reactor. Hirota (2020) investigated production of methane in wet and semi-dry
anaerobic digesters. Maximum levels of methane gas were seen for 30 days in
both thermophilic conditions. In anaerobic digestion, new studies of using hypermesophilic temperatures were reported by Moestedt et al. (2014). The range of
organic loading rate is 3–5 kg VS/m
3 /d. Hyper-mesophilic temperatures between
40 and 44
C have been explored for different kinds of substrates (Westerholm et al.
2015). van Lier et al. (1993) and Lindorfer et al. (2008) have earlier observed process
instability when hyper-mesophilic conditions were used for anaerobic digestion
process for mesophilic microorganism. However, Moestedt (2015) has reported
higher biogas yield in digestion of food and slaughterhouse waste in Linköping
biogas plant. Biogas produced during the process of anaerobic digestion are made up
of material such as PVC-coated fiber fabric, etc.. Labtut et al. (2014) have done a
comparative study between mesophilic and thermophilic processes and concluded
that a mesophilic digester was stable regardless of the organic and influent composition, while thermophilic digester performed better at high organic loading rates.
They have also observed that the stability of thermophilic digester was dependent on
influent composition when compared to mesophilic digester. Performing anaerobic
co-digestion of food waste with lignocellulosic wastes can overcome the limitations
of their respective mono-digestions. Mahdy et al. (2020) evaluated the influence of
hyper-thermophilic pre-hydrolysis stage on methane recovery using sewage sludge
and microbial populations present in them. Bacteroidetes and Cloacimonetes
populations were more, while there was reduction in the population of Firmicutes.
Prem et al. (2020) studied the microbial community dynamics when proteinaceous
wastes were treated in mesophilic and thermophilic batch reactors. They have
observed that in mesophilic samples, acetoclastic methanogenesis took place
where phenylacetate (PAA) levels favored the growth of Psychrobacter spp.,
while phenylpropionate (PPA) favored the growth of Haloimpatiens spp. Lopez
et al. (2020) assessed the microbial quality of sewage sludge which was treated in
three different plants: two anaerobic and one aerobic plant. Out of the three, one was
anaerobic mesophilic, one was anaerobic thermophilic, and the last plant was aerobic
thermophilic. They have observed that anaerobic thermophilic treatment could
decrease the concentration of the Enterococcus sp., while aerobic thermophilic
could decrease the concentrations of E. coli.
8.3 Mechanism of Biogas Production
The groups of microbes involved in anaerobic digestion are poorly understood.
Angelidaki et al. (2011) have reported that the bacterial communities involved in
anaerobic digestion can be divided into fermenting bacteria, anaerobic bacteria, and
methanogens. The oxidizing microorganisms oxidize these reduced substances to
hydrogen, formate, acetate, and carbon dioxide (Angelidaki et al. 2011). Propionate
accumulation is seen in cases of process imbalance (Angelidaki et al. 2006). Wang
et al. (2012) have reported that ratio of 1.25 between propionate and acetate may lead
230
R. Kumar et al.
anaerobic digesters. Maximum levels of methane gas were seen for 30 days in
both thermophilic conditions. In anaerobic digestion, new studies of using hypermesophilic temperatures were reported by Moestedt et al. (2014). The range of
organic loading rate is 3–5 kg VS/m
3 /d. Hyper-mesophilic temperatures between
40 and 44
C have been explored for different kinds of substrates (Westerholm et al.
2015). van Lier et al. (1993) and Lindorfer et al. (2008) have earlier observed process
instability when hyper-mesophilic conditions were used for anaerobic digestion
process for mesophilic microorganism. However, Moestedt (2015) has reported
higher biogas yield in digestion of food and slaughterhouse waste in Linköping
biogas plant. Biogas produced during the process of anaerobic digestion are made up
of material such as PVC-coated fiber fabric, etc.. Labtut et al. (2014) have done a
comparative study between mesophilic and thermophilic processes and concluded
that a mesophilic digester was stable regardless of the organic and influent composition, while thermophilic digester performed better at high organic loading rates.
They have also observed that the stability of thermophilic digester was dependent on
influent composition when compared to mesophilic digester. Performing anaerobic
co-digestion of food waste with lignocellulosic wastes can overcome the limitations
of their respective mono-digestions. Mahdy et al. (2020) evaluated the influence of
hyper-thermophilic pre-hydrolysis stage on methane recovery using sewage sludge
and microbial populations present in them. Bacteroidetes and Cloacimonetes
populations were more, while there was reduction in the population of Firmicutes.
Prem et al. (2020) studied the microbial community dynamics when proteinaceous
wastes were treated in mesophilic and thermophilic batch reactors. They have
observed that in mesophilic samples, acetoclastic methanogenesis took place
where phenylacetate (PAA) levels favored the growth of Psychrobacter spp.,
while phenylpropionate (PPA) favored the growth of Haloimpatiens spp. Lopez
et al. (2020) assessed the microbial quality of sewage sludge which was treated in
three different plants: two anaerobic and one aerobic plant. Out of the three, one was
anaerobic mesophilic, one was anaerobic thermophilic, and the last plant was aerobic
thermophilic. They have observed that anaerobic thermophilic treatment could
decrease the concentration of the Enterococcus sp., while aerobic thermophilic
could decrease the concentrations of E. coli.
8.3 Mechanism of Biogas Production
The groups of microbes involved in anaerobic digestion are poorly understood.
Angelidaki et al. (2011) have reported that the bacterial communities involved in
anaerobic digestion can be divided into fermenting bacteria, anaerobic bacteria, and
methanogens. The oxidizing microorganisms oxidize these reduced substances to
hydrogen, formate, acetate, and carbon dioxide (Angelidaki et al. 2011). Propionate
accumulation is seen in cases of process imbalance (Angelidaki et al. 2006). Wang
et al. (2012) have reported that ratio of 1.25 between propionate and acetate may lead
230
R. Kumar et al.