Anaerobic digestion transforms manure into methane that can generate energy, and marketable compost, fiber and water high in nutrient content but low in odor and bacteria. The 1990s saw a doubling of successful farm digesters, bringing the total to nearly 40 projects nationwide by 2002. Digesters can also help Northwest farmers respond positively to tightening water quality and salmon protection regulations.

Deciding whether to install a digester and what kind to use requires addressing a number of issues, including:

• Methods of manure removal and water management
• Degree to which animals are pastured
• Local electrical and tax rates
• Projected life of the facility

Surplus power can be sold back to the grid, helping utilities reduce power line investments. Utilities can resell the power under Green Tag premium programs. Utilities in some areas have raised roadblocks to connecting generators, so be sure to check on their policies.

Many consultants will offer to prepare a project assessment free-of-charge. You can do it yourself with EPA’s AgSTAR Program, or ask for guidance from state contacts or your local Conservation District. Financial assistance may be available through EQIP and other federal programs. See Resources section for contact information.

How can “biogas” be used?

The biogas from digesters is 50-70% methane, with the remainder mostly carbon dioxide. Biogas typically contains around 600 BTU of energy per cubic foot, a lower value than natural gas which is nearly all methane. But all natural gas-burning equipment can be modified to use biogas, including electrical generators, heaters and vehicles. Heat and steam recovered from electrical engine or turbine operation can be recycled to the digester to maintain required temperatures.

You should also consider on-site uses (e.g., heating, electricity, refrigeration), nearby facilities that could use biogas, and whether electric power distribution systems in your area will buy power generated by biogas.

What digester technologies are available?

Most digesters use one, or a hybrid of three major technologies depending on farm practices and local conditions:

Covered Lagoon – With 2% or less solids, this design is the least expensive to operate but requires large throughput. Driven by atmospheric heat, it needs a warm climate and is most common in the Southern states.

Complete Mix – Manure with 2-10% solids is heated and actively mixed in a silo-like tank to keep solids suspended. This is the most expensive to build and operate, but it works well on farms that use rinse systems for manure removal.

Plug Flow – A tube-like structure in which manure fed into one end pushes processed product out the other, this design can handle 11-13% solids. Hot water pipes running through the plug keep it heated. This type of digester is appropriate for operations that mechanically remove manure from barns, rather than washing it out. It usually runs at 95-105° F and processes manure in 20-30 days.

What conditions are most favorable for digestion?

Cow and pig waste is the easiest to process. Although used in some areas, poultry manure has a higher concentration of fine solids that can cause problems in digesters. Generally speaking, at least 300 cows or 2,000 pigs should be present. Not surprisingly, economies of scale favor larger installations. Manure should be relatively free of clumps of bedding or other material, collected every day or two, and delivered to a common location.

Other factors which may make a digester more attractive include the need to reduce odors or runoff, high local power or fertilizer costs, and regional markets for compost and other soil amendments.

What other benefits do digesters offer?

Biogas processes are an important part of total resource management on many Northwest farms. In addition to producing energy for on-farm use, digesters can help resolve manure management problems including odor and runoff, and open the way to herd expansion.

Odor Control – Odor management has been a major driver of US farm digester growth. Studies show digesters reduce odors by more than 90%.

Pathogens, Pests and Weeds – Digestion at 95° F and above destroys 99% of fecal coliform bacteria. It also alleviates the clouds of flies that can plague animal operations, and virtually eliminates weed seeds that pass through animal digestive tracks.

Nutrient Management – Separation of solids during the digestion process removes around 25% of nutrients. Odor reduction and weed seed elimination make the solids marketable as compost in watersheds where nutrient overloading is not a problem.

Regulatory Solutions – Because of water quality and salmon recovery concerns, Northwest livestock operators face tightening federal and state regulations. Digesters can provide cost-effective solutions that help meet these regulations.

Cleaner Air – Current farm digesters keep more than 5,000 metric tons of methane out of the atmosphere each year. Digesters at the 3,000 farms where they would be cost-effective could increase that figure to 426,000 tons. The evolution of carbon trading systems may provide a market for reductions sold to parties seeking to offset emissions.

Fertilizer and Fiber – Digested product is more stable than untreated manure and may be more readily taken up by plants. Digested manure can be composted and sold for $4-8/yard. Fiber separated from manure can be recovered for animal bedding.