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Making the Case for On Farm Anaerobic Digesters

by Chris de Morsella
Sustainable Farming, November 9, 2009

On 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.

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.

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.

Chris de Morsella
Making the Case for On Farm Anaerobic Digesters
Sustainable Farming, November 9, 2009

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