Processing Biomass Into Biogas

Table Of Contents

Anaerobic Digestion of Animal Manures: Understanding the Basic Processes

Animation showing breakdown of manure.

Organic Compounds Breaking Down into Biogas

The process of producing methane from manure is fairly straight forward – seal manure in an airtight container at a favorable temperature and it will break down into biogas: a mixture of Methane (CH4), Carbon dioxide (CO2), and trace amounts of other gases. Behind the apparent simplicity, though, lie complicated interactions involving several communities of microorganisms.

The Bioconversion Process (How Microbes Turn Organic Materials into Fuel)

Anaerobic digestion is a multi-stage process (Figure 1) involving two to four steps, depending on where you want to draw lines in the process.

  • Hydrolysis is the phase of anaerobic digestion where complex organic molecules are broken down into simpler organic molecules.
  • Acidogenisis is where simple organic molecules are converted into fatty acids.
  • Acetogenisis is where fatty acids are converted into acetate. This, along with acidogenisis, represents the transition from simple organic molecules to the methanogenic substrates as shown in the figure below.
  • Methanogenisis is where molecules that have been converted into a suitable food source (or substrate) for methanogenic microorganisms are converted into methane by those organisms.

Figure 1. Steps in anaerobic digestion

More than 100 different anaerobic microbes are known to contribute to the production of biogas. These microbes are organized into a number of interlinked communities. Communities of hydrolytic bacteria break complex organic matter down to simpler compounds. Acid-forming bacteria convert the simple compounds to volatile fatty acids – principally acetic acid (vinegar). Hydrolysis and acidogenisis are commonly lumped together and called anaerobic fermentation. Some microbiologists also distinguish between formation of mixed volatile fatty acids (acidosis) and reduction to Acetic acid (acetogenesis).

Figure 2. Major pathways of methane (CH4) formation

Methane forming Archaeans (very simple, single cell organisms similar to bacteria), called methanogens, take the end products of fermentation – volatile fatty acids, hydrogen gas (H2), CO2, and water (H2O) – and use them to form methane. Methanogens fall into two main camps depending on the pathway they use to produce methane (Figure 2). All methanogens can reduce CO2 and H2 into CH4 and H2O; those that use this pathway exclusively are called hydrotrophic methanogens. Methanogens that convert volatile fatty acids (and a number of other simple organic compounds) to CH4 and CO2 are called acetotrophic methanogens.

Limitations on Biological Processes

Anaerobic digestion is a living process, and like all biological communities, the microorganisms that carry out anaerobic fermentation and methanogensis do so under certain operating conditions.

Phases of Microbial Community Growth

Given an ample food supply, sufficient room to expand, the absence of predators or competing organisms, all communities of organisms – be they methanogens or human beings – grow in a pattern similar to the one shown in Figure 3. The Lag Time occurs as the organisms acclimate to their environment.

Figure 3. Generalized microbial growth curve
  • During the Growth Phase, food is not limiting, and population expands rapidly. Sometimes the Growth phase is called the Log Growth or Exponential Growth phase because the growth pattern follows an exponential curve. *Population growth slows down in the Decline Phase as the organisms meet the limit of their food supply.
  • During the Stationary Phase, the community has met the limits of its food supply. Bear in mind that reproduction does not necessarily stop during the decline and stationary phases, only that the death rate approaches the reproduction rate. In some cases the community becomes dormant or goes into hibernation during the Stationary phase.
  • Communities enter a Death or Endogenous Growth phase once a limited food supply is exhausted or an inhibiting element limits the further growth of organisms. During Endogenous Growth, the death rate exceeds the birth rate.

The inhibitory elements that cause endogenous growth are often the end products of a community’s metabolism.

The beauty of anaerobic digestion is that it is the work of a mixed community of organisms. The toxic end product of one community is the food supply of another. Acid forming bacteria consume the simple sugars that might inhibit hydrolytic communities. Methanogens use the acids formed in fermentation to produce CH4 and CO2. And in the end, CH4 and CO2 leave the digester as biogas.

Reproduction Time

Most anaerobic digesters are designed so that the microbial communities remain in exponential growth. An important concept to grasp is the doubling rate or reproduction time of an organism. This is the time needed for a population to double in size during exponential growth. In simple terms it is the time required for the organisms to replace themselves.

Hydraulic Retention Time

Anaerobic digestion generally takes place in a liquid, continuous flow reactor (Figure 4). If the reactor volume does not change, then flow out of the reactor equals flow into the reactor, and the average time that liquid remains in the reactor is its volume divided by the flow rate. This is called the hydraulic retention time of the reactor (HRT).

If the reactor is completely mixed, that is, the microorganisms are completely suspended in the reactor, then the time the cells remain in the reactor equals the HRT. If the HRT of a completely mixed reactor equals the reproduction time of the organisms living in the rector, a new cell is formed to replace each cell leaving the reactor, and the population within the reactor remains stable. If the HRT is shorter than the reproduction time, a new cell will not be there to replace the one leaving, and the population will decline, or “wash out.”

Figure 4. Relationships between Reactor Volume, Flow, Solids Mass, and Retention Times in a Completely Mixed Reactor.

