Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 115

are suitable for AD of diluted beef cattle manure and poultry litter [20]. However,
in a pilot study [9], upflow anaerobic sludge blanket (UASB) reactors were shown
to be suitable and more efficient for the AD of diluted poultry feces. Co-digestion
and thermophilic AD are also shown to improve digestion of poultry litter [16].
Thermophilic AD of poultry litter can be difficult due to the resultant high ammonia
concentrations that render the AD process unstable [16]. The future will probably
see more application of AD to both beef cattle manure and poultry litter, either alone
or in co-digestion with other feedstocks, to both harvest bioenergy and produce
fertilizer.

3.2.2 Dairy and Swine Manure Slurry
Some small dairy and swine farms do not use water to flush their barns so they
produce manure with low water contents. These manures can be digested using dry
AD [27]. Large dairy and swine farms, however, use water to flush the manure
out of the barns and hog houses, respectively, generating manure slurries of mod-
erate solid contents (>8%). Traditionally, both types of slurries are stored in waste
lagoons built on the farms. By installing a flexible or floating gas-impermeable plas-
tic cover, such lagoons can be easily converted to a unique type of digesters, covered
lagoon digesters [68]. Covered lagoons typically have a long retention time (several
months or longer) and high dilution rates [11]. Because of impracticality in temper-
ature control, covered lagoons are left to operate at ambient temperatures and can
produce biogas efficiently only in areas with moderate and elevated year round tem-
peratures. Covered lagoons are simple and cheap to construct, operate, and maintain,
which justifies their low AD efficiency. Another disadvantage is the slow but contin-
uous accumulation of undigested solids at the bottom of the lagoons, which is costly
to remove. One example of covered lagoons is located at Royal Farms in Tulare,
California. It has three cells with a surface area of nearly 2,800 m 2 . Supported
by the US EPA AgSTAR Program ( http://www.epa.gov/agstar/index.html ), it was
started in 1982 and has been in operation ever since. The biogas produced has been
enough to fuel two Waukesha engine-generators to generate electricity to meet all
of the farm's electricity needs with excess being sold to the local utility. The heat
recovered from the generators is used as supplemental heat in the nursery barns,
and the stabilized effluent is used as fertilizer. Barham Farm in North Carolina also
operates a covered lagoon that has an effective volume of 24,500 m 3 . It digests the
manure slurry generated from 4,000 sows. Baumgartner Environics, Inc. and MPC
Containment Systems, LLC are two providers and installers of anaerobic lagoon
covers.
Another type of digester that has been successfully and commonly used in AD of
dairy manure slurry is non-mixing plug-flow reactors [15], which can successfully
digest manure slurries with high solid contents (up to 11-14%). With a HRT of
21 to 40 days, methane biogas containing more than 60% CH 4 can be produced
at rates from 0.37 to 0.79 m 3 /m 3 reactor volume/d. As estimated from the bio-
gas yields of three such digesters, the daily biogas production ranged from 1.16
to 2.41 m 3 per cow per day [88]. Although non-mixing plug-flow reactors are