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 C50% (concentration at 50% inhibition) and CRC (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 C50% was FAA (5.2 mM) > FBA (8.3 mM) > FPA (8.5 mM), while for CRC 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.