Abstract

Anaerobic digestion at mesophilic and thermophilic temperatures have been widely studied and evaluated for the purpose of sludge stabilization. However, limited extensive research has been conducted on anaerobic digestion in the intermediate zone of 45°C, mainly due to the notion that limited microbial activity occurs within this zone. The objectives of this research were to evaluate the performance and the capability of anaerobic digestion at 45°C in producing class A biosolids, in comparison to a mesophilic and thermophilic anaerobic digestion system operated at 35°C and 55°C, respectively. 45°C anaerobic digestion systems were not able to achieve comparable methane yield and high quality effluent as mesophilic system, though the systems produced biogas with 66.08±2.83% methane. No ammonia inhibition was observed and the digesters were able to achieve volatile solids (VS) reduction of 47.79±1.86%. Moreover, the pathogen counts were less than 1,000 MPN/g dry solids, thus, producing Class A biosolids. However, the 45°C systems suffered from high acetate accumulation, but sufficient buffering capacity was observed. Correspondingly, the dominant methanogen existed in 45°C system was thermo-tolerant acetate-utilizing methanogen of <em>Methanosarcinaceae </em>species.