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Pathogen Reduction in Anaerobic Digestion of Manure

Benefits of Anaerobic Digestion of Manure in Reducing Pathogens

Manure is a biologically active material that hosts and supports many microorganisms and thus can seldom be considered “pathogen free.” Certain manure handling techniques and methods, however, can limit the production and multiplication of such pathogens. Common sense must be used when making manure handling decisions. Pathogens are microbes such as bacteria, viruses, protozoa, and other organisms that cause disease. These pathogens persist commonly in animal manures. For more information about pathogens and zoonotic pathogens, see Pathogens and Potential Risks Related to Livestock or Poultry Manure. A list of animal related microorganisms (including some that are pathogens) are listed in Table 1.

Table 1. Animal Related Microorganisms
Fecal coliforms (an indicator bacteria, not all coliforms are pathogenic)
Salmonella spp. (pathogen)
Generic E. coli (not all E. coli are pathogens), including O157:H7 (pathogen)
Enterococci (not generally considered pathogenic)
Listeria (pathogen)
Clostridium (pathogen)
Mycobacterium paratuberculosis (MAP or Johne’s) (pathogen)
Enterovirus (pathogen)
Campylobacter (pathogen)
Cryptosporidium (C. parvum is the only one related to animal manure that is considered pathogenic)
Bovine Spongiform Encephalopathy (BSE) (The prions that cause BSE are not a true pathogen, but are considered an “infectious agent”)

Excessive or careless land application of manure and livestock facility runoff can contaminate surface water. This manure laden runoff can pose significant risk to human and animal health. Stored or fresh manure can be applied to land with minimal reduction of harmful pathogens, as some microorganisms can persist for long periods outside an animal’s body.

Treatment through anaerobic digestion can greatly reduce the number of pathogens within the manure and therefore limit the number of pathogens entering the environment. Anaerobic digestion (AD) of manure has a pathogen reducing effect with as much as 95-98% of common pathogens eliminated in mesophillic (~ 100 degrees Fahrenheit) digesters. The reduction in pathogens has the potential to be of benefit for: manure application in impaired watersheds when trying to manage certain pathogens such as Mycobacterium paratuberculosis (MAP or Johne’s) or salmonella, and when considering a community- based anaerobic digester where manure from multiple farms is combined, treated, and AD solids and AD effluent returned back to the farms.

Supporting Research-What We’ve Learned

There is a growing body of research which demonstrates the anaerobic digestion process can vastly reduce if not eliminate the concentration or presence of numerous organisms. Current research in this area is summarized below in Table 2.

Table 2. Potential for microbial (including pathogen) and infectious agent reduction by anaerobic digestion
Microbes Reduced By Anaerobic Digestion Microbes Not Reduced By Anaerobic Digestion
Salmonella Bovine Spongiform Encephalopathy (BSE) (Infectious agent–not a microbe)
Generic Escherichia coli  
Escherichia coli O157:H7  
Mycobacterium paratuberculosis (Johne’s)  
Bovine enterovirus (BEV)  
Enterovirus  
Fecal coliform  
Cryptosporidium  

Anaerobic digestion of manure has been shown to reduce the Johne’s-causing organism, Mycobacterium avium a subspecies of paratuberculosis. Thermophilic digesters operating at 135 degrees F. have shown complete elimination of Johne’s bacteria, while digesters operating at 99 degrees F with a 20-day retention time have demonstrated significant reduction [3]. Other potentially harmful pathogens to humans include Escherichia coli O157:H7, Salmonella, and the protozoan parasite Cryptosporidium parvum. These bacteria and protozoa have all been reduced in number of viable and infectious organisms after passing through a digester. Pathogen reduction of 95% is possible with a 20-day retention time under mesophilic conditions (95-105 degrees F.) with a digester [3].

Anaerobic digestion under mesophilic or thermophilic conditions has not been shown to reduce or eliminate Bovine Spongiform Encephalopathy (BSE), or Mad Cow Disease. Although little is known about this disease, it is accepted that the protein-infecting prions are resistant to heat. Even thermophilic conditions (135 degrees F.) are not sufficient to destroy BSE prions [3].

In a study in New York state, samples were taken from a plug-flow digester over a 14-month period and tested for fecal coliform and Mycobacterium avium paratuberculosis (MAP), or Johne’s disease. It was found (see Table 3) that anaerobic digestion has the potential to reduce the number of fecal indicator bacteria in dairy effluent, including in this study, by 100% reduction of MAP CFU/gram. The substantial reduction of pathogen concentrations led the authors to recommend anaerobic digestion of dairy manure when concentration of pathogens is a concern [4].

