Anaerobic digestion of wastewater from hydrothermal liquefaction of sewage sludge and combined wheat straw-manure

[Display omitted] •Anaerobic digestion of HTL process water was investigated over long-term.•Methane yields were lower for sewage sludge than for straw-manure mixtures.•The composition of HTL process water was a crucial factor in methane inhibition.•Microbial community shifts play a significant role in biogas production.•Mainly methanogens where inhibited, while fermenters were functionally redundant. Hydrothermal liquefaction (HTL) shows promise for converting wet biomass waste into biofuel, but the resulting high-strength process water (PW) requires treatment. This study explored enhancing energy recovery by anaerobic digestion using semi-batch reactors. Co-digesting manure with HTL-PW from wheat straw-manure co-HTL yielded methane (43–49% of the chemical oxygen demand, COD) at concentrations up to 17.8 gCOD·L-1, whereas HTL-PW from sewage sludge yielded methane (43% of the COD) up to only 12.8 gCOD·L-1 and complete inhibition occurred at 17 gCOD·L-1. Microbial community shifts confirmed inhibition of methanogenic archaea, while hydrolytic-fermentative bacteria were resilient. Differences in chemical composition, particularly higher levels of N-containing heterocyclic compounds in PW of sewage sludge, likely caused the microbial inhibition. The considerable potential of combining HTL with anaerobic digestion for enhanced energy recovery from straw-manure in an agricultural context is demonstrated, yet sewage sludge HTL-PW requires more advanced approaches to deal with methanogenesis inhibitors.