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Unlike most companies, we are equipment supplier/vendor neutral. This means we help our clients select the best equipment for their specific application. This approach provides our customers with superior performance, decreased operating expenses and increased return on investment. 

What is an Anaerobic Digester?

An Anaerobic Digester is a device for optimizing the anaerobic digestion of biomass and/or animal manure, and possibly to recover biogas also referred to as BioMethane for energy production. Digester types include batch, complete mix, continuous flow (horizontal or plug-flow, multiple-tank, and vertical tank), and covered lagoon.

What is Anaerobic Digestion?

Anaerobic digestion is a biological process that produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas. These gases are produced from organic wastes such as livestock manure, food processing waste, etc. 

Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant. Organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so the process could occur. Depending on the waste feedstock and the system design, biogas is typically 55 to 75 percent pure methane. State-of-the-art systems report producing biogas that is more than 95 percent pure methane. 

The U.S. EPA AgSTAR Program Background

The U.S. EPA AgSTAR is an outreach program designed to reduce methane emissions from livestock waste management operations by promoting the use of biogas recovery systems. A biogas recovery system is an anaerobic digester with biogas capture and combustion to produce electricity, heat or hot water. Biogas recovery systems are effective at confined livestock facilities that handle manure as liquids and slurries, typically swine and dairy farms. Anaerobic digester technologies provide enhanced environmental and financial performance when compared to traditional waste management systems such as manure storages and lagoons. Anaerobic digesters are particularly effective in reducing methane emissions but also provide other air and water pollution control opportunities. AgSTAR provides an array of information and tools designed to assist producers in the evaluation and implementation these systems, including:

  • Conducting farm digester extension events and conferences
  • Providing “How-To” project development tools and industry listings
  • Conducting performance characterizations for digesters and conventional waste management systems
  • Operating a toll free hotline
  • Providing farm recognition for voluntary environmental initiatives
  • Collaborating with federal and state renewable energy, agricultural, and environmental programs

Methane Emissions from Animal Waste Management

Methane emissions occur whenever animal waste is managed in anaerobic conditions. Liquid manure management systems, such as ponds, anaerobic lagoons, and holding tanks create oxygen free environments that promote methane production. Manure deposited on fields and pastures, or otherwise handled in a dry form, produces insignificant amounts of methane. Currently, livestock waste contributes about 8 percent of human-related methane emissions in the U.S. Given the trend toward larger farms, liquid manure management is expected to increase. For more information on international emissions, projections, and mitigation costs, see International Analyses.

Methane (Biogas) from Anaerobic Digesters

Methane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4 ). It is the major component of the "natural" gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 British Thermal Units (Btu) [252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic meters) when burned. Natural gas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal.

The same types of anaerobic bacteria that produced natural gas also produce methane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or "digest" organic material in the absence of oxygen and produce "biogas" as a waste product. (Aerobic decomposition, or composting, requires large amounts of oxygen and produces heat.) Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managed in a "digester" (an airtight tank) or a covered lagoon (a pond used to store manure) for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit.

Biogas produced in anaerobic digesters consists of methane (50%-80%), carbon dioxide (20%-50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The relative percentage of these gases in biogas depends on the feed material and management of the process. When burned, a cubic foot (0.028 cubic meters) of biogas yields about 10 Btu (2.52 kcal) of heat energy per percentage of methane composition. For example, biogas composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).

Digester Designs

Anaerobic digesters are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. All designs incorporate the same basic components: a pre-mixing area or tank, a digester vessel(s), a system for using the biogas, and a system for distributing or spreading the effluent (the remaining digested material).

There are two basic types of digesters: batch and continuous. Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.

In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. They may be better suited for large-scale operations. There are three types of continuous digesters: vertical tank systems, horizontal tank or plug-flow systems, and multiple tank systems. Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas.

Many livestock operations store the manure they produce in waste lagoons, or ponds. A growing number of these operations are placing floating covers on their lagoons to capture the biogas. They use it to run an engine/generator to produce electricity.

The Digestion Process

Anaerobic decomposition is a complex process. It occurs in three basic stages as the result of the activity of a variety of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.

A variety of factors affect the rate of digestion and biogas production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135° Fahrenheit (F) (57.2° Centigrade [C]), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic). Bacteria activity, and thus biogas production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).

In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range. However, the process is highly sensitive to disturbances such as changes in feed materials or temperature. While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction. Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.

To optimize the digestion process, the digester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. In most areas of the United States, digestion vessels require some level of insulation and/or heating. Some installations circulate the coolant from their biogas-powered engines in or around the digester to keep it warm, while others burn part of the biogas to heat the digester. In a properly designed system, heating generally results in an increase in biogas production during colder periods. The trade-offs in maintaining optimum digester temperatures to maximize gas production while minimizing expenses are somewhat complex. Studies on digesters in the north-central areas of the country indicate that maximum net biogas production can occur in digesters maintained at temperatures as low as 72°F (22.2°C).

