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Anaerobic Digestion Potential of Coproducts Associated with Ethanol Production from Sweetpotato: A Review

Mussoline, Wendy A., Wilkie, Ann C.
Industrial biotechnology 2015 v.11 no.2 pp. 113-126
agricultural wastes, anaerobic digestion, biogas, biorefining, coproducts, distillery effluents, electricity generation, energy recovery, energy requirements, ethanol, ethanol production, feedstocks, heat, membrane bioreactors, methane, methane production, microbial growth, roots, sludge, sweet potatoes, vines
The synergy between biofuels and biogas can be realized effectively with the use of a high-yielding sweetpotato as a feedstock for ethanol production, while the coproducts are anaerobically digested for energy recovery to achieve sustainable operation of a biorefinery. The focus of this review article is to characterize, quantify, and evaluate the methane potential of coproducts associated with dry-type, industrial sweetpotatoes, including culls, aerial vines, and distillery waste. Reported biomass yields of vines and culls combined with biomethane potential assays suggest that agricultural residues from sweetpotatoes have untapped energy potential that can be recovered by anaerobic digestion. Both pilot- and full-scale digesters of various types have demonstrated successful and stable biogas production from sweetpotato alcohol distillery wastewater. Higher methane yields were observed in thermophilic compared to mesophilic conditions. Reactors with fixed media such as fixed-bed and membrane bioreactors had better performance than the upflow anaerobic sludge bed, which is attributable to improved microbial growth and adherence to the media. An overall energy analysis was conducted for generating one metric ton (t) of anhydrous ethanol from industrial sweetpotatoes by incorporating (1) biomass yields for roots, vines, and culls; (2) methane recovery from associated coproducts; and (3) energy requirements for the ethanol-conversion process. Results of the theoretical analysis show that 12,500 megajoules (MJ) are required to convert 7.9 t of industrial sweetpotatoes into 1 t of ethanol, and the methane potential associated with the sweetpotato coproducts is 35,600 MJ. Therefore, nearly two-thirds of the energy produced from the coproducts is excess energy available for heat or electricity generation in an integrated biorefinery.