability to float in water and digesters.

Composting, lime, and certain enzymes break down lignin so that it no longer is a scum problem. But not only does composting break down lignin, it also allows aerobic bacteria to use the biogas bacteria's food. So if Composting is used at all, seriously think about complete composting instead of partial Composting A simple way to prepare plant waste for biogas digesters is to feed the plants to cud-chewing animals such as cattle or sheep and use the manure to fuel the digesters.

Lignin as a percentage of total solids in animal manure and plants (animal manure percentages will change depending on what they eat):










dairy cow





rice straw


wheat straw


rice hulls






Sample recipes for digester slurries: These recipes are estimates, based on carbon/nitrogen ratios, experience, and educated guesses. All of these combinations need some buffering if they use plants. Newspaper that has been wetted and sun dried will be easier to shred and ground into tiny pieces. Remember, newspaper is a plant waste; it will need grinding and partial composting like any plant before it goes into a digester.

The quantities of waste listed below are the amounts needed for one cubic meter of digester space. Notice the difference between amounts of manure and amounts of plant waste needed per cubic meter of digester space. This is due in part because all wastes have different percentages of water in them, while digester slurry should always be about 90 percent water.

1) leaves and cattle manure (1:5.5:7.3):

air dried leaves

1.8 kilograms

fresh cattle manure

10.0 kilograms

water, no less than

13.2 liters/kilograms

2) leaves and kitchen garbage (1:2.4:15.8):

air dried leaves

1.3 kilograms

fat free kitchen garbage

3.1 kilograms

water, no less than

20.6 liters/kilograms

3) newspaper and kitchen garbage (1:1.6:10.5):

shredded newspaper

1.9 kilograms

fat free kitchen garbage

3.1 kilograms

water, no less than

20.0 liters/kilograms

4) young grass and leaves (1:3.9:8.2):

air dried leaves

1.9 kilograms

young grass

7.5 kilograms

water, no less than

15.6 liters/kilograms

The following manure-based slurry recipes come from many different countries and were used in many different types of digesters. Use them as starting points. Experiment with the ratios; improving them will result in higher gas production rates.

Chicken : water

chicken : grass : water

chicken: newspaper: mango + banana + papaya waste (no seeds or stems):water

pig : cornstalks : water

pig : fallen leaves : water

pig : soy bean stalks : water

pig : rice straw : water

Cubic feet of biogas produced by volatile solids of combined wastes (Merrill and Fry, 1973):



cubic feet of gas per
pound vol. solids

methane content
of the biogas

chicken manure




chicken manure


+ paper pulp




chicken manure


+ newspaper




chicken manure


+ grass clippings




cattle manure




cattle manure


+ grass clippings




pig manure




Notice the trade-off between biogas volume and biogas quality for the different combinations. A standard combination in China is: 10 percent human waste, 30 percent animal waste, 10 percent crop waste, and 50 percent water.

Digester Temperature Information

1) The "greenhouse effect" refers to the fact that a building made of glass or plastic will let sunlight in and out but will keep most of the sun's heat energy from escaping. Greenhouses are used in cold climates to grow plants indoors when the weather outside is too cold.

The more a digester is surrounded with a roof and walls of plastic or glass, the hotter the digester will get, and the more biogas it will produce. Up to a point. Do not let a digester slurry get hotter than 35 degrees centigrade (96° F) on a regular basis or less gas will be produced. A complete greenhouse around a digester is best, but even a plastic or glass roof will help heat the digester.

2) The lower the temperature is below 35 degrees centigrade (96° F), the harder it will be for the bacteria in the digester to digest plant and animal waste. During cold seasons and rainy seasons when the temperature drops below 21 degrees centigrade (70° F), it is a good idea to reduce the daily slurry load by as much as 50 percent (one-half) if the digester is unheated. Any extra waste can be completely composted for use as compost fertilizer.

Doing this will not bring biogas production rates back up to hot weather levels, but it will keep the quality of the fertilizer from dropping. People often eat more in cold weather, but the opposite is true for biogas bacteria.

Finding the Biogas Production Rate

It is very important to know what is happening inside a digester. Checking for changes in the biogas production rate is an easy way to discover problems when they are small, before the digestion process is seriously harmed.

There is a simple math formula for finding the rate at which biogas is produced. It is: volume = 3.14 x radius x radius x height. This is the same formula as the one used for finding the volume of round digesters and gas storage tanks.

V = volume of gas produced in a fixed time period
R = radius of gas tank (half of diameter)
H = height the gas tank rises in that fixed amount of time
h1 = height at start of test
h2 = height at end of test

Time period is one hour in this example.

Measure the height of the floating gas tank from the water seal level to the top of the gas tank (h1). After one hour, during which time no biogas is used, measure the same distance again (h2). If the gas tank reaches the cross bar at the top before the time period is up, or if gas bubbles out from under the water seal for any reason, the result of the test will not be accurate.

An example:

R = 0.75 meters (75 cm)
h1 = 0.08 meters (8 cm)
h2 = 0.65 meters (65 cm)
H = h2 - h1, H = .65 - .08, H = .57
V = 3.14 x .75 x .75 x .57
V = 1.0067

Volume = 1.0 cubic meter of gas per hour or 24 cubic meters per 24 hour day (at a constant pressure such as 6.0 column incheses of water)

Sources & Resources

A Bibliography

1) Practical Building of Methane Power Plants for Rural Energy Independence by L. John Fry, 1974. Costs US$ 14.25 shipping included, from META Publications, P.O. Box 128, Marblemount, Washington 98267, U.S.A. This book was my primary source for biogas theory and design concepts. Mr. Fry has been working with biogas systems since 1958.

2) Methane Digesters for Fuel Gas and Fertilizer by Richard Merrill and John Fry, 1973. This book was available from Volunteers in Technical Assistance (VITA).

