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close this bookThe Biogas/Biofertilizer Business Handbook (Peace Corps, 1982, 186 p.)
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View the documentNew ideas
View the documentComposting
View the documentBioinsecticides
View the documentFerrocement
View the documentFacts & Figures
View the documentSources & Resources
View the documentFeasibility Studies
View the documentProblem solving
View the documentVocabulary

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)