Other factors affect the rate and amount of biogas output. These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time. Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH, for example when too much "green" or material high in nitrogen content is added. It may be necessary to add water to the feed material if it is too dry, or if the nitrogen content is very high. A carbon/nitrogen ratio of 20/1 to 30/1 is best. Occasional mixing or agitation of the digesting material can aid the digestion process. Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. Complete digestion, and retention times, depend on all of the above factors.

Producing and Using Biogas

As long as proper conditions are present, anaerobic bacteria will continuously produce biogas. Minor fluctuations may occur that reflect the loading routine. Biogas can be used for heating, cooking, and to operate an internal combustion engine for mechanical and electric power. For engine applications, it may be advisable to scrub out hydrogen sulfide (a highly corrosive and toxic gas). Very large-scale systems/producers may be able to sell the gas to natural gas companies, but this may require scrubbing out the carbon dioxide.

Using the Effluent

The material drawn from the digester is called sludge, or effluent. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner. It can also be used as a livestock feed additive when dried. Any toxic compounds (pesticides, etc.) that are in the digester feedstock material may become concentrated in the effluent. Therefore, it is important to test the effluent before using it on a large scale.


Anaerobic digester system costs vary widely. Systems can be put together using off-the-shelf materials. There are also a few companies that build system components. Sophisticated systems have been designed by professionals whose major focus is research, not low cost. Factors to consider when building a digester are cost, size, the local climate, and the availability and type of organic feedstock material.

In the United States, the availability of inexpensive fossil fuels has limited the use of digesters solely for biogas production. However, the waste treatment and odor reduction benefits of controlled anaerobic digestion are receiving increasing interest, especially for large-scale livestock operations such as dairies, feedlots, and slaughterhouses. Where costs are high for sewage, agricultural, or animal waste disposal, and the effluent has economic value, anaerobic digestion and biogas production can reduce overall operating costs. Biogas production for generating cost effective electricity requires manure from more than 150 large animals.

Below-ground, concrete anaerobic digesters have proven to be especially useful to agricultural communities in parts of the world such as China, where fossil fuels and electricity are expensive or unavailable. The primary purpose of these anaerobic digesters is waste (sewage) treatment and fertilizer production. Biogas production is secondary.


The AgSTAR Program has been very successful in encouraging the development and adoption of anaerobic digestion technology. Since the establishment of the program in 1994, the number of operational digester systems has doubled. This has produced significant environmental and energy benefits, including methane emission reductions of approximately 124,000 metric tons of carbon equivalent and annual energy generation of about 30 million kWh. The graph below shows the historical use of biogas recovery technology for animal waste management.

Chart showing how many farms have biogas recovery systems in place.

The development of anaerobic digesters for livestock manure treatment and energy production has accelerated at a very fast pace over the past few years. Factors influencing this market demand include: increased technical reliability of anaerobic digesters through the deployment of successful operating systems over the past five years; growing concern of farm owners about environmental quality; an increasing number of state and federal programs designed to cost share in the development of these systems; and the emergence of new state energy policies (such as net metering legislation) designed to expand growth in reliable renewable energy and green power markets.

In the past 2 years alone, the number of operational digester systems has increased by 30%. For more detailed information on anaerobic digester use in the U.S. , go to the Guide to Operational Systems or see the AgSTAR 2003 Digest


The process of anaerobic digestion consists of three steps. 

The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar. The second step is the conversion of decomposed matter to organic acids. And finally, the acids are converted to methane gas. 

Process temperature affects the rate of digestion and should be maintained in the mesophillic range (95 to 105 degrees Fahrenheit) with an optimum of 100 degrees F. It is possible to operate in the thermophillic range (135 to 145 degrees F), but the digestion process is subject to upset if not closely monitored. 

Many anaerobic digestion technologies are commercially available and have been demonstrated for use with agricultural wastes and for treating municipal and industrial wastewater. 

At Royal Farms No. 1 in Tulare, California, hog manure is slurried and sent to a Hypalon-covered lagoon for biogas generation. The collected biogas fuels a 70 kilowatt (kW) engine-generator and a 100 kW engine-generator. The electricity generated on the farm is able to meet monthly electric and heat energy demand.

