farm anaerobic digestorOn farm anaerobic digesters can transform noxious polluting waste streams of farming operations into useful products like biogas and high quality fertilizer, promoting sustainable agricultural practices and helping to combat global warming.

by Chris de Morsella, Green Economy Post

Anaerobic Digestion is one of the more promising biological technologies for sustainable waste management and has the potential to turn a large and worsening agro-headache into a growing opportunity for sustainable farming. It can extract useful biogas energy and high quality fertilizer from manure and other problematic agro waste products while also reducing the air and water pollution and emissions of greenhouse gases from a farming operation. Anaerobic Digestion harnesses natural living biological processes to maintain the natural carbon cycle and extract useful energy and fertilizer byproducts from what had been problematic waste streams. It is well suited for many types of farming operations and is an important sustainable farming practice.

UPDATE: Anaerobic Digesters to Help Cut Dairy Greenhouse Gas Emissions by 25% by 2020

The Agriculture Secretary Tom Vilsack announced an agreement with U.S. dairy producers to accelerate adoption of innovative manure to energy projects on American dairy farms. This will lead to a reduction of their greenhouse gas emissions by 25% by 2020 while also extracting electricity from the raw manure using anaerobic digesters. Under a Memorandum of Understanding signed by the Innovation Center for U.S. Dairy , the USDA, and dairy producers, the groups agreed to work together — in a public/private partnership — to reach this target. USDA will contribute by undertaking research initiatives, allowing implementation flexibility, and enhancing efforts to market anaerobic digesters to dairy producers.

Currently, only about 2% of U.S. dairies that could support a profitable digester are using the technology. This in spite of the fat that dairy operations that do operate anaerobic digesters routinely generate enough electricity to power 200 homes. Through the agreement, USDA and the Innovation Center for U.S. Dairy will increase the number of anaerobic digesters supported by USDA programs. Beyond promoting the digesters, the agreement will encourage the research and development of new technologies to help dairies reduce their greenhouse gas emissions. See the USDA press release and the DOE description of anaerobic digesters on DOE’s Energy Savers Web site.

Current Agro Practices Waste Energy and Nutrients, Pollute and Contribute to Global Warming

Currently the byproducts of many farming operations are poorly and only partially utilized and often become part of an unexploited polluting waste stream. Furthermore the scale of this pollution problem is very large. A single large scale feedlot operation generates the waste equivalent of a small city, which is pretty amazing and cause for concern.

Farming operations produce byproducts such as manure, animal bedding, feed waste, and runoff from silos that in many cases contribute to pollution of waterways, smell bad, attract rodents and flies and can carry dangerous pathogens within them posing a public health risk. As anyone who has ever been in the general vicinity of a feedlot or other type of animal farming operation knows the smell of rank manure can become overpowering.

On most animal farming operations, the animals that are being raised are crowded into small areas and their manure and urine are funneled into massive noxious waste lagoons. These animal waste cesspits can and often do break, leak or overflow, sending dangerous pathogenic microbes, including Salmonella, E. coli, Cryptosporidium, and fecal coliform; nitrate pollution and drug-resistant bacteria into waterways and aquifers. These foul lagoons also emit toxic gases such as ammonia, hydrogen sulfide and methane into the air, polluting the atmosphere and contributing to global warming.

One of the principle current practices is to spray the manure onto land, ostensibly as fertilizer — these “sprayfields” as they are called have the downstream consequence of bringing still more of these harmful substances into our air and water.

Clearly the current way of doing things is not an example of sustainable agriculture and is very damaging for the environment as well as wasteful of both energy and other inputs such as fertilizer. Anaerobic digesters can help mitigate this problem while producing renewable energy as well

What Is an On Farm Anaerobic Digestion System?

A farm-based anaerobic digester (AD) —also known as a biodigester— is essentially an air tight sealed, and heated container, located on a farm in which organic waste materials are broken down to produce biogas, effluent and solid remnants. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. The biogas produced by the AD is approximately 60% methane with the most of the rest being comprised of CO2.

AD systems are either batch type systems, which are simpler to build and maintain and are more suitable for most on farm scenarios, or they are continuous types. In a continuous digester, organic material is constantly or regularly fed into the digester.