Solids Retention Time

Reducing HRT decreases reactor size, and smaller reactors reduce costs; therefore, many digestion systems are designed so that microorganisms remain in the reactor longer than its HRT. For instance, we could put a screen on the outlet of the reactor in Figure 4 so that some of the microorganisms are trapped inside. We can calculate cell retention time by dividing the mass of microorganisms trapped in the reactor by the mass of organisms leaving the reactor. The microbial population is kept stable by setting cell retention time equal to reproduction time.

It is easier to measure the total mass of solid particles suspended in liquid rather than the mass of viable cells; therefore, cell retention time is approximated by Solids Retention Time (SRT), or the mass of solids in the reactor divided by the mass of solids leaving.

Steady Food Supply

Microorganisms need food to reproduce and grow. Methanogens have the uncanny ability to go dormant during periods of food shortage. This is good because you can easily restart an anaerobic digester after long periods of inactivity. On the other hand, sudden increases in feeding can cause bursts of gas production, which lead to foaming and scum formation in the digester.


Methanogens thrive in two temperature ranges. Thermophyllic (heat loving) methanogens are fast growing (reproduction time 10 to 15 days), but they operate in a fairly narrow band of temperature centered on 55 degrees C. Mesophyllic methanogens are slower growing (reproduction time up to 30 days), but they tolerate a wider range of temperatures. 35 degrees C. is optimal for mesophyllic methanogens, but they can tolerate much lower temperatures. Provided they have sufficiently long SRT, digesters operated at 20 degrees C. do not show substantial loses in gas production when compared to those operated at 35 degrees C.


Methanogens are strict anaerobes, meaning the least amount of oxygen is poison to them. Acid forming bacteria are more tolerant of oxygen. So, if oxygen gets into an anaerobic digester, methane concentration will drop and carbon dioxide concentration will increase in the biogas.


pH is an major indicator of reactor health. A complex set of naturally occurring buffers control pH within an anaerobic digester, with organic acid-ammonia and carbonate-bicarbonate being the most important buffers. The optimum pH range for anaerobic digestion is neutral to slightly basic (pH 6.6 to 7.6).

Digester Start-up

To ensure that there is a living population of methane producing microbes in a digester, materials that contain methanogens are commonly added. When these materials come from an outside source, such as sludge from an active manure treatment lagoon or biosolids from a sewage treatment plant, it is called a “hot start.” Sometimes, digesters are brought on line with a “cold start,” meaning manure is slowly added to a liquid filled digester until gas begins to form. Hot starts are generally quicker than cold starts. Depending on digester type, a hot start can take between one and six months to bring a digester on line. It may take six months to a year for a digester to reach steady-state under a cold start.

Inhibitory Substances Commonly Found in Manure

Inhibiting elements are often called toxic, but toxicity is a misnomer. Low doses of most inhibitory agents actually stimulate biological activity. The following sets of chemicals are the major inhibitory agents found in animal manures.


Antibiotics given therapeutically or fed sub-therapeutically to animals have the potential to alter the communities of microorganisms digesting manure from treated animals. The inhibitory effect of antibiotics on gas production has been shown to be minimal if high SRT is maintained and the microorganisms are given time to acclimate.


Disinfectants and cleaning agents can also cause digester imbalance, although reports of upsets have not been reported in the literature. Dilution of cleaning water or bypassing the digester after cleaning is the safest tactic for handling disinfectants. Some doses may prove to have minimal effect on performance if high SRT is maintained.


Ammonium ion (NH4+) and its gaseous relative, Ammonia (NH3), are byproducts of protein digestion and reduction of urea. Concentrations at which Ammonical Nitrogen (NH4+NH3-N) is beneficial, inhibitory, or toxic to anaerobic digestion are given in Table 1. The toxicity of ammonia is highly dependent on pH. NH3, which is the predominant form at higher pH, is more toxic than NH4+. Usually, ammonia is not a problem in manure digesters, except for reactors treating highly nitrogenous materials such as poultry manure; and some poultry manure digesters have been able to tolerate levels as high as 6,000 mg/l if the microorganisms are given time to acclimate.

Table 1. Effect of Ammonia and Sulfide Concentrations on Anaerobic Treatment
Effect on Anaerobic Treatment NH4+NH3-N mg/l S mg/l
Beneficial 50-200 <50
No Adverse Effect 200-1000 50-100
Inhibitory at higher pH values 1500-3000 100-200
Toxic >3000 >200

Sulfate and Sulfide

Sulfate (SO4) is not an inhibitory substance, per se. Its presence can reduce CH4 production because a group of bacteria called sulfate reducers can out-compete hydrotrophic methanogens for available H2. Competition with sulfate reducers does not usually reduce production from manure digesters since there is plenty of acetic acid around for acetotrophic methanogens to produce gas. The end product of sulfate reduction, Sulfide (S), can be quite toxic to anaerobic digestion. Like ammonia, the toxicity of S is dependent on digester pH. Stimulatory and inhibitory concentrations of sulfide are given in Table 1. Also, like ammonia, S- exists in a gaseous form, Hydrogen sulfide (H2S), so its control is a balance between source reduction, gas production, and pH.


The higher the soluble salt content in a digester, the harder microorganisms have to work in order to transport water in and out of their cells. Research has shown that it is mainly the cations (positive ions in solution) that inhibit microbial activity. Stimulatory and inhibitory concentrations of base cations are given in Table 2.

