The AD system has been widely
applied in the environmental management of wastewater sludge,
municipal solid waste, and livestock manure, with the primary
objective of waste reduction. The produced biogas as renewable
energy can also improve the operational profitability, reduce the
greenhouse gas (GHG) emission, and mitigate climate change
(Coats et al., 2013). It is reported that more than 2000 AD plants
are currently operated in the United States to produce biogas,
and an additional 11,000 sites could be developed in the future,
which in total could provide power supply for over 3 million
households and reduce between 4 and 54 million metric tons
CH4 equivalent of GHG emissions by 2030 (USDA et al., 2014).
Parawira et al. (2007) compared the biogas production from PPW
using two-stage digesters with mesophilic and thermophilic configurations,
and results showed that higher CH4 yield was obtained
under mesophilic condition