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

•The inhibition of butyrate and propionate on methanogenesis is investigated.•Free propionic acid (FPA) and free butyric acid (FBA) are the key factors.•Good correlation of CoM and concentrations of FPA and FBA are obtained.•The toxicities of FPA, FBA and free acetic acid to methanogens are similar.