Table 2. Stimulatory and Inhibitory Concentrations of Base Cations
Cation Stimulatory (mg/l) Moderately Inhibitory (mg/l) Strongly Inhibitory (mg/l)
Sodium, Na+ 100-200 3500-5500 8000
Potassium, K+ 200-400 2500-4500 12,000
Calcium, Ca+ 100-200 2500-4500 8000
Magnesium, Mg+ 75-150 1000-1500 3000

Precipitation of Inhibitory Substances

It should be noted that most inhibitory substances must be in solution to reduce biological activity in digesters. Therefore, the inhibitory effect of S is reduced through precipitation of insoluble metal sulfides. Soluble Mg and NH3 are reduced by precipitation of Struvite (MgNH4PO4).

Upset Conditions and Their Control

Fermentative bacteria are generally more robust and faster growing than methanogens. The first indication that something is wrong with a digester occurs when the acid formers start to overpower the methane formers. This may show up as a drop in biogas production. First, however, the CO2 concentration in the biogas will increase, and the organic acid concentration of the reactor liquid increases. Both of these will cause a drop in pH. So, daily measurement of pH is a good method to monitor digester health. The relationship between fermentative and methanogenic communities can become so unbalanced, that even the acid formers can no longer tolerate the low pH conditions. At this point we say the digester is “stuck,” or it has “soured” or become “pickled.” Basic steps to follow in case of digester upset are:

  1. Reduce the feeding rate.
  2. Stabilize pH.
  3. Determine and correct the cause of the imbalance.
  4. Slowly increase feeding rate while maintaining neutral pH.


  • Cundiff, J.S., and K.R. Mankin. 2003. Dynamics of Biological Systems. St Joseph, MI: ASABE.
  • Darling, D. 2009. The Internet Encyclopedia of Science. Accessed May 16, 2009
  • Hamilton, D.W., P.R. Sharp, and R.J. Smith. 1985. The Operational characteristics of a manure digester for 60 beef cattle. pp 500-508, in Agricultural Waste Utilization and Management, the Proceedings of the 5th International Symposium on Agricultural Wastes. St Joseph MI: ASABE.
  • McCarty, M.L. 1964. Anaerobic waste treatment fundamentals. Public Works, Sept-Dec. 1964.
  • Mohr, M.A. 1983. Inhibition of methane production by ammonia nitrogen in the anaerobic digestion of poultry manure. MS Thesis. Madison, WS: University of Wisconsin.
  • Ndegwa, P.M., D.W. Hamilton, J.A. Lalman, and H.J. Cumba. 2007. Effects of cycle frequency and temperature on the performance of anaerobic sequencing batch reactors treating swine waste. Bioresource Technology. 99:1972-1980
  • Strauch, D. and K. Winterhalder. 1985. Effect of disinfectants, additives and antimicrobial drugs on anaerobic digestion. pp 516-522 in, Agricultural Waste Utilization and Management, the Proceedings of the 5th International Symposium on Agricultural Wastes. St Joseph MI: ASABE.
  • Varel, V.H. and A.G. Hashimoto. 1982. Methane production by fermentor cultures acclimated to waste from cattle fed Monenesin, Lasalocid, Salinomycin, or Avoparcin. Applied and Environmental Microbiology. 44(6):1415.


This document was adapted from Factsheet BAE-1747, Oklahoma State University, authored by

Peer Reviewers

Biogas Utilization and Cleanup


Biogas generated from anaerobic digestion processes is a clean and environmentally friendly renewable fuel. But it is important to clean, or upgrade, biogas before using it to increase its heating value and to make it useable in some gas appliances such as engines and boilers.

Biogas Utilization

While most large farms use their biogas for heat and power, it is worthwhile to consider all the options before deciding which path to take, including direct sale of biogas to an off-farm buyer.

Raw animal manure biogas contains 55 to 65% methane (CH4), 30 to 45% carbon dioxide (CO2), traces of hydrogen sulfide (H2S) and hydrogen (H2), and fractions of water vapor. For the anaerobic digestion of sludge or landfill processes, traces of siloxanes may also be found in biogas. These siloxanes mainly originate from silicon-containing compounds widely used in various industrial material or frequently added to consumer products such as detergents and personal care products. This article will not address the cleanup of biogas of siloxanes.

Biogas is about 20% lighter than air and has an ignition temperature in the range of 650 to 750 degrees C. (1,200-1,380 degrees F.). It is an odorless and colorless gas that burns with a clear blue flame similar to that of natural gas. However, biogas has a calorific value of 20-26 MJ/m3 (537-700 Btu/ft3) compared to commercial quality natural gas’ caloric value of 39 MJ/m3 (1,028 Btu/ft3).

Biogas can potentially be used in many types of equipment, including:

  • Internal Combustion (Piston) Engine – Electrical Power Generation, Shaft Power
  • Gas Turbine Engine (Large) – Electrical Power Generation, Shaft Power
  • Microturbine Engine (Small) – Electrical Power Generation
  • Stirling Heat Engine – Electrical Power Generation
  • Boiler (Steam) Systems
  • Hot Water Systems
  • Process Heaters (Furnaces)
  • Space or Air Heaters
  • Gas Fired Chiller – Refrigeration
  • Absorption Chiller – Refrigeration
  • Combined Heat and Power (CHP) – Large and Small Scale – Electrical Power and Heat
  • Fuel Cells – Electrical Power, Some Heat

There are a variety of end uses for biogas. Except for the simplest thermal uses such as odor flaring or some types of heating, biogas needs to be cleaned or processed prior to use. With appropriate cleaning or upgrade, biogas can be used in all applications that were developed for natural gas.

