Anaerobic digestion (the decomposition of organic matter by bacteria in the absence of oxygen) occurs naturally in liquid manure systems. The lack of oxygen and abundance of organic matter in liquid manure provide the proper conditions for anaerobic bacteria to survive. Unfortunately, uncontrolled anaerobic decomposition can cause the foul odors sometimes associated with liquid manure storage and spreading.

However, controlled anaerobic decomposition not only can reduce the odors in liquid manure systems, but also can turn odorous compounds and organic matter into energy. The effluent remaining after controlled anaerobic decomposition, equal in volume to the influent material, is liquefied, low in odor and rich in nutrients. This digested material is biologically stable and will resist further breakdown and odor production, when stored under normal conditions.

Anaerobic bacteria transform manure and other organic material into biogas and a liquefied effluent during the three stages of biogas production (see Figure 1*). In the liquefaction stage, liquefying bacteria convert insoluble, fibrous materials such as carbohydrates, fats and proteins into soluble substances.

However, some fibrous material cannot be liquefied and can accumulate in the digester or can pass through the digester intact. Water and other inorganic material can also accumulate in the digester or pass through the digester unchanged. Undigested materials make up the low-odor, liquefied effluent. Most of the liquefied, soluble compounds are converted to biogas by the acid- and methane-forming bacteria during steps 2 and 3 of biogas production.

In the second stage of anaerobic digestion, acid-forming bacteria convert the soluble organic matter into volatile acids (the organic acids that can cause odor production from stored liquid manure).

Finally, methane-forming bacteria convert those volatile acids into biogas (a gas composed of about 60 percent methane, 40 percent carbon dioxide and trace amounts of water vapor, hydrogen sulfide and ammonia). Not all volatile acids and soluble organic compounds are converted to biogas; some become part of the effluent.

Methane-forming bacteria are more sensitive to their environment than acid-forming bacteria. Acid-forming bacteria can survive under a wide range of conditions, while methane-forming bacteria are more demanding (see Figure 2*). Under the conditions typical of liquid manure storages, more acid-forming bacteria can survive than methane-forming bacteria. Therefore, acids are formed and are not converted to biogas. This excess of volatile acids can result in a putrid odor.

In a controlled, optimum environment, methane-forming bacteria survive and convert most of the odor-producing volatile acids into biogas. Conditions that encourage activity of both acid- and methane-forming bacteria include:

•An oxygen-free environment
•A relatively constant temperature of about 95ºF
•A pH between 6.6 and 7.6
•A consistent supply of organic matter to “feed” upon

For consistent operation of an anaerobic digester, the manure that “feeds” the bacteria should be:

•A flowable liquid, about 12 percent solids or less (for pump or flow requirements)
•Not frozen
•Free from excess amounts of medication, feed additives or chemical washes
•Supplied fresh to the digester at least twice a day
•A uniform slurry of manure that does not separate easily

Anaerobic digestion is simply a continuation of the animal’s digestive system – a process to turn manure into energy and effluent, just like an animal turns feed into energy and manure.

Anaerobic digestion system
An anaerobic digestion system (see Figure 3*) can provide an optimal environment for controlled anaerobic digestion. A typical system consists of liquid manure-handling equipment, a heated anaerobic digester, gas utilization equipment, safety equipment and effluent storage and handling systems. The anaerobic digestion system is an addition to the manure-handling scheme, a step for manure processing between the barn and the storage facility. It does not replace any part of a typical manure-handling system.

Liquid manure-handling system
A liquid manure-handling system (such as the system used to transport liquid manure from a barn to a storage facility) transports manure from the animal housing facility to the anaerobic digester and from the digester to the storage facility or spreader. When possible, the use of gravity flow is encouraged to reduce the energy consumption and complexity of the handling system.

A bypass line routes manure around the digester when the manure is unsuitable for digestion or the digester is not operating.