3) Biogas and Waste Recycling--The Philippine Experience by Felix D. Maramba, Sr., 1978. This book is available from Maya Farms Division, Liberty Flour Mills, Inc., Pasay Road, Makati, Philippines * telephone 86-50-11 * P.O. Box 459 MCC, Philippines, Cable: LIBFLOUR MANILA, Telex: 7-22490-EEC-PH. The book costs approximately US$ 4.00. It is an excellent book, especially for businesses which are thinking about starting a large scale biogas system. The book describes what Maya Farms learned in their profitable ten year, one million peso investment in biogas technology. The addresses given here can also be used to write to the Maya Farms Bio-Energy Division.

4) The Compleat Biogas Handbook for Farm and Home by D. House, 1978. This book costs US$ 9.25 shipping included, from META Publications. It is interesting reading for people who want a lot of technical information.

5) A Chinese Biogas Manual translated by Michael Cook, edited by Ariane van Buren, published by Intermediate Technology Publications Ltd., 1976, and was available from VITA.

6) Construction of Fixed Top Enclosed Biogas Plant prepared by Mien Chu District, Province of Szechuan, China, and was available from VITA.

7) Methane Digesters is the title I gave a large collection of articles on biogas systems, primarily from projects in India and China, which were available from VITA.

8) Biogas Technology in the Third World: A Multidisciplinary Review by Andrew Barnett, Leo Pyle, and S.K. Subramanian, 1978. This book cost US$ 10.00 from IDRC, Box 8500, Ottawa, Canada K1G 3H9. An informative but very technical book, not for the beginner.

9) An Inexpensive Anaerobic Digester for Small Farms by John Martin and Philip Lichtenberger, May, 1981. Published by the consulting firm of Sheaffer and Roland, Inc., 6308 Buffie Ct., Burke, Virginia 20015, U.S.A.

10) Guidebook on Biogas Development, Energy Resource Series No. 21 of the United Nations Economic and Social Commission for Asia and the Pacific. Published: New York, 1980, ST/ESCAP/96, sales number: E-80-II-F-10, price: US$ 11.00. A good collection of standard biogas literature based on experience gained from the Indian and Chinese digester designs.

11) Renewable Energy by Daniel Deudney and Christopher Flavin, published by Worldwatch Institute, 1983. In their own words, the "Worldwatch Institute (1776 Massachusetts Avenue, N.W., Washington, D.C. 20036, U.S.A.) is an independent, nonprofit research organization created to analyze and to focus attention on global is funded by private organizations and the United Nations." Renewable Energy is a well-written, easy-to-read "textbook" on the subject of renewable energy. It is not a "how-to" manual; it is a study of the social issues of energy development. How a technology like biogas fits into a community often has much more to do with its success or failure than the quality of the technology.

12) Chinese Biogas Digester: A Potential Model for Small-Scale, Rural Applications (A Manual for Construction and Operation), prepared by Charles H. Nakagawa with Q.L. Honquilada. A joint project of The Philippine Rural Reconstruction Movement, The U.S. Peace Corps/Philippines, and The German Freedom From Hunger/Agro-Action. Draft: December 1981.

13) "Peace Corps/Philippines," the magazine of the United States Peace Corps in the Philippines. It is available upon request from the U.S. Peace Corps, P.O. Box 7013, M.I.A., Manila, Philippines 3120. The December, 1979, issue had an article on a demonstration model biogas digester built by Peace Corps Volunteers and the Philippines Rural Reconstruction Movement at the PRRM Farm in San Leonardo, Nueva Ecija. It was a digester of that design that I built and operated for a year.

14) "Philippine Farmers' Journal," 113 West Avenue, Quezon City, Philippines. The price is 3.50 pesos (approximately US$ 0.50) per copy, per month. A good magazine that sometimes has biogas articles.

15) "Tropical Animal Production" is a technical agricultural magazine with editions in English and Spanish, costs US$ 10.00 per year for 3 issues and is available from: Consejo Estatal de Azucar, DR, Universidad de Yucatan, Escuela de Medicina Veterinaria y Zootecnia, Apdo. 116D, Merida, Yucatan, Mexico.

16) Ferrocement Water Tanks and Their Construction by S.B. Watt, 1978. Published by Intermediate Technology Publications, Ltd., and available from ISBS Inc., P.O. Box 555, Forest Grove, Oregon 97116, U.S.A. This is the best basic book I have found on the ferrocement concrete technique.

17) Solar Water Heater by Dale Fritz, 1979. This book is published by VITA and is available in English for US$ 2.95 and in French for US$ 3.95. It describes how to build and operate a simple, low-cost solar collector. For the beginner, those who have a small budget, and those who do not need large quantities of hot water, this is an ideal water heater. The manual includes step-by-step illustrated construction plans.

18) Solar Energy Handbook by Dr. Jan F. Kreider and Dr. Frank Kreith, 1981. This book costs US$ 50.00 and is published by McGraw-Hill, Inc., which has a bookstore at 14th and Constitution Avenue, N.W., Washington, D.C., U.S.A. This is a very thick book written for experts and engineers. It discusses in great technical detail the many different types of solar energy applications. Rankine engines are the only solar-powered heat engines described in detail because the authors believe Rankine engines are the only currently practical heat engines for temperatures below 300 degrees centigrade. This is an excellent, but expensive and very technical, book.

19) Energy for Rural Development: Renewable Resources and Alternative Technologies for Developing Countries is a two book series. The original book was published in 1976 and the supplement in 1981. The subjects covered include solar collectors, photovoltaics, wind energy, water power, biogas, and pedal power. While the supply lasts, a free copy of each book can be ordered by people affiliated with institutions in government, education, or research. When writing for the books, indicate name, title, and institutional address. The address to write to is: Commission on International Relations (JH 214), National Academy of Science--National Research Council, 2101 Constitution Avenue, Washington, D.C. 20418, U.S.A.

20) Report and Recommendations on Organic Farming, United States Department of Agriculture, Washington, D.C., July, 1980.

