Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 113

digested and the methane biogas yields are relatively low. This is largely attributable
to the relatively small net amounts of energy that can be produced. However,
when municipal sludge is co-digested with carbohydrate-rich yet nitrogen-poor
biomass wastes (e.g., OFMSW and food-processing wastes), the energy yields can
increase substantially [4]. For example, in a full-scale two-staged AD system, a
25% increase in organic load rate (OLR) with OFMSW resulted in an increase in
biogas yield by 80% and overall degradation efficiency by 10%, which resulted in an
increase in electrical energy production by 130% and heat production by 55% [94].
Additionally, when co-digested with carbohydrate-rich yet nitrogen-poor biomass
wastes, municipal sludge can stabilize the AD process of the former [46].
Municipal sludge is among the most studied feedstocks in AD. Numerous topics
and reviews have been published on AD of municipal sludge (e.g. [79]). In gen-
eral, because of the presence of high levels of suspended solid (SS), most AD
technologies are not suitable for the AD of municipal sludge. Continuously stirred
tank reactors (CSTR) and completely mixed contact reactors (CMCR) are most
commonly used in AD of municipal sludge [79]. For example, the CSTR with a
total volume of 1,350 m 3 in Karlsruhe, Germany digests municipal sludge at 37 ◦ C
and produces approximately 3,800 m 3 of biogas of 62-70% methane daily [33].
More recent research efforts have been directed at pretreatment to enhance degra-
dation of the solid found in municipal sludge and production of methane biogas
(see [28, 45] for reviews). Thermophilic AD, in single- or two-staged systems,
is also being increasingly used to enhance biogas production and sanitation [92].
Additionally, because of the low solid contents (1-5%) and low BMP, large digesters
are required for the conventional “wet” AD. Currently, “dry” AD technology is
being evaluated to produce methane biogas from dewatered biosolids, which have
significantly reduced water contents (70-85%) and thus reduced digester volumes
[64]. Dewatered biosolids are also ideal feedstocks to be co-digested with other solid
feedstocks, such as OFMSW and crop residues.
3.2 Anaerobic Digestion of Animal Manures
Animal manures represent a huge methane biogas potential. As estimated, 106
million dry tons of animal manures are produced each year in the USA, with approx-
imately 87 million dry tons being available for methane biogas production [69].
Given a BMP of 200-400 m 3 CH 4 /dry ton [8], the amount of animal manures
available for AD provides a potential of 17-35 billion m 3 of CH 4 per year in
the USA. The animal manures produced from confined animal feeding operations
(CAFOs) offer one of the most abundant single feedstocks available for large-scale
methane biogas productions. The composition and physical features (e.g., water
contents) of animal manures vary widely from species to species and from oper-
ation to operation [58]. In general, animals manures have relatively high water
contents, ranging from 75% (poultry manure) to 92% (beef cattle manure). Most
of the animal manure is organic matter, with VS contents ranging from 72% (poul-
try manure) to 93% (beef cattle manure) of TS. Inorganic nutrients, including N,