Reversibility of propionic acid inhibition to anaerobic digestion: Inhibition kinetics and microbial mechanism.

Anaerobic digestion is a technology that simultaneously treats waste and generates energy in the form of biogas. Unfortunately, when a high organic loading rate is applied, anaerobic digestion can suffer from volatile fatty acid accumulation that results in pH drop and decreased biogas production. In particular, propionic acid has shown to inhibit biogas production even at a very low concentration. Therefore, the kinetics of biogas production in relation to propionic acid concentration needs to be investigated. In batch experiments on anaerobic co-digestion of food waste and dairy manure in the present study, cumulative biogas production showed little inhibition by propionic acid in the concentration range of 6.5-14.6 mM, but a lag phase of 9.4, 16.3 and 60.8 d was detected in the digesters with initial propionic acid concentrations of 22.7, 36.2, and 56.4 mM, respectively. After the lag phase, these digesters accelerated to specific biogas yields of 0.59-0.70 L g-VS-1. The similar specific biogas yields across all of the digesters at initial propionic acid concentrations of 6.5-56.4 mM indicated reversibility of the inhibition. The reversibility was made possible by microbial acclimation and the shift to hydrogenotrophic methanogenesis in syntrophy with acetogenic bacteria. Evidently, an increase of hydrogenotrophic Methanobacterium and Methanoculleus abundances was found at 36.2 and 56.4 mM. Batch digestion experiments must be extended beyond the lag phase in order to fully reveal the inhibition kinetics. This paper highlights the need for a standard protocol that experimentally evaluates inhibition in anaerobic digestion.