Continuous semi-dry anaerobic co-digestion of cattle manure and food waste: Performance evaluation and microbial community dynamics

This study compared two routes to improve semi-dry anaerobic digestion (AD) of cattle manure (CM): thermal pretreatment of CM and co-digestion with food waste (FW), which were targeted to overcome low C/N ratio and biodegradability of CM. In batch tests, a CM:FW ratio of 7:3 maximized methane yield. Although thermal pretreatment increased methane yield and production rate, its substantial energy demand exceeded the produced energy from methane, limiting applicability. The effect of co-digestion with FW was further investigated by continuous digestion using of two identical horizontal tubular reactors that were operated for 322 days under semi-dry conditions (17 % total solids) and organic loading rates ranging 3.66–8.34 g VS/L.d. At every investigated SRT (40, 25, 20, and 16 days), co-digestion with FW resulted in the higher methane yield to BMP values (63.4–79.1 % vs. 55.9–77.7 %) as well as the methane yield (171.6–214.3 mL/g VS vs. 106.0–147.3 mL/g VS) and methane production rate (0.79–1.43 L/L.d vs. 0.52–0.92 L/L.d) than single digestion. With increasing organic loading rate, the bacterial community shifted from Firmicutes toward Bacteroidetes and the dominant archaeal genus from Methanobacterium to Methanobrevibacter . Co-digestion also showed higher archaeal abundance, with 16S rRNA gene copies rising from 8.4 to 9.6 log 10 (copy no.)/g VS. Overall, semi-dry co-digestion with high biodegradability and C/N ratio such as FW provides practical and sustainability advantages, including lower water demand and improved process stability, making it a more feasible improvement pathway for CM-based AD than thermal pretreatment. • Thermal pretreatment improved methane yield but was economically inefficient. • Optimal methane yield obtained at 7:3 ratio of cattle manure and food waste. • Semi-dry co-digestion maintained stability even at high organic loading rates. • Tracer study confirmed uniform reactor mixing conditions under high solids content. • Microbial shift reflected adaptation to high ammonia, sustaining methanogenesis.