Given the success of this project, three other swine farms (Sharp Ranch, Fresno and Prison Farm) have also installed floating covers on lagoons. The Knudsen and Sons project in Chico, California, treated wastewater which contained organic matter from fruit crushing and wash down in a covered and lined lagoon. The biogas produce is burned in a boiler. And at Langerwerf Dairy in Durham, California, cow manure is scraped and fed into a plug flow digester. The biogas produced is used to fire an 85 kW gas engine. The engine operates at 35 kW capacity level and drives a generator to produce electricity. Electricity and heat generated is able to offest all dairy energy demand. The system has been in operation since 1982. 

Most anaerobic digestion technologies are commercially available. Where unprocessed wastes cause odor and water pollution such as in large dairies, anaerobic digestion reduces the odor and liquid waste disposal problems and produces a biogas fuel that can be used for process heating and/or electricity generation. 

Technology assessment

This section describes the anaerobic digestion (AD) process, outlines guidelines for assessing the feasibility of AD and biogas usage at a swine facility and provides summary information on AD system performance and reliability.

Anaerobic Digestion Technology Description

AD promotes the bacterial decomposition of the volatile solids (VS) in animal wastes to biogas, thereby reducing lagoon loading rates and odor. The primary component of an AD system is the anaerobic digester, a waste vessel containing bacteria that digest the organic matter in waste streams under controlled conditions to produce biogas. As an effluent, AD yields nearly all of the liquid that is fed to the digester. This remaining fluid consists of mostly water and is allowed to evaporate from a secondary lagoon, land-applied for irrigation and fertilizer value or recycled to flush manure from the swine building to the digester.

The benefits of AD include:

  • Odor reduction;
  • Reduction in the biological oxygen demand of treated effluent by up to 90 percent, reducing the risk for water contamination;
  • Improved nutrient application control, because up to 70 percent of the nitrogen in the waste is converted to ammonia, the primary nitrogen constituent of fertilizer;
  • Reduced pathogens, viruses, protozoa and other disease-causing organisms in lagoon water, resulting in improved herd health and possible reduced water requirements; and
  • Potential to generate electricity and process heat.

AD takes place in three steps: hydrolysis, acid formation, and methane generation. During the first step, hydrolysis, bacterial enzymes break down proteins, fats and sugars in the waste to simple sugars. During acid formation, bacteria convert the sugars to acetic acid, carbon dioxide and hydrogen. Then the bacteria convert the acetic acid to methane and carbon dioxide, and combine carbon dioxide and hydrogen to form methane and water.

Digester technologies that can be used to collect biogas from swine facilities include:


  • Covered anaerobic lagoons,
  • Complete mix digesters and
  • Sequencing batch reactors.

Although a sequencing batch reactor has been used for AD at one swine facility in the United States , this technology is considered to be experimental, and thus is not included in this report. This report focuses on technologies that have verifiable performance characteristics, namely, covered anaerobic lagoons and complete mix digesters. Appendix B provides contact information that can help producers find AD system designers/installers, odor control technologies, generators, heating and cooling equipment, and other information to help manage air and water quality at hog facilities.

Covered lagoon digesters are the simplest AD system. These systems typically consist of an anaerobic combined storage and treatment lagoon, an anaerobic lagoon cover, an evaporative pond for the digester effluent, and a gas treatment and/or energy conversion system. Figure 1 shows a typical schematic for a floating covered anaerobic lagoon.

Figure 1 . Covered anaerobic lagoon digester

Covered lagoon digesters typically have a hydraulic retention time (HRT) of 40 to 60 days. The HRT is the amount of time a given volume of waste remains in the treatment lagoon. A collection pipe leading from the digester carries the biogas to either a gas treatment system such as a combustion flare, or to an engine/generator or boiler that uses the biogas to produce electricity and heat. Following treatment, the digester effluent is often transferred to an evaporative pond or to a storage lagoon prior to land application.

Climate affects the feasibility of using covered lagoon digesters to generate electricity. Engine/generator systems typically do not produce sufficient waste heat to maintain temperatures high enough in covered lagoon digesters in the winter to sustain consistently high biogas production rates. Using propane or natural gas to provide additional heat for the lagoon contents is typically not an economically viable option. Without that additional heat, most covered lagoon digesters produce less biogas in colder temperatures, and little or no gas below 39 FACE= "Symbol">° F. As a result, covered lagoon digesters are most appropriate for use in warm climates if the biogas is to be used for energy or heating purposes.

Complete mix digester systems consist of a mix tank, a complete mix digester and a secondary storage or evaporative pond. The mix tank is either an aboveground tank or concrete in-ground tank that is fed regularly from underfloor waste storage below the animal feedlot. Waste is stirred in the mix tank to prevent solids from settling in the waste prior to being fed to the digester. The complete mix digester is essentially a constant-volume aboveground tank or in-ground covered lagoon that is fed daily from the mix tank. Complete mix digesters with in-ground lagoons often employ covers similar to those used in covered lagoon digesters. In the digester, a mix pump circulates waste material slowly around the heater to maintain a uniform temperature. Hot water from an engine/generator cogeneration water jacket or boiler is used to heat the digester. A cylindrical aboveground tank, such as that shown in Figure , optimizes biogas production, but is more capital intensive than in-ground tanks. The only operating AD system in Colorado that recovers methane for energy use is a complete mix digester, located at Colorado Pork LLC near Lamar , Colorado .

