Microbial community restructuring and anaerobic digestion performance under mesophilic (35°C) and intermediate-temperature (45°C) conditions: effects of feedstock and inoculum from start-up to long-term operation.

Temperature effects on anaerobic digestion (AD) have been reported to be feedstock dependent, and intermediate temperature has been suggested as a general optimum for lignocellulosic energy crops. To investigate whether this finding extends to feedstocks with distinct characteristics, this study compared AD performance and microbial community dynamics at mesophilic (35°C) and intermediate-temperature (45°C) using food waste (FW) and sewage sludge (SS) as readily digestible and lower-digestibility waste-derived feedstocks. SS-fed digesters maintained comparatively stable performance at 35°C and 45°C with varying operational conditions. In contrast, FW45 maintained more stable operation and higher methane production at 45°C even though significant microbial restructuring regarding functional microorganisms, while FW35 experienced longer start-up period, and showed greater sensitivity to operational changes at 35°C. As the inoculum to FW35 was from a full-scale digester at 45°C, the instability observed in FW35 might be attributed to either longer adaptation of the 45°C inoculum to mesophilic condition or intrinsic high FW digestion sensitivity at 35°C or the combination of both factors. Overall, the findings suggest that 45°C is a viable intermediate-temperature option for AD of both readily digestible FW and much less readily digestible SS, achieving good performance and operational stability and thereby challenging the view that AD should be operated primarily under mesophilic or thermophilic conditions. However, inoculum effects require further investigation, particularly for FW.