Humic acids promotion or inhibition of sludge anaerobic digestion depends on their redox potentials

Thermal hydrolysis pretreatment has been widely employed to improve sludge anaerobic digestion (AD), but also results in excessive releases of humic acids (HAs) from sludge. The influences of HAs on sludge AD recently have resulted in ever-increasing concerns, but those reported results are not consistent and sometimes even contradictory. The results in the present study indicated whether HAs promote or inhibit sludge AD mainly depends on their redox potentials. Generally, the HAs derived from sludge with low redox potentials were seemingly facilitative to sludge AD, whereas commercial HAs with high redox potentials presented inhibitory effects. In fact, the influences of HAs on each stage of sludge AD, namely hydrolysis, acidification and methanogenesis, were also quite different, and mostly the differences were related to HAs redox potentials. Mechanism analysis indicated HAs with different redox potentials did not evidently influence microbial community profile, but resulted in noticeable changes in microbial activities. Protease and α-glucosidase activities were substantially inhibited by HAs, which became more pronounced as HAs redox potentials decreased from 154 to −238 mV. The relative activity of F420 was shifted to 89.1 %, 104.1 %, 121.3 %, 92.4 % and 73.2 % with the additions of HAs at redox potentials of 154, −63, −392, −419 and −458 mV, respectively. Furthermore, the presence of HAs accelerated interspecies electron transfer and assisted ferredoxin (Fd) in intracellular electron transfer of methanogens. HAs with too high (≫ −398 mV) or too low (≪ −398 mV) redox potentials were not conducive to methanogenesis. The findings of this study could forcefully clarify those contradictory results reported recently about the influences of HAs on sludge AD. Overall, all the findings in this present study indicate that it is of great necessity to regulate sludge thermal hydrolysis based on the redox potentials of released HAs.