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could be achieved by enhancing the intensity of biomass pretreatment (Sun et al.,
2019). The process of acidification comprises two different phases—the acidogenic
and the acetogenic.
In the acidogenic phase, acidogens classes of microbes convert the hydrolyzed
simpler molecules, such as amino acids, sugars, peptides, and others into volatile
fatty acids. This phase results in the production of alcohols and organic acids
(Gerardi, 2003).
The acetogenic microbial populations reduce hydrogen and carbon dioxide to
acetic acid. The organic acids and alcohols produced in the previous step are also
converted to acetate in this phase, which again acts as a substrate for the methane-
producing microorganisms in the methanogenic phase.

13.7.3.2 Methanogenesis

The methanogenic group of microorganisms generates methane by a process
wherein carbon dioxide is used as a terminal electron acceptor (Cheremisinoff,
1997). This category of organisms includes bacteria such as Methanosarcina bar-
keri, Methanothrix soehngenii, and Methanococcus mazei (Weiland, 2010).
Methane is produced in this phase via anaerobic digestion that occurs through two
different ways—acetoclastic methanogenesis and hydrogenic methanogenesis. The
acetoclastic methanogenesis converts acetic acid to methane, while carbon dioxide
is converted to methane in association with hydrogen by the hydrogenic methano-
genesis (Chen et al., 2008).

13.8 Socioeconomic aspects of biofuel generation from
postharvest rice crop residues

The rice crop productivity and generation of rice straw as biofuel feedstock are
mostly confined to kharif crop (June July to November December) and rabi crop
(January February to April May) (MoA, 2012). After productivity and harvest of
rice grains, the quick disposal of rice straw is essential for making the land avail-
able for the next agricultural production. The storage of rice straw demands a place
to be protected from contact with humid and moist conditions due to its biodegrad-
able nature. Rice straw is produced in bulk amount in all the rice-growing countries
of the world. Storage of bulk amount of rice straw for a long span is difficult on the
part of the marginal farmers.
The agricultural residues such as postharvest rice crop residues are renewable
hydrocarbon sources, produced in million tonnes per annum (Claassen et al., 1999).
Rice straw has the potential to be treated as a biofuel feedstock for its hydrocarbon
constituents having many glycosidic bonds. Rice straw gains its strength as a bio-
fuel feedstock, because of its cellulose, hemicellulose and lignin contents. Burning
or biodegradation of rice straw in the open agricultural fields leads to the emission
of hydrocarbons, suspended particulate matters, greenhouse gases such as oxides of