All biodigesters share the following basic components: A pre-mixing area or tank, which mixes the feedstock into a properly dilute slurry; A digester vessel or series of specialized vessels designed to each provide optimum environments for the specialized microorganisms required for each stage of the complex digestion process; a system for collecting, storing and using the biogas produced in the digester and a system as well as a system for storing and using the effluent and solid residue that is produced by the anaerobic digestion process.

What is Anaerobic Digestion?

Anaerobic digestion is a biological process similar in many ways to composting. Like composting it relies on microorganisms to decompose agricultural waste such as manure, processing by-products, and other materials that are being digested into effluent and biogas. 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.

Anaerobic decomposition is a process that occurs naturally in swamps, water-logged soils and other anaerobic environments such as the guts of large animals. It can be managed and promoted in a “digester”, which is basically an airtight tank to process manure and other waste. It is a complex process, occurring in three basic stages as the result of the activity of a variety of different types of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Finally methane-producing (methanogenic) anaerobic bacteria utilize these organic acids to complete the decomposition process.

The Many Benefits of On Farm Anaerobic Digesters

The primary benefits of anaerobic digestion are: on farm nutrient recycling, waste treatment, and odor control. In addition biogas is produced and can be use to provide energy and heat for on farm use and for selling surplus electricity onto the grid. Each of these beneficial outcomes contributes towards making the farming operation become more sustainable. It promotes sustainability by significantly increasing the energy efficiency of the operation; retaining nutrients that are currently largely being lost and re-using them on the farm; reducing the environmental impact of the operation and its potential for spreading disease and by promoting re-use and useful extraction of energy and nutrient content from the waste stream enabling the operation to significantly reduce its reliance upon external energy and fertilizer and feed inputs.

Effluent from Digesters Is a Lot More Beneficial than the Word Makes it Sound

The material drawn from the anaerobic digester often called sludge, or effluent is a mix of solids suspended in a thick liquid solution. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner.

The solid residue or effluent that is produced in the digester is a stable, odorless, and depending on the retention time and operating temperature is a largely pathogen-free fertilizer, which can be stored much more easily than raw manure and does not attract flies and rodents.

In the process of anaerobic digestion, the organic nitrogen in the manure is largely converted to ammonium, which is readily available and taken up by plants and the ease of storage allows the fertilizer to be applied onto fields when they are most needed. It is important to note that any toxic compounds (pesticides, etc.) that are in the digester feedstock material will become concentrated in this effluent and in the solid co-products; so care must be taken to minimize pesticides or other toxic compounds from the feedstock that is feeding the AD reactors.

The separated, digested solids contain most of the phosphorus in the manure, and operators may choose to export those solids or continue to apply them to their land, depending on their available acreage for manure application. The solid digested manure co-products may also be used for animal bedding, composted on-farm or exported from the farm for use as compost. These fibrous solids can also be used to fabricate low grade fiber boards.

Less Smell, Less Pollution, less Global Warming and Fewer Pathogens

Anaerobic digesters allow a greater portion of odor-causing volatile acids to be converted to biogas, resulting in less odor-causing compounds than would be present in manure in a typical liquid storage system. The overpowering odor of animal raising operations is one of the more immediately apparent forms of pollution that these operations cause and bioreactors can go a long way to eliminating these noxious odors that can lay over the land for miles around a large feedlot operation.

The potential for pollution of ground and surface water sources is also reduced due to several co-factors. Firstly, the digester extracts much of the nitrogen, phosphorous and potassium nutrient load out of the feedstock as easily re-cycled co-products i.e. the liquid and solid components of the effluent and by doing so greatly reduces the potential for this type of water pollution – currently a major issue causing eutrophication of lakes, rivers and waterways and dead zones in the river fan zones in the oceans – for example the famous and growing dead zone at the mouth of the Mississippi river delta. Secondly the heat in the reactor significantly reduces the number of pathogens existing in the effluent effectively acting to sterilize it to a pretty high degree, making it easier to handle, store and posing less of a pollution liability than untreated sludge.

