Page 53 - Advances in Eco-Fuels for a Sustainable Environment
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30 Advances in Eco-Fuels for a Sustainable Environment
Lipase
Lipids!Fatty acids,glycerol (2.1)
cellulase,cellobiase,xylanase,amylase
Polysaccharide !monosaccharide (2.2)
protease
Proteins!amino acids (2.3)
methanogenic
Acetic acid!methane + carbon dioxide (2.4)
methanogenic
Hydrogen + Carbon dioxide !methane + water (2.5)
Biogas is produced from wastewater treatment facilities, landfills and codigestion
facilities, where the process of fermentation is monitored to ensure that methane con-
stitutes at least 50% of produced biogas, with percentages up to 75%.
The main advantage of biogas production is the extensive range of available sub-
strates that can be used. The criteria of choice for a substrate are its sustainability as
well as its energy, environmental, and economic values. Among different feedstocks,
the most fitting are agricultural biomass residues (i.e., animal manure), human and
livestock excreta, the organic fraction of MSWs, and food industry wastewater.
Organic matter in MSW accounts for about 55% of its contents. Pretreatment is
required to remove inorganic fractions and operate particle size reduction and
codigestion with sludges or animal manure (containing more nutrients and readily fer-
mentable substrates) is operated to increase CH 4 production. Thermophilic conditions
present several advantages on mesophilic conditions, such as faster degradation rates,
higher biogas production rate, lower viscosity, and higher pathogen destruction. Ani-
mal manure as AD feedstock presents 75%–92% moisture content, 72%–93% volatile
solids, and great microbial diversity able to speed up the process inoculation. How-
ever, the slow digestion and poisoning effect of the ammonia generated make this sub-
strate alone less effective, suggesting coupling manure digestion to carbohydrate-rich,
nitrogen-free sources such as food processing wastes and dairy wastewater. It has been
estimated that in North America, using all available landfill, animal, and sewage waste
production for AD would generate electricity to meet about 3% of the country’s needs.
Poultry droppings contain more easily degradable organic materials than other agri-
cultural waste products, but also a high content of lignocellulosic fibers (up to 50%).
Hence, to increase their biogas production yield, codigestion with other organic wastes
rich in biodegradable organics (e.g., rice and wheat straws, whey, MSW, dairy manure,
and sewage sludge) is suggested in order to achieve an improved nutrient balance and
more suitable C/N ratio for methane production (Table 2.9). Studies on the correct ratio
of C/N to maximize methane yield in biogas suggest an optimal range of 20–30 [33].
Industrial effluents can also be processed by AD to reduce the organic load and
recover energy. Vinasse from the ethanol production process has serious disposal
challenges because to produce 1L ethanol, 12L vinasse with a BOD load of
17000–50000mg/L, are produced. AD of digested vinasse can reduce its BOD content
by up to 90%, producing additional energy to the ethanol-extracted one.
