on solid content present. Later an even more effective technology came into being
combining both the modes called co-digestion. Wet anaerobic digestion systems are
used to treat sewage water and industrial effluents which contain low amounts of
solids. In dry digestion, high solid content substrates (25–40%) are treated (Verma
2002). Heat and nutrient transfer is good in wet processes when compared to dry
processes (Luning et al. 2003; Wellinger et al. 1993). In the process of dry digestion,
municipal solid waste (MSW) and energy crop residue digestion are generally done.
These systems could reach higher organic loading rate values resulting in smaller
volumes of digestate and hence are more economical when compared to wet
digestion processes. Co-digestion is the process of transformation of various feedstocks. In contrast to conventional methodology used for anaerobic digestion process, mixtures of substrates are used as feedstock. Of late, this procedure was
adopted by many countries. Mathias (2014) proposed the use of four types of
anaerobic digesters, namely, “continuously stirred tank reactors (CSTR); upflow
anaerobic sludge blanket (UASB) reactors, upflow anaerobic filter (UAF) digesters,
and baffled digesters.” The digester to be used in the process is dependent on the
major type of the substrate which would be treated in the process. Substrates with
more amounts of total solids are treated in continuously stirred tank reactors
(CSTRs). Other types of feedstocks especially dissolved organic solids are treated
in upflow anaerobic sludge blanket (UASB) reactors, anaerobic filters, and fluidized
bed reactors (Mathias 2014). The process takes place in a single step in which the
substrates are digested till we reach a solid dry content between 8% and 15%.
According to Langeveld et al. (2016), the major advantages of co-digestion when
compared to other types of digestion strategies are enhanced biogas yields and lower
emission of greenhouse gases, process stability, homogenization, high nutrient
recycling, and continuous production of biogas in all season.
The feedstocks are treated at very high temperatures for hydrolysis of substrate to
make it more homogeneous. Figure 8.1 shows the conversion of food waste to
biogas and the intermediate steps involved in it. It also removes contaminants
present in the feedstock and to produce a uniform biomass. The refined organic
substances are treated at high temperatures to enhance biogas generation. This
process also helps in the pasteurization of the waste. The process generally involving
treatment at temperatures about 70
C with hydraulic retention time (HRT) of 1 h is
done to pasteurize the waste as required by national and international regulations.
The slurry obtained after pasteurization is cooled. The temperature should be equal
to that of the digester operating temperature. Using a heat recovery system, the
excess heat is recovered. It will be then used to treat the unpasteurized organic waste.
Pathogenic microorganisms are eliminated through the process of thermal treatment.
Thermal treatment of high lignocellulosic contents will result in higher organic
transformation efficiencies especially when the organic waste is heated up to
165–170
C for half an hour. In anaerobic contact process, the limitations are high
space requirement and not suitable for high organic rate loading. Moreover, no phase
separation takes place, and the tank must be always closed to prevent foul smell. In
case of fluidized bed reactor (FBR), difficulties in maintaining optimum mixing and
difficult to start-up conditions are seen. It would also be difficult to scale up the
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 227
combining both the modes called co-digestion. Wet anaerobic digestion systems are
used to treat sewage water and industrial effluents which contain low amounts of
solids. In dry digestion, high solid content substrates (25–40%) are treated (Verma
2002). Heat and nutrient transfer is good in wet processes when compared to dry
processes (Luning et al. 2003; Wellinger et al. 1993). In the process of dry digestion,
municipal solid waste (MSW) and energy crop residue digestion are generally done.
These systems could reach higher organic loading rate values resulting in smaller
volumes of digestate and hence are more economical when compared to wet
digestion processes. Co-digestion is the process of transformation of various feedstocks. In contrast to conventional methodology used for anaerobic digestion process, mixtures of substrates are used as feedstock. Of late, this procedure was
adopted by many countries. Mathias (2014) proposed the use of four types of
anaerobic digesters, namely, “continuously stirred tank reactors (CSTR); upflow
anaerobic sludge blanket (UASB) reactors, upflow anaerobic filter (UAF) digesters,
and baffled digesters.” The digester to be used in the process is dependent on the
major type of the substrate which would be treated in the process. Substrates with
more amounts of total solids are treated in continuously stirred tank reactors
(CSTRs). Other types of feedstocks especially dissolved organic solids are treated
in upflow anaerobic sludge blanket (UASB) reactors, anaerobic filters, and fluidized
bed reactors (Mathias 2014). The process takes place in a single step in which the
substrates are digested till we reach a solid dry content between 8% and 15%.
According to Langeveld et al. (2016), the major advantages of co-digestion when
compared to other types of digestion strategies are enhanced biogas yields and lower
emission of greenhouse gases, process stability, homogenization, high nutrient
recycling, and continuous production of biogas in all season.
The feedstocks are treated at very high temperatures for hydrolysis of substrate to
make it more homogeneous. Figure 8.1 shows the conversion of food waste to
biogas and the intermediate steps involved in it. It also removes contaminants
present in the feedstock and to produce a uniform biomass. The refined organic
substances are treated at high temperatures to enhance biogas generation. This
process also helps in the pasteurization of the waste. The process generally involving
treatment at temperatures about 70
C with hydraulic retention time (HRT) of 1 h is
done to pasteurize the waste as required by national and international regulations.
The slurry obtained after pasteurization is cooled. The temperature should be equal
to that of the digester operating temperature. Using a heat recovery system, the
excess heat is recovered. It will be then used to treat the unpasteurized organic waste.
Pathogenic microorganisms are eliminated through the process of thermal treatment.
Thermal treatment of high lignocellulosic contents will result in higher organic
transformation efficiencies especially when the organic waste is heated up to
165–170
C for half an hour. In anaerobic contact process, the limitations are high
space requirement and not suitable for high organic rate loading. Moreover, no phase
separation takes place, and the tank must be always closed to prevent foul smell. In
case of fluidized bed reactor (FBR), difficulties in maintaining optimum mixing and
difficult to start-up conditions are seen. It would also be difficult to scale up the
8 Bioprocess Parameters for Thermophilic and Mesophilic Biogas Production: Recent. . . 227