21) The Samaka Guide to Homesite Farming by Colin Hoskin, 1973. Published by the Samaka Service Center, P.O. Box 2310, Manila, Philippines. The Samaka Guide is a good, basic book on family farming practices in the Philippines. It has chapters on water supply, fertilizer, vegetables, fruits, chickens, ducks, pigs, goats, fish ponds, carabao, food preserving, and many other interesting subjects. The book is out of print now, but libraries may have it.

22) "An Insect Control Method--Too Good to Be True," October, 1976; "The Bug Juice Method: How Safe? How Effective?", May, 1977; "More Backyard Blender Sprays," April, 1978; "The Safe Pesticides in Your Own Backyard," July, 1979, all by Jeff Cox, and "Testing Organic Pest Controls," August, 1981, by Michael Lafavore, are from the magazine "Organic Gardening and Farming." The magazine would like to get letters from people who try the bug juice and plant juice biological insect control methods. Good news and bad news; it is all welcomed. The address to write to is: Reader Service, Bug Juice Test, Organic Gardening, Emmaus, Pennsylvania 18049, U.S.A.

In 1979, Rodale Press, the publishers of "Organic Gardening," started a new magazine called, "The New Farm." It looks like an excellent magazine for family farmers who are interested in producing profitable, high-quality crops by working with nature, not against it. The address to write to for subscriptions is: "The New Farm," 33 East Minor St., Emmaus, Pennsylvania 18049, U.S.A. The magazine costs US$ 11.00 per year in the United States and US$ 15.00 per year in all other countries.

23) Lik-lik Buk, a Rural Development Handbook Catalogue for Papua New Guinea. Published and distributed by: Lik-lik Buk Information Centre, P.O. Box 1920, Lae, Papua New Guinea. The book costs US$ 5.95 from VITA, or K 5.50 surface mail and K 9.00 air mail from Lik-lik Buk. The book contains hundreds of practical appropriate technology ideas from the South Pacific island nation of Papua New Guinea. The Appropriate Technology Sourcebook highly recommends Lik-lik Buk, and so do I. The first edition was published in 1977 and a revised second edition has Just been published.

24) Volunteers in Technical Assistance (VITA) is a treasure chest of resources, information, and people. Their address in the United States is: 1815 North Lynn Street, Arlington, Virginia 22209, U.S.A. (telephone: 703-2761800). VITA also has field representatives in many parts of the world. Their names and addresses are available from VITA's main office. When writing VITA, ask for a copy of their magazine "VITA NEWS." The easiest way to explain what VITA is and what VITA does is to quote from their own literature:

"VITA is a private, nonprofit development organization based in the United States. Since 1960 VITA has supplied information and assistance, primarily by mail, to people seeking help with technical problems in more than 100 developing countries. Providing its services in response to requests from individuals and groups working to improve homes, farms, communities, businesses, and lives, VITA helps select and implement technologies appropriate to the situation. VITA participates with local institutions in problem-solving relationships and in efforts to design and carry out local solutions to local problems.

"There is no fixed charge for VITA charges; often there is no charge at all. Each type of assistance provided by VITA is handled differently and may or may not involve a fee. Each month VITA handles case requests from individuals and local groups with little or no access to funds for development projects or to foreign currency. VITA is pleased to be able to support these efforts and in such cases provides its services at no cost to the requester. When an organization requires access to VITA's services over an extended length of time, VITA and the organization can make arrangements to suit both their needs. Such arrangements usually include an agreement for payment of a fee.

"VITA's technical experts often provide on-site consulting services as part of VITA's project involvement. Areas of particular interest to VITA are: agriculture and animal husbandry, alternative energy systems (wind-solar-biogas-water), water and sanitation, food processing, small-scale industries, equipment design, project feasibility and evaluation, low-cost housing and construction, crafts production and marketing, and appropriate management technologies.

"(To put VITA to work for you), send VITA a complete description of the situation or problem. Tell what you are trying to do and how you feel VITA can help. Include drawings, diagrams, photographs, if possible. Be as specific as necessary for clear understanding of the situation. Your description should include the following kinds of information if possible: technical details (location-measurements-symptoms), scale (size and capacity of equipment or machinery, planned annual or seasonal output, size, nature and location of markets, amount of funding available), pertinent area background (social structure-climate-terrain, type of leadership, local laws or regulations, labor and energy resources, tools-materials-workshop facilities), status of project or problem (result of previous attempts to implement or solve it), other participating individuals or groups."

25) Two schools that have interests and experience in biogas systems are:

(1) E.J. Kemsley, Vice Principal, Private Bag 0027, Botswana
Agricultural College, Gaborone, Botswana (phone 52381 and 52384).

(2) Reverend Palpito D. Dumanig, Director, Mindanao Institute, UCCP,
Cabadbaran, Agusan del Norte, Philippines.

26) The International Bio-Energy Directory is published by the Bio-Energy Council, Suite 825A, 1625 Eye St., N.W. Washington, D.C. 20006, U.S.A. (telephone 202-833-5656). The directory can be found in some libraries, including the U.S. International Communication (U.S. Information Service) libraries in many countries. The following list was taken from the descriptions of 182 biogas system projects from around the world that were listed in the 1981 issue of the International Bio-Energy Directory.

(1) Latin American Energy Organization (OLADE), Casilla 119-A, Quito, Ecuador (telephone 544-800). OLADE has held seminars and workshops to train biogas technicians from Guatemala, E1 Salvador, Honduras, Nicaragua, Haiti, Jamaica, Dominican Republic, Grenada, Guyana, Bolivia, Ecuador, Mexico, and Peru.

(2) Coordenadoria de Energia, Secretaria de Transportes, Energia e Communicacoes (STEC), Avenida Cruz Cabuga 1419, 50000 Recife, Pernambuco, Brazil (telephone 222-4161). STEC promotes biogas technology, using agricultural wastes to produce fuel and fertilizer in rural areas.

(3) Ogunlade R. Davidson, University of Sierra Leone, Department of Mechanical Engineering, Private Mail Bag, Freetown, Sierra Leone 27390. The school is studying the technical and economic feasibility of biogas systems in rural African villages.

