Anaerobic digestion involves the microbial fermentation of cellulosic/lignocellu — losic biomass in the absence of oxygen for about 2-8 weeks. A similar process involving municipal solid waste, which may, in addition to cellulosic components, contain polymeric substances, fats, proteins, etc. is termed as anaerobic digestion. This is the most commonly used, commercially viable process under the bio­chemical methods of biomass conversion. The product of anaerobic fermentation comprises 65-70% methane, 30-35% carbon dioxide, and traces of other gases such as H2S and hydrogen. This product has a heating value of 26 MJ/m3 [23].

Lignocellulose is very refractory in nature and requires harsh pretreatment procedures before it can be used for fermentation. The pretreatment procedures usually used are described in ‘‘Forest Biorefinery’’. Following the pretreatment procedure, lignocellulose breaks down to cellulose, hemicellulose, and lignin. The cellulose and hemicellulose subsequently undergo anaerobic, or alcoholic fer­mentation to give biogas or ethanol. Alcoholic fermentation is discussed in the following section.

Anaerobic fermentation/digestion is also called biomethanation as methane is the major end product of the process. It can be carried out according to a two-stage scheme or a four-stage scheme. The two-stage scheme originated in the 1930s and involves two major metabolic groups of bacteria- the acid forming and the methane forming bacteria. The acid forming bacteria are a complex species of bacteria which hydrolyze the primary substrate polymers such as polysaccharides, proteins, and lipids, and ferment these to mainly fatty acids and other organic acids, alcohols, ammonia, sulfide, carbon dioxide, and methane. The methane forming bacteria involve a group of bacteria which degrade the products of the first stage to methane and carbon dioxide.

The current processes used (Fig. 1.19), involve four stages which are executed by four different groups of organisms, and end up with different end products [24].

The first phase involves facultative, or strictly anaerobic bacteria such as Streptococcus, Peptococcus, Micrococcus, and Clostridium (a thermophilic spe­cies). These convert the polymers/polysaccharides into the biomass to small monomers or oligomers such as glucose, cellobiose, amino acids, short chain fatty acids, and glycerol. Carbon dioxide and hydrogen are released in the process. In

other words, in this phase, the biomass is hydrolyzed to give smaller fragments, which are then processed further by other groups of microbes. This process is catalyzed by extracellular hydrolases such as cellulase, xylanase, protease, and lipase. This stage is relatively slow and may often be the rate limiting stage in the process.

The second phase is the acidogenesis phase which again involves facultative and strictly anaerobic microbes such as Bacteriodes, Clostridium, Butyribacteri — um, Propionibacterium, Pseudomonas, and Ruminococcus. Some of these are hydrolytic and others, non-hydrolytic. The products of this phase are short chain fatty acids such as formate, acetate, butyrate, isobutyrate, and succinate. Among these, acetate is the major short chain fatty acid. Small quantities of alcohols (methanol, ethanol, glycerol, and butanol) and acetone may also be produced depending on the nature of the initial feedstock and the anaerobic digestion

process used. This stage of fermentative acidogenesis is rapid and often leads to accumulation of the short chain fatty acids. If this happens, it usually leads to reduced methane formation, followed by methanation failure in the subsequent methanation stage. This usually happens when the feedstock contains large amounts of readily fermentable carbohydrates, and when the loading rate is high.

The third phase involves strictly anaerobic bacteria, which are syntrophic acetogens—Syntrophomonas wolfei and Syntrophobacteri wolinii. They are called syntrophs because they are in close proximity to methanogens, the group of microbes that are responsible for methanogenesis, which is the fourth phase of anaerobic digestion (the syntrophy occurs through interspecies hydrogen transfer). The syntrophic acetogens convert substances such as ethanol, and esters such as propionate, butyrate, valerate, and other short chain fatty acids containing three or more than three carbons to acetate, hydrogen, and carbon dioxide. The hydrogen produced is rapidly consumed by the methanogens, which reduces the partial pressure of hydrogen and takes the processes forward toward generation of methane in the next phase. Syntrophic acetogens grow very slowly (require more than one week to grow to sufficient numbers), making this stage another possible rate limiting step (in addition to the first hydrolytic stage).

In case of inefficiency of this acetogenesis phase, accumulation of the non­acetic acid small chain fatty acids, i. e., the products of the second phase occurs, which reduces the pH of the fermentation reactor. This inhibits the methanogens, thus reducing the formation of methane, which eventually ceases completely.

The fourth phase involves a group of microbes called the methanogens that are completely different from bacteria and belong to a class called Archaea. These are strict anaerobes, which require a very narrow range of environmental conditions of pH and temperature and a reduction potential of <300 mV. These are present naturally in anaerobic environments such as swamps and wetlands and grow slowly. Hence, this is a third possible rate limiting step in the process of anaerobic digestion. These microbes use acetate, hydrogen, carbon dioxide (the products of the third phase), in addition to methanol, formate, methylamines, and methyl sulfides as their substrate. Depending on the substrate specificity and methano — genesis pathway, these microorganisms are classified into two categories—the Hydrogenotrophic methanogens and the Acetoclastic or acetotrophic methano — gens. The Hydrogenotrophic methanogens use hydrogen produced by both the acidogens, as well as the syntrophic acetogens to reduce carbon dioxide to methane or to convert methanol, methylamines and methyl sulfides to methane. A wide diversity of species is found in this category. Acetoclastic or acetotrophic methanogens convert acetate to methane. As acetate is the major product of the preceding stage of anaerobic digestion, two-thirds of the methane produced is by this class of microorganisms. Only two genera in this category have been identi — fied—Methanosaeta (formerly called methanothrix) and Methanosarcina. The latter is both acetotrophic as well as hydrogenotrophic. All these processes are endothermic processes. The yield of the anaerobic digestion processes depend on the nature of feedstock used. Yu Zhongtang et al. [24] have listed the Biochemical methane potential (BMP) of a variety of feedstock such as livestock manure, food­processing wastes, municipal solid wastes, crop residues, and energy crops. From among the livestock manure evaluated, poultry manure had the highest BMP of 460 m3/dry ton, whereas beef and dairy cattle manure had a BMP of 148-250 m3/dry ton. Among the food-producing wastes, fresh fruit and vegetable waste had a BMP of 228-495 m3/dry ton and municipal solid (organic fraction) waste had a BMP of 300-550 m3/dry ton. Agricultural residue such as corn stover and wheat straw had a BMP of 250 and 161-241 m3/dry ton, respectively. Energy crops such as sugar beet and grass silage had a BMP of 380 and 390 m3/dry ton respectively. The technologies currently used for anaerobic fermentation/ digestion, along with their salient features are shown in Table 1.7.