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 experimen ts were conducted at various butyrate concentrations and under the influenc e of increased partial pressure of hydrogen (P-H2). 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 but yrate was through acetate by beta-oxidation, and the rate of butyrotrophic acetogenesis was considerably higher than that of acetotrophic methanogenes is. Assuming the degradation followed the Monod equation, the maximum-rate constant mu(max) was found in the range of 3.4-6.0 mg (mg VSS d)(-1) and th e half-rate concentration K-s was 700-1150 mg l(-1). Other interim by-produ cts 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 carbo xylic acids (up to 21 mg l(-1)). All these by-products became fully degrade d eventually after reaching the maximum levels, except propionate which rem ained at the peak concentration. This suggests that the butyrate-degrading sludge was incapable of degrading propionate. Addition of formate to the fe ed solution increased the P-H2, which enhanced the formation of propionate but did not affect butyrate degradation. The effect of P-H2 on propionate w as in accordance with the chemical energy analysis. The chemical energy ana lysis also suggests that the formation of caproate, one of the higher-molec ular-weight acids, was independent of the increase of P-H2. (C) 1999 Elsevi er Science Ltd. All rights reserved.