Hydrothermal pretreatment of dewatered sewage sludge: Effects of process conditions on carbon and phosphorus fate and methane production

Abstract The effect of hydrothermal pretreatment (HTP) of dewatered sewage sludge (DSS) under optimized hydrothermal conditions and the fate of carbon and phosphorus (P) during anaerobic digestion (AD) of hydrochar and process water were studied. The results showed that the methane yield reached 241 mL CH4/g COD when the hydrothermal conditions were 200°C-2 h-10% (A4), which was 78.28% higher than that without hydrothermal pretreatment (A0) and 29.62% higher than that of unoptimized A1 (140°C-1 h-5%). Protein, polysaccharides, and volatile fatty acids (VFAs) were the main hydrothermal products of DSS, and polysaccharides were first absorbed and utilized by microorganisms. By 3D-EEM analysis, tyrosine, tryptophan proteins, and fulvic acids decreased after HTP, but the content of humic acid-like substances increased, and this phenomenon was more obvious after AD. A lag phase in protein degradation occurred during AD while polysaccharides are degraded first. The solid-organic P is converted into liquid-P in the hydrothermal process, and non-apatite inorganic P is converted to organic P during AD. A lower methane production rate occurred in the early AD, suggesting that the inoculum needed some time to adapt to the substrate. Microbial analysis showed that the composition of the microbial community of anaerobic degradation was different due to the various organic components.