Production of Methane Biogas as Fuel Through Anaerobic Digestion - Sustainable Biotechnology: Sources of Renewable Energy - page 109

renewable bioenergy and protecting the environment. Indeed, a diverse range of
feedstocks (e.g., municipal sludge, food-processing wastes and wastewaters, live-
stock manures, the organic fraction of municipal solid wastes (OFMSW), crop
residues, and some energy crops) are being diverted to AD for increasing biogas
production [4]. Although AD is a relatively slow process and its operation and per-
formance are sometimes unstable, the methane biogas derived from biomass wastes
has become competitive, in both efficiency and cost, with heat (via burning), steam,
and ethanol production [31]. In this chapter, the microbiological underpinning of
the AD process as well as the recent understanding of the microbial communities
driving AD will be discussed from a biotechnological perspective. This chapter will
also provide an overview of the common characteristics of feedstocks that have great
biogas potentials and the AD technologies suitable for each of these types of feed-
stocks. The drivers and barriers for commercial AD implementation as well as the
AD technology gaps and the research needs will also be discussed.
2 The Microbiology Underpinning Anaerobic Digestion
A very complex community of bacteria and archaeal methanogens drives the entire
AD process [36, 65]. Fungi and protozoa are also found in anaerobic digesters [60]
although their functions and contributions to the AD process are not known. The cell
densities of microbes in anaerobic digesters are among the highest in managed envi-
ronments, with bacteria being the most predominant (up to 10 10 cells/mL of digester
content) followed by methanogens. The entire AD process can be described as a
synergistic process of four sequential phases: hydrolysis, acidogenesis, syntrophic
acetogenesis, and methanogenesis (Fig. 1). Each phase is mediated by a distinct
functional group, or guild, of microbes [36, 91]. During the first phase, some fac-
ultative or strictly anaerobic bacteria (e.g., Clostridium spp.) hydrolyze the biomass
polymers (e.g., polysaccharides, proteins, and lipids) present in the feedstocks, giv-
ing rise to monomers or oligomers (e.g., glucose, cellobiose, amino acids, peptides,
fatty acids, and glycerol). This hydrolysis step is catalyzed by the extracellular
hydrolytic enzymes such as amylases, cellulases, xylanases, proteases, and lipases
secreted by the hydrolytic bacteria. Kinetically, the hydrolysis step can proceed
rapidly for soluble feedstocks such as starch. However, for insoluble lignocellulosic
feedstocks that contain recalcitrant embedded lignin, the hydrolysis phase is rather
slow and often becomes a major rate-limiting step of the entire AD process [2].
The resulting hydrolytic products are immediately fermented to short chain
fatty acids (SCFA), CO 2 , and H 2 during the subsequent fermentative acido-
genesis by another guild of facultative or strictly anaerobic bacteria (e.g.,
Bacteroides, Clostridium, Butyribacterium, Propionibacterium, Pseudomonas, and
Ruminococcus ). The major SCFA formed include acetate, propionate, butyrate,
formate, lactate, isobutyrate, and succinate, with acetate predominating. Small
quantities of alcohols (e.g., ethanol and glycerol) are also produced. The fermen-
tative acidogenesis typically proceeds rather rapidly [10]. In fact, when feedstocks