Source: EPA. (February 1997). AgStar Technical Series: Complete Mix Digesters – A Methane Recovery

Option for All Climates. EPA 430-F-97-004. Washington , DC .

Figure 2 . Complete mix digester schematic

Complete mix digesters have an HRT of 15 to 20 days, which means that complete mix digesters can reduce the overall lagoon volume required for waste storage and treatment. This makes complete mix digesters comparable to covered lagoon digesters in cost, despite the increased complexity of stirring, mixing and plumbing components. In addition, biogas production rates, and therefore heat and electricity production, are greater and more consistent than for covered lagoons. This can help reduce system payback periods compared to covered lagoon systems. Like covered lagoon systems, digester effluent from complete mix digesters is frequently stored in evaporative ponds or storage lagoons.

System Requirements

This section provides guidelines for conducting a preliminary assessment of the feasibility of using AD at a swine facility. Although AD system requirements will vary depending on the application and system design, there are some rule-of-thumb measures that should be noted when assessing the feasibility of AD at a given location. For AD to potentially be technically feasible and cost-effective, a swine facility should:

  • Simultaneously house at least 2,000 animals with a total live animal weight of at least 110,000 pounds,
  • Have no more than 20 percent variation in animal population throughout the year,
  • Collect waste at one central location such as an underfloor pit,
  • Collect waste daily or every other day, or can convert to an equivalent collection system,
  • Have manure free of large amounts of bedding or other foreign materials, and
  • Have some manure storage capability to maintain a steady digester feedstock supply

If the above characteristics are present, the facility is a possible candidate for AD. Many pre-existing waste storage and treatment lagoons are too large to practically or cost-effectively employ covers over their entire area. Partial covers may be an option to recover methane from these older systems, as an alternative to installing a completely new storage and treatment lagoon system. If energy recovery is to be employed, methane production and gas quality should be considered and compared to energy requirements at the facility. Daily biogas production at installed farm-based anaerobic digesters in the United States varies from 24,000 to 75,000 cubic feet, or an energy equivalent of 13 to 42 million British thermal units (Btu) (assuming 55 percent methane content for biogas). Covered lagoon digesters and complete mix digesters differ in their methane production characteristics, and energy conversion systems that rely on methane from anaerobic digesters should be chosen according to the end-use objective for the system. Complete mix digesters can produce heat and electricity at a constant rate throughout the year because heat recovery can be used to heat the digesters in the winter. Covered lagoon digesters can consistently produce biogas only in months when the temperature exceeds 39 degrees Fahrenheit.

Facilities that are located south of the line of climate limitation in Figure 3 are usually warm enough for cost-effective energy recovery from covered lagoon digesters. In most cases, facilities north of the climate line in Figure 3 are too cold for cost-effective energy recovery from covered lagoon digesters. Complete mix digesters can be used in cold or warm climates. If odor control is the only objective, either covered lagoon or complete mix digesters may be used, but odor control will be less effective in the winter for covered lagoon digesters south of the line of climate limitation in Figure 3. In general, complete mix digesters are the most appropriate choice for use in Colorado . 

Source: EPA. (July 1997). AgStar Handbook: A Manual for Developing Biogas Systems at Commercial Farms in the United States . EPA 430-B-97-015. pp. 4-12.

Figure 3 . Line of climate limitation for biogas energy recovery

Table 2 shows which digesters are appropriate for the waste collection strategies at covered swine facilities. Complete mix digesters can operate with a waste total solids (TS) percentage between 3 and 10 percent, while covered lagoon digesters can use waste with a TS percentage less than 2 percent.

Table 2 . Matching a digester to existing waste collection practices

Collection system

Percent TS required

Digester type

Suitable climate



Complete mix

Warm or cold

Pit storage


Complete mix

Warm or cold



Covered lagoon


Pit recharge


Covered lagoon


Gravity drainage




Pull plug


Covered lagoon


Managed pull-plug


Complete mix

Warm or cold

Source – Adapted from: EPA. (July 1997). AgStar Handbook: A Manual for Developing Biogas Systems at Commercial

Farms in the United States . EPA 430-B-97-015. pp. 4-15.

Appendix C describes each of the various waste collection technologies listed in Table 2.

