Age-old phenomena of spontaneous combustion of natural gas, continuously or intermittently, were called “will-o-wisp” or “fool’s fire.” Later, these phe­nomena were assigned to “marsh gas” and mainly methane by H. Tappeiner (1882) [7]. Almost a century passed, through which different postulates had to be verified in order to unveil the mechanism behind this natural methanogenesis or biogas formation. First, one-step microbial degradation of cellulose to methane was proposed. This was replaced by a two-step con­cept, where lower-molecular-weight organic acids are produced as interme­diates, which further undergo conversion to methane. Finally, the three-step concept has been prevailing (the entire process is anoxic):

Hydrolytic Acetogenic stage Methane, organic fermentive stage S (Mesophilic) S (Thermophilic)

Organic matter (35°C, pH 5-6) Acetic acid

Organic matter ^ ——— >

Alcohols, H2, CO2 H2,CO2

Methane _ (45°C, pH 4-6)

CO2 *

An oversimplified mass balance may be written as C6H12O6 ^ 3CH4 + 3CO2

The technical values of yield coefficient, biological efficiency, chemical/ biological oxygen demand (COD/BOD), biological efficiency in productivity/ ecologic efficiency rate (BEP/EER) ratios, and so forth are yet to be estab­lished for each setup or system. Mostly obligate anaerobes and a few fac­ultative microbes contributing to these conversions belong to different genera. A few may be mentioned: Actinomyces, Aerobacter, Aeromonas, Arthrobacter, Bacillus, Bacteroides, Cellulomonas, Citrobacter, Clostridium, Corynebacterium, Enterobacter, Escherichia, Klebsiella, Lactobacillus, Laptospira, Micrococcus, Nocardia, Peptococeus, Proteus, Pseudomonas, Ruminococcus, Sarcina, Staphylococcus, Streptococcus, Streptomyces, and many others. A few methanogenic species are also known: Methano- bacterium bryantii, Methanococcus vanniellii, Methano-genum aggre — gans, Methanomicro-bium mobile, Methanosarcina barkeri, Methano — thrix concillii, usually eukaryotic organisms, and blue-green algae are incapable of performing such bioconversions [8].

Morphologically, the organisms belong to wide groups: coccus, sarcina (flower-like), rod, filamentous, and other shapes. G + C (guanine-cytosine) values of DNAof these organisms also suggest that they all have varied origin and hence are likely to have different metabolic patterns. Khan (1980) found that Acetivibrio cellulolyticus producing acetic acid and hydrogen from cellulose are readily utilized by M. Barkeri to produce methane and carbon dioxide. It has been established beyond doubt that the process is chemolithotrophic metabolism, favored by strict anaero­bic condition, and facilitated by the absence of sulfates, abundance of mois­ture, approximate temperature range of 25-40oC (37°C), and pH 6.2-8.0 (pH 6.8). The organic materials on which these organisms survive and grow are usually cellulose in nature. Crop residues, agricultural residues, animal excreta, municipal sewage, and other organic materials derived from terrestrial and aquatic origin are also considered as good sub­strates. Plant materials with high lignin content are an inferior type of feed for such reactions.

A pretreatment or partial putrefaction or degradation makes the process easy. In this respect, animal excreta appear to be a ready-made substrate. The art of producing gaseous fuel out of cattle excreta is well known in the Indian Subcontinent as the gobargas plant, and will be dis­cussed subsequently.

Sargassum tenerrimum, an abundant variety of marine algae found on the Indian coast of the Arabian Sea, shows promising results in lab­oratory experiments by anaerobic digestion. A mixed culture of marine bacteria and methanogens happens to be a better choice. In a prototype experiment, the partially treated marine algal biomass mixed with cattle dung could be the initial feed for a digester. In a mixed culture, the entire process is a complex one. The organisms which are very effi­cient in cellulolytic activities degrade higher-carbohydrate materials into simpler products as lower organic acids, including CO2 and less fre­quently H2, along with other products, but very seldom show a significant amount of reduction reactions. In absence of methanogens, they usually produce H2, CO2 (even CO), formate, acetate, and less favorably other fatty acids and alcohols. It has been established that many methanogens utilize NH4 as their nitrogen source, either H2S or cysteine for their sulfur requirement, and other growth-stimulating amino acids, vita­mins, and some trace minerals.

Uncommon in many other anaerobic organisms, methanogens have shown presence of a cofactor (coenzyme) named CoM, identified as HSCH2CH2SO3 (2-mercapto-ethanesulfonic acid), and also another low — molecular-weight factor called F420, as of yet unidentified. This F420 in an oxidized state fluoresces at 420 nm but loses all optical activity when reduced. This compound is neither a ferredoxin nor can it be substituted by ferredoxin. Another interesting part is its dependence on Co II (NADP) and it cannot be substituted by Co I (NAD system). Occurrence of oxidative or substrate-level phosphorylation in methanogens could not be established, and the presence of quinines or cytochrome b/c systems could not be observed. The involvement of methylcobalamin also could not be substantiated. So, a large part of the information is yet to be derived by the next-generation scientists. It will be useful to summarize some of the metabolic steps, so far understood (see Fig. 1.10).

The ecologic role of biogas is manifold. Chemical anoxic transforma­tion reduces the BOD value of the organic residues, which in turn are enriched, proportionately in its C, N, P, and mineral ratios. In lignocel — lulosics, after the anoxic process, enrichment of lignin occurs and may lead to peat formation. This may be the origin of coal; natural gas and coal deposits are likely to be found within a reasonable stretch. This is a built-in machinery of nature for BOD and pollution control.