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.

DIAGRAM 18

CARBON DIOXIDE SCRUBBER

-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

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.

DIAGRAM 19

SOLAR GREENHOUSES TYPICAL HEAT GAINS AND LOSSES

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.

DlAGRAM 20

MANUFACTURE OF DRY ICE

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.

DIAGRAM 21

SOLAR COLLECTOR PLATE

-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.

DIAGRAM 22

VITA PORTABLE SOLAR COLLECTOR AND STORAGE TANK

DIAGRAM 23

MOVEMENT OF WATER THROUGH A COT COLLECTOR

"(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.

DIAGRAM 24

SOLAR COLLECTOR AND RANKINE ENGINE

"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.

DIAGRAM 25: INDIAN SOLAR COLLECTOR AND RANKINE ENGINE

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."

DIAGRAM 26

PRINCIPLE OF THE CENTRIFUGE