Integrating hydrothermal pretreatment and co-digestion of activated sludge and kitchen waste for enhanced biogas production and microbial mechanism explanation

Waste activated sludge (AS) and kitchen waste (KW) recently emerged as major environmental problems. Aiming at sustainable development and carbon emission reduction, AS and KW could be transformed to resourceful substance through anaerobic digestion (AD), and energy (i.e., methane) could be consequently recovered. Pretreatment integrated with AD was believed to enhance AD effects and efficiencies. This study used both AS and KW as inocula and their hydrothermal-pretreated counterparts as substrates (HT_AS and HT_KW) to establish mono-AD and anaerobic co-digestion (ACoD) systems. ACoD of AS + HT_KW exhibited efficient biogas and acetic acid production, which maximized to 330.5 mL and 40.68 g/L on the third day, respectively. Three-dimensional excitation-emission-matrix examination demonstrated increased protein-like substances (combining tyrosine and tryptophan, 56.56 %–65.72 %) and decreased fulvic acid (16.74 %–12.96 %) and humic acid (10.76 %–6.13 %) in AS + HT_KW digestate. AS + HT_KW developed distinct community compositions and structure compared with other systems, and distinguished Lactobacillus, Bacillus, Romboutsia, Sarcina, and Clostridium_sensu_stricto_1 as the dominant genera, which significantly contributed to biogas and acetic acid production. TAX4FUN prediction revealed that Glycolysis/Gluconeogenesis (ko00010), Pentose phosphate pathway (ko00030), and Starch and sucrose metabolism (ko00500) were significantly enriched in AS + HT_KW communities favoring biogas and acetic acid generation. Complexed correlation networks, constructed amongst AS + HT_KW communities as well as between communities and environmental parameters, identified the key correlations and communities that could be referred as microbial targets to further regulate, manipulate, and enhance AD effects.