Biogas Utilization Options

This section discusses some of the biogas utilization options that are available for use with AD. Electricity generation with waste heat recovery (cogeneration) and direct combustion and use in equipment that normally uses propane or natural gas are the two primary options for biogas utilization. Electricity generated using biogas can be generated for on-farm use or for sale to the electric power grid if an economically attractive power purchase agreement can be negotiated through the local utility or rural electric cooperative. Direct combustion allows the gas to be used in existing equipment that normally uses propane or natural gas such as boilers or forced air furnaces with minor equipment modifications. Combustion is usually a seasonal use for biogas, as most boiler and furnace applications are only required during the winter. The EPA FarmWare manual describes some characteristics of engine/generator and direct combustion systems that can be used with biogas. The following subsections draw from the FarmWare manual to provide some basic information about the use of these systems at covered swine facilities and other farm applications.

Electricity Generation

Commercial electricity generation systems that use biogas typically consist of an internal combustion (IC) engine, a generator, a control system and an optional heat recovery system.

IC engines designed to burn propane or natural gas are easily converted to burn biogas by adjusting carburation and ignition systems. Such engines are available in nearly any capacity, but the most successful varieties are industrial engines that are designed to work with wellhead natural gas. A biogas-fueled engine will normally convert 18 to 25 percent of the biogas Btu value to electricity.

Two types of generators are used on farms: induction generators and synchronous generators. Induction generators operate in parallel with the utility and cannot operate as a stand-alone power source. Induction generators derive their phase, frequency and voltage from the utility. Synchronous generators operate as an isolated system or in parallel to the utility, and require more sophisticated intertie systems to match output to utility phase, frequency and voltage.

Control systems are required to protect the engine and the utility. Control packages are available that can shut the engine off due to mechanical problems, utility power outage or utility voltage and frequency fluctuations, or in the event that excess power is generated that the utility will not accept. Generators that operate in parallel with the utility system, such as induction generators, require an intertie system with safety relays to shut off the engine and disconnect from the utility in the event of a problem. Intertie negotiations with a utility for induction generators are typically much easier than for a synchronous generator, due to the level of control the utility has over the characteristics of power entering the grid from an induction generator. The primary advantage of a synchronous generator is its ability to act as a stand-alone power source. However, if operated as an isolated system, a synchronous generator must be oversized to meet the highest electrical demand, while operating less efficiently at average or partial loads. Due to the system size and more complicated control requirements, a synchronous generator operating as an isolated system is typically more expensive than an induction generator.

Biogas engines reject approximately 75 to 82 percent of the energy input as waste heat. This waste heat can be used to heat the digester and/or provide water or space heat to the facility. Commercial heat exchangers can recover waste heat from the engine water cooling system and the engine exhaust, recovering up to 7,000 Btu/hour for each kW of generator load. Waste heat recovery increases the energy efficiency of the system to 40 to 50 percent.

Ongoing research and development is focusing on the use of microturbines and fuel cells for converting biogas to electricity. Microturbines are high-speed, small-scale (typically less than 100 kW) gas-driven turbine systems that produce electricity efficiently, have low emissions and require little maintenance. Reflective Energies in Viejo , California in partnership with Capstone Microturbine Corporation is working on developing the Flex-Microturbine, a power generation technology that can use biogas from animal waste, landfill gas and biomass gasification as its fuel source. Fuel cells are an emerging technology that operate, in principle, like a battery, but do not run out of charge. Instead, fuel cells equipped with a fuel reformer can use any type of hydrocarbon fuel, and run continuously as long as fuel is available. Fuel cells can convert fuel to electricity at efficiencies close to 40 percent, compared to 30 percent for the most efficient engine. In addition, fuel cell emissions include heat, some of which can be recovered for other applications, water, and carbon dioxide.

The Department of Energy’s WRBEP funded a project in fiscal year 2000 in San Luis Obispo , California that will demonstrate electricity generation from methane using a prototype microturbine at a 350-cow farm. The project will be using a 25 kW Capstone microturbine prototype to generate electricity at the California Polytechnic State University ’s demonstration farm.

Direct Combustion

Direct combustion of biogas on-site in a boiler or forced air furnace can provide seasonal heat to nurseries, farrowing rooms and other facilities at a swine facility. A cast iron natural gas boiler can be used for most farm boiler applications. The air-fuel mixture will require adjustment and burner jets will need to be enlarged for use with low-Btu gas. Cast iron boilers are available in many sizes, from 45,000 Btu/hour and up. Untreated biogas may be used, but all metal surfaces of the boiler housing should be painted to prevent corrosion. Flame tube boilers with heavy gauge flame tubes may be used if the exhaust temperature is maintained above 300 FACE= "Symbol">° F to prevent condensation. Forced air furnaces can be used in place of direct fire room heaters, but biogas must be treated to remove hydrogen sulfide because of potential corrosion problems in metal ductwork.