(4) E.R. Ela Evina, Director, Department Energies Renouvelables, CENEEMA, Ministry of Agriculture, Cameroon. Mailing address: B.P. Box 1040, Yaounde, Cameroon (telephone 22-32-50).

(5) Rural Industries Innovation Centre (RIIC), Private Bag 11, Kanye, Botswana (telephone 39213). RIIC is developing biogas systems to run modified diesel engines.

(6) Dr. Richard K. Solly, University of the South Pacific, P.O. Box 1168, Suva, Fiji Islands, South Pacific (telephone: Fiji 313-900, telex: FJ2276). The university is developing biogas systems appropriate to conditions in the islands of the South Pacific.

(7) Ministry of Agriculture, Office of the National Leading Group for Bio-gas Construction, Biejing, China. The Group studies biogas systems in rural villages and the possibilities of future developments in biogas technology.

(8) Department of Science and Technology, Technology Bhawan, New Meherauli Road, New Delhi 110029, India. The Department conducts research on biogas technology.

27) Appropriate Technology Sourcebook by Ken Darrow and Rick Pam. This book has now been published in two volumes, and it can be ordered from: Appropriate Technology Project, Volunteers in Asia, Box 4543, Stanford, California 94305, U.S.A. The regular price is US$ 5.50 for Volume One (published 1976, revised 1981) and US$ 6.50 for Volume Two (published 1981). There are discount prices for Third World Groups, and discounts are also available for purchases of ten copies or more.

To quote from the abstract in Volume One, the Appropriate Technology Sourcebook is a "guide to practical plans and books...on alternative sources of energy, farm implements, shop tools, agriculture, low-cost housing, health care, water supply, pedal power, philosophy of appropriate technology...small-scale systems using local skills and resources...entries selected on the basis of low price, clarity of presentation, easily understandable, non-technical language...more than 375 publications listed (with) prices and addresses given for each publication...250 illustrations." Volume Two lists 500 more publications and has 300 more illustrations.

28) A Handbook on Appropriate Technology published in 1979 and costing US$ 14.50 and its sequel, Experiences in Appropriate Technology published in 1980 and costing US$ 7.95, are both available from The Canadian Hunger Foundation, 232 Chapel Street, Ottawa, Canada, KIN 7Z2. These two excellent books describe not only the technical aspects of several appropriate technologies but also appropriate technology purposes and results.

To quote from an advertisement for the books, Appropriate technology {AT) "is much more complex than providing blueprints or equipment...the very use of AT questions standard development theories. The case studies (in these books) go beyond this to question the current practice of AT....Unless these communities have some control over the process, access to more resources and markets, power to make decisions, and are supported by government policies which promote community-scale production and services, a large percentage of men and women will remain on the periphery (outside edge) of both development and any new technologies, however appropriate."

29) Paper Heroes: A Review of Appropriate Technology by Witold Rybczynski, 1980. This book is a critical review of appropriate technology and costs US$ 5.00 in the paperback edition by Anchor Books, Garden City, New York, U.S.A.

30) The Book of Think by Mary Burns and illustrated by Martha Weston, 1976. The Book of Think, which costs US$ 4.95, is part of a series of school books written by authors who "believe learning only happens when it is wanted; that it can happen anywhere, and does not require fancy tools." Other books in the Brown Paper School Book series include, The I Hate Mathematics! Book and Blood and Guts (about what is inside the human body and how it works). Books in the Brown Paper School Book series can be ordered from the publisher. The address is: Little, Brown and Company, 200 West Street, Waltham, Massachusetts 02154, U.S.A.

31) Economics for a Developing World: an Introduction to Principles, Problems, and Policies for Development by Michael P. Todaro, 1977, and a revised second edition in 1981. The book is available in many, if not most, developing countries for approximately US$ 3.00. The authorized Philippine edition is published by Phoenix Press, Inc., 927 Quezon Boulevard Ext., Quezon City, Philippines. The book is available in the United States as a textbook for US$ 23.00 under the title, Economic Development in the Third World. For information about local editions, write the publisher: Longman Group Ltd., 5 Bentinck Street, London W1, Great Britain. Inquiries about the U.S. textbook edition can be addressed to Longman Inc., 1560 Broadway, New York, New York 10036, U.S.A. Longman Group also has "associated companies, branches, and representatives throughout the world" which could be contacted for information on the book's availability.

The book Economics for a Developing World is not about biogas. But, just as biogas technology is part of appropriate technology, appropriate technology is part of what could be called appropriate economics for a developing world. When reading the following quotes from the book, if you find the ideas and questions interesting, I believe you will find the book interesting and useful. Understanding something about the "forest" makes the study of any one "tree," such as a biogas business, much easier.

"This book has been written for use by first-year economics students at universities throughout Africa, Asia, Latin America and the Middle East. For too many years, students in these developing nations have had to rely on "Western" economics textbooks written primarily for their counterparts in North America and Western Europe. Although such books may claim to be universal in scope, in reality they are typically oriented towards the unique institutional, social and economic structures of the industrially advanced economies of the West. Students in developing countries are often therefore forced to absorb a broad spectrum of economic concepts, principles and theories and to analyse a wide range of contemporary problems and issues which may have little or no relationship to the institutional and economic realities of their own societies...

"The book is organised into four Parts. Part One focuses on the nature and meaning of underdevelopment and its manifestations in different Third World nations...Parts Two and Three form the core of the book. Part Two focuses on major domestic development problems and policies while Part Three examines the place of Third World nations in the international economy...Finally, Part Four reviews the possibilities and prospects for Third World development...

"In our discussion and analysis of critical development we give the diverse and often conflicting viewpoints of development economists, other social scientists, planners and those actually on the "firing line" in Third World government ministries and/or departments. If we reveal a bias, it is probably in trying always to put forward the viewpoints of Third World social scientists and development practitioners who recently have begun to articulate their shared perceptions of the meaning of development as never before...