Anaerobic digesters also work to reduce the greenhouse gas emissions in a farming operation and in this manner increase the sustainability of the practice. Methane is a powerful greenhouse gas that remains in the atmosphere for approximately 9 to 15 years. By capturing methane and converting it to heat or electricity, methane digesters reduce greenhouse gas emissions from manure produced by farming operations.

Bioreactors Promote Greater Overall Energy Efficiency and Can Generate Electricity

Anaerobic digestion reduces a farming operations overall energy throughput by combining manure with other high energy waste streams that might otherwise be landfilled and processing them through the AD system. The bioreactor produced biogas that can be used on farm to generate electricity as well as co-generating heat required for the anaerobic digesters themselves and that can be used wherever heat is needed on the farm. This co-generation both saves energy on the farm and produces electricity for on farm use and potentially a surplus that can be sold onto the grid. The biogas can be stored on site so that electric generation can be timed for periods of peak demand when it is most valuable.

Many farms have the potential to create sufficient energy to run their operations (power and heat) and also provide power onto the local grid.

Incentives, Grants and Certification Programs for On Farm Anaerobic Digesters

Financial institutions are generally well disposed to finance projects that help to diversify an operations revenue stream while at the same time reducing the operations costs and mitigating risk exposure. It is a win win situation and is the case with anaerobic digester systems.
In addition to these systems being a relatively easy sell to raise money on the private capital market or to borrow money from lending institutions many states and federal programs exist, which can help to defray the capital costs to build these systems.

Some examples of these incentive programs are:

The Rural Energy for America Program (REAP) provides grants and loan guarantees for renewable energy projects, including anaerobic digesters.

The USDA-Natural Resources Conservation Service may have cost-share funding available for digesters through the Environmental Quality Incentives Program (EQIP). For more information, contact your local USDA Service Center.

Oregon’s Business Energy Tax Credit (BETC) provides a 50 percent credit for the capital costs of biofuels and bioenergy projects and anaerobic digester systems qualify. Oregon also offers 5 to 20 year loans of up to $20 million for these systems.

North of the border Ontario, Canada provides a feed-in tariff for renewable energy that allows farms to sell electricity to the grid at a premium price, and the farming ministry provides construction grants. There are consequently quite a few systems under construction. Perhaps US government agencies including the DOE, EPA, and the USDA should study the success of this program and model programs for the US based on their experience.

Anaerobic digester operator certification is becoming increasingly common. For example Michigan Department of Agriculture offers an Agricultural Anaerobic Digester Operator Training Program


Systems to make better use of manure do not have the same sex appeal as say solar power or wind energy, but it does not mean that the problems are not important and in need of a solution or that the contributions these systems can make towards promoting a new green sustainable agriculture are not as vital as say the renewable energy sector or the smart grid for our future.

Reducing the energy throughput and other external resource requirements – such as fertilizers and water — of farming systems is going to be as important and as vital to building the new green economy as other better known icons of this new paradigm. The electric car or the solar array may capture the loin’s share of public attention, but the humble bioreactor processing manure and other high nitrogen containing agro waste may be as important although unsung.

Of course it is really really hard to sex up a song singing the praises of digesting manure, but it is a song… a message… that is important to get out there. A sustainable green economy will require us to become much better at how we handle our crap…. literally.

© 2009 – 2010, Chris de Morsella. All rights reserved. Do not republish.

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Author: Chris de Morsella (146 Articles)

After a decade performing as a lead guitarist for rock bands, Chris de Morsella decided to return to the career his uncle mentored him in as a youth....Software Engineering. Since that time he has thrown himself into his work. He has designed a compound document publishing architecture for regulatory submissions capable of handling very large multi-document FDA regulatory drug approval submissions, for Liquent, a division of Thompson Publishing. At the Associated Press, Chris worked with senior editors at facilities around the world, to develop a solution for replacing existing editorial systems with an integrated international content management solution. He lead the design effort at Microsoft for a help system for mobile devices designed to provide contextual help for users. Chris also helped to develop the web assisted installer for LifeCam2.0, the software for Microsoft’s web cam and developed late breaking features for the product He also served with the Rhapsody client team to redesign and build a major new release of Real Networks Rhapsody client product. His most recent assignment has been Working with the Outlook Mobile Time Management team for the next release of Outlook Mobile for the SmartPhone. Chris' interests are in green building and architecture, smart grid, the cloud, geo-thermal energy, solar energy, smart growth, organic farming and permaculture. Follow Chris on Twitter.