The three basic end uses for biogas are:

  • production of heat and steam
  • electricity generation
  • vehicle fuel

Production of heat or steam

The most straightforward use of biogas is for thermal (heat) energy. In areas where fuels are scarce, small biogas systems can provide the heat energy for basic cooking and water heating. Gas lighting systems can also use biogas for illumination.

Conventional gas burners are easily adjusted for biogas by simply changing the air-to-gas ratio. The demand for biogas quality in gas burners is low, only requiring a gas pressure of 8 to 25 mbar and maintaining H2S levels to below 100 ppm to achieve a dew point of 150 degrees C.

Electricity Generation or Combined Heat and Power (CHP)

Combined heat and power systems use both the power producing ability of a fuel and the inevitable waste heat. Some CHP systems produce primarily heat, and electrical power is secondary (bottoming cycle). Other CHP systems produce primarily electrical power and the waste heat is used to heat process water (topping cycle). In either case, the overall (combined) efficiency of the power and heat produced and used gives a much higher efficiency than using the fuel (biogas) to produce only power or heat.

Other than high initial investments, gas turbines (micro-turbines, 25-100 kW; large turbines, >100 kW) with comparable efficiencies to spark-ignition engines and low maintenance can be used for production of both heat and power. However, internal combustion engines are most cmmonly used in CHP applications. The use of biogas in these systems requires removal of both H2S (to below 100 ppm) and water vapor.

Fuel cells are considered the small-scale power plants of the future for production of power and heat with efficiencies exceeding 60% and low emissions. One of the largest digester/fuel cell units is located in Washington State. The fuel cell, located at the South Treatment Plant in Renton, WA, can consume about 154,000 ft3 of biogas a day to produce up to 1 megawatt (1,000,000 watts) of electricity. That’s enough to power 1,000 households, but it’s being used instead for the operation of the plant.

Vehicle fuel

Gasoline vehicles can use biogas as a fuel provided the biogas is upgraded to natural gas quality in vehicles that have been adjusted to using natural gas. Most vehicles in this category have been retro-fitted with a gas tank and a gas supply system in addition to the normal petrol fuel system. However, dedicated vehicles (using only biogas) are more efficient than these retro-fits.

Biogas Cleanup Or Upgrading

Biogas cleaning is important for two reasons: (1) to increase the heating value of biogas, and (2) to meet requirements for some gas appliances (engines, boilers, fuel cells, vehicles, etc). Desired biogas cleaning or upgrading purposes are summarized in Figure 1. “Full treatment” implies that biogas is cleaned of CO2, water vapor, and other trace gases, while “reforming” is conversion of methane to hydrogen.

Figure 1. Alternative biogas utilization and required cleanup

CO2 Removal

For many of the simpler biogas applications such as heaters or internal combustion engines or generator systems, carbon dioxide (CO2) removal from biogas is not necessary and CO2 simply passes through the burner or engine. For more demanding biogas/engine applications, such as vehicles that require higher energy density fuels, CO2 is routinely removed. Removing CO2 increases the heating value and leads to a consistent gas quality similar to the natural gas. Carbon dioxide can be removed from biogas economically through absorption or adsorption. Membrane and cryogenic separations are other possible processes.

Pressurized counter-current scrubbing of CO2 and H2S from biogas can be accomplished in water. For removal of CO2 in particular; pH, pressure, and temperatures are critical. High pressures, low temperature, and high pH increases CO2 scrubbing from biogas. Use of Ca(OH)2 solutions can completely remove both CO2 and H2S. Both CO2 and H2S are more soluble in some organic solvents such as polyethyleneglycol and alkanol amines that do not dissolve methane. These organic solvents can thus be used to scrub these gases from biogas even at low pressures. Systems using these kinds of organic solvents can remove CO2 down to 0.5% from the biogas.

However, use of organic solvents is much more expensive than water systems. Adsorption of CO2 on solids such as activated carbon or molecular sieves is possible although it requires high temperatures and pressures. These processes may not be cost-effective because of associated high temperature and pressure drops. Cryogenic separation is possible because at 1 atm, methane has a boiling point of -106oC, whereas CO2 has a boiling point of -78oC. Fractional condensation and distillation at low temperatures can thus separate pure methane in liquid form, which is convenient for transportation. Up to 97% pure methane can be obtained, but the process requires high initial and operational investments. Membrane or molecular sieves depend on the differences in permeability of individual gas components through a thin membrane. Membrane separations are quickly gaining in popularity. Other chemical conversions are technically viable, but their economics are poor for practical biogas-cleaning.

Water Vapor Removal

Straight from the digester, biogas will generally be saturated with vapor. Besides reducing the energy value of biogas, water can react with H2S to create ionic hydrogen and/or sulfuric acid, which is corrosive to metals. Refrigeration or sensible pipe-work design can condense and remove the water. The biogas is normally compressed before cooling to achieve high dew points. Alternative water vapor removal mechanisms include adsorption on: (1) silica gel and Al2O3 at low dew points, (2) glycol and hygroscopic salts at elevated temperatures, and 3) molecular sieves.