Table 3. Pathogen results from dairy manure treatment
  Fecal coliform CFU/Gram MAP CFU/Gram
Raw Manure 3,836,000 20,640
Digested Effluent 3,400 136
Wright et al. 2001

In a study conducted by Washington State University on two operating anaerobic digesters in Oregon (2004), pre-digested and post-digested samples were taken bi-weekly, for six sampling events. Samples were obtained from: manure prior to the AD system, and solids and liquids post-AD. The design of the two digesters was different: one was a plug-flow and the other, a continuous mix, each operating at 100 degrees F. and with expected retention times of ~ 21 days and 24 hours, respectively. Specific organisms selected for evaluation were: Salmonella, Generic E. coli (including 0157:H7), enterococci, Mycobacterium paratuberculosis (Johne’s), and enterovirus.

 

Figure 1. Generic E.Coli concentration in anaerobic digester samples


 

Figure 2. Enterococci concentration in anaerobic digester samples


The data indicated reductions in fecal indicator bacterial concentration was > 98% (generic E. coli, enterococci, and enterovirus) in most cases (see figure 1 and 2). While the detection of Mycobacterium paratuberculosis was reduced in post digested samples, greater than 50% of samples had detectable levels. The data from this study suggests that AD treatment of dairy manure does not completly remove all biosecurity hazards [2].

Additional Resources

Bibliography

  1. Spiehs, Mindy; Goyal, Sagar. Best Management Practices for Pathogen Control in Manure Management Systems. University of Minnesota Extension. 2007.
  2. Harrison, J.H., D. Hancock, M. Gamroth, D. Davidson, J.L. Oaks, J. Evermann, and T. Nennich. 2005. Evaluation of the pathogen reduction from plug flow and continuous feed anaerobic digesters. Symposium – State of the Science Animal Manure and Waste Management. San Antonio, TX. Jan. 5-7
  3. [3.0][3.1][3.2]Topper, Patrick; Graves, Robert; Richard, Thomas. The Fate of Nutrients and Pathogens during Anaerobic Digestion of Dairy Manure. Penn State Cooperative Extension. Agriculture and Biological Engineering. Extension Bulletin. 2006.
  4. Wright, P. E., S. F. Inglis, S. M. Stehman, and J. Bonhotal. “Reduction of selected pathogens in anaerobic digestion.” 5th Annual NYSERDA Innovations in Agriculture Conference (2001): 1-11.
  5. “Pathogen Overview.” Information Collection Rule. US Environmental Protection Agency, 10 Apr. 2009. Web. 7 Dec. 2009.

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Authors

  • Olivia Saunders, Crop and Soil Science, Washington State University
  • Joe Harrison, Professor, Nutrient Management Specialist, PAS, Washington State University

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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.

 

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Feedstocks for Biogas

Anaerobic digestion of manure and other feedstocks produces biogas which can be burner to make energy on farms. Learn how to evaluate a feedstock and which ones to exclude for biogas production.

Anaerobic Digester in Charlotte, VT.  Photo: Caragh Fitzgerald, University of Maine.

Contents

Introduction

A great variety of organic material can be used in anaerobic digesters as a feedstock for generating biogas. However, there are scientific, engineering and legal limits to what can be added successfully to a digester. In addition, the feedstock needs to be a liquid mixture with an appropriate moisture content. For example, mesophilic complete mix tank digesters (the type most commonly used today) typically operate best with a mixture of 4 to 8% solids in water. Digesters require various moisture contents, depending on the design and operation of the system.

Feedstocks for Anaerobic Digestion

Manure in dairy barn.

Most easily biodegradable biomass materials are acceptable as feedstocks for anaerobic digestion. Common feedstocks include livestock manure, food-processing waste, and sewage sludge. The energy production potential of feedstocks varies depending on the type, level of processing/pretreatment and concentration of biodegradable material. Listed below are feedstocks that can be commonly used in anaerobic digesters:

  • Livestock manures
  • Waste feed
  • Food-processing wastes
  • Slaughterhouse wastes
  • Farm mortality
  • Corn silage (energy crop)
  • Ethanol stillage
  • Glycerine as the product from biodiesel production
  • Milkhouse wash water
  • Fresh produce wastes
  • Industrial wastes
  • Food cafeteria wastes
  • Sewage sludge

Livestock manures are generally lower-energy feedstocks because they are predigested in the gastrointestinal tracts of the animals. Manure, however, is an easy choice for anaerobic digestion because it generally has a neutral pH and a high buffering capacity (the ability to resist changes in pH); contains a naturally occurring mix of microbes responsible for anaerobic degradation; provides an array of nutrients, micronutrients, and trace metals; is available in large quantities; and can be transferred by pump.

Animal wastes containing bedding such as chicken litter with substantial quantities of wood chips or sawdust can be used successfully in anaerobic digestion. The woody material, which degrades very slowly because of its lignin structure, is essentially passed through without digestion, and retention times are based on digestion of the manure.