  • John D. Wilson

    This is a very good overview. However, issues we are seeing in the field are the lack of organization to develop a market and a lack of clarity regarding environmental performance of these systems.

    The revenue stream for these systems basically has three components: on-farm savings (cost offsets), electricity sales, and renewable energy/carbon credits. In order to achieve the electric sales and credit revenue streams, the digester system has to be “sold” to the utility and its regulator as having value in those respects. This requires a lot of paperwork and, ideally, an economy of scale by bundling multiple systems together.

    Who’s doing work to help address this issue?

    • Chris de Morsella

      John ~ You raise a critically important and practical point. There is a lot of work that needs to be done in order to make the environmental benefits of anaerobic digesters more widely understood and better appreciated, not only by farmers themselves, but also by policy makers and society at large. The mitigation of air and water pollution and the reduction in greenhouse gas emissions that can be achieved by bioreactors has a significant benefit or value for the commons but whose cost is currently born mostly by the farmer. Some states, recognizing the larger societal benefits of AG systems are beginning to offer tax and other incentives to farming operations that build these systems. I mentioned a few examples I was able to find in the article. However much more needs to be done.

      I am impressed by the support for on farm anaerobic digester systems and the well thought out provincial government programs of Ontario, Canada. One aspect that they have addressed and that in my opinion should be a model for analogous US programs is mandating a clear market base for selling surplus electricity up onto the grid. The Renewable Energy Standard Offer Program (RESOP) — in Ontario province — gives biogas electricity producers the option to sell or replace power at fixed rates for a period of 20 years. At the time of writing, the value of the power is approximately 11 ¢/kilowatt hour (kWh) for non-peak periods and 14.52 ¢/kWh for peak periods. This guarantee makes it much easier for farming operations to raise capital by providing a fixed rate guaranteed market for any electric power that is produced.

      Are you aware of any similar programs in the southeast region of the US, which you are most familiar with? I have looked for incentive programs that are tied to the carbon offset market, but have not been able to find any. If you know of any existing tie ins I would love to hear about them.

      As I alluded to in my post one of the problems that anaerobic digesters face is that they are poorly understood by the public and by policy makers and the technology is not sexy like say solar power is and is thus often relegated to the back of the policy agenda. I am on the lookout for finding grants, tax incentives such as accelerated capital depreciation, training programs, and market guarantees similar to those existing in Ontario, Canada. Bioreactors are an important technology for making many types of farming operations more sustainable.

  • joelsk44039

    The central problem facing large dairies is dealing with the overwhelming volumes of manure they produce. Anaerobic digestion does nothing to remedy this problem. At the end of the process, the volume of digestate remains massively large. Okay, it doesn’t smell as much, has better overall characteristics than raw manure slurry, etc., etc., but the fact remains, the volume has changed very little.

    Additionally, digesters and their bacteria, are a finicky breed. A sudden drop or rise in the operating temperature can ruin a processing cycle.

    My company manufactures an integrated pyrolysis and conventional engine/generator set system that not only addresses the problem of overall manure volumes, but generates much higher power capacities while also creating biochar and oils which can be sold. We believe that our solution represents a significantly better means of manure processing and disposal than anaerobic digestion.

    • Chris de Morsella

      Joel, You raise an interesting perspective. Are pyrolyisis systems superior to AG systems as you suggest, because they succeed in reducing the volume of the outputs. The answer, depends on how these outputs are viewed. If they are viewed as a waste product, as a problem then certainly reducing both the volume and weight of the end output is an important advantage. However if value can be extracted — or more accurately preserved — from the outputs then it is not as clear. The effluent from Anaerobic digesters is an easily stored nitrogen, phosphorous and potassium fertilizer. It is valuable. The solids can be composted and processed into rich humus.

