Inhibition of methanogenesis and its reversal during biogas formation from cattle manure

The composition of volatile fatty acids in the biogas digester based on cattle manure as substrate and stabilized at 25 degree C showed that it contained 87-88% branched chain fatty acids, comprising of isobutyric and isovaleric acids, in comparison to 38% observed in the digester operating at 35 degree C. Mixed cellulolytic cultures equilibrated at 25 degree C (C-25) and 35 degree C (C-35) showed similar properties, but rates of hydrolysis were three times higher than that observed in a standard biogas digester. The porportion of isobutyric and isovaleric were drastically reduced when C-25 was grown with glucose or filter paper as substrates. The volatile fatty acids recovered from C-25 (at 25 degree C) inhibited growth of methanogens on acetate, whereas that from C-35 was not inhibitory. The inhibitory effects were due to the branched chain fatty acids and were observed with isobutyric acid at concentrations as low as 50 ppm. Addition of another micro-organism Rhodotorula selected for growth on isobutyric completely reversed this inhibition. Results indicate that the aceticlastic methanogenes are very sensitive to inhibition by branched chain fatty acids and reduction in methane formation in biogas digester at lower temperature may be due to this effect.