Removal of Hydrogen Sulfide

Hydrogen sulfide in biogas needs to be removed for all but the most simple burner applications. Hydrogen sulfide in combination with the water vapor in raw biogas can form sulfuric acid (H2SO4), which is very corrosive to engines and components. At concentrations above 100 parts per million by volume (ppmv), H2S is also very toxic. Activated carbon can be used to remove both H2S and CO2. Activated carbon catalytically converts H2S to elemental sulfur. Hydrogen sulfide can also be scrubbed out from biogas in either: NaOH, water, or iron salt solutions. A simple and inexpensive process is dosing a stream of biogas with O2, which oxidizes H2S to elemental sulfur. Oxygen dosing can reduce H2S to below 50ppm levels from biogas [Warning: IMPROPERLY DOSING A BIOGAS STREAM WITH O2 CAN CREATE AN EXPLOSION HAZARD]. Iron oxide also removes H2S as iron sulfide. This method can be sensitive to high water vapor content of the biogas. In addition to clean up of biogas of H2S after it has been produced, available methods of reducing H2S content from produced biogas include: co-digestions, multiphase digestion, reactor pH buffering, and removal of sulfur from feed substrates.


Biogas produced from animal waste can be a valuable energy resource. By combusting waste methane (biogas), a powerful greenhouse gas is eliminated that would otherwise be released. If used in simple burners for cooking or lighting the gas may not need to be treated prior to use. However, for uses that require the gas to be used in internal combustion engines, boilers or fuel cells, the biogas will probably need to be pretreated in order to remove corrosive or dangerous contaminants. The primary contaminant of biogas is hydrogen sulfide. This chemical will also react with water to form corrosive acids that can attack metals and plastics. Hydrogen sulfide is also toxic and sufficient quantities also present a possible health hazard if not treated.

Additional Resources

Intro | Feedstocks | Processing | Utilization


  • Appels, L., J. Baeyens, J. Degre`ve, R. Dewil. 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Progress in Energy and Combustion Science, 34:755–781.
  • Drewitz, M., P.Goodrich. 2005. Minnesota dairy runs hydrogen fuel cell on biogas.
  • EMG International, Inc. 2007. Biogas Clean-Up Technologies. Presentation to NYS ERDA Innovations in Agriculture.
  • FAO. 1997. A system approach to biogas technology,
  • Glub, J.C., L.F. Diaz. 1991. Biogas purification process. Biogas and alcohol fuels production, vol. II. The JP Press.
  • Hagen, M., E. Polman. 2001. Adding gas from biomass to the gas grid. Final report submitted to Danish Gas Agency, pp. 26-47.
  • Harasmowicz, M., P. Orluk, G. zakrzewska-Trznadel, A.G. Chemielewski. 2007. Application of polyimide membranes for biogas purification and enrichment. Journal of Hazardous Materials 144:698-702.
  • Kapdi, S.S., V.K. Vijay, S.K. Rajesh, R. Prasad. 2005. Biogas scrubbing, compression and storage: perspective and prospectus in Indian context. Renewable Energy 30:1195-1202.
  • Kayhanian, D.J. Hills. 1988. Membrane purification of anaerobic digester gas. Biological Wastes 23: 1-15.
  • Lastella, G., C. Testa, G. Cornacchia, M. Notornicola, F. Voltasio, V.K. Sharma. 2002. Anaerobic digestion of semi-solid organic waste: biogas production and its purification. Energy Convers. Manage., 43:63–75.
  • Leposky, G. 2005. Fuel Cell Uses Biogas from Sewage to Generate Electricity.
  • Li, K., W.K. Teo. 1993. Use of an internally staged permeator in the enrichment of methane from biogas. J. Membr Sci 1993;78:181–90.
  • Martin, J. H. 2008. A method to Evaluate Hydrogen Sulfide Removal from Biogas. MS Thesis: Biological and Agricultural Engineering, North Carolina State University, Raleigh, North Carolina.
  • Pandey, D.R., C. Fabian. 1989. Feasibility studies on the use of naturally accruing molecular sieves for methane enrichment from biogas. Gas Separation and Purification, 3:143-147.
  • Sarkar, S.C., A. Bose. 1997. Role of activated carbon pellets in carbon dioxide removal. Energy Conversions Management 38:S105-S110.
  • Stern, S.A., B. Krishnakumar, S.G. Charati, W.S. Amato, A.A. Friedmann, D.J. Fuess. 1998. Performance of a bench-scale membrane pilot plant for the upgrading of biogas in a wastewater treatment plant. J. Membr Sci, 151:63–74.
  • Walls, J.L., C. Ross, M.S. Smith, S.R. Harper. 1989. Utilization of biogas. Biomass 20: 277-290.
  • Wellinger, A., A. Lindberg. 2005. Biogas Upgrading and Utilization. Task 24: Energy From Biological Conversion of Organic Waste. IEA Bioenergy.
  • Wise, D.L. 1981. Analysis of systems for purification of fuel gas. Fuel gas production from biomas, vol. 2. Boca Raton, FL. The CRC Press.



Peer Reviewers

Environmental Benefits of Anaerobic Digestion

The manure handling system of any farm is made up of many different components, each with a different function and purpose. An anaerobic digester, although only one component of the system, can greatly improve the environmental performance and efficiency of the overall system. The main effect of anaerobic digestion is conversion of organic matter to biogas. This conversion has many potentially beneficial environmental and management side effects.