"Students, therefore, often choose to study economics in hope of finding answers to vital questions such as the following:

1) What do we really mean by "development" and how can economic principles and theories contribute to a better understanding of the development process?

2) What are the sources of national and international economic growth? Who benefits most from such growth and why? Why do some countries and groups of people continue to get richer while others remain poor?

3) Why is there so much unemployment, especially in the cities, and why do people continue to flock into the cities from rural areas even though their chances of finding a job are very slim?

4) Should the rich be taxed more than the poor and how should government tax revenues be spent in order to improve standards of living for all people?

5) What is development planning all about? Why plan at all?

6) Should foreign private corporations be encouraged to invest in the economies of poor nations and, if so, under what conditions?

7) What about "foreign aid" from rich country governments? Should it be sought after, under what conditions, and for what purposes?

8) Should exports of primary products such as agricultural commodities be promoted or should all Less Developed Countries attempt to industrialize by developing their own heavy manufacturing industries as rapidly as possible?

9) What is a "balance of payments" problem? When and under what conditions should the government adopt a policy of exchange control, raise tariffs and/or set quotas on the importation of certain goods in order to improve balance of payments deficits?

10) Is international trade desirable from the point of view of the development of poor nations? Who really gains from trade and how are the advantages distributed among nations?

11) What has been the impact of the rapid rise in international oil prices on the economies of less developed nations? And, what future role might the now wealthy OPEC oil nations play in furthering the development of other Third World nations?

12) What is the best way to promote agricultural and rural development where 80 to 90 percent of most Less Developed Country populations still reside?

13) How does the spread of inflation and unemployment among the economies of rich nations affect the levels of living of people in poor nations? Do poor nations have any recourse, or must they be passive but vulnerable spectators at an international economic power game?

14) Are there economic factors influencing levels of fertility (birth rates) in poor nations? What are the economic and social consequences of rapid population growth? Is the population problem simply a question of numbers or is it also related to the impact of rising affluence in developing nations on resource depletion throughout the world?

15) Will there be chronic world food shortages? If so, which nations will be most adversely affected and how much such shortages best be avoided in the future?

16) Do contemporary Third World educational systems really promote economic development or do they simply act as a rationing or screening device by which certain select groups or classes of people are perpetuated in positions of wealth, power and influence?

17) What is the origin and basis of growing Third World demands for a "new international economic order"? Is such a new world order possible, and, if so, what might be its main features?

These and many other similar questions are analyzed and explored in the following chapters of Economics for a Developing World."

Feasibility Studies

This section is adapted from the article "Guidelines for Preparing Feasibility Studies" by Brain Fekety. The article was published in the May, 1981, edition of the magazine, Peace Corps/Philippines. For a biogas business or any kind of business, a feasibility study may be the most important first step.

A feasibility study's main purpose is to prove that the project is worth investing money in, to the people, businesses, banks, private, and government programs that can make grants and loans. Feasibility studies can be used to interest investors and partners.

Writing a feasibility study is a good way to organize ideas and help the prospective owner(s) decide if investing the considerable time and money involved in building and operating a biogas business is worth it, and if the answer is yes, then what design, size of operation, and type of business organization would be best.

A feasibility study is a report outlining a project that a group hopes to undertake. It shows in detail whether or not a project will work. It also shows, step-by-step, plans for the development of a successful project. Lenders, investors, and partners want to know that their money will be well spent. They will be impressed with a detailed and professional feasibility study outlining the proposed project. It must outline the nature of the project, the costs and resources required, the expected profits and results, and a social-economic evaluation of the project.

Project summary:

1) A brief description of the project and its purpose.

2) Summarize the history of the group, cooperative, or company that is proposing the project.

3) Summarize the conclusions and findings of the study regarding the technical, management, marketing, and financial characteristics of the proposed project.

4) The timetable and current status of the project.

5) Say something about why you think the project will be successful and something about the project's social-economic implications.

Description of project site:

1) Location of the project and area served by the project. A map which identifies provinces, municipalities, cities, and towns would be helpful.

2) Climate, land, rivers, and other characteristics of the area as they relate to the project.

3) The area's infrastructure (communication systems, roads, ports, airports, banks, schools, electricity, and so on) and economic conditions as they relate to the project.


1) Size, capacity, efficiency, and various processes involved in the project should be outlined.

2) Purpose or service of the project (for biogas: fuel, fertilizer, improved sanitation).

3) Raw material requirements (for biogas: water, plant, and animal waste).

4) Requirements and costs of utilities and fuel in terms of output.

5) Equipment and other fixed asset requirements (for biogas: digester, gas storage tank, and so on). The number, specifications, rated capacities, costs, and life expectancy of newly acquired and existing machinery, land, equipment, buildings, and other assets.

6) Describe the quantity and quality of all labor requirements. Describe sources and number of workers according to skill. Indicate whether labor requirements are full-time, part-time, or seasonal. Include wage rates, salaries, fringe benefits, and the details and costs involved for any special training programs.

7) Description of all construction costs.

8) Outline the project timetable and present position of the project including target dates for such items as preparation of special studies, land improvements, and the construction or purchase or equipment, buildings, and the start of operations.


1) The background and skill requirements of the project manager position should be presented (big-data of the manager should be included in the appendix).

2) The form of organization and an organizational chart.

3) Determine the availability of manpower required to fill the various positions in the project.


1) Supply and demand conditions for the output (products) of the project and the raw material inputs required to produce the product and complete the project.

2) Production costs, price policies, marketing policies and strategies for methods of product distribution.

3) Evaluate the contribution of the proposed project to the national, provincial. and local economy.

4) Estimate the annual volume and value of the proposed sales and/or savings.

5) Identify the guaranteed markets and any existing or proposed contracts for marketing of the project's products.


1) Detailed projection of all financing requirements and the use of capital (loans, grants, investments).

2) Sources selected or proposed for short-term financing, long-term financing, and credit from suppliers.

3) The amount and terms of selected or proposed financing according to each source, including security offered, repayment schedules, and interest rates.

4) For equity financing (money that owners invest in their own business), indicate subscriptions made and proposed.

