Anaerobic Digestion of Aqueous Waste for Methane and Hydrogen
219
increased hygienic requirement and restriction on the available land use. Finally, its
economics is very much dependent on the crop cost and yield.
Recent research results by Wu et al. [82] and numerous others [53,54,73,78,79,80,
83–85,88,95–108] demonstrate that using co-substrates in the anaerobic digestive
systems improves the biogas yields through positive synergisms established in the
digestion medium and the supply of missing nutrients by the co-substances. This subject is under an extensive investigation in the anaerobic digestion industry.
Historically, anaerobic digestion was carried out for animal manure and sewage
sludge from aerobic wastewater treatment. In the recent years, agricultural biogas
plants use pig, cow, and chicken manure with co-substrates, which increase the
organic content of the total substrate. The co-wastes can be organic wastes from
the agriculture-related industries, food waste, collected municipal biowaste from
households, energy crops, tops and leaves of sugar beets, and so on. Fats provide the
largest biogas yield but require high retention time. Carbohydrates and proteins have
faster conversion rates but lower yields. If pathogens or other organisms are present, pasteurization at 70°C and sterilization at 130°C of feed materials are needed
prior to fermentation. The carbon/nitrogen (C/N) ratio should be between 15 and
30 to avoid the process failure by ammonia accumulation. The fermentation residue
should be used as an fertilizer.
8.6 eFFeCts OF harVestinG, stOraGe, and Pretreatment
8.6.1 eFFeCT oF hArveSTing
The specific methane yield obtained from this material depends on its age
[5,109,110,111]. Harvesting time and its frequency are important for biogas yield.
Crops can be grown as preceding crop, main crop, or succeeding crop, each leading to a different biogas yield [5]. Weiland [5] pointed out that maize crops harvested after 97 days of milk ripeness produced 37% more methane yield than those
at full ripeness.
8.6.2 STorAge
Easy storage is an important factor in the selection of energy crops. The storage of
energy crops by ensiling converts soluble carbohydrates into lactic acid, acetate, propionate, and butyrate, which inhibit the growth of detrimental microorganisms by a
strong drop in pH between 3 and 4 [112]. The starter cultures, enzymes, and easily
degradable carbohydrates can control and accelerate the acid formation. The optimum ensiling conditions are obtained by cutting particle length between 10–20 mm,
and maintaining the total solid contents between 25% and 35%. Often, the storage by
ensiling can be considered as a pretreatment process [5,113].
The structural polysaccharides of plant material are partly degraded during storage. They lose about 8%–20% of energy due to aerobic degradation, which is largely
caused by oxygen, pH, and growth of yeasts that are responsible for heat upon exposure to oxygen. During storage, a plastic wrap should cover the plant material to
minimize degradation.
219
increased hygienic requirement and restriction on the available land use. Finally, its
economics is very much dependent on the crop cost and yield.
Recent research results by Wu et al. [82] and numerous others [53,54,73,78,79,80,
83–85,88,95–108] demonstrate that using co-substrates in the anaerobic digestive
systems improves the biogas yields through positive synergisms established in the
digestion medium and the supply of missing nutrients by the co-substances. This subject is under an extensive investigation in the anaerobic digestion industry.
Historically, anaerobic digestion was carried out for animal manure and sewage
sludge from aerobic wastewater treatment. In the recent years, agricultural biogas
plants use pig, cow, and chicken manure with co-substrates, which increase the
organic content of the total substrate. The co-wastes can be organic wastes from
the agriculture-related industries, food waste, collected municipal biowaste from
households, energy crops, tops and leaves of sugar beets, and so on. Fats provide the
largest biogas yield but require high retention time. Carbohydrates and proteins have
faster conversion rates but lower yields. If pathogens or other organisms are present, pasteurization at 70°C and sterilization at 130°C of feed materials are needed
prior to fermentation. The carbon/nitrogen (C/N) ratio should be between 15 and
30 to avoid the process failure by ammonia accumulation. The fermentation residue
should be used as an fertilizer.
8.6 eFFeCts OF harVestinG, stOraGe, and Pretreatment
8.6.1 eFFeCT oF hArveSTing
The specific methane yield obtained from this material depends on its age
[5,109,110,111]. Harvesting time and its frequency are important for biogas yield.
Crops can be grown as preceding crop, main crop, or succeeding crop, each leading to a different biogas yield [5]. Weiland [5] pointed out that maize crops harvested after 97 days of milk ripeness produced 37% more methane yield than those
at full ripeness.
8.6.2 STorAge
Easy storage is an important factor in the selection of energy crops. The storage of
energy crops by ensiling converts soluble carbohydrates into lactic acid, acetate, propionate, and butyrate, which inhibit the growth of detrimental microorganisms by a
strong drop in pH between 3 and 4 [112]. The starter cultures, enzymes, and easily
degradable carbohydrates can control and accelerate the acid formation. The optimum ensiling conditions are obtained by cutting particle length between 10–20 mm,
and maintaining the total solid contents between 25% and 35%. Often, the storage by
ensiling can be considered as a pretreatment process [5,113].
The structural polysaccharides of plant material are partly degraded during storage. They lose about 8%–20% of energy due to aerobic degradation, which is largely
caused by oxygen, pH, and growth of yeasts that are responsible for heat upon exposure to oxygen. During storage, a plastic wrap should cover the plant material to
minimize degradation.