221
Anaerobic Digestion of Aqueous Waste for Methane and Hydrogen
of the slurry. The stirring can be slow if it is continuous or it can be high if it is
intermittent.
Most wet fermenters are operated at temperature between 38°C and 42°C (mesophilic condition). Since higher temperature gives faster degradation rate requiring
lower HRT and reactor volume, some fermenters do operate under thermophilic conditions. Ultimate methane yield is, however, not influenced by higher temperature.
Under mesophilic conditions, both ammonia toxicity and growth rate of microorganisms are reduced which can lead to the washout problem of microbial population
[5,120]. In general, mesophilic processes are more energy efficient than thermophilic
processes. The energy crops require very high retention time (weeks to months) and
are generally fed at lower solids concentration (2–4 wt%) in wet fermenter [5,121].
8.7.2 dry FermenTATion
Dry solids fermentation is carried out with solids concentration between 15% and
35% [5]. Dry fermentation is operated either batchwise or in a continuous mode. For
dry fermentation of slurry containing >25% solids, a horizontal mechanically mixed
fermenter or a vertical plug flow reactor can be used [5]. These have been used for the
anaerobic treatment of municipal organic solids [5,122,123]. In the vertical fermenter, the substrate flows from top to bottom by gravity only. The substrate fed at the
top is mixed with the digestate coming from the bottom. This recycling and mixing
of digestate with fresh feedstock prevents the accumulation of VFA and allows the
high organic loading rate in the fermenter. The typical vertical fermenter volume in
dry fermentation varies from 1000 to 4000 m 3 [5,122,123].
8.7.3 BATCh FermenTATion
For energy crops, dry fermentation in batch processes is preferred. For these processes, sometimes no mixing is required and solid inoculum up to 70% is necessary
[5,124]. The batch process is operated with gas-tight lids, and operated for several
weeks of digestion period.
8.7.4 TWo-STAge FermenTATion
Horizontal digesters are generally a part of a two-stage system in which high solids
concentration flows in a horizontal plug flow mode with a low rotating horizontal paddle mixer. The reactor volume of such a reactor is limited to 700 m 3 . For
energy crops and processing of high solids concentration slurry, a two-stage digester
system includes a high-loaded main fermenter followed by a low-loaded secondary
fermenter.
The two-stage process generally gives higher biogas production and a lower methane potential of the final digestate [5,121,125]. In the two-stage process, hydrolysis
and methanation take place in both reactors, although it is possible to use the first
bed only for hydrolysis and treat the leachate coming out of the first bed in a second fixed-bed methanation reactor [5,126,127]. For achieving better metabolization
of solid organic compounds, a two-stage reactor system with a separate hydrolysis
Anaerobic Digestion of Aqueous Waste for Methane and Hydrogen
of the slurry. The stirring can be slow if it is continuous or it can be high if it is
intermittent.
Most wet fermenters are operated at temperature between 38°C and 42°C (mesophilic condition). Since higher temperature gives faster degradation rate requiring
lower HRT and reactor volume, some fermenters do operate under thermophilic conditions. Ultimate methane yield is, however, not influenced by higher temperature.
Under mesophilic conditions, both ammonia toxicity and growth rate of microorganisms are reduced which can lead to the washout problem of microbial population
[5,120]. In general, mesophilic processes are more energy efficient than thermophilic
processes. The energy crops require very high retention time (weeks to months) and
are generally fed at lower solids concentration (2–4 wt%) in wet fermenter [5,121].
8.7.2 dry FermenTATion
Dry solids fermentation is carried out with solids concentration between 15% and
35% [5]. Dry fermentation is operated either batchwise or in a continuous mode. For
dry fermentation of slurry containing >25% solids, a horizontal mechanically mixed
fermenter or a vertical plug flow reactor can be used [5]. These have been used for the
anaerobic treatment of municipal organic solids [5,122,123]. In the vertical fermenter, the substrate flows from top to bottom by gravity only. The substrate fed at the
top is mixed with the digestate coming from the bottom. This recycling and mixing
of digestate with fresh feedstock prevents the accumulation of VFA and allows the
high organic loading rate in the fermenter. The typical vertical fermenter volume in
dry fermentation varies from 1000 to 4000 m 3 [5,122,123].
8.7.3 BATCh FermenTATion
For energy crops, dry fermentation in batch processes is preferred. For these processes, sometimes no mixing is required and solid inoculum up to 70% is necessary
[5,124]. The batch process is operated with gas-tight lids, and operated for several
weeks of digestion period.
8.7.4 TWo-STAge FermenTATion
Horizontal digesters are generally a part of a two-stage system in which high solids
concentration flows in a horizontal plug flow mode with a low rotating horizontal paddle mixer. The reactor volume of such a reactor is limited to 700 m 3 . For
energy crops and processing of high solids concentration slurry, a two-stage digester
system includes a high-loaded main fermenter followed by a low-loaded secondary
fermenter.
The two-stage process generally gives higher biogas production and a lower methane potential of the final digestate [5,121,125]. In the two-stage process, hydrolysis
and methanation take place in both reactors, although it is possible to use the first
bed only for hydrolysis and treat the leachate coming out of the first bed in a second fixed-bed methanation reactor [5,126,127]. For achieving better metabolization
of solid organic compounds, a two-stage reactor system with a separate hydrolysis