Blending of energy-dense feedstocks with livestock manure is a common practice to maximize biogas production by optimizing nutrient levels and providing buffering capacity. The use of manure as a base for anaerobic digestion is important because many of the energy-dense feedstocks, such as food-processing waste and ethanol stillage, are acidic, contain little if any naturally occurring microbes, and oftentimes lack the nutrients (nitrogen, trace elements, vitamins, etc.) necessary for anaerobic digestion. Potentially, farms operating anaerobic digestion systems could take on additional wastes and benefit from increased gas production as well as tipping fees.

Materials to Be Excluded from Anaerobic Digesters

Materials that should be excluded as feedstock from anaerobic digesters include those containing compounds known to be toxic to anaerobic bacteria, poorly degradable material, and biomass containing significant concentrations of inorganic material. Poorly biodegradable materials require higher retention times, meaning they must spend more time in the anaerobic digester to be broken down and converted into biogas.

Biogas equipment for electricity generation. Photo: Daniel Ciolkosz, Extension Associate, Penn State.

Inorganic materials, on the other hand, contain no carbon and cannot be converted into biogas. Materials such as sand bedding do not contribute to the biogas potentialo and may cause operational problems such as pipe clogging, premature equipment wear and volume reduction due to sludge accumulation. Also, the feedstock containing too much ammonium or sulfur should be avoided, because ammonium and sulfur inhibit anaerobic organisms.

Evaluating Feedstock Biogas Potential

The biogas potential of different feedstock materials or feedstock combinations is often difficult to predict due to differences in the source, processing, volatile solids concentration, chemical oxygen demand, moisture content, and/or inclusion of toxic compounds. The total biogas potential assay, also known as the biochemical methane potential (BMP) assay, provides an efficient and economic method for estimating biomass conversion and biogas yield of feedstocks or feedstock blends.

BMP assays are a multifaceted approach to evaluating the potential to produce biogas. BMPs are a practical, lab-based approach to identifying and evaluating potential feedstocks for anaerobic digestion. Potential anaerobic digestion feedstocks are commonly evaluated by three criteria.

  1. Feedstock characterization: Both before and after BMP assay, includes pH, chemical oxygen demand (COD), total solids (TS), and volatile solids (VS). Characterization results found prior to the experiment are used to determine the quantity of feedstock needed to maintain the BMP assay for as much as 30 days. Characterization results following the completion of the BMP assay are used to evaluate the anaerobic digestion process in terms of the destruction of the organic material.
  2. Total biogas production: Is measured throughout the BMP either through manual means or continuously by commercial software designed for tracking gas production. Biogas can be scrubbed of the carbon dioxide by running it through a potassium/sodium hydroxide solution to monitor only methane production or can be left unscrubbed to monitor the total biogas production.
  3. Biogas analysis: Biogas composition can be investigated by means of a gas chromatograph during the BMP assay. Though the capital investment is large, gas chromatographs provide accurate measurements of the constituents of the biogas produced during the BMP. Gas chromatographs can be set up to determine the concentrations of methane, carbon dioxide, nitrogen, and hydrogen sulfide gases.

The BMP assay is a combination of a single feedstock or feedstock blend, inoculum, and stock solutions in a batch system. Inoculum is used to seed the feedstock with an active anaerobic culture to initiate activity and reduce any lag time required for establishment of a culture. Stock solutions are added to assure that macronutrients, micronutrients, and vitamin deficiencies do not limit biogas production. BMP evaluations should always be completed in replication and results should be verified at pilot or full-scale, and it is strongly recommended that full-scale designs not be based on BMP results because full-scale digesters are often run at continuous mode while BMP tests are batch mode.

Considerations

The biogas potential of feedstocks is an important factor when considering anaerobic digestion on your farm. But other considerations, such as economics, regulatory issues, feedstock availability on and off the farm, and end use of the biogas, should also be evaluated.

References

  • Chynoweth, D.P., C.E. Turick, J.M. Owens, D.E. Jerger and M.W. Peck. 1993. Biochemical Methane Potential of Biomass and Waste Feedstocks. Biomass & Bioenergy 5:95-111.
  • Liu, Y., S.I. Miller, and S.A. Safferman. 2009. Screening co-digestion of food waste water with manure for biogas production. Biofuels, Bioproducts, Biorefining 3:11–19
  • Owen, W.F., D.C. Stuckey, J.B. Healy, L.Y. Young, and P.L. McCarty. 1979. Bioassay for monitoring biochemical methane potential and anaerobic toxicity. Water Research 13:485-492.
  • Steffen, R., Szolar, O., and Braun, R. 1998. Feedstocks for Anaerobic Digestion. Institute for Agrobiotechnology Tulln, University of Agricultural Sciences, Vienna.
  • Speece, R.E. 1996. Anaerobic Biotechnology for Industrial Wastewater. Archae Press, Nashville.

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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

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Peer Reviewers

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