Odor reduction

By removing organic matter, the digester reduces the organic matter-loading and associated oxygen demand on downstream manure handling components. This may allow the downstream components to be smaller, operate more efficiently and function with less environmental impact. Anaerobic pretreatment may be a more economical method of converting an anaerobic lagoon to an aerobic lagoon, compared to mechanical aeration. Digester effluent is more stable than raw manure. It contains more stable organic material and less volatile odorants. Thus, storage and land application of digester effluent greatly reduces odor nuisance compared to raw manure.

Uses for digested solids

Manure solids are stabilized through anaerobic digestion. What was once reactive, partially digested material has been processed into stable microbial biomass and precipitated nutrients, although the majority of nutrients remain with the liquid. The potential to dry and transport digester solids is greatly improved over raw manure. The solids can be recycled and used for bedding or a soil amendment on the farm. The reduction in moisture content also increases the feasibility of selling the solids to farms that are greater distances away. In the right market conditions, composting the digested solids can result in a value-added product that can be sold to homeowners, gardeners or the landscape industry.

Plant nutrients

Plant nutrients are conserved and transformed during anaerobic digestion. Ammonium is created from manure proteins. This can be a benefit or a nuisance. If injected immediately into the soil, ammonium-rich effluent is highly available for plant growth. On the other hand, if digester effluent is stored under anaerobic conditions, ammonium will convert to ammonia gas and escape to the atmosphere. Since digesters are also a reducing environment, the potential exists for capture of ammonium and soluble phosphorus through precipitation as struvite.

Many metals are precipitated during anaerobic digestion. Sulfur is reduced to sulfide, which is generally a bad thing since it can escape as hydrogen sulfide gas. However, the digester environment can be manipulated so that sulfides are precipitated along with potentially harmful metals such as Ni and Zn.

Greenhouse gases

Anaerobic digestion results in the reduced emission of greenhouse gases. This may seem ironic, since the methane contained in the resulting biogas is a powerful greenhouse gas. An anaerobic digester is a controlled environment that captures the methane. After capture, it is either flared or used to generate electricity and/or heat.

When flared, the carbon dioxide formed in the combustion has less heat trapping potential than the original methane, and it is essentially recycled atmospheric carbon. What is released to the atmosphere through combustion of methane was once plant material formed through photosynthesis from atmospheric carbon dioxide.

When used for energy generation, the biogas replaces power that might have otherwise been created through conversion of fossil fuel. Regardless, if the biogas is flared or used for energy generation, the farmer may be eligible for carbon credit payments.

Anaerobic Digestion on Farms

With all of the potential benefits, one might wonder why relatively few farms utilize these systems. One major reason is that anaerobic digesters are expensive to install and operate. The economic benefits have, in the past, been limited to a reduction in electricity purchased by the farm, which is not enough to offset the costs of the system.

As the interest in renewable energy sources increases, farms are increasingly able to apply and receive carbon credits. Some farms also accept off-farm waste, collecting tipping fees, to co-digest with manure. In many states, more favorable net-metering laws have also made the economics more favorable. Power generated by the digester is valued at retail costs rather than wholesale costs.

The decision to install a digester is often driven by additional considerations, such as nuisance issues. A digester greatly reduces the odor potential of the manure, which also greatly reduces neighbors’ complaints and the potential for lawsuits.

At the current time, anaerobic digestion is slowly but surely increasing as a manure treatment method in the United States. Additional information is available at: Economics of Anaerobic Digesters for Processing Animal Manure.

Contributors To This Document

Author: Doug Hamilton, Oklahoma State University Waste Management Specialist

Contributors: Jill Heemstra, University of Nebraska

Reviewers: Mark Rice, North Carolina State University and Karl Vandevender, University of Arkansas

Types of Anaerobic Digesters

Table Of Contents
Passive Systems
Low Rate Systems
High Rate Systems
Contributors To This Article

All anaerobic digesters perform the same basic function. They hold manure in the absence of oxygen and maintain the proper conditions for methane forming microorganisms to grow. There is a wide variety of anaerobic digesters, each performing this basic function in a subtly different way. Seven of the most common digesters are described in this article. Construction and material handling techniques can vary greatly within the main categories.

For clarity, we can divide digesters into three categories:

  • Passive Systems: Biogas recovery is added to an existing treatment component.
  • Low Rate Systems: Manure flowing through the digester is the main source of methane-forming microorganisms.
  • High Rate Systems: Methane-forming microorganisms are trapped in the digester to increase efficiency.

Passive Systems

Covered lagoon

Figure 1. First Covered Cell of a Lagoon Located on the Oklahoma State University Swine Research and Education Center.

Figure 2. Schematic Drawing of Covered Lagoon Digestion System.

This system takes advantage of the low maintenance requirement of a lagoon while capturing biogas under an impermeable cover (Figure 1). The first cell of a two-cell lagoon is covered, and the second cell is uncovered (Figure 2). Both cells are needed for the system to operate efficiently. A lagoon is a storage as well as a treatment system; the liquid level on the second cell must rise and fall to create storage, while the level on the first cell remains constant to promote manure breakdown.

