Control of Ammonia Sensitivity in Thermophilic and High Solids Anaerobic Digestion

Advanced anaerobic digestion processes that apply high temperatures or high solids loading rates create distinct chemical environments that are not routinely encountered by conventional digestion communities. Among the most important chemical changes that occur in high temperature and high solids digestion processes relative to conventional digestion are unfavorable shifts in the unionized fractions of both acetic acid and ammonia, the latter of which imparts inhibition on the acetic acid degrading methanogenic community. The objective of this paper is to identify and, where possible, to quantify the impact of digestion temperature and solids loading on the performance of advanced anaerobic digestion in the context of these shifts in ionized/unionized chemical species.Two case studies are discussed in order to reveal operational considerations to control ammonia sensitivity and to potentially improve digestion performance in these advanced digestion systems. In the first study, the operating temperature of a full-scale digestion process is reduced by several degrees Celsius, resulting in a rapid increase in biogas production during process start-up. In a second case study, bench scale digestion investigations of a moderately and a highly loaded digestion process are compared. The stoichiometric release of ammonia and alkalinity in the highly loaded process results in a calculated free ammonia concentration in excess of 8 times that of the moderately loaded process, resulting in an apparent inhibition of aceticlastic methanogenesis and a high residual concentration of volatile acids. The impacts observed in these case studies result in several suggested strategies to control ammonia sensitivity in anaerobic digestion processes where it is expected to occur.