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Anaerobic Digesters

Anaerobic digesters compost (or "digest") organic waste in a machine that
limits access to oxygen (hence the "anaerobic" part), encouraging the
generation of methane and carbon dioxide by microbes in the waste. This
digester gas (which also comes contaminated with hydrogen sulfide) is then
burned as fuel to make electricity. Digesters aren't widely used yet, but
tend to be used for sewage sludge at sewage treatment plants and for animal
waste on farms. Digesting manure doesn't avoid the need to apply that
manure as fertilizer, since the manure still exists and has to go somewhere
(it doesn't magically disappear and turn into energy).

Digesters are only marginally effective at reducing problems with odors,
pathogens and greenhouse gas emissions from animal waste or sewage sludge,
but they are incapable of making any chemical contaminants in the wastes go
away. Digesters aren't emissions-free. They are known to emit nitrogen
and sulfur oxides, particulate matter, carbon monoxide and ammonia.

Living next to a digester could be unpleasant, particularly if located in a
residential neighborhood or if the facility would be large -- attracting
manure-hauling trucks from around the region. Some proposals for digesters
have been fought off by community opposition.

Large anaerobic digesters are used to make factory farms more
viable. Consequently, advocates of small family farms and of sustainable
agriculture see digesters as a Trojan horse that pretends to solve a waste
management problem while enabling factory farms to invade the community.

Estimated air emissions data on digesters can be found here on page 3-12:

Lusk, P. (1998). Methane Recovery from Animal Manures: A Current Opportunities Casebook. (3rd Edition. NREL/SR-25145. Golden, CO: National Renewable Energy Laboratory. Work performed by Resource Development Associates, Washington, DC.

You can learn more about what digester industry enthusiasts have to say about digesters via the digestion email list and the list's archives.

There are also attempts to use digesters for municipal solid waste (household and commercial trash). See our page on MSW digesters for more info on this "drygester" technology.

Critiques of Anaerobic Digesters:

  • Methane Digesters and Concentrated Animal Feeding Operation (CAFO) Waste (Sierra Club Guidance Oct 20, 2004)

  • GRACE Factory Farm Project's June 27, 2003 Position Statement on Methane Digesters [MS Word file]

    [Reprinted below, for those who can't read the MS Word file]

    The GRACE viewpoint on methane digesters. The US government has started to hand out subsidies to farmers to install methane digesters--which extract methane from animal manure to create energy--as a way to deal with problems associated with factory farm waste. Digesters have greatly varying efficiencies, sometimes using all of the methane produced to sustain the production of more methane. The process is prohibitively expensive unless subsidized by taxpayer money and requires a constant supply of manure, but does not diminish the amount of manure present after methane has been extracted. Thus, methane digesters should not be viewed as a sustainable solution for factory farm waste, but rather a mechanism for perpetuating the production of excess manure...

Methane Digesters

(by the GRACE Factory Farm Project)

The US Department of Agriculture (USDA) estimates animals in the US meat industry produced 1.4 billion tons of waste in 1997 -- 130 times the nation’s volume of human waste and five tons of animal waste for every US citizen. The use of manure for fertilizer occurs naturally on traditional livestock farms and, assuming the manure is applied at the appropriate agronomic rates, this is good management. Production of more manure than the land on which the animals are housed can absorb is unsustainable, unless a large amount of additional land can be found for manure spreading.

The number of animals on factory farms often exceeds the ability of the factory farm operator to find enough cropland to responsibly spread the manure. This invariably leads to attempts to increase the quantity of manure spread on each plot of land by calculating the nutrient requirements using only the most generous assumptions -- or by spreading manure in excess of the legal requirements. When these practices are combined with the fact that virtually all manure spreading is calculated on nitrogen content only, even though many manures are enriched in phosphorus, the result is pollution of ground water from over application, run-off of nutrients into streams and lakes, and eutrophication of water ways.

Today’s confined animal feeding operations (CAFOs) use various methods of waste disposal and storage: giant cesspools, known as lagoons; waste dispersion through injection of liquid manure into the soil; spraying the liquefied manure over the surrounding fields; or disposal of solid waste in landfills after liquid waste has evaporated from the lagoon. These practices create odor nuisances and health hazards for the neighbors of the CAFO. Pollution from animal waste can cause respiratory problems, skin infections, nausea, depression and other serious illnesses for people who live near factory farms.

Given these potential problems and the community resistance to CAFOs they have spawned, mitigating the environmental impacts of this massive amount of manure has become a major concern for CAFOs, agribusiness and the USDA. Several companies claim to have invented systems that consume parts of the manure and create valuable byproducts such as methane gas and fertilizer. One of the more popular ideas is a very old concept: the methane digester. The methane given off during the decomposition of the manure is captured and burned, providing either heat or power for electrical generation. Proponents of digesters claim they remove offensive odors from manure, while producing low-cost energy.

While the act of covering the manure used by the digester to capture the methane will reduce odors, it has no effect on the odors from manure kept in additional lagoons or on the major source of CAFO odors—the exhaust fans from the CAFO barns (these fans account for about 60% of all odors emanating from a CAFO). Further, because methane burns at such a low temperature, the digester continually emits ammonia in excess of air pollution standards unless a separate ammonia stripper is employed.

