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.

The land application of manure, litter, or wastewater at rates of application in accordance with a plan for nutrient management designed to enhance soil productivity and provide the crop or forage growth with needed nutrients for optimum health and growth.

A lot or facility (other than an aquatic animal production facility) where animals have been, are, or will be stabled or confined and fed or maintained for a total of 45 days or more in any 12-month period, and the animal confinement areas do not sustain crops, vegetation, forage growth, or post harvest residues in the normal growing season over any portion of the lot or facility. Two or more AFOs under common ownership are a single AFO if they adjoin each other, or if they use a common area or system for beneficial use of wastes. A land management unit is not part of an AFO.

A unit of measurement for any animal feeding operation calculated by adding the following numbers: the number of slaughter and feeder cattle and dairy heifers multiplied by 1.0, plus the number of mature dairy cattle multiplied by 1.4, plus the number of swine weighing over 55 pounds multiplied by 0.4, plus the number of weaned swine weighing 55 pounds or less multiplied by 0.1, plus the number of sheep multiplied by 0.1, plus the number of horses/mules multiplied by 2.0.

Any animal feeding operation (AFO) is defined as follows:

(A) Large CAFO – Any AFO that stables or confines and feeds or maintains for a total of 45 days or more in any 12-month period equal to or more than the numbers of animals specified in any of the following categories:

(i) 1,000 cattle other than mature dairy cattle or veal calves. Cattle includes, but is not limited to, heifers, steers, bulls, and cow/calf pairs;
(ii) 1,000 veal calves;
(iii) 700 mature dairy cattle (whether milkers or dry cows);
(iv) 2,500 swine weighing more than 55 pounds or 10,000 swine weighing less than 55 pounds;
(v) 500 horses;
(vi) 10,000 sheep or lambs;
(vii) 55,000 turkeys;
(viii) 125,000 chickens (other than laying hens, if the operation does not use a liquid waste handling system);
(ix) 30,000 laying hens or broilers (if a liquid manure handling system), or 82,000 laying hens (if the operation does not use a liquid manure handling system); or
(x) 5,000 ducks (a liquid manure handling system), or 30,000 ducks (if the operation does not use a liquid manure handling system);

(B) Medium CAFO – Any AFO with the following number of animals that discharges pollutants into water in the state either through a man-made ditch, flushing system, or other similar man-made device, or directly into water in the state that originates outside of and passes over, across, or through the facility or otherwise comes into direct contact with animals confined in the operation:

(i) 300 to 999 cattle other than mature dairy cattle or veal calves. Cattle includes, but is not limited to, heifers, steers, bulls, and cow/calf pairs;
(ii) 200 to 699 mature dairy cattle (whether milking or dry cows);
(iii) 300 to 999 veal calves;
(iv) 750 to 2,499 swine each weighing 55 pounds or more, or 3,000 to 9,999 swine each weighing less than 55 pounds;
(v) 150 to 499 horses;
(vi) 3,000 to 9,999 sheep or lambs;
(vii) 16,500 to 54,999 turkeys;
(viii) 37,500 to 124,999 chickens (other than laying hens and other than a liquid manure handling system);
(ix) 9,000 to 29,999 laying hens or broilers (if a liquid manure handling system), or 25,000 to 81,999 laying hens (if other than a liquid manure handling system); or
(x) 1,500 to 4,999 ducks (if a liquid manure handling system), or 10,000 to 29,999 ducks (if other than a liquid manure handling system).

(C) Small CAFO – An AFO that is designated by the executive director as a CAFO because it is a significant contributor of pollutants into or adjacent to water in the state and is not a large or medium CAFO.

(D) State-only CAFO – An AFO that falls within the range of animals in subparagraph (B) of this paragraph and that is either located in the dairy outreach program areas or designated by the executive director as a CAFO because it is a significant contributor of pollutants into water in the state. A state-only CAFO is authorized under state law.

A retention control structure used for the biological treatment of liquid organic wastes. Lagoons can be aerobic, anaerobic, or facultative depending on their design and can be used in a series to produce a higher quality effluent. Treatment volume must be included in the lagoon design.

The act of applying manure, litter, or wastewater associated with the animal feeding operation including distribution to, or incorporation into, the soil mantle primarily for beneficial use purposes.

Any barrier in the form of a layer, membrane or blanket, naturally existing, constructed or installed to prevent a significant hydrologic connection between liquids contained in retention structures and waters in the state.