5) Status of financing from each source indicating actual releases already made, applications already approved, pending applications, and proposed applications.

Social-economic justification (very important in grant applications):

1) Government revenues in terms of tax receipts (such as income and sales taxes), if applicable.

2) The benefit to people directly affected by the project (for biogas include the impact of the fertilizer on the businesses of fish pond owners and farmers).

3) The effect on the provincial, municipal, town, and village level economies by the project (for biogas this would include improved sanitation).

4) Identify any short-term and/or long-term benefits to suppliers of raw materials.

5) The amount of employment generated by the project as well as skills and/or training provided to employees.

Appendix A (Attached Enclosures):

1) Bio-data of management and Board of Directors.

2) Certified copies of all sales and supply contracts.

3) Copies of price quotations for machinery, structures, and other inputs.

4) Copies of plans and drawings for structures and for the physical layout of the project site indicating provisions for expansion (use of photographs can be impressive).

5) Any required resolutions from the group's Board of Directors.

6) Latest audited report of the group's financial statements, if applicable.

7) Certified true copy of Articles of Incorporation and bylaws, together with all amendments, if applicable.

Appendix B (The Project's Financial Data Schedules):

1) Projected Income Statement.

2) Projected Balance Sheet.

3) Projected Cash Flow Statement.

4) Projected Costs of Sales Statement.

5) Projected Operating Expenses.

6) Projected Amortization Schedule (paying off of debts).

7) Financial Evaluation:

(a) discounted rate of return on total investment,

(b) profitability ratios (net operating income as a ratio of sales and net operating income for each year and the average amount of net income over the projected operating period,

(c) debt ratios, calculating debt to equity ratios and repayment schedules of loans,

(d) break-even analysis,

(e) internal rate of return, and

(f) payback period.

Problem solving

How do you get a problem? There is not just one answer to that question. Problems come in different ways. There is no telling when one will appear. But there is a moment when you realize there is a problem.

It is not always enough to know you have a problem. Sometimes it helps to see where it came from. That is not always easy; problems sneak up in different ways.

One Way: You notice something is not quite right. Not the way it should be. The fertilizer from the biogas digester has always increased the quantity and quality of your crops in the past. But not now. It is as though you were using no fertilizer at all. Using the biofertilizer has become a waste of time.

Okay, you know there is a problem. Now that you have it, are you sure what it is?

· Is the fertilizer no good?

· Could it be that you have been using too much fertilizer too often?

· Is the fertilizer being used at the best time in the plant growth cycle?

· Is the digester being overloaded?

· Is the digester sludge left in the aging pond long enough? e Has rain diluted the sludge?

A second way: You open the outlet gate valve on the digester and nothing comes out, even though the digester is full.

· Is the digester so full of solids that the outlet is blocked? How would you find out if that is the problem? If it is a too high percentage of solids what would you do? How did the percentage of solids in the slurry go over the ten percent in the first place? Is the problem the valve, the solids, or how the slurry is prepared? Or is the real problem something else?

A third way: Something has been bothering you for a while. Your five cubic meter family digester is working, producing biogas and fertilizer, but you feel it requires more time and more work than the biogas and fertilizer are worth. It costs more to produce your own gas and fertilizer than it does to buy bottles of pressurized gas and bags of chemical fertilizer. Not every consumer should be a producer. Is the digester temperature 20 degrees centigrade? The biogas bacteria would see that as a problem and not produce much gas. Have you sat down and figured out what would be the best combination of available plant and animal wastes to feed the digester? Maybe you should have joined a biogas business or cooperative instead of building a family digester. Where is the problem?

It is not always easy to decide where the problem is. But that is the important first step. No use solving a symptom of the problem and leaving the problem untouched.

Your first idea may not be your best idea. Go comparison shopping for solutions. Take a good look at several ideas before you choose one. If you want 5.0 cubic meters of biogas every day, the solution is to build a 5.0 or 6.0 cubic meter capacity digester, right? Maybe. Maybe you should start a neighborhood biogas cooperative and own a 25 percent share in a 20 cubic meter system. Maybe you should invest in a biogas business and make enough money to buy all the bottled gas you want.

Making lists is one way to study a problem. Lists can be used to help remember things, but they can also be used to help you think. Lists can help you look at things in different ways. You can list your fuel and fertilizer needs. You say you are not a farmer and you do not need fertilizer. Do you have the land or the desire to build a fish pond? You could list the ways you could sell the fertilizer. You could list your resources: money, skills, organic wastes, and so on. You could list possible solutions: family biogas system, business biogas system, or cooperative biogas system; and under each possible solution you could list what your role (active partner, investor, etc.), costs in time and money, and potential profits might be. Remember that trap of looking at something in only one way.

Be logical. That is a method you have heard about for solving problems. When using logic you pick a direction for your thinking to go in. Then you go step-by-step. You check each step along the way that makes sense. Hopefully you get what you are looking for.

The problem with logic is, in which direction should you go? If you go down the wrong road, logic takes you to the wrong place. If you cannot decide which direction to take, logic will not take you anywhere.

Logically, fresh sludge from a digester looks very watery; you see very little solid matter floating on top of the bucket and very little left in the bucket when you use the sludge to dilute fresh waste going into the digester. Everything is fine until a few months later when biogas production drops off and nothing comes out of the outlet. What your logical eyes did not see was solid matter suspended, floating in the sludge, not much (only about ten percent), but enough to make a difference in time--if the solids are not filtered out of the sludge before it is used to make fresh slurry. Learning to solve problems does not mean learning to be more logical. It means learning to think in different ways. Not always in a straight line.

Sometimes you know where you need to end up. But, it is knowing how to get there that is the problem. Going backwards is sometimes a better way to go. It can save time; it can make a problem a lot easier to solve. Suppose you have two slurry buckets. One holds ten liters. One holds six liters. You need to know where the eight liter line is in the big bucket for when you mix fresh slurry. How can you do it? Do not start by pouring. That may get you going in a useless direction, and you would be wasting a lot of slurry. Picture where you want to end up. Now try to back up one step at a time to see how to get there. (The answer is at the end of this section.)

