Additionally, the utilization of a mixed microbial culture creates an inherent competition for the H2 between the hydrogenotrophic methanogens and homoacetogens. The proportion of the H2 that was consumed and accumulated as acetate by the homoacetogenic pathway was quantified to evaluate the methanogenic degree of conversion. The concentrations of dissolved volatile fatty acids (VFA) (C2-C6) were analyzed with a GC (System 7890A, Agilent Technologies, USA) equipped with a HP-INNOWAX column (Agilent Technologies, USA), where the employed carrier gas was helium, and the analytes were measured with a flame ionization detector. The liquid samples were pretreated by acidification with 0.3M oxalic acid containing an internal standard of pivalic acid in a 1:4 ratio between the sample and the oxalic acid on a rotation mixer for 10 min.