Influence of thermal hydrolysis on sludge anaerobic digestion: Release of humic acid promotes electron transport of methanogen

[Display omitted] •Methanogenesis was unexpectedly promoted by humic acids (HAs) released from sludge thermal hydrolysis.•A novel mechanism was developed to explain HAs influence on methanogenesis.•Electroactive groups in HAs included Q1, Q2 and NQ but only Q1 transferred electrons during methanogenesis.•RT-qPCR confirmed HAs could transfer electron to energy conversion hydrogenase. Thermal hydrolysis pretreatment (THP) has been widely applied to promote anaerobic digestion (AD). Humic acids (HAs) are a group of insoluble organics and often present in the conventional AD of sludge, but are reportedly substantially released during THP. Thus, the influences of the released HAs on AD shall be investigated, in particular on methanogenesis. Our results indicated that the presence of HAs extracted from thermally hydrolyzed sludge improved methanogenesis by 15.8–80.8%. However, HAs harvested at different temperatures presented distinctively different capabilities in enhancing methane production. Seemingly, the promotion efficiency of HAs to methanogenesis did not significantly respond with any individual characteristics of HAs in aspects of molecular weight, electrical activity and electroactive group compositions. However, the efficiency presented an evident correlation with the superimposed effects of the characteristics aforementioned. Therefore, a hypothesis on the mechanism involved in the phenomenon observed above was proposed that only the HAs with low molecular weight, allowing the molecules to penetrate cells could affect methanogenesis, and not all of the electroactive groups of HAs were involved in microbial electron transfer. Moreover, it was found that only the group of quinones (Q1) with side chain of electron withdrawing group played key roles in accelerating methyl acetyl-CoA degradation and promoting methanogenesis. Directional regulation of methanogenesis promotion assisted with HAs was realized by selectively removing or retaining the Q1 groups in HAs, which also evidently sustained the proposed mechanism aforementioned. The findings of this study demonstrate that high temperature was conducive to the release of organics during sludge thermal hydrolysis, whereas the temperature above 160℃ led to changes of HAs structure and then subsequently undermined methane production from sludge.