Costs such as those associated with ammonia stripping when coupled with the short life-span of digesters (usually 10 years or less) are two of the reasons that even though methane digester technology has been available for many years, it has never been shown to be an economical way to generate power. Studies show that the installation and operating costs invariably exceed the savings the digester produces by replacing other energy sources. The cost/savings differential is better for large operations, but it can never overcome the requirement that methane power must be a secondary power system. Building regulations usually require that farm-operated systems like methane digesters have a back-up system which is normally the standard electrical grid connection that the farm would use if it was not generating methane. Further, because the gasses given off by manure in the barns are so lethal, no large CAFO can allow barn ventilation systems to fail for even limited periods. As a result, the installation costs for both the electrical system and the digester must be absorbed by the owner. For these reasons, no methane digester in the United States has been an acceptable investment unless it was partially supported by a government subsidy.

Even when supported by subsidies, digesters have a high initial cost and a long payback period. They also require the waste of at least 150-200 animals to maintain efficient operation. Digester operation is labor intensive and expensive, and in the end analysis, farmers have little economic need to produce their own energy. Energy expenditures amount to only about 3-5% of total operational costs on many farms. According to California Polytechnic University, subsidies are not the make-or-break factor in the use of biomass as long as the environmental advantages can be shown to be greater than the benefit of energy production. But are the environmental benefits of digesters real?

Digesters have greatly varying efficiencies. They are unreliable and inefficient when operated at ambient temperatures. Heating the manure improves the generation of methane, but even small amounts of manure heating will consume approximately 35% of the biogas produced by the digester temperature . Heating the manure enough to speed decomposition significantly takes far more energy. For example, Big Sky Farms proposed an operation using 6 digesters in Idaho, each accommodating a 330,000-pound operating load of manure. In order to get the digesters to consume this amount of manure (from a 50,000 sow farrow-to-finish operation) as rapidly as it was produced, all the heat from the methane generated by the digesters was needed to heat the manure for digestion -- in other words, all the digester methane was used to create more digester methane. These digesters still required a huge supply of manure to be retained in the surrounding lagoons, and it did not reduce fossil fuel use at the CAFO.

Because of their excessive ammonia emissions, digesters are not likely to solve any greenhouse gas problems. A recent National Academy of Sciences report on air quality surrounding factory farms shows that ammonia released into the atmosphere, and the "nitrogen cascade" that follows this release, is the major concern for green house gasses. Air pollutants with a nitrogen base (NOx) released by a digester are similar to those from an internal combustion engine. Further, since the burning temperatures of methane are so low, the ammonia in the gasses coming from the manure will not be oxidized and will be released from the digester stacks directly into the atmosphere. In the case of Big Sky, ammonia emissions from the digesters vastly exceeded those allowed at industrial sites in Idaho. Even if the ammonia was oxidized this would simply add other Nitrogen/oxygen compounds to the waste gasses that have worse greenhouse effects than the ammonia.

Proponents of methane digesters originally touted these systems as an alternative source of energy. When it became obvious this was not the case, many proponents began to claim that the benefit of digestion was actually the reduction of odor from CAFO manure. However, while digesters mitigate some of the odor from manure, they do not make CAFOs good neighbors. In order to use a digester manure must be stored in anaerobic lagoons, which emit foul odors unless covered. Further, a study reported by the National Pork Board found that lagoon covers only reduce odor by about 45 percent. And finally, CAFO barns produce the majority of the odors at the site and this is unchanged when digesters are used.

After the methane has been produced there is still the issue of the waste that remains behind. The quantity of manure is not reduced by digestion. It still must be put somewhere, and all the chemicals and heavy metals that were in the manure prior to digestion remain after the digester has operated must be disposed of. As the National Academy of Sciences report emphasizes, the only reliable way to limit emissions to the atmosphere is to incorporate the manure into the land as rapidly as possible and to keep it covered and contained until that time. This is only possible if sufficient spreadable acreage is located adjacent to the CAFO. Obviously, to really solve the problems of odor and waste there is only one answer—never keep more animals than the available land will accommodate.

Anaerobic digesters do almost nothing to make a very serious problem less serious. Anaerobic digesters operate at temperatures too low to destroy pathogens. The concentration of oxygen-demanding carbon compounds exceeds concentrations in untreated municipal waters. Digestion converts organic forms of nitrogen to ammonia nitrogen, but it does not reduce phosphorus. And finally, the lagoons themselves continue to pose dangers to surrounding residents--leaking, emitting dangerous gases, and threatening to overflow.

Less Manure is the Solution

Manure accumulation is not the problem, it is a symptom of the real problem: CAFOs concentrate so much manure in such a small area that it cannot be realistically used for fertilizer. The GRACE Factory Farm Project does not endorse any manure treatment, recycling, or utilization technology. In our view, the solution is to allow unconfined animals to spread their own manure and to employ sustainable farming practices, not to rely on the application of endless and increasingly expensive technological fixes.


Further reading

Test and Quality Assurance Plan: Swine Waste Electric Power and Heat Production Systems: Capstone MicroTurbine and Martin Machinery Internal Combustion Engine (Prepared by Greenhouse Gas Technology Center, in cooperation with EPA. November 2002. 166pp.)