Anaerobic digester
An anaerobic digester is a sealed, heated tank which provides a suitable environment for naturally-occurring anaerobic bacteria to grow, multiply and convert manure to biogas and a low-odor effluent. Typical digesters that have been insulated are squat, silo-like structures or in-ground rectangular or round concrete tanks.

Rigid or flexible covers have been used. They are designed to hold about 20 days of manure and a small supply of biogas. Manure, added daily to the digester, remains inside for about 20 days (the retention time) before flowing to the storage facility or spreader. Because there is no volume reduction with anaerobic digestion, the same amount of material added daily to the digester is also removed daily. While manure is flowing through the digester, the bacteria convert organic matter to biogas and effluent.

During the retention time, lightweight materials such as bedding or animal hair can float to the top of the digester, forming a crusty scum, and heavy or insoluble materials such as dirt can settle to the bottom. Settling reduces the effective volume of the digester and can cause incomplete digestion and odor problems, while crusting can keep gas from escaping the surface of the digesting manure. To control settling and scum formations, material in the digester can be agitated by a slurry pump, a mechanical stirrer or strategic placement of the heating pipes.

Slurry pumps are an effective way to keep material in the digester well-mixed. Mechanical mixing adds complexity to the system, but can aid thermal uniformity, reduce settling and break up crust formation. Mechanical mixing may be necessary for certain manure-handling systems (such as flush systems) where solid and liquid portions may separate easily into distinct layers within the digester. Strategic placement of the heating pipes will encourage thermal circulation and reduce settling problems.

The heating system is a critical part of the anaerobic digester. Heating pipes in which hot water circulates must be able to heat all material entering the digester to 95ºF and to resist corrosion from manure. Adding manure to the digester as soon as possible after it is excreted from the animal will help minimize heating requirements.

Gas utilization equipment
Biogas is collected in the head space of the anaerobic digester (or under the flexible cover) and has about 60 percent of the energy density of natural gas (methane) or about 600 British Thermal Units (BTU) per square foot. With minor equipment modifications, biogas can be used in the same applications as liquid petroleum gas, propane or natural gas. Biogas is best suited for stationary continuous operation because of its low-energy density, the corrosive nature of some of the impurities, and the constant production rate.

Biogas utilization equipment typically consists of either an engine generator set with electric utility hook-up, an engine operating hydraulic or air pumps or a gas boiler. Utilization equipment should be housed in a separate equipment shed apart from the digester to prevent corrosion.

Operating biogas-powered equipment continuously keeps the equipment temperature high enough to prevent condensation and sulfuric acid formation. Sulfuric acid is highly corrosive and can ruin expensive engines or boilers. Because biogas is a gas and not a liquid fuel, it is not practical for fueling vehicles. It would take 240 square feet of biogas to produce the same energy as one gallon of fuel oil. Biogas cannot feasibly be compressed to a liquid fuel due to its low-energy density.

For electricity production, biogas is piped to an internal combustion engine. The engine drives a generator to produce electricity that can be used on the farm or sold. To maintain continuous operation, the engine throttle is adjusted to balance biogas use with production. Waste heat from the engine is used to heat the digester and for other farm heating needs.

Most systems produce about 2 kilowatt-hours per day per 1,400-pound cow. Many utility companies pay much less than the consumer price for a kilowatt-hour. Therefore, maximizing the replacement of purchased energy with farm-produced energy will improve the economics of on-farm electricity generation.

Safety equipment
Because biogas is a potentially dangerous gas, safety devices such as gas detectors, flame traps, physical barriers and warning signs control and minimize the hazards of biogas and manure storage. Safety hazards Anaerobic digesters are confined spaces which pose a potential immediate threat to human life. They are designed to seal out oxygen, making death by asphyxiation possible within seconds of entry. Toxic gases (such as hydrogen sulfide and ammonia) accumulate inside a digester. Never enter an empty digester without extensive venting with mechanical fans, checking for toxic gases with gas detection equipment and following safe entry procedures.