      One of the key ideas underlying the movement towards sustainable agriculture is the notion of closing loops that is in capturing and re-cycling valuable co-products from what are currently treated as waste streams. As you point out pyrolysis does have the advantage of significantly reducing the volume of the biomass being processed by thermally decomposing it. In instances where the biomass is contaminated with toxic components I think that pyrolysis is the best means of re-covering energy and (especially in high temperature systems) destroying many of the organic toxics such as pesticide residues etc.

      However I disagree that pyrolosis is always a better answer as you seem to imply. When the feedstock is relatively free of toxins — i.e. pesticide residues, antibiotic residues, or other non organic pollutants — the large volume of co-products produced by AG systems is a valuable fertilizer and the solids residues when composted are a marketable product — can be sold to landscapers for example.

      I think there is room for both pyrolysis and for Anaerobic Digester systems and the relative merits need to be weighed on a case by case basis.

      • joelsk44039

        “The effluent from Anaerobic digesters is an easily stored nitrogen, phosphorous and potassium fertilizer. It is valuable. The solids can be composted and processed into rich humus.”

        Manure management is the number one problem associated with large dairy, swine and other animal operations. How exactly does one store hundreds of thousands of gallons of digestate? It is my contention that pyrolysis does more than just reduce the huge and unmanageable volumes of manures. It produces a sterile material with most of the nutrients still present. It is dry, so can be economically transported much further than digestate. As a “terra preta” material, biochar will loosen denser soils and retain water. And lastly, it can be field applied any time of year, unlike manures.

        • Chris de Morsella

          For starters I would contend that large feedlot operations are to sustainable agriculture as coal is to clean energy. The very large feedlot operations, which you are referring to are unsustainable on many levels, besides being huge sources of pollution. They are prime abusers of antibiotic over usage, which presents many problems including environmental and health problems (it encourages the evolution of anti-biotic resistance); these types of industrial scale operations have huge energy requirements — especially when one looks at the total ERoEI of the operation including the energy required for growing, transporting and storing the feed.

          Trying to make these animal factories sustainable is akin to trying to make coal clean — which is to say it cannot be done.

          I remain convinced that anaerobic digesters fit very well on smaller scale operations where the effluent can be more easily stored and used on surrounding fields as needed and that for these types of operations they make better sense than pyrolysis systems. While I agree that the biochar produced by pyrloysis is much more compact this is not a determining issue — IMO — on small to medium scale operations. The effluent from a ADS is much easier to store and handle than raw manure and the much of the fertilizer value is lost in the process of thermal decomposition.

          Sustainable agriculture seeks to close open cycles and recapture as much value as it can from what were waste streams. It also seeks to be as energy efficient and other external input efficient as possible. This is the crux of sustainability and will become increasingly determinant as energy and resource commodity prices continue on the long term trajectory of ever increasing price due to increasing global scarcity of available supplies.

          Although it may not seem like it I do happen to think that pyrolysis is a good green technology — where applicable. I see it being particularly applicable for recovering energy from and for carbon capture (i.e. with the biochar) for waste streams that are not suitable for anaerobic digestion, because they contain excessive levels of pollutants that would make the resultant effluent and solid remnants unsuitable for use as re-inputs into agricultural systems. A lot of urban generated waste streams fall into this category, because they contain a lot of organic pollutants that can be decompsed by pryloysis, but not by AD.

          If the source waste stream feed stock is contaminated with heavy metal pollution it is my understanding that these heavy metals remain in the biochar making it unsuitable for use as a soil additive unless these meals can be removed. And that this is a problem with a lot of municipal waste.

          • biogas!

            The debate between pyrolisis vs anaerobic digestion can be answered by more sophisticated understanding of manure management. Pyrolisis is appropriate for dry materials with low water content (water will absorb much energy to heat up) and AD is very suitable for more liquid manure management. Manure can be removed from stalls using numerous methods scraped stalls produce a high solids waste stream while many flushed systems and swine systems produce a manure stream with lower solids higher water content. The former may be appropriate for pyrolisis but the latter certainly is not.