Any process generated wastewater directly or indirectly used in the operation of a CAFO (such as spillage or overflow from animal or poultry watering systems which comes in contact with waste); washing, cleaning or flushing pens, barns, manure pits, direct contact swimming, washing, or spray cooling of animals; and dust control), and precipitation which comes into contact with any manure or litter, bedding, or any other raw material or intermediate or final material or product used in or resulting from the production of animals or poultry or direct products (e.g. milk, meat or eggs).

Any water directly or indirectly used in the operation of a CAFO (such as spillage or overflow from animal or poultry watering systems which comes in contact with waste; washing, cleaning or flushing pens, barns, manure pits, direct contact swimming, washing, or spray cooling of animals; and dust control) which is produced as wastewater.

The maximum rainfall event with a probable recurrence interval of once in 25-years, with a duration of 24 hours, as defined by the National Weather Service in Technical Paper Number 40, “Rainfall Frequency Atlas of the United States,” May 1961, and subsequent amendments, or equivalent regional or state rainfall information developed there from.

The law passed in 1970 (and subsequent amendments) that created federal air quality standards and established the Environmental Protection Agency as the lead federal agency protecting and improving the nation’s air quality.

One of six air pollutants that may adversely affect human health. These “criteria pollutants” are ozone (O₃), carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), particulate matter (PM), and lead (Pb). EPA established maximum concentrations for these substances.

The United States Environmental Protection Agency, created in 1970 by the Clean Air act to set and enforce air quality standards and guide state regulatory agencies.

Air quality standards set by the EPA for the criteria pollutants. Found in 40 CFR 50 (Title 40 of the Code of Federal Regulations, Part 50).

A geographic region that does not meet the NAAQS for one of the criteria pollutants.

A term describing the settling velocity of different size particles in air compared to a particle of water.

A term that categorizes the properties of irregularly shaped particles based on a correction to a spherical shape. Particles with an ESD of 10 all behave as spherical particles of 10-micron diameter.

One millionth of a meter, sometimes referred to as a micro-meter or µm. This is the typical unit when referring to particle diameters.

A pollution source that emits 10 tons per year of any of the listed toxic air pollutants, or 25 tons per year of a mixture of pollutants. Major sources may release air pollutants when there are equipment leaks, when materials are transferred from one location to another, or when materials are discharged through emission stacks or vents.

A smaller facility that releases lesser quantities of toxic pollutants into the air (less than 10 tons per year of a single pollutant, or less than 25 tons per year of a combination of pollutants). Though emissions from individual area sources are often relatively small, collectively their emissions can be of concern, particularly where a large number of sources are located in heavily populated areas.

A source of pollution that is not stationary, such as a bus, automobile, airplane, or train.

A single, identifiable, fixed source of pollution such as a stack, cyclone exhaust, facility or mine.

Air pollutants that escape unplanned, as from equipment leaks, the bulk handling or processing of raw materials, windblown dust, natural airflow over area sources, and a number of other specific processes.

Procedures or controls that can be implemented to prevent or reduce pollution.

Under the Clean Air Act, the rate of emissions that reflects 1) the most stringent emission limitation in the implementation plan of any state for such source unless the owner or operator demonstrates such limitations are not achievable; or 2) the most stringent emission limitation achieved in practice, whichever is more stringent. A proposed new or modified source may not emit pollutants in escess of existing new source standards. LAER is accomplished by using the maximum achievable control technology (MACT).

The level of control associated with the lowest achievable emission rate (LAER). It is used for polluters in non-attainment areas of a regulated pollutant. There is generally no consideration for economic reasonableness for this level of control.

The range of particle sizes in a given air sample, typically expressed as a frequency function such as a lognormal curve.

EPA program that requires state and/or federal permits in order to restrict emissions from new or modified sources in places where air quality already meets or exceeds primary and secondary ambient air quality standards.

The amendment to the Clean Air Act that establishes the federal operating permit program administered by the states.

Any substance designated by the EPA for which releases of a designated quantity into the environment must be reported.

In addition to the six criteria pollutants, 187 other air pollutants that are not covered by ambient air quality standards but that, as defined in the Clean Air Act, may reasonably be expected to cause or contribute to irreversible illness or death. Such pollutants include asbestos, beryllium, mercury, benzene, coke oven emissions, radionuclides and vinyl chloride.