Influence mechanisms of released humic substances during sludge thermal hydrolysis on anaerobic digestion: Roles of humic and fulvic acids

Thermal hydrolysis pretreatment (THP) is widely applied to sludge anaerobic digestion (AD). However, a large number of humic substances (HSs), mainly humic acids (HAs) and fulvic acids (FAs), are released into the liquid phase during THP. The complex structures of HSs would influence sludge AD in ways that are not fully understood and sometimes contradictory. This study investigates the influence mechanisms of HSs on sludge AD by analyzing the divergent roles of HAs and FAs. The findings indicated that FAs exhibited a more positive influence on sludge AD than HAs, specifically in enhancing the acidogenesis and methanogenesis, while HAs promoted methanogenesis but inhibited hydrolysis and acidogenesis. These differences are attributed to their distinct effects on key enzymes. Although FAs had a stronger electrostatic potential, its inhibitory effect on extracellular enzymes was weaker than that of HAs, resulting from its weaker net trapping effect on enzymes due to its smaller molecular weights. The primary influence of FAs and HAs on intracellular enzymes is promoting electron transfer. However, the powerful electron donor capacity of HAs, as high as 352% of FAs, made it inhibitory to acidificationase. Moreover, it was found that the effects of HSs on pyruvate ferredoxin oxidoreductase (PFOR), acetate kinase (AK), and phosphotransacetylase (PTA) would be easily underestimated if only the total ultimate acidification activity is considered due to microbial enzyme self-regulation. The study revealed the essential reason for the differentiated effects of HAs and FAs on sludge AD, assisting in understanding how complex HSs influence sludge AD.