Biochemical properties of sludge derived hydrothermal liquid products and microbial response of wastewater treatment

Sludge hydrothermal treatment can achieve efficient reduction and resource utilization, and the hydrothermal liquid products (HLPs) generated in the hydrothermal process may be returned to beginning of the wastewater plant for resource utilization. In this study, dehydrated sludge was treated by a hydrothermal treatment process, and hydrothermal products were separated by centrifugation. Then the HLPs were returned to the front end of the anaerobic-oxic (A/O) reactor to explore their potential role as a source of denitrified carbon. Study focuses on the HLPs' changes of Carbon (C), Nitrogen (N), and Phosphorus (P) speciation and distribution, and clarifying the organic matter forms and their impacts on the microbial community composition and functional expression in biological process after recirculation. The results show C and N components are dissolved to HLPs, existing in forms of easily biodegraded components, such as acetic acid (28.04 %), tryptophan, and humic substances, while P mainly precipitates in the solid residue, avoiding accumulation in the wastewater treatment system. The dominant bacteria Proteobacteria accounts for 30 %–40 % of the total sequence, and the abundance of metabolism-related genes maintain the highest level. The wastewater treatment process can withstand the shock of the recirculation of HLPs, and the microbial community composition remains stable.