|Biogas is around 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
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.
|Most biodigesters use one, or a hybrid of three
major technologies depending on farm practices and local
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.
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.
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 biodigester 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.
|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
biodigesters. 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 biodigester 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.
|Biogas processes are an important part of total
resource management on many Northwest farms. In addition to
producing energy for on-farm use, biodigesters 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 biodigester
growth. Studies show biodigesters reduce odors by more than 90%.
Pathogens, Pests and Weeds
– Biodigestion 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.
– Separation of solids during the biodigestion 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.
– Because of water quality and salmon recovery concerns,
Northwest livestock operators face tightening federal and state
regulations. Biodigesters can provide cost-effective solutions
that help meet these regulations.
Cleaner Air –
Current farm biodigesters keep more than 5,000 metric tons of
methane out of the atmosphere each year. Biodigesters 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
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.