Coupling energy-production processes: The use of residues from bioethanol production to improve the anaerobic digestion of corn stover

Highlights: • Bioethanol production and anaerobic digestion of corn stover were productively linked. • Wastewater from bioethanol production improved the CH₄ yield from corn stover by 42%. • This improvement was due to the lignocellulolytic enzymes present in the wastewater. • This is a simple strategy for parallel waste management and bioenergy production. -- Abstract: Currently, industry is migrating towards sustainable practises, where the wastes produced in one process are used as feedstock for other productive purposes. In the present study, we show that a by-product from the fabrication of second-generation bioethanol can be used to improve the production of biogas from corn stover. The washing water generated during the pretreatment stage of bioethanol production was recovered and reused as a co-substrate for the anaerobic digestion of corn stover. This water contained high concentrations of lignocellulolytic enzymes that once added to the digester improved the methane production kinetics. In comparison with digestion in the absence of these extra enzymes, the ultimate methane yield (P) increased 42%, with a reduction in the lag-phase time (λ) of 67% and an increase in the methane production rate (R_m) of almost 30%. These results were compared with those obtained during the co-digestion of corn stover and cattle manure, which is a well-known strategy to improve methane yields from lignocellulosic residues. However, despite the fact that the use of cattle manure improved methane yields, the results obtained using the enzyme-rich washing water were significantly better. The present study describes a simple strategy for the revalorisation of a by-product from the production of second-generation bioethanol.