CONVERSION OF ACETATE, PROPIONATE AND BUTYRATE TO METHANE UNDER THERMOPHILIC CONDITIONS IN BATCH REACTORS

Abstract

These experiments were performed to determine the degradation of VFA (acetate, propionate and butyrate) and the maximum methanogenic activity of granular sludge from the thermophilic anaerobic digestion of pure molasses. The compositions of acetate, propionate and butyrate used as substrate were 25:35:40. The tests were performed at constant temperature (55-degrees-C) and pH 7 on two duplicate batch reactors (I and II) running in parallel and were repeated to show the effect of acclimatization. During the first feeding, there was a significant lag phase and after about 23 h incubation the volumes of CH4 gases produced from two reactors were only about 20 and 490 ml, respectively. In this experiment, propionate was converted to acetate only after the initial concentrations of butyrate and acetate had completely degraded. Acetate formed from propionate was immediately converted to methane and carbon dioxide. The maximum methanogenic activities of the first feeding were not high because the natural populations of the propionate-degrading bacteria were low and the sludge adapted itself to the VFAs' substrate very slowly. In a second experiment with the same sludge, the maximum methanogenic activities of the second feedings were about 1.60 times higher than those of the first feedings because of the adaptation of the sludge and increase of populations of the propionate-degrading bacteria.