Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 112

and presence and concentrations of inhibitory compounds. Feedstocks rich in starch
and/or proteins are easier to digest than lignocellulosic feedstocks. Reduction of par-
ticle size of insoluble feedstocks can significantly speed up AD and increase CH 4
yields. Microbes need numerous nutrients to grow, with nitrogen and phosphorous
being the most important. The optimal carbon (expressed as chemical oxidation
demand, COD) to N to P ratios (COD:N:P) for efficient AD differ with different
feedstocks and the AD technologies used. For most feedstocks, a C:N ratio of 25-32
is suitable for most AD processes [8].
3 Methane Biogas Production from Different Feedstocks
Any biomass can be used as feedstocks for AD. However, biomass wastes, espe-
cially those with a relatively high water content (>50%), are the most common
feedstocks suitable for AD. In fact, methane biogas has been produced from mil-
lions of tons of biomass wastes arising from municipal, industrial and agricultural
sources [91]. The characteristics of biomass wastes vary widely. The common feed-
stocks suitable for AD have been discussed with respect to features pertinent to
AD and biogas potentials by Yu et al. [91]. The AD of several types of feedstocks
has also been reviewed recently (e.g., [15, 66, 85]). Anaerobic digesters can be cat-
egorized in many different ways (see [80, 91] for an overview). No AD reactor is
universally ideal or superior because each type of reactor has certain advantages and
disadvantages that make it appropriate for particular type(s) of feedstocks. In this
chapter, the features of individual feedstocks that have substantial methane biogas
potentials and the AD technologies that are suitable for their AD will be discussed.
3.1 Anaerobic Digestion of Municipal Sludge (Biosolids)
Municipal sludge includes primary sludge and waste activated sludge derived from
centralized wastewater treatment plants that employ biological treatment of sewage.
It is probably the first type of feedstock subjected to AD. It has very high contents
(95-99%) of water, low contents (15-20%) of volatile solid (VS, representing the
biodegradable portion of total solid, TS), and low contents of readily fermentable
carbohydrates [8, 94]. However, most municipal sludge has rich and balanced nutri-
ents (nitrogen: 3-6%; phosphorus: 1.0-1.2%; of TS). The biochemical methane
potential (BMP) of municipal sludge is relatively small, ranging from 85 to 390 m 3
CH 4 /dry ton. Municipal sludge contains a high density of bacterial cells (mostly
aerobic and facultative anaerobic bacteria), some of which may be pathogenic to
humans and/or animals. Toxic compounds may also be present in some munici-
pal sludge, especially those derived from large metropolitan areas. Approximately
6.2 million dry tons of municipal sludge are produced annually in the USA (based
on 1999 data [39]), representing an annual potential of at least 6 billion m 3 of
methane biogas. At present, however, only a portion of the municipal sludge is