System Performance and Benefits of AD

There are several measures of waste management system performance that are relevant for producers considering the use of AD. These include:

  • Odor control,
  • Water quality protection
  • Energy production.

AD is the only waste management strategy available that provides the option to recover methane for energy production.
The APCD has determined that the minimum standard for compliance with odor control regulations for waste vessels and impoundments is an 80 percent reduction in all odor-causing gases, including hydrogen sulfide, ammonia and volatile organic compounds from waste vessels or impoundments. Table 3 compares the effectiveness of some of the odor control methods being implemented at covered swine facilities in Colorado . Lagoon covers and AD are among the most effective means of reducing odors from waste storage and treatment systems. However, several strategies may be combined to increase the effectiveness of individual odor control strategies at a facility. As an example, feed additives can be used in conjunction with biofilters, surface aeration or solids separation to increase overall odor control from waste storage and treatment lagoons. In addition, any lagoon odor control technology should be accompanied by an overall odor management program using best management practices as described in Appendix D.

Table 3 . Odor control effectiveness of management strategies for anaerobic lagoons

Odor control technology

Percent (%) odorous gas emissions reduction

Feed processing/additives


Grinding feed


Wet-feeding hogs (3:1 water to feed)


Reducing sulfur-containing amino acids


Adding fiber (soybeans, hulls to diet)

Up to 68



Solids separation


Soil injection of waste upon land application

50-80 (land application odors only)

Surface aeration

Up to 85

Aerobic cap

Up to 90

Lagoon additives

Up to 90

Lagoon covers


Anaerobic digestion



Up to 100 for well-managed systems

Source: Iversen, Kirk and Jessica Davis. (February 1999). Innovations in odor management technology. Colorado State University . Agricultural and Resource Policy Report. APR-99-02. Fort Collins , CO .

In addition to regulating odors from waste lagoons, the new odor control regulations have requirements for waste that is applied to agricultural land. The new regulations for waste treatment at covered swine facilities require that waste applied to agricultural land and not injected be treated to remove at least 65 percent of the TS and over 90 percent of the total volatile fatty acids or 60 percent of total VS. If not treated, waste applied to agricultural land must be injected or knifed into the soil upon application. Land application is not permitted between November 1 and February 28. Of the waste management strategies in Table 3, four will help reduce the TS and VS content prior to land application.

  • Wet-feeding,
  • Solids separation,
  • AD and
  • Composting.

Wet feeding can reduce the TS and VS by a value equal to the dilution rate of the feed (i.e., 3:1 ratio of water to feed). However, introducing this type of feeding system increases water requirements and may increase required anaerobic lagoon volumes. Solids separation can reduce TS by 30 to 45 percent. Solids separation methods include screen separators, mechanical presses, settling tanks, settling basins, vacuum filters and many other means. An efficient AD installation will reduce the TS percentage by up to 76 percent and VS by up to 90 percent. Of the above technologies, AD with covered anaerobic lagoons is the only one the APCD considers a proven technology because of their odor control effectiveness. Therefore, unlike the other options above, covered anaerobic digesters do not have to meet the additional testing requirements for technologies that the APCD considers experimental.

Composting may or may not meet the TS requirement because it often involves the addition of a bulking agent to increase TS to optimize waste decomposition. However, composting can be effective at controlling odors and reducing pathogens. The APCD is presently reviewing the compliance status of one facility that uses composting. Composting has applications besides manure treatment for livestock facilities. The Colorado Governor’s Office of Energy Management and Conservation is currently supporting the demonstration of composting technology for hog mortality disposal at a hog farm in Colorado .

In an AD system, most of the organic nitrogen (N) from the digester is converted to ammonium, an easily manageable fertilizer with slow release properties when compared to mineralized fertilizers. This is an advantage over anaerobic lagoons alone. Organic N in the form of protein and urea is mineralized in soil solution after land application. This mineralized N can pose a groundwater problem when land-applied because mineralized N can be converted to nitrates and leach into groundwater in the spring and fall when plant uptake of N is low.

A disadvantage of reducing the nutrient content of lagoon effluent via AD is the loss of the value of nutrients. Reducing the use of lagoon effluent as fertilizer increases the need for industrial fertilizers, the manufacture and transportation of which uses significant quantities of petroleum. However, this loss is balanced by the benefits of increased control farmers have over the nutrient content of effluent used for irrigation purposes.