Since they are not heated, the temperature of covered lagoons follows seasonal patterns. Methane production drops when lagoon temperatures dip below 20 degrees C. A covered lagoon located in the tropics will produce gas year-round, but gas production will drop considerably during the winter farther north. Since sludge is stored in lagoons for up to 20 years, methane-forming microorganisms also remain in the covered lagoon for up to 20 years. This means that much of the fertilizer nutrients, particularly phosphorus, also remain trapped in the covered lagoon for a long time. If lagoon effluent is recycled to remove manure from buildings, liquid retention time is generally 30 to 60 days – depending on the size and age of the lagoon.

Low Rate Systems

Complete Mix Digester

Figure 3. Complete Mix Digester Located on the Crave Brothers Farm in Waterloo Wisconsin (photo: Crave Brothers Farm/USEPA).

Figure 4. Schematic Drawing of a Complete Mix Digester

A complete mix digester (Figure 3) is basically a tank in which manure is heated and mixed with an active mass of microorganisms (Figure 4.). Incoming liquid displaces volume in the digester, and an equal amount of liquid flows out. Methane forming microorganisms flow out of the digester with the displaced liquid. Biogas production is maintained by adjusting volume so that liquids remain in the digester for 20 to 30 days. Retention times can be shorter for thermophyllic systems. The digester can be continuously or intermittently mixed. Intermittent mixing means the tank is stirred during feeding and only occasionally between feedings. Sometimes the process takes place in more than one tank. For instance, acid formers can break down manure in one tank, and then methane formers convert organic acids to biogas in a second tank. Complete mix digesters work best when manure contains 3 percent to 6 percent solids. Digester size can be an issue at lower solids concentrations. Lower solids mean greater volume, which means you need a larger digester to retain the microbes in the digester for 20 to 30 days.

Plug Flow Digester

Figure 5. Plug Flow Digester located on the Emerling Farm in Perry, NY (Photo Courtesy of Cornell University/USEPA).

Figure 6. Schematic Drawing of a Plug Flow Digester.

The idea behind a plug flow digester (Figure 5) is the same as a complete mix digester – manure flowing into the digester displaces digester volume, and an equal amount of material flows out (Figure 6). However, the contents of a plug flow digester manure are thick enough to keep particles from settling to the bottom. Very little mixing occurs, so manure moves through the digester as a plug – hence the name “plug flow.” Plug flow digesters do not require mechanical mixing. Total solids (TS) content of manure should be at least 10 or 15 percent, and some operators recommend feeding manure with solids as high as 20 percent. This means you may need to add extra material to manure to use a plug flow digester. This is not always a bad thing if you consider the added material may also be biodegradable. More degradable material means more biogas. Plug flow digesters are usually five times longer than they are wide. Recommended retention time is 15 to 20 days.

High Rate Systems

Solids Recycling

Figure 7. Schematic Drawing of Contact Stabilization Digester.

Returning some of the active organisms to the digester decreases digestion time. This is done in plug flow systems by pumping some of the effluent leaving the digester to the front of the digester. In complete mix systems, solids are settled in an external clarifier, and the microbe-rich slurry is recycled back to the digester. The systems are called contact stabilization digesters or anaerobic contact digesters (Figure 7).

Fixed Film Digester

Figure 8. Fixed Film Digester located on the University of Florida Dairy Research Farm (photo courtesy of Ann Wilkie, University of Florida) .

A fixed film digester (Figure 8) is essentially a column packed with media, such as wood chips or small plastic rings. Methane-forming microorganisms grow on the media. Manure liquids pass through the media (Figure 9). These digesters are also called attached growth digesters or anaerobic filters. The slimy growth coating the media is called a biofilm. Retention times of fixed film digesters can be less than five days, making for relatively small digesters. Usually, effluent is recycled to maintain a constant upward flow. One drawback to fixed film digesters is that manure solids can plug the media. A solid separator is needed to remove particles from the manure before feeding the digester. Efficiency of the system depends on the efficiency of the solid separator; therefore, influent manure concentration should be adjusted to maximize separator performance, usually 1 percent to 5 percent total solids). Some potential biogas is lost due to removing manure solids.

Figure 9. Schematic Drawing of a Fixed Film Digestion System.


Suspended Media Digesters

Figure 10. Schematic Drawing of an Upflow Anaerobic Sludge Blanket (UASB) Digester.

In these types of digesters, microbes are suspended in a constant upward flow of liquid. Flow is adjusted to allow smaller particles to wash out, while allowing larger ones to remain in the digester. Microorganisms form biofilms around the larger particles, and methane formers stay in the digester. Effluent is sometimes recycled to provide steady upward flow. Some designs incorporate an artificial media such as sand for microbes to form a biofilm; these are called fluidized bed digesters.

Suspended media digesters that rely on manure particles to provide attachment surfaces come in many variations. Two common types of suspended media digesters are the upflow anaerobic sludge blanket digester, or UASB digester (Figure 10), and the induced blanket reactor, or IBR digester (Figures 11 and 12). The main difference between these two systems is that UASB digesters are better suited for dilute waste streams (<3-percent total suspended solids); whereas, the IBR digester works best with highly concentrated wastes (6 percent to 12 percent TS).

Figure 11. Schematic Drawing of Induced Bed Reactor (IBR) Digester (Courtesy of Conly Hansen, Utah State University).


Figure12. Battery of Induced Bed Reactor (IBR) Digesters (photo courtesy of Conly Hansen, Utah State University).


Sequencing Batch Digester

Figure 13. Anaerobic Sequencing Batch Reactor (ASBR) Digester Located on the Oklahoma State University Swine Research and Education Center.

