Steady-state and transient behavior in microbial methanification: I. Experimental results.

The process of methanification of volatile fatty acids (VFA) was studied to elucidate its kinetics. An upflow anaerobic sludge bed (UASB) system was used to perform the experiments. At residence times of less than 2.5 h the UASB system was found to exhibit hysteresis with respect to acetic and propionic acid consumption but not with respect to butyric acid consumption. These hysteretic effects could be attributed to the manner in which the various VFA-consuming cultures were structured inside the flocculated biomass in light of the cross-inhibitory effects of the acetic- and propionic-acid-consuming fractions of the total culture. (