Methanogenesis and sulfate reduction in chemostats—I. Kinetic studies and experiments

Six anaerobic chemostats containing mixed microbial cultures were used to investigate the interactions between sulfate reduction and methanogenesis for three substrates: acetic acid, methanol and formic acid. Sulfate reducers outcompeted methanogens in acetate-fed chemostats while methanol was not utilized by sulfate reducers. In the formic acid-fed chemostats, competition was observed between methanogens and sulfate reducers with 62 and 24% of the substrate utilized through sulfate reduction and methanogenesis, respectively. Iron was added to the sulfate-reducing chemostats to precipitate the hydrogen sulfide produced, thus eliminating sulfide inhibition and ensuring stable chemostat operation. This study involved the measurement of the oxidation-reduction potential (ORP) of the chemostats using a novel technique. Batch spike tests were also conducted to evaluate kinetic parameters for the degradation of different substrates. Although both methanol-fed chemostats were exclusively methanogenic, scanning electron microscopy (SEM) analysis revealed the presence of two different strains of methanogens. This difference was also manifested by the ORP values and the kinetic parameters.