Microbial insights towards understanding the role of hydrochar in enhancing phenol degradation in anaerobic digestion

Abstract

Anaerobic digestion (AD) of wastewater is the most promising bioprocess for organic conversion, however, phenol is toxic and resistant to anaerobic degradation. The current study compared the effect of hydrochar and granular activated carbon (GAC) on AD of phenol at four concentrations (100 mg/L, 250 mg/L, 500 mg/L and 1000 mg/L). Results demonstrated that hydrochar significantly improved the methane production rate and reduced the lag phase at all concentrations of phenol. The methane production rate was improved by about 50% at both 100 mg/L and 250 mg/L phenol, while it was raised by >160% at 500 mg/L and 1000 mg/L phenol by hydrochar. The GAC only increased the methane production rate at 500 mg/L and 1000 mg/L due to high adsorption capacity. Further, the adsorption of phenol by hydrochar had no apparent impact on the methane production rate, even though certain amounts of phenol were adsorbed. At 500 mg/L, the amount of methane produced significantly increased, so 16S rRNA transcripts sequencing and metabolomic analysis were conducted. 16S rRNA transcripts sequencing analysis indicated that hydrochar resulted in the enrichment of syntrophic bacteria (e.g., Syntrophorhabdus &Syntrophobacter) and Methanosaeta, which might be related with direct interspecies electron transfer. Further, it was noticed that the growth of Methanobacterium was repressed at 500 mg/L phenol, while hydrochar promoted its growth. Phenol was degraded into L-tyrosine and then followed the benzoate degradation pathway for methane production as revealed by metabolomic analysis. In addition, metabolomic analysis also revealed that hydrochar promoted the degradation of all metabolites and enhanced the phenol degradation into methane.