Formation of interim by-products in methanogenic degradation of butyrate

The formation of interim by-products during the methanogenic degradation of butyrate was monitored and analyzed in this study. Two series of experiments were conducted at various butyrate concentrations and under the influence of increased partial pressure of hydrogen (PH2). In all batches, acetate was found most abundant in the mixed liquor, accounting for over 52–83% of carbon in the original butyrate. This confirms that the degradation of butyrate was through acetate by β-oxidation, and the rate of butyrotrophic acetogenesis was considerably higher than that of acetotrophic methanogenesis. Assuming the degradation followed the Monod equation, the maximum-rate constant μmax was found in the range of 3.4–6.0 mg (mg VSS d)−1 and the half-rate concentration Ks was 700–1150 mg l−1. Other interim by-products included hydrogen (up to 10−3.8 atm), propionate (up to 25 mg l−1), i-butyrate (up to 780 mg l−1), and several higher-molecular-weight carboxylic acids (up to 21 mg l−1). All these by-products became fully degraded eventually after reaching the maximum levels, except propionate which remained at the peak concentration. This suggests that the butyrate-degrading sludge was incapable of degrading propionate. Addition of formate to the feed solution increased the PH2, which enhanced the formation of propionate but did not affect butyrate degradation. The effect of PH2 on propionate was in accordance with the chemical energy analysis. The chemical energy analysis also suggests that the formation of caproate, one of the higher-molecular-weight acids, was independent of the increase of PH2.