An anaerobic sequencing batch reactor (Figure 13), or ASBR digester, is a variation on an intermittently mixed digester. Methane forming microorganisms are kept in the digester by settling solids and decanting liquid. An ASBR operates in a cycle of four phases (Figure 14). The digester is fed during the fill stage, manure and microbes are mixed during the react phase, solids are settled during the settle stage, and effluent is drawn off during the decant stage. The cycle is repeated up to four times a day for nearly constant gas production. Liquid retention times can be as short as five days. Although ASBR digesters work well with manure in a wide range of solids concentrations, they are particularly well suited for very dilute manures (< 1 percent TS), and if filled with active microbes during start-up, can even produce biogas with completely soluble organic liquids. Sludge must be removed from the ASBR digester periodically. Concentrated nutrients are harvested during sludge removal.

Figure 14. Four phases of an ASBR Digester Cycle.


Contributors To This Article


Peer Reviewers

Introduction to Biogas and Anaerobic Digestion

Harnessing energy from livestock waste.

Intro | Feedstocks | Processing | Utilization

On-farm biogas production has long been a topic of interest for farmers, with historical records of biogas production going back several hundreds of years. In modern livestock production systems, for example, the benefits of producing biogas are significant and include:

  • provision of supplemental renewable energy
  • odor reduction
  • reduction of emissions of greenhouse gases
  • pathogen control
  • waste biostabilization
  • nutrients are preserved and transformed into plant-available forms

The economics of biogas production, however, are sometimes difficult to justify unless the accompanying environmental benefits and other by-products are considered.

What Is a Biogas?

Biogas is a by-product of the anaerobic (without oxygen) breakdown of organic matter. The organic matter could be any of a number of materials, but on the farm, it most often comprises animal manure or other agricultural waste.

The most important component in biogas is methane, a flammable gas that can be used in furnaces, for cooking, or even as an engine fuel. However, biogas also contains carbon dioxide and small amounts of hydrogen, hydrogen sulfide, nitrogen, and water vapor.

What Is a Digester?

A digester is a sealed vessel or container in which anaerobic digestion of organic matter occurs. The bacteria “feed” off the manure and, in the process, release biogas as a by-product. This process is referred to as anaerobic digestion, and the sealed vessel or container is thus usually referred to as an anaerobic digester. Anaerobic digestion also occurs in the anaerobic zones of open or unsealed swamps, bogs, and wastewater lagoons.

Today, farmers in developed countries are using digesters primarily to improve the quality of their manure and to reduce manure odors, the energy content of the methane being simply a by-product. However, as the price of energy increases, more farmers are looking at using anaerobic digestion as a way to generate supplemental heat and electricity for their farms. Digesters are a popular technology in rural areas of the developing world, where electricity and petroleum fuels are often unavailable or unaffordable.

What Does a Digester Look Like?

This is a 600,000-gallon plug-flow digester that creates biogas using the manure from 1,000 dairy cows.

Physically, digesters can come in many different shapes and sizes, varying from simple earthen lagoons to complex steel and concrete structures. In North America, the most common commercial farm digesters are usually buried concrete tanks with heavy plastic covers.

Take a virtual tour of one regional digester (hydraulic mix type) located in Cayuga County, NY. More…

How Does a Digester Work?

Fresh biomass entering a digester is supplied with anaerobic bacteria by the existing digested biomass, which is tremendously rich in these microbes. The digester tank provides a conducive environment for anaerobic microbes to “digest” the biomass, resulting in digested solids, liquids, and biogas. In general, the anaerobic digestion is a living process, requiring favorable conditions (temperature, moisture content, oxygen exclusion,and pH) and a steady food supply in order to flourish.

Manure from this dairy barn is automatically collected and delivered to a nearby anaerobic digester.

What Goes into a Digester?

Livestock manure is the most popular material, or feedstock, for anaerobic digestion on the farm, but almost any type of organic matter can be digested, including food waste, forestry residue, animal processing waste, and field crops.

What Can Go Wrong?

Probably the biggest problem in a digester occurs when the digester’s pH drops too low. In general, acid-forming bacteria grow much faster than methane-forming bacteria. This can reduce the pH to an unfavorable level for methane-forming bacteria, thus inhibiting the activity of methanogens. This is referred to souring and may result in failure or crashing of the anaerobic digester. In most cases, however, the pH is self-regulating, but bicarbonates are sometimes used to maintain consistent pH. The optimal pH range is between 6.8 to 8.5. Restarting a digester that has “soured” is not an easy task. Typically, the approach is to open the digester, excavate the soured material, then refill and restart the digester. This is a costly and unpleasant task and should be avoided whenever possible.

There are safety risks in dealing with biogas, including explosion, asphyxiation, disease, or hydrogen sulfide poisoning. Operators must be aware of the potential hazards and take preventative measures.

How Is Biogas Used?

Biogas generated from anaerobic digestion processes is a clean and environmentally friendly renewable fuel. There are many uses for this fuel, including use in engines, generation of electricity, heat and hot water systems, and even refrigeration.

This generator makes electricity using biogas from a digester on a 1,000 cow dairy farm.


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Recommended Reading About Anaerobic Digestion

Contributors to This Document


Peer Reviewers

  • Patricia A. Westenbroek, Cornell Cooperative Extension
  • William F. Lazarus, Professor and Extension Economist, University of Minnesota Extension