Page 207 - Advances in Eco-Fuels for a Sustainable Environment

Anaerobic digestion of various feedstocks for second-generation biofuel production 171

6.9 Feedstock

For anaerobic digestion, all decomposable biomass materials are well suited. Feed-
stock might be both dilute and concentrated, slurries, and solid liquids [69]. Common
substrate includes crop residues and agricultural wastes, forest residues, municipal
solid wastes, animal wastes, and aquatic waste [69].

6.9.1 Agricultural wastes and crop residues

For biogas generation, numerous agricultural processes and crops yield residues that
can be used as a substrate. India stands second among the countries of the world in
producing wheat, rice, tea, cashew nuts, cotton, and some other crops. In India, to pro-
duce biogas no crop has been grown specifically for anaerobic digestion. However, for
anaerobic digestion, there is a plentiful biomass source available in the form of crop
residue and agricultural waste, which can be used as a substrate [70]. Spoiled fodder,
straw, crop stubbles, weeds, oil cakes, fruit and vegetable processing wastes, tobacco
waste, rice and coffee husks, bagasse, and sugarcane trash are some of the types of
crop residues and agricultural wastes used as feedstock in anaerobic digestion [71, 72].

6.9.2 Animal waste

An extensive amount of waste is produced by animals, as animal breeding activity is
highly developed. India stands first with 281.7 million cattle among the world’s cattle
population, which is estimated at 1300 million [73]. Cattle dung is used as one of the
solid waste feedstocks for anaerobic digestion in rural India. The buffalo and cattle
populations were 97 and 224 million, respectively, for the year 2010 [61]. It has been
premeditated that around 354million tons of dung are available per year from a cattle
population of about 458 million. Korea, the Philippines, Taiwan, and China frequently
use pig manure as a substrate. Worldwide, regularly used animal wastes are poultry
manure, cow dung, horse dung, pig waste, fishery waste, camel dung, slaughterhouse
wastes, elephant dung, etc. [74]. Poultry wastes are a relatively lower carbon source
and rich in organic nitrogen out of the above mentioned. This leads to the process
being unsuccessful and slow. This problem can be overcome by codigestion of various
substrates having high and low nitrogen values [74].

6.9.3 Aquatic plants
The best sources for biogas production are marine plants such as the water hyacinth,
seaweed, various species of macro and microalgae, etc. Because they have easily
hydrolyzable sugars and low lignin content, they do not compete with land resources
used in food crop cultivation [75]. Microalgae and water hyacinths are mostly used as
substrate material due to the higher gas yield. The efficiency of biomass production is
considered as per hectare of microalgae, which is about 5–30 times compared to that
of plants of the crop. For methane production, the absence of lignin and the relatively