8.2 Thermophilic and Mesophilic Anaerobic Digestion
Thermophilic anaerobic digestion will take place at temperatures above 50
C. The
biggest advantage of thermophilic digestion is the decrease in retention time which
could be as low as 10 days when compared to mesophilic reactors where the
retention time is about 20 days. The advantages are that mixing energy requirements
are less and overall heat loss per unit volume of material processed also is less apart
from pathogen reduction. In the third process, hydrolysis stage is the rate-limiting
step. This is overcome in the thermophilic digester which operates at high temperature range so that hydrolysis takes place efficiently. Thermophilic anaerobic digestion generated higher amounts of biogas production. The disadvantage is that there
tends to be accumulation of volatile fatty acids which decrease the biogas yield. The
thermophilic anaerobic digestion process is also instable. Other limitations are that
the water quality gets worse, fluctuation in temperatures, and sensitivity to toxic
heavy metals (Khemkhao et al. 2012). The process is energy intensive as more
energy is required for raising the initial temperature. The anaerobic process which
operates at mesophilic temperature range (35–38 degree centigrade) is called
mesophilic digestion. This temperature range can produce class A biosolids. Thermophilic digesters need lesser time to process feedstocks but are difficult to operate
and are expensive. Kushkevych et al. (2020) have investigated the diversity of
various thermophiles which are occurring in mesophilic biogas plants located in
Czech Republic. They found 19 thermophilic genera using 16S rRNA gene sequencing. Most of the thermophilic population was found in substrate containing primary
sludge and biological sludge, and less were found in maize silage and liquid pig
manure. Bolzonella et al. (2020) have treated agrowaste using a thermophilic posthydrolysis process in a digester operated for 3 days to increase the production of
biogas by 30%. Dai et al. (2020) have proposed a thermophilic mixed culture
fermentation (TMCF) for enhancing the production of methane and hydrogen with
a high substrate degradation rate and low gas solubility. Lei et al. (2020) have
investigated thermophilic anaerobic digestion (TAD) of Arundo donax, an energy
crop with high cold tolerance to understand the relation among microbial population
and their functions during the process of fermentation. They have observed
Firmicutes with three dominant genera of Tepidiphilus, Sedimentibacter, and Gelria
during the thermophilic anaerobic digestion process apart from Methanoculleus and
Methanosarcina. Wu et al. (2020) compared the process of anaerobic digestion of
municipal sludge with high (10%) solid content under both mesophilic and thermophilic conditions. Thermophilic digestion was better than mesophilic anaerobic
digestion for biogas production. Mesophilic anaerobic digestion showed more
microbial diversity than thermophilic anaerobic digestion.
Ryue et al. (2020) reviewed the usual and promising methods for improving
process stability in thermophilic anaerobic digestion. Zhang et al. (2020) used a
mixing strategy for treating food waste and chicken manure under thermophilic
conditions using a mesophilic innoculum. They observed that methane yield in the
continuous stirred reactor was 71.3% more when compared to intermittent agitated
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 229
Thermophilic anaerobic digestion will take place at temperatures above 50
C. The
biggest advantage of thermophilic digestion is the decrease in retention time which
could be as low as 10 days when compared to mesophilic reactors where the
retention time is about 20 days. The advantages are that mixing energy requirements
are less and overall heat loss per unit volume of material processed also is less apart
from pathogen reduction. In the third process, hydrolysis stage is the rate-limiting
step. This is overcome in the thermophilic digester which operates at high temperature range so that hydrolysis takes place efficiently. Thermophilic anaerobic digestion generated higher amounts of biogas production. The disadvantage is that there
tends to be accumulation of volatile fatty acids which decrease the biogas yield. The
thermophilic anaerobic digestion process is also instable. Other limitations are that
the water quality gets worse, fluctuation in temperatures, and sensitivity to toxic
heavy metals (Khemkhao et al. 2012). The process is energy intensive as more
energy is required for raising the initial temperature. The anaerobic process which
operates at mesophilic temperature range (35–38 degree centigrade) is called
mesophilic digestion. This temperature range can produce class A biosolids. Thermophilic digesters need lesser time to process feedstocks but are difficult to operate
and are expensive. Kushkevych et al. (2020) have investigated the diversity of
various thermophiles which are occurring in mesophilic biogas plants located in
Czech Republic. They found 19 thermophilic genera using 16S rRNA gene sequencing. Most of the thermophilic population was found in substrate containing primary
sludge and biological sludge, and less were found in maize silage and liquid pig
manure. Bolzonella et al. (2020) have treated agrowaste using a thermophilic posthydrolysis process in a digester operated for 3 days to increase the production of
biogas by 30%. Dai et al. (2020) have proposed a thermophilic mixed culture
fermentation (TMCF) for enhancing the production of methane and hydrogen with
a high substrate degradation rate and low gas solubility. Lei et al. (2020) have
investigated thermophilic anaerobic digestion (TAD) of Arundo donax, an energy
crop with high cold tolerance to understand the relation among microbial population
and their functions during the process of fermentation. They have observed
Firmicutes with three dominant genera of Tepidiphilus, Sedimentibacter, and Gelria
during the thermophilic anaerobic digestion process apart from Methanoculleus and
Methanosarcina. Wu et al. (2020) compared the process of anaerobic digestion of
municipal sludge with high (10%) solid content under both mesophilic and thermophilic conditions. Thermophilic digestion was better than mesophilic anaerobic
digestion for biogas production. Mesophilic anaerobic digestion showed more
microbial diversity than thermophilic anaerobic digestion.
Ryue et al. (2020) reviewed the usual and promising methods for improving
process stability in thermophilic anaerobic digestion. Zhang et al. (2020) used a
mixing strategy for treating food waste and chicken manure under thermophilic
conditions using a mesophilic innoculum. They observed that methane yield in the
continuous stirred reactor was 71.3% more when compared to intermittent agitated
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 229