            That said… there are numerous high solid AD technologies out there and if the goal was to use manure as a resource rather than a problem, the digestate has a higher value for nutrients (conserves nitrates and nutrients are in a bioavailable “mineralized” form) as opposed to biochar where the nutrients that haven’t been volatilized are left in an immoble form. Though pyrolisis does make manure “disappear” which is more attractive to animal husbandry operations than high quality organic fertilizer (unfortunately IMO).

  • George Hoguet

    This is a ggod articel as you both have mentioend. Sadly, there are only a few hundred systems yet in the US, whereas Europe and South America each have thousands of digesters. I worked on helping to get a reasonable net metering provision for dairy digesters in Pennyslvanai, and our company has been successful in helping bring front-end funds forthe REC and carbon offsets to several family dairies there as well as in a few other states. We are now beginning to pilot a concept of mutli-farm clusters that can help reduce the start-up costs and make these systems viable for smaller farms. We do not build the digesters, but rather help with financing and marketing the emission credits. You can learn more on our website: Click on “Our Projects”.

    • Chris de Morsella

      George ~ You raise what seems to me to be the single most important road block that is preventing wider adoption of anaerobic digester systems in this country. This point was also raised in a previous comment by John Wilson. If the government (working with the utilities) can mandate a reasonable long term guaranteed market for electric power and preferably one that emphasizes peak load pricing; just this single factor would make it much easier for farming operations to raise the capital needed in order to build AD systems by providing the financial lending institutions with a known and quantifiable long term revenue stream that would be generated (literally) by the project.
      As you point out by tying these systems into the larger carbon offset market an additional and critically for raising capital a quantifiable revenue stream can be added. Muliti-farm clusters also seem like an excellent idea, especially for smaller scale operations.

  • Alison Tottenham

    It is a good article as far as it goes. But for those in a position to install the technology, the crucial data is missing i.e. Number of Stock (n) related to Vol. of Digester related to Heat produced or equivalent KWh of energy; together with an idea of the cost of installing the digester to deal with the output of n stock; and the need for other materials to feed the digester and maintain a suitable consistency in the digestate.

    • Chris de Morsella

      Alison ~ you raise valid points. I am surprised by the level of interest this post has generated I had figured it would be hard to get anyone to read an article on the subject of digesting… well mostly crap. I would like to do a more detailed follow up article on this that provides greater details and specificity, perhaps by focusing on some specific example.
      If you have specific real world numbers that quantify AD system inputs, outputs, capital expenditures, revenue streams, environmental mitigation — I am currently researching them and would love to hear from you.
      Thanks, Chris


    I am a CDM Consultant in Penang,Malaysia.
    Malaysia and Indonesia Have more than 1000 over Palm Oil Mills.Palm Oil Mill Effluent ponds generate GHG Methane.
    I worked for US Based AES Agriverdi Company in Kuala Lumpur Malaysia.We have under taken 50 over CDM Projects in Malaysia and in Indonesia.But We are not successful in getting CER Certificates and the CDM Projects are not successful.I think that it is due to the poor technology and If any good technology is available using Anaerobic Digester Technology and capture Methane Gas,Monitor and report for CER Certification. This will be a multi Billion Doller Business Oil palm Plantation Companies to get actively involved in CDM Projects.given the whole year-round availability of Palm Oil Mill Effluents.

    • Shahed Khan

      Interesting to know about it.
      Have u solved your problems now.
      I work on CDM in Bangladesh so it is always interesting to know new cases.
      Shahedkhandhk at

  • James McPartlan


    You are absolutely correct in that this is a totally misunderstood technology and that it is not a “sexy solution” to renewable energy or waste stream reduction issues. Having said that, I find it to be a very simple solution to one facet of pollutants converted to energy. Recycling of Ag waste on-site is very efficient as well as making a strong financial case. Ontario Province attitudes & incentives could be applied to more U.S. Ag & feedlot operations and create new sources of clean domestic energy not to mention, a whole new creation of U.S. jobs that could not be exported.

    It sounds like a potentially great concept that I for one, would explore for development possibilities. Also, the Pyrolosis Systems can have a place for voluminous & municipal waste situations. I am interested in more debate on these issues.

    Keep up the good work!