Genome-Centric Metatranscriptomics Analysis Reveals the Role of Hydrochar in Anaerobic Digestion of Waste Activated Sludge

Anaerobic
digestion (AD) of waste activated sludge (WAS) has been
widely used, while it poses problems including low methane yield and
production rate. Hydrochar is produced by hydrothermal liquefaction
of biomass; however, little is known about the role of hydrochar in
promoting AD of WAS. The present study showed that hydrochar increased
the methane production rate by 30.8% and yield by 31.4% of hydrothermal
pretreated dewatered WAS. Hydrochar increased the methane production
rate and yield by enhancing the acidification and methanogenesis processes.
Genomic-centric metatranscriptomics were used to identify the metabolic
activities and transcriptomic response of individual metagenome-assembled
genomes that were enriched by hydrochar. Although Methanosarcina sp. FDU0106 had been shown unable to used H2, it had
the complete pathway for the reduction of CO2 to methane. Syntrophomonas sp. FDU0164 expressed genes for extracellular
electron transfer via electrically pili, suggesting that Syntrophomonas sp. FDU0164 and Methanosarcina sp. FDU0106 were exchanging electrons via direct interspecies electron
transfer. The expression of pili was decreased, indicating that hydrochar
could replace its roles. Additionally, Firmicutes sp. FDU0048, Proteiniphilum sp. FDU0082,
and Aminobacterium mobile FDU0089 were
related to the degradation of organics, which could be related to
the enhanced methane yield.