System Reliability

System reliability is a key concern for swine producers that are considering AD with energy recovery as an objective. AD systems first began to be used extensively after World War II in Europe when energy supplies were reduced. Today there are over 600 digesters in Europe alone. Farm-based anaerobic digesters are the most common application of AD technology worldwide. In the U.S. , livestock producers have less experience working with anaerobic digesters, with a total of approximately 160 digesters either planned or installed in 1998. Of these, 36 employ technology that is suitable for use at swine facilities.

A recent survey of anaerobic digesters yielded mixed results for system reliability (Table 4). At farms across the U.S. , the percentage of installed digesters that are not operating is nearly 46 percent. However, one encouraging note is that the reliability of digesters constructed since 1984 is much greater than for those constructed between 1972 and 1984.

Table 4 . Status of farm-based digesters at swine facilities in the United States


Covered lagoon digesters

Complete mix digesters






Not operating




Facility closed




Planned/Under construction



Planned but not built








Source: Lusk, Phil (September 1998). Methane Recovery from Animal Manures: the Current Opportunities Casebook. NREL/SR-25145. NREL. Golden, CO. pp. 1-2.

The most common reasons that systems are not operating include poor design and installation and poor equipment specification. The lessons learned that should be kept in mind for future systems include the need to select qualified contractors and the fact that amortizing the cost of appropriate equipment is less costly than a system failure. The improved reliability of newer systems and increased understanding of the biological systems that operate in an anaerobic digester suggest that the reliability of systems will continue to improve as long as the lessons of past system failures are heeded.

What is BioMethane?

BioMethane is a renewable energy/fuel, with properties similar to natural gas, produced from "biomass." Unlike natural gas, BioMethane is a renewable energy. 

The cost of producing BioMethane, after installation of the BioMass Gasification equipment used to produce BioMethane (the process of making BioMethane is called "BioMethanation") is called is essentially free. 

Again, unlike the price of natural gas, which has been around $6.00/mmbtu for the past year. 


More About Biomass Gasification and BioMethanation Technology 

The production and disposal of large quantities of organic and biodegradable waste without adequate or proper treatment results in widespread environmental pollution. Some waste streams can be treated by conventional methods like aeration. Compared to the aerobic method, the use of anaerobic digesters in processing these waste streams provides greater economic and environmental benefits and advantages.

As previously stated, Biomethanation is the process of conversion of organic matter in the waste (liquid or solid) to BioMethane (sometimes referred to as "BioGas) and manure by microbial action in the absence of air, known as "anaerobic digestion."

Conventional digesters such as sludge digesters and anaerobic CSTR (Continuous Stirred Tank Reactors) have been used for many decades in sewage treatment plants for stabilizing the activated sludge and sewage solids. 

Interest in BioMethanation as an economic, environmental and energy-saving waste treatment continues to gain greater interest world-wide and has led to the development of a range of anaerobic reactor designs. These high-rate, high-efficiency anaerobic digesters are also referred to as "retained biomass reactors" since they are based on the concept of retaining viable biomass by sludge immobilization.


Biomass Gasification and the Production of BioMethane

Biomass is a renewable energy resource which includes a wide variety if organic resources. A few of these include wood, agricultural residue/waste, and animal manure. 

Biomass Gasification is the process in which BioMethane is produced in the BioMass Gasification process. The BioMethane is then used like any other fuel, such as natural gas, which is not a renewable fuel.

Historically, biomass use has been characterized by low btu and low efficiencies. However, today biomass gasification is gaining world-wide recognition and favor due to the economic and environmental benefits. In terms of economic benefits, the cost of the BioMethane is essentially free, after the cost of the equipment is installed. BioMethane, probably the most important and efficient energy-conversion technology for a wide variety of biomass fuels. The large-scale deployment of efficient technology along with interventions to enhance the sustainable supply of biomass fuels can transform the energy supply situation in rural areas. 

It has the potential to become the growth engine for rural development in the country. 

Principles of Biomass Gasification

Biomass fuels such as firewood and agriculture-generated residues and wastes are generally organic. They contain carbon, hydrogen, and oxygen along with some moisture. Under controlled conditions, characterized by low oxygen supply and high temperatures, most biomass materials can be converted into a gaseous fuel known as producer gas, which consists of carbon monoxide, hydrogen, carbon dioxide, methane and nitrogen. This thermo-chemical conversion of solid biomass into gaseous fuel is called biomass gasification. The producer gas so produced has low a calorific value (1000-1200 Kcal/Nm3), but can be burned with a high efficiency and a good degree of control without emitting smoke. Each kilogram of air-dry biomass (10% moisture content) yields about 2.5 Nm3 of producer gas. In energy terms, the conversion efficiency of the gasification process is in the range of 60%-70%.

