Combined microbial transcript and metabolic analysis reveals the different roles of hydrochar and biochar in promoting anaerobic digestion of waste activated sludge

Hydrothermal pretreatment of waste activated sludge (WAS) could eliminate the rate limiting step of anaerobic digestion (AD)-hydrolysis. However, the high organic loading rate may cause acid accumulation, thus leading to an unstable system. This study compared the effect of different hydrochar (HC2-260 degrees C and HC3-320 degrees C) and biochar (BC5-500 degrees C and BC7-700 degrees C) on AD of hydrothermal pretreated WAS (HPS). Results demonstrated that hydrochar was superior to biochar in the methane yield and production rate, especially HC2. HC2 had the highest surface oxygen-containing functional groups that could facilitate direct interspecies electron transfer (DIET). The enhanced methane yield was related with the increased protein utilization, and hydrochar and biochar enriched different microbes related to protein degradation. Metabolomic analysis showed the significantly changed metabolites induced by hydrochar and biochar were involved in fatty acids and amino acids related metabolism, indicating the rapid conversion of intermediated products, which was consistent with the microbial community structure results. Hydrochar and biochar also induced upregulation of metabolites related to microbial metabolic activity and extracellular electron transfer. Although biochar induced the same metabolic changes, the alterations of these metabolites were weaker than those of hydrochar. The results of this study offered new insights into the molecular mechanisms of enhanced AD of HPS by hydrochar and biochar.

Automated summary

The study found that hydrochar outperformed biochar in enhancing anaerobic digestion of hydrothermal pretreated waste activated sludge, especially HC2, due to its higher methane yield and production rate. Both hydrochar and biochar facilitated protein utilization and enriched different microbes involved in protein degradation, leading to increased methane production. Metabolomic analysis revealed that hydrochar and biochar induced significant changes in metabolites related to fatty acids and amino acids metabolism, showing rapid conversion of intermediate products and microbial community structure alignment.