A mixture of microorganisms breaks down the complex organic material in manure or other substrates such as crop residues, grains, sugars, and lignocellulose (wood and plant materials) into simple sugars, organic acids, carbon dioxide and hydrogen when no oxygen is present. Other microorganisms in the mixture then convert these simple compounds into methane. The process has to take place in a container that does not allow air to enter and that has an outlet for the gas produced. The process takes place at about 100°F in most digesters. This is called a mesophilic temperature range. It also proceeds at ambient conditions in lagoons and marshes where anaerobic conditions occur; however, since the temperature is somewhat lower, the rate of gas production is also lower. In some digesters, the temperature is elevated to about 140°F, which is in the thermophilic temperature range.

The methane produced in the digester is collected in the top of the digester and can be drawn off and burned as a fuel much like natural gas. The methane is diluted with carbon dioxide so the biogas mixture has a lower energy content than natural gas. The biogas energy content is about 600 Btu/ft3 whereas natural gas has an energy content of about 1000 Btu/ft3. The easiest way to use biogas is as fuel for a boiler or hot water heater. It also can be used as a fuel in an internal combustion engine. In our demonstration project, we are using the gas to fuel an internal combustion engine that drives a generator. The electricity produced will help meet the dairy’s power needs.

About 60% of the carbon in the organic matter is converted to methane and the other 40% ends up as carbon dioxide. However, not all of the organic matter can be converted in the anaerobic digestion process. Lignin is not attacked by the bacteria present, and some of the cellulose is difficult to digest. Overall, I would expect 30 – 50% of the organic matter is not converted in the process. This will vary depending on how long the material is allowed to digest. If the biogas is used to fuel an engine/generator system, the efficiency of energy conversion would probably be about 25%; i.e., based on the energy of the fuel entering the engine, the energy output from the generator is about 25% of that.

A properly operating anaerobic digestion process does not produce much odor. If the biogas is collected for use as fuel, there is essentially no odor from the process. Any odorous compounds are captured and burned. Ammonia and hydrogen sulfide are both present in digesters in small quantities. Most of the ammonia will remain dissolved in the liquid phase, but the hydrogen sulfide is collected with the biogas. Hydrogen sulfide can be scrubbed out of the gas, but it also can be burned along with the methane.

Changing of the seasons will affect anaerobic lagoons. As temperatures get colder, activity in the lagoon will decrease. When temperatures start warming in the spring, density gradients in a lagoon may cause the contents to turn over so that material on the bottom rises to the top. This can lead to odor problems since the material is not completely digested because of the low winter temperatures. This generally is not a problem in Texas but it is in colder climates such as Iowa. Anaerobic digesters normally have heat exchangers included in the design so that temperature is maintained at a constant value all year. The only affect of changing seasons would be on the amount of heating required to maintain the temperature. In our demonstration project, we use waste heat from the engine to heat the digester, and there is more waste heat available than needed.

Temperature needs to be maintained at about 100°F for mesophilic operation or 140°F for thermophilic operation. The pH of the digester needs to be at about 7 (neutral). If the pH drops into the acidic range, the methanogens (bacteria which produce methane) stop functioning. None of the bacteria like a high pH. The buffering capacity of the digester is given by the alkalinity measurement, which determines the amount of calcium carbonate present. The alkalinity should be about 2500 mg/L as calcium carbonate. The retention time (time material stays in the digester) normally is 10 – 30 days, but shorter times can be used with easily digested materials.

Most digesters are custom installations consisting of a large tank or covered lagoon in which the digestion takes place, plumbing, pumps, compressors and something to use the biogas (engine, boiler, etc.) The digester itself (tank or lagoon) needs to be properly designed for the manure load, heating, mixing and gas collection. In addition, design of the system for collection and preparation of the manure to feed into the digester is important and can present difficult problems.

The major expense is the capital equipment cost of setting up the process; i.e., the cost of the digester tank, pumps, plumbing, compressor, engine, etc. Once the system is in place, operating costs should be fairly minimal. Labor costs should be low, with some supervisory labor required as well as some labor to check that the manure handling system is working properly. No chemicals are required unless the pH or alkalinity get out of the desired operating range. Power consumption by the system is low so utility costs will be minimal.

Manure scraped from dirt lots is not a good feedstock since it contains a large amount of inorganic material (dirt, rocks, etc.) that can plug lines and settle out in tanks. With proper design, manure from any type of animal can be digested. Also, the feed ration for the animals can affect the design or operation of a digester.

The yield for anaerobic digesters generally falls in the range of 3 to 8 standard cubic feet of biogas per pound of manure. The biogas usually contains 60 to 70% methane. The yield will depend on how long the manure is allowed to digest, type of manure, and type of feed given to the animals.

Manure from animals fed high energy diets generally would be easier to digest than manure from animals fed a high forage diet. Manure from animals on a high forage diet will have a high content of partially digested lignocellulosic material. The animals will have utilized the most easily digested parts of the forage, leaving the most recalcitrant parts in the manure.

No. Occasionally the pH or alkalinity may need to be adjusted which would require addition of lime or sodium hydroxide. Otherwise, no additional chemicals are needed.

The greatest benefit to the environment is that using biogas as a fuel does not increase the carbon dioxide content of the atmosphere. Carbon dioxide is a greenhouse gas and its accumulation in the atmosphere from burning fossil fuels is causing global warming. Although carbon dioxide is released when biogas is burned, it is simply being recycled rather than adding new carbon to the atmosphere. (The carbon in the biogas originated in the atmosphere and was fixed by plants that became feed for the animals.) Methane is another greenhouse gas and is much more potent than carbon dioxide. By capturing the biogas and burning it as fuel, greenhouse gas emissions are reduced since that methane would likely have been released to the atmosphere from an anaerobic lagoon treating the manure.

Design of an anaerobic digestion system requires knowledge of the process so that the system is properly sized. Also, some technical expertise is needed to supervise the operation and keep it producing biogas at the optimal level.

The process has been implemented in a number of commercial operations. Unfortunately, many of them did not have the technical expertise available to maintain good operation of the system and it was abandoned. Also, the process is not economical based only on the energy produced. Economic credits for environmental benefits have not been developed to date.

Animal manure could provide only a small percentage of the energy used in the US. However, anaerobic digestion or other renewable energy processes that use manure also can use other types of organic wastes such as municipal solid waste and crop residues. When all of these are considered, there is a greater potential for substantial contribution to the US energy requirements.

Horse manure makes very good raw material for compost especially when it contains bedding material such as straw. With the presence of bedding in it, the carbon nitrogen ratio (desired ratio is between 25 and 30, for example 25 parts of carbon to every part of nitrogen) is high and the moisture is normally about 60 to 70 percent or so. This bedding and manure mixture makes excellent compost. If manure comes with out the bedding then a carbon source such as straw, wood chips, tree trimmings, saw dust or wood shaving must be added prior to starting a compost pile. Compost pile should not be more than 4 to 5 feet high. You may have to turn the pile once or twice during the first 2 weeks or so and you must also check the temperature of the pile. The temps must rise to about 140 deg. F or higher with in 7-10 days of a properly working compost pile and the pile should not have bad odor by then. Compost pile should be done with in about 30 to 40 days or when the temps go back down to the surrounding temps. The pile should then be “cured” for another month or so before applying to the land as nutrient for garden or crops. Avoid spreading the compost directly on the garden vegetables. Apply the compost and cover it or mix it with the soil. Compost manure at a place that drains well and does not stay wet all the time or where it may contaminate surface or ground water. keep it from too much direct sun light.