Multiple Advantages of Biomass Gasification

Conversion of solid biomass into combustible gas has all the advantages associated with using gaseous and liquid fuels such as clean combustion, compact burning equipment, 

high thermal efficiency and a good degree of control. In locations, where biomass is already available at reasonable low prices (e.g. rice mills) or in industries using fuel wood, gasifier systems offer definite economic advantages. Biomass gasification technology is also environment-friendly, because of the firewood savings and reduction in CO2 emissions.

Biomass gasification technology has the potential to replace diesel and other petroleum products in several applications, foreign exchange.

Applications for Biomass Gasification

Thermal applications: cooking, water boiling, steam generation, drying etc. Motive power applications: Using producer gas as a fuel in IC engines for applications such as water pumping Electricity generation: Using producer gas in dual-fuel mode in diesel engines/as the only fuel in spark ignition engines/in gas turbines.

Publicly Owned Treatment Works ("POTW's") or Wastewater Treatment Systems

More and more, cities, counties and municipalities are faced with greater environmental compliance issues relating to their municipally-owned landfills, Publicly Owned Treatment Works ("POTW's") or Wastewater Treatment Systems. A city's landfill and/or POTW provides an excellent opportunity for cities to reduce their emissions as well as provide an additional revenue stream. These facilities may have valuable gases that our company recovers and pipes to one of our clean, environmentally-friendly cogeneration or trigeneration energy systems. We solve a city's environmental liabilities (air emissions) and provide a new cash flow simultaneously. We offer turn-key solutions for cities that includes the preliminary feasibility analysis, engineering and design, project management, permitting and commissioning. We provide very attractive financing packages for cities that does not add to a city's liability, yet provides a valuable new revenue stream. And, we are also able to offer a turn-key solution for qualified municipalities that includes our company owning, operating and maintaining the onsite power and energy plant.

At the heart of the system is a (Bio) Methane Gas Recovery system similar those used in Flare Gas Recovery or Vapor Recovery Units. Methane Gas Recovery, Flare Gas Recovery, Vapor Recovery, Waste to Energy and Vapor Recovery Units all recover valuable "waste" or vented fuels that can be used to provide fuel for an onsite power generation plant. Our waste-to-energy and waste to fuel systems significantly or entirely, reduces your facility's emissions (such as NOx , SOx, H2S, CO , CO2 and other Hazardous Air Pollutants/Greenhouse Gases) and convert these valuable emissions from an environmental problem into a new cash revenue stream and profit center.

Methane Gas Recovery and vapor recovery units can be located in hundreds of applications and locations. At a landfill, Wastewaster Treatment System (or Publicly Owned Treatment Works – "POTW") gases from the facility can be captured from the anaerobic digesters, and manifolded/piped to one of our onsite power generation plants, and make, essentially, "free" electricity for your facility's use. These associated "biogases" that are generated from municipally owned landfills or wastewater treatment plants have low btu content or heating values, ranging around 550-650 btu's. This makes them unsuitable for use in natural gas applications. When burned as fuel to generate electricity, however, these gases become a valuable source of "renewable" power and energy for the facility's use or resale to the electric grid. 

Additionally, if heat (steam and/or hot water) is required, we will incorporate our cogeneration or trigeneration system into the project and provide some, or all, of your hot water/steam requirements. Similarly, at crude oil refineries, gas processing plants, exploration and production sites, and gasoline storage/tank farm site, we convert your facility's "waste fuel" and environmental liabilities into profitable, environmentally-friendly solutions.

Our Methane Gas Recovery systems are designed and engineered for these specific applications. It is important to note that there are many internal combustion engines or combustion turbines that are NOT suited for these applications. Our systems are engineered precisely for your facility's application, and our engineers know the engines and turbines that will work as well as those that don't. More importantly, we are vendor and supplier neutral! Our only concerns are for the optimum system solution for your company, and we look past brand names and sales propaganda to determine the optimum system, which may incorporate either one or more; gas engine genset(s) or gas turbine genset(s), in cogeneration or trigeneration mode – in trigeneration mode, we incorporate absorption chillers to make chilled water for process or air-conditioning, fuel gas conditioning equipment and gas compressor(s). 

Our turn-key systems includes design, engineering, permitting, project management, commissioning, as well as financing for our qualified customers. Additionally, we may be interested in owning and operating the flare gas recovery or vapor recovery units. For these applications, there is no investment required from the customer.

Chromatograph Fuel/Gas analysis which provides us with the btu's (heating value) and the composition of the gas and its' impurities such as methane (and the percentage of methane), soloxanes, carbon dioxide, hydrogen, hydrogen sulfide, and any other hydrocarbons. 

Total amount of gas available, from all sources, at the facility. 

Anaerobic lagoons are perhaps the most trouble free, low maintenance systems available for treatment of animal waste. This is particularly true in the southern U.S.