Natural ventilation is not enough to remove toxic gases from the digester or to provide sufficient breathable air. Dense hydrogen sulfide gas will sink to the bottom of the tank, lighter ammonia will linger in the top of the tank and neither gas will escape without mechanical ventilation. Moreover, methane is explosive when mixed with air in concentrations of 5 to 15 percent. A leak in a gas line will create a fire hazard.

Potential advantages of controlled anaerobic digestion

•Substantially less odor with digested manure than with stored liquid manure.
•Energy produced from biogas offsets the cost of the investment.
•Digested manure is more liquefied than raw manure, making it easier to pump long distances.
•Emissions of methane from liquid manure storage areas are reduced.
•The nutrient content of digested manure is equal to that of raw manure.
•Digested manure is biologically stabilized, making it easier to store for long periods without odor problems.
•Homogenous digested manure performs well in liquid application systems.
•Rodents and flies are less likely to be attracted to digested manure.

Potential disadvantages of anaerobic digestion

•Initial investment may be costly for a digestion system.
•Bankers and lenders may be wary of lending money for these systems.
•The digester requires proper care and feeding, just like an animal.
•Technical knowledge of the digestion process and good management are required.
•Labor is required for preventive and unscheduled maintenance. Ideally, one person will be in charge of the digester, and the digester takes precedence over that person’s other farm duties.
•Daily maintenance tasks are minimal, but weekly oil changes, regular engine overhauls and periodic digester clean-outs are required.
•There is no reduction in the amount of manure to be handled. If water is added to the system, the volume is increased.
•Nutrient conservation may be undesirable on a farm with excess nutrients to manage.
•Much of the nitrogen in raw manure is converted from its organic form to ammonium. Ammonium can be transformed to either ammonia or nitrate. Ammonia can be lost from unincorporated, field-applied manure.
•Nitrate can be leached through the soil and may eventually reach groundwater.
•Field application and management to reduce nitrogen losses may be more demanding for digester effluent than for untreated liquid manure. •Anaerobic digesters can be a farm safety hazard.

Alternatives to electric generation from anaerobic digesters
With minor equipment modifications, biogas can be used as a substitute for natural gas. Running a gas-fired boiler is an inexpensive and efficient method to use biogas. The obstacle will be finding uses for the heat, especially in the summer. Absorption (heat-activated) cooling systems are a promising technology for using excess heat, but currently these systems have a high initial cost.

Another option is to remove carbon dioxide and hydrogen sulfide from the biogas and sell it as natural gas. Scrubbing the gas, finding a market, providing the buyer with a dependable supply of gas and maintaining the distribution equipment require money, time, maintenance and management. Additionally, natural gas will sell for a much lower price than electricity. Although other options are available for biogas utilization, electricity is the most versatile and valuable energy product from biogas.

Planning for future changes
If expansion of an animal production operation or a new facility is planned but an anaerobic digestion system is not included in the layout, leaving adequate space and installing a compatible manure handling system could add to the flexibility for the future. There may be a time when investing in a digester is just the right step for a farm.

Options with solids separation
Separating solids prior to anaerobic digestion and digesting only the organic matter in the liquid portion of the manure may produce a higher quality biogas (70 percent methane has been observed) and typically will reduce crusting and settling problems.

The solids can be field-applied, sold or composted and used for animal bedding. Separation and marketing of solids can generate farm income. Replacing bedding with composted solids could be a money-saver if a substantial amount of bedding currently is purchased and a solids separator is owned. However, if a solids separator needs to be purchased, the savings in bedding costs may not cover the cost of solids separation.  ANM

References omitted due to space but are available upon request.

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—From Penn State University, Agricultural and Biological Engineering website Copywrite 2006 Progressive Dairy Publishing. All Rights Reserved.

Jeannie Leggett, Extension Assistant; Robert E. Graves, Professor of Agricultural Engineering; and Les E. Lanyon, Associate Professor of Soil Fertility; Penn State University