Investigation of the Performance and Kinetics of Anaerobic Digestion at 45°C

Abstract

The objective of this study was to evaluate the performance of anaerobic digestion (AD) within the intermediate zone, specifically at 45°C. Single-stage batch anaerobic digestion system was developed in the lab and performance was monitored for more than 2 years. The AD system was able to achieve high biogas production with about 62% - 67% methane content. The digester exhibited high acetate accumulation, but sufficient buffering capacity was observed as the pH, alkalinity and volatile fatty acids-to-alkalinity ratio were within recommended values. The system achieved 36.5% reduction of total solids (TS) and 47.8% reduction of volatile solids (VS), which exceeded the required VS destruction efficiency for Class A biosolids. The pathogen counts were less than 1000 MPN/g total solids in the effluent, which also satisfied Class A biosolids requirements. The accumulation of acetate was presumably due to the high temperature which contributed to high hydrolysis rate. Consequently, it produced large amount of toxic salts that combined with the acetate, making them not readily available to be consumed by methanogens. The slower degradation of acetate was observed by the kinetic parameters. Accumulation of acetate contributed to 52% to 71% reduction in acetate degradation process, but was not completely inhibitory. The methanogens existing in the system were mostly thermo-tolerant acetate-utilizing methanogens, and specifically from Methanosarcinaceae species.