Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 114

P and K, are rich in animal manures, especially poultry manure. Because most of
the readily degradable substances, especially carbohydrates, have been digested and
absorbed by the animals, animal manures have very little readily fermentable sub-
strates. Additionally, animal manures have high concentrations of amino nitrogen
such as urea and ammonia and a large pH buffering capacity against acids. Thus, the
fermentative acidogenesis during AD of animal manures typically does not result
in significant pH decline, but high concentrations of ammonia can result, causing
toxicity to methanogens, especially in thermophilic digesters where methanogens
are very susceptible [43]. Furthermore, animal manures contain large amounts of
microbial biomass, including bacteria and methanogens. Consequently, AD reac-
tors digesting animal manures, especially livestock manures, can be started without
the addition of external digested sludge as a start culture or inoculum.
Because of the relatively low contents of readily degradable substances, the
methane biogas production from animal manures is generally slow. Thus, when
digested alone a long retention time is needed. Co-digestion with nitrogen-poor
yet carbohydrate-rich feedstocks, such as food-processing wastes and OFMSW,
can substantially enhance CH 4 production and stabilize the AD process of animal
manures [59, 94]. Some animal manures, especially dairy cattle manure, contain
sand from the sand bedding [42], which settles in AD reactors and can cause oper-
ational problems if not dealt with properly. Due to the large differences in many
physicochemical characteristics and degradability, different manures may require
different AD technologies for efficient and cost-effective AD. Here the AD tech-
nologies suitable for beef manure, diary manure, swine manure, and poultry litter
will be discussed.
3.2.1 Animal Manure Dung and Poultry Litter
The manures from beef cattle feedlots (or barns that do not use water to flush the
animal manure) and poultry barns have relatively low water contents. They are
often applied to farmland as fertilizer and thus have not been commonly subjected
to AD. However, these two types of manures can be digested using dry AD pro-
cesses [37] such as the dry anaerobic combustion (DRANCO) process, ECOCORP
process, BEKON process, Kompogas process, and Linde process. These dry AD
processes have several advantages over wet AD technologies and are described later
in this chapter. Although not demonstrated on either type of manures [27], anaero-
bic leaching bed reactors may be suitable for the AD of these manures without any
dilution.
Both types of manures can be diluted to slurry and digested in conventional wet
AD reactors. For beef cattle manure, a slurry containing 12% TS can be digested,
but for poultry litter, a higher dilution (TS <3%) is needed to minimize inhibition by
ammonia [44]. Inevitably, such dilutions create the need for large reactor volumes
and high capital and operating cost. Pretreatment may be needed to remove the un-
ingested diet (e.g., hay in beef cattle manure) or course materials (e.g., bedding
materials and feather in poultry litter) prior to AD. Because of the presence of high
solid contents, only CSTR, CMCR, and mixed plug-flow loop reactor (MPFLR)