to failure of biomethanation process. Clostridium and Megasphaera species have
been reported to convert lactic acid to propionic acid (Prabhu et al. 2012; Tracy et al.
2012). Biogas has lower emission rates compared to that of any other fossil fuel,
subsequently leading to less environmental pollution (Vijay et al. 2006). The need
for international sustainable waste management has resulted in renewed research
interest in agro-waste and biowaste-based biofuels (Weiland et al. 2009; Deublein
and Steinhauser 2008). Boe et al. (2012) reported that the feedstock composition
with excessive lipid or protein content shows high correlation with foam formation
during anaerobic digestion. Other parameters, like temperature, digester design, and
form of the mixing, are responsible for foam formation (Barber 2005). Foaming may
cause blockage of mixing systems due to the presence of solids in the foam (Ganidi
et al. 2009). Excess financial costs are incurred due to foaming (Barjenbruch et al.
2000). In anaerobic digestion method, four processes are involved (Bharathiraja
et al. 2014), namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In
the hydrolysis step, carbohydrates, proteins, and lipids are hydrolyzed to single chain
monomers and dimers like sugars, amino acids, and fatty acids. In step
2 (acidogenesis), the monomers and dimers from hydrolysis are turned into
propionic acid, butyric acids, and valeric acids. In the case of step 3 (acetogenesis),
acetic acid, hydrogen, and carbon dioxide are formed. In the last stage
(methanogenesis), acetate is converted into methane and CO 2 ; whole hydrogen is
used up. Methanogenic microorganisms are sensitive to oxygen and are less versatile
when it comes to substrate utilization. Methane is generated through acetoclastic
methanogenesis using acetate. Hydrogen produced will be the remaining 1/3 of the
total biogas produced. Belay et al. (1986) and Lovely and Klug (1983) have
observed methane production from substrates such as formate methanol and methylamines. Wolfe (2011) reported that methanogens need a higher pH at later stages of
the process compared to initial stages. Richards et al. (2016) reported that
Methanococcus maripaludis has a doubling time of just 2 h. Research by De
Vrieze et al. (2012) found that Methanosarcina spp. is a more robust methanogen
when compared to other methanogenic populations which are involved in
methanogenesis. They have reported that it is capable of variations in pH and also
concentrations of acetate, ammonia, and sodium. Dhamodharan et al. (2015) and Li
et al. (2015) have developed many kinetic models to describe the processes involved
in anaerobic digestion.
Anaerobic digestion takes place in three stages, that is, hydrolysis, acidification,
and methane formation. The acidogens produce hydrolytic enzymes and transform
soluble organics to volatile fatty acids and alcohols. Breakdown of carbohydrates,
proteins, and lipids into sugars, amino acids, and fatty acids takes place in hydrolysis. This is carried out by specific enzymes of hydrolytic bacteria. In the hydrolysis
stage, these microorganisms were observed, namely, Peptococcus, Ruminococcus,
Eubacterium, Bacillus, Butyrivibrio, Proteus vulgaris, Micrococcus, Staphylococcus, Acetovibrio, Clostridium, Lactobacillus, Streptococcus, etc. The monomers
released during hydrolysis are converted by fermentative bacteria into carbon dioxide, pyruvate, hydrogen or formate, ammonia, volatile fatty acids, lactic acid, and
alcohols. In acetogenesis, some compounds generated during acidogenesis are
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 231
been reported to convert lactic acid to propionic acid (Prabhu et al. 2012; Tracy et al.
2012). Biogas has lower emission rates compared to that of any other fossil fuel,
subsequently leading to less environmental pollution (Vijay et al. 2006). The need
for international sustainable waste management has resulted in renewed research
interest in agro-waste and biowaste-based biofuels (Weiland et al. 2009; Deublein
and Steinhauser 2008). Boe et al. (2012) reported that the feedstock composition
with excessive lipid or protein content shows high correlation with foam formation
during anaerobic digestion. Other parameters, like temperature, digester design, and
form of the mixing, are responsible for foam formation (Barber 2005). Foaming may
cause blockage of mixing systems due to the presence of solids in the foam (Ganidi
et al. 2009). Excess financial costs are incurred due to foaming (Barjenbruch et al.
2000). In anaerobic digestion method, four processes are involved (Bharathiraja
et al. 2014), namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In
the hydrolysis step, carbohydrates, proteins, and lipids are hydrolyzed to single chain
monomers and dimers like sugars, amino acids, and fatty acids. In step
2 (acidogenesis), the monomers and dimers from hydrolysis are turned into
propionic acid, butyric acids, and valeric acids. In the case of step 3 (acetogenesis),
acetic acid, hydrogen, and carbon dioxide are formed. In the last stage
(methanogenesis), acetate is converted into methane and CO 2 ; whole hydrogen is
used up. Methanogenic microorganisms are sensitive to oxygen and are less versatile
when it comes to substrate utilization. Methane is generated through acetoclastic
methanogenesis using acetate. Hydrogen produced will be the remaining 1/3 of the
total biogas produced. Belay et al. (1986) and Lovely and Klug (1983) have
observed methane production from substrates such as formate methanol and methylamines. Wolfe (2011) reported that methanogens need a higher pH at later stages of
the process compared to initial stages. Richards et al. (2016) reported that
Methanococcus maripaludis has a doubling time of just 2 h. Research by De
Vrieze et al. (2012) found that Methanosarcina spp. is a more robust methanogen
when compared to other methanogenic populations which are involved in
methanogenesis. They have reported that it is capable of variations in pH and also
concentrations of acetate, ammonia, and sodium. Dhamodharan et al. (2015) and Li
et al. (2015) have developed many kinetic models to describe the processes involved
in anaerobic digestion.
Anaerobic digestion takes place in three stages, that is, hydrolysis, acidification,
and methane formation. The acidogens produce hydrolytic enzymes and transform
soluble organics to volatile fatty acids and alcohols. Breakdown of carbohydrates,
proteins, and lipids into sugars, amino acids, and fatty acids takes place in hydrolysis. This is carried out by specific enzymes of hydrolytic bacteria. In the hydrolysis
stage, these microorganisms were observed, namely, Peptococcus, Ruminococcus,
Eubacterium, Bacillus, Butyrivibrio, Proteus vulgaris, Micrococcus, Staphylococcus, Acetovibrio, Clostridium, Lactobacillus, Streptococcus, etc. The monomers
released during hydrolysis are converted by fermentative bacteria into carbon dioxide, pyruvate, hydrogen or formate, ammonia, volatile fatty acids, lactic acid, and
alcohols. In acetogenesis, some compounds generated during acidogenesis are
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 231