Comparison of mono- and co-digestion of moderately thermo-alkaline pretreated sewage sludge with food waste: Effects on methane production and microbial communities

This study investigates the synergistic effects of co-digesting sewage sludge, waste-activated sludge (WAS) and primary sludge (PS) with food waste (FW) using moderate thermo-alkaline pretreatment (TAP; pH 10, 121 °C, 60 min). While TAP is known to improve sewage sludge digestion, a systematic comparison of different sludge types and operational modes (mono-vs. co-digestion) under these moderate conditions, and their impact on microbial communities, has not been previously investigated. This study fills that gap by linking process performance with microbial community dynamics in batch bioreactors, comparing mono- and co-digestion scenarios using TAP-treated WAS and PS with FW. Moderate TAP enhanced protein solubilization (leading to ∼90 % protein degradation in WAS mono-digestion) and methane yield. Co-digestion further improved performance, with the TAP-treated PS and FW combination achieving the highest methane yield (359.49 mL CH4/g VS). Furthermore, co-digestion with TAP-treated sludge shortened the lag phase and improved process stability by enhancing buffering capacity. This enhanced performance correlated with key microbial shifts, involving an enrichment of fermentative Firmicutes and Bacteroidota to drive initial VFA production and the dominance of the acetoclastic methanogen Methanothrix for efficient methane conversion. These findings demonstrate that combining moderate TAP with the co-digestion of PS and FW is a highly efficient strategy for enhancing methane production by fostering a microbial community structure that ensures process stability and high yields.