Revealing the occurrence of refractory melanoidins and inhibitions on anaerobic fermentation via thermal hydrolysis pretreatment

Thermal hydrolysis pretreatment (THP) is widely used to promote the hydrolysis of polysaccharides and proteins in waste activated sludge (WAS). However, it also generates unwanted refractory organics of melanoidins products (MPs), which should not be overlooked, especially concerning the occurrence and inhibition of hydrolases and anaerobes. In this study, the MPs occurrence by THP was investigated and inhibition mechanism on anaerobic fermentation was revealed for the first time. The results showed that SCOD in WAS significantly increased from 0.5±0.02 g/L in control to 4.1±0.02 g/L in THP at 180 °C. However, the color of MPs under a higher temperature (>120 °C) changed from colourless to dark brown. The components of MPs were mainly nitrogenous and humic acid-like refractory organics. Unwanted MPs generated above 160 °C reduced methane production by 49.1 %–89.8 %, which was due to the inhibition of protease, acidogenic bacteria, and acetoclastic methanogens. Inhibition of acetoclastic methanogens also resulted in the accumulation of VFAs. The microbial diversity clearly showed that the toxicity of MPs altered the microbial community structure, reducing the proportion of both acidogenic bacteria and acetoclastic methanogens. Therefore, the negative effect of MPs on anaerobic digestion demonstrated that THP should be re-evaluated in the production of biochemicals, which is important for achieving carbon neutrality of WWTPs in the future.