Sometimes you do not ask a question because it seems stupid, silly, or because you feel it would be embarrassing to ask. Questions such as: Why is biogas bubbling up through the water that the gas storage tank floats in?

Successful problem solvers cannot go around hoping someone else will ask the questions. Asking questions is a good way to find out something. Practice questioning what you read, as well as questioning other people. But before you can ask what you need to know, separate what you know from what you do not know about a situation.

It has been said that two heads are better than one. Then maybe twenty heads are even better. For some problems, it helps to have other people's opinions. Then you can pick and choose among many ideas and maybe even come up with a completely new idea. Before building a biogas system, you might ask all the farmers and fish pond owners you know if they would buy biofertilizer and at what price for both the liquid and the solid kinds.

You would want to know what the local prices of chemical fertilizers are, if they are used much, and if anybody is already using organic fertilizers such as fresh manure (which is a health hazard) or compost. You might find that you have to give the biofertilizer away for free for a few months in order to convince farmers of the fertilizer's quality. But that within one or two growing seasons you can make a profit selling the biofertilizer at a price equal to or less than that of chemical fertilizers of equal value.

You might have questions about biogas system operations that are not answered in this book and cannot be answered even with the help of friends. You could write letters to VITA, Maya Farms, or the groups listed in the International Bio-Energy Directory for their opinions and help. You could ask a nearby Peace Corps Volunteer, if they are working in your country. If they cannot help, they should know where to ask for help.

You want to build a biogas system and you have enough plant and animal waste to feed the size digester you have chosen. The problem is: What is the easiest way to get the waste into the digester? Explain the situation to your friends and see what they suggest. No one friend may have the best solution for your situation. The answer may be a combination of several suggestions. You can pretend to be different people you know and ask imaginary people how they would answer your questions. Whether you are asking real or imaginary people, you will see how different people think differently. The more you do this, the easier it will be to think differently when you need different viewpoints.

There are times when thinking crazy on purpose is a good idea. Not acting crazy, just thinking crazy. Let your mind run wild in different directions. For example: If you decide to go into the biogas business, how could you persuade other people to join you? Whom would you ask? Should the biogas system be a private business or a cooperative? How big should the system be? How should the system be operated? How should the gas and fertilizer be used? How could composting and bioinsecticide be added to a biogas business? Could the waste from a town's market place support a biogas business? How much per kilogram should you pay people to collect water lilies for the digester? Let the possible questions and answers run wild.

Another approach to problems is to find a similar problem to the one you have. It may be a problem you have already solved. It may be a problem that you could solve more easily. In either case, you will get a helpful clue for your thinking, a push in the right direction. Anyone who understands how to build ferrocement water tanks knows how to build strong, inexpensive concrete biogas digesters and the water tanks for gas storage tanks.

In real life, very often the same problem shows up over and over. Sometimes it is disguised so that it does not look the same. If you do not learn from your first solution, you will just keep getting the same problem again and again. That is no way to live. If you have solved one problem having to do with the biogas pipes, think about what you learned; it may help solve a new gas pipe problem. The same is true no matter what the problem is: liquid fertilizer distribution, scum layers, or waste to water slurry ratios. Sometimes what is a help in solving a new problem is not remembering a similar solved problem but remembering a method of problem solving that worked well for you.

There are some problems that are totally confusing. For example: You want a biogas system but do not know where to start; there are so many decisions to be made. You have no idea what the answers are. Not even wrong answers. One big blank. What you need is inspiration. A brand new way of looking at the problem. A little hope. You need to relax and dream about the problem without worrying about it. Sleep on it. Give the problem a night to just roll around in your head without any pressure to find a solution. You might be surprised. The answers, or at least a few important clues, might just appear at the strangest time.

No matter which way you go about solving a problem, try to do it in such a way that the challenge is enjoyable. Many times there are no clear solutions. There may be several possible answers, not just one. It is not always easy to know when you are right. Thinking about thinking can help you get a good start.

The answer to the two buckets problem: You want eight liters in the 10-liter bucket. So two liters have to be subtracted by pouring it into the 6-liter bucket. That is the last step. To know how much two liters are, you must already have four liters in the 6-liter bucket. You can do that by filling the 6-liter bucket from a full 10-liter bucket, pouring out the six liters, and pouring the four liters that are left in the 10-liter bucket into the 6-liter bucket. To complete the solution, pour two more liters from a full 10-liter bucket into the 6-liter bucket that has four liters in it. Now that you have found the eight-liter level, do not forget to mark it. Working backwards can be fun. (This section was adapted from The Book of Think.)


ACID: A sour substance. A usually water-soluble chemical compound that has a sour taste, reacts with a base substance to form a salt, and turns litmus paper red. Acids, depending on their strength, can chemically burn or dissolve other substances. Stomach acids help dissolve food.

AERATION: To supply with or mix with air.

AEROBIC: With oxygen. An aerobic organism can live only where there is oxygen.

ANAEROBIC: Without oxygen. An anaerobic organism can live only where there is no oxygen.

BACTERIA: Simple, primitive types of life which reproduce by dividing. Bacteria occur as single cells, groups of cells, chains or strings of cells, and cannot be seen without a microscope because of their small size. Three types of bacteria are: (1) Aerobic bacteria which need oxygen from the air in order to live; (2) anaerobic bacteria that can only live where there is no air with oxygen in it; and (3) pathogenic bacteria, some aerobic, some anaerobic, which can cause disease in plants and animals. Not all, or even most, bacteria can cause diseases.

BASE: A bitter substance. An alkaline chemical compound such as lime or ammonia that can react with acids to form a salt and turns litmus paper blue. Bases, depending on their strength, can chemically burn or dissolve other substances.

BIOFERTILIZER: A fertilizer made from decomposed organic matter. In this book biofertilizer usually refers to the organic waste, called sludge, that has been taken from the outlet of a biogas digester. This sludge is 90 percent liquid. Another biofertilizer mentioned in this book is the traditional organic fertilizer called compost.

BIOGAS: A mixture of gases produced by the actions of living organisms. People produce carbon dioxide; plants produce oxygen; and some types of anaerobic bacteria produce methane, carbon dioxide, a little hydrogen sulfide, and traces of a few other gases. Though technically any gas produced by a living organism is a biogas, it is only the mixture of gases produced by anaerobic bacteria that is commonly called biogas.

CARBON DIOXIDE: A heavy, colorless gas that does not burn. Carbon dioxide can be formed by the burning or decomposition of organic matter.

C/N RATIO: The ratio in organic matter between the amounts of the two basic building blocks of life: the elements carbon and nitrogen.

CONDENSATION TRAP: A trap which collects a gas such as steam (water vapor) and changes it into a liquid such as water.

DECOMPOSE: To reduce from complex to simple. To rot, to break down, to decay--a process in which bacteria often play an important role.

DETENTION TIME: It is the number of days that organic waste is supposed to remain inside a biogas digester from the day it is put into the inlet to the day it comes out of the outlet.

DIGESTER: An airtight, watertight tank in which organic matter is stored for the purpose of permitting anaerobic digestion until the organic matter no longer carries any disease-causing organisms and most of the potential for producing methane gas has been used up.

DIGESTION: The biological breakdown, by living organisms, of organic matter into forms that can be absorbed by living cells. Digestion takes place in the bacteria of biogas digesters and in the stomachs of animals and people.

DUAL-FUEL ENGINES: Engines that have been adapted to operate on a mixture of two kinds of fuel--in this case--diesel fuel and biogas.

ENZYMES: Chemicals which help to break down or form organic molecules.

FERROCEMENT: A type of reinforced concrete made of wire mesh, sand, water, and cement. It is strong and long lasting. In comparison to other types of concrete, ferrocement is flexible. It is usually only 1.0 cm (0.5 inch) to 5.0 cm (2.0 inches) thick. Used for many years to build boats, ferrocement is now being used to build water tanks, grain storage bins, and biogas digesters.

FIBERGLASS: A very strong type of plastic containing hair-like fibers of glass.

GALVANIZED IRON (GI SHEETS): Iron that has been coated with zinc in order to resist rust.

GAS STORAGE TANK: A container with holes in the bottom, floating in an open-top water tank. Lighter-than-air gases such as biogas are trapped in the gas tank by the water. When gas is piped into the gas tank, the tank floats up. When gas is piped out of the tank, the gas tank sinks into the water. In this way, the gas pressure remains fairly constant, making the gas easy and safe to use.

HORSEPOWER: One horsepower is a unit of power equal to 746 watts of electricity. Brake horsepower is very different. It is the power of an engine as calculated from the force exerted on a friction brake or absorption dynamometer applied to the flywheel or shaft or the engine.

HUMUS: Organic matter formed by the decay of dead plants. Humus is essential to the fertility of soil.

METABOLISM: The process of using biological energy. The sum of all physical and chemical changes which take place within an organism. The changing of the chemical energy of food into mechanical energy (your arm moving) and heat (your body's temperature).

METHANE: A compound of the elements carbon and hydrogen. Methane is a colorless, odorless gas that can burn. Methane is 60 to 70 percent of biogas. Methane is also found underground, often with deposts of oil. It is sold by oil companies as a fuel called natural gas.

MOLECULE: The smallest unit into which a substance can be divided without a change in its chemical nature.

NUTRIENT: A substance that has food value to a living organism.

NUTRITION: Nutrition is the process of taking in and using food.

ORGANIC: All plant and animal matter, living and dead. All organic matter is made in large part of the basic elements--carbon, nitrogen, oxygen, and hydrogen.

ORGANISM: Any living thing, such as animal, plant, or bacteria.

OXYGEN: A nonmetallic element occurring free in the air as an odorless, colorless, tasteless gas. Animals breathe air for the oxygen in it. Oxygen in combination with other elements is three-fourths of the animal world by weight, four-fifths of the plant world by weight, one-half of the mineral world by weight, eight-ninths of all water by weight, and one-fifth of the air by volume. pH: A measure of how acid or base (alkaline) a solution or substance is. Most all living things prefer a balance of acid and base--a natural pH. Both strong acids and strong bases can chemically burn and destroy living matter.

PRESSURE GAUGE: It is a measure of how tightly packed gas in a system is, including the gas storage tank. It is not a direct measure of how much gas is in the system. The same amount of gas can occupy a large space or a small space. The smaller the space that the same amount of gas occupies, the higher the gas pressure.

RADIATION: The process by which energy is given off by one body (such as the sun), travels through space, water, or something else, and is absorbed by another body (such as the water in a solar collector).

RUST: Rust is the result of iron combining with the oxygen in air and forms a red or orange coating on the surface of iron.

SCRUBBING: Separating and removing unwanted gases from biogas.

SLUDGE: The organic plant and animal slurry when it is removed from a biogas digester after the production of biogas. Sludge is 90 percent liquid, 10 percent solids (mostly humus). Biogas sludge makes an excellent fertilizer.

SLURRY: The plant and animal matter, mixed with water, that is put into a biogas digester. Slurry should be about 90 percent water.

SOLDER: Solder (pronounced sod-er) is a soft metal alloy that joins harder metal objects together without heating them to the melting point. The solder is applied in a melted state.

SPARK-IGNITION ENGINES: Ordinary gasoline-powered engines in which spark plugs are used to ignite the gasoline.

STARTER: Fresh sludge from a working digester which because it is full of biogas-producing bacteria, can be used to speed up the start of biogas production in a new digester.

THERMOSYPHON: Moving liquid from one place to another by changes in heat. The thermosyphon is based on the principle that hot liquids and gases want to rise, while cold liquids and gases want to fall.

WASTE: The word waste is used in this book to refer to organic matter that does not have a purpose more important than being used as "food" for a biogas digester.