Page 385 - Refining Biomass Residues for Sustainable Energy and Bioproducts

348 Refining Biomass Residues for Sustainable Energy and Bioproducts

AD is a biochemical process that is performed in oxygen-free environment with the help
of different microorganisms that degrade the organic components into biogas. Biogas is a
mixture of carbon dioxide and methane produced from biodegradable material by the
enzymatic action of microorganisms under anaerobic condition and also traces of other
gases, such as H 2 S(Bala et al., 2019). Quality of biogas varies from digester to digester
according to the type of agricultural waste. There are four steps in AD process: hydroly-
sis, acidogenesis, acetogenesis, and methanogenesis. Each step is carried out by a group
of different microorganisms. In the first step, Hydrolytic bacteria (Streptococcus, Bacillus,
Enterobacteria, etc.) convert the complex polysaccharides, protein, and fat into sugar,
amino acids, and fatty acid with their enzymatic action. The enzymes involved are cellu-
lase, amylase, protease, lipase, etc. (Divya et al., 2015). Further, the degradation of hydro-
lysis end products is carried out by facultative and obligatory anaerobes (Micrococcus,
Syntrophomonas, Pseudomonas, etc.) into fatty acids and organic acids by secreting
enzymes, such as acetate kinase, formate hydrogen lyase, and acetaldehyde dehydrogenase
(Divya et al., 2015). The organic acids formed in acedogenesis are converted into acetic
acid by acetogenic bacteria (Syntrophomonas, Clostridium, Syntrophobacter) in acetogen-
esis phase by secreting hydrogenases. Methanogenesis is carried out strictly by anaerobes.
The methanogenic bacteria (Methanosarcina, Methanococcus, Methanobacteria, etc.) con-
vert the acetic acid into CH 4 ,CO 2 , traces of H 2 S, N 2 ,H 2 , siloxanes, etc. known as biogas
by secreting the enzymes formylmethanofuran dehydrogenase, methyl coenzyme
m-methyl transferase, etc. (Divya et al., 2015). The AD is a widely used technology for
wastes with high moisture content (80% 90%). Different types of substrates, such as ani-
mal manure, fruit and vegetable waste, municipal solid waste, agricultural residues, micro-
algae, and industrial waste water, can be used as substrate for biogas production. AD
requires an adequate amount of carbon and nitrogen content to balance the C/N ratio for
process stability. The optimum ranges of C/N ratio, pH, and temperature are 20 30,
5.5 8.5, and 30 40 (mesophilic), 50 55 (thermophilic), respectively (Zhang et al.,
2016). Biogas yield and composition vary according to the substrate and inoculum used.
The main constituents of biogas are CH 4 (50% 70%) and CO 2 (30% 50%) with other
trace gases (Bala et al., 2019). The methane yield achieved from AD of sugarcane bagasse
was 299.3 mL/g volatile solids (VS) using 10% ammonia pretreatment (Hashemi et al.,
2019), while the biogas production from wheat straw was 615 N mL/g VS after pretreat-
ment at 180 C(Rajput et al., 2018). Biogas produced can be used in spark engines or in

turbines. Further, it can also be upgraded to natural gas with the removal of CO 2 . The
left-over solid, that is, digestate, can be used as soil conditioner as it has important nutri-
ents content.

Agricultural waste 1 microorganism ! Biogas 1 digestate

Fermentation is another biological process that operates in the absence of oxygen and
involves the help of microorganisms to convert sugar to basically alcohol, acid, or mixture
of gases (CO and H 2 S). The biomass is converted to smaller particles and then starch is
converted to sugar and then to alcohol with the help of enzymes. A variety of substrates,
such as starch biomass, lignocellulosic biomass, and algal biomass, is converted into
fermentable sugar. However, fermentation of agricultural wastes is difficult as it has com-
plex long-chain polymeric molecules and requires acid or enzymatic hydrolysis before the
sugar is fermented to alcohol. Fermentation is lengthy time-consuming process, with
chances of contamination of other foreign microorganisms. It is a promising approach but
high cost is preventing its movement from laboratory to commercial world. Generally,