Inhibitory effects of free propionic and butyric acids on the activities of hydrogenotrophic methanogens in mesophilic mixed culture fermentation

The aim of this work was to study the inhibitory of free propionic acid (FPA) and free butyric acid (FBA) on enriched hydrogenotrophic methanogens. It demonstrated that concentrations of FPA and FBA were correlated well with the specific methanogenic activity. Coenzyme M concentrations also agreed well with the trends of FPA and FBA. Two fators of C-50% (concentration at 50% inhibition) and C-RC (recoverable concentration from inhibition) were used to quantitively analyze the inhibitory order using the former result of free acetic acid (FAA) and the results of FBA and FPA. The order according to C-50% was FAA (5.2 mM) > FBA (8.3 mM) > FPA (8.5 mM), while for C-RC it was FPA (9.3 mM) > FAA=FBA (13.5 mM). After comparing with literatue, it suggests that the toxicities of these three organic acids are similar. Thus, accumulating free organic acid offers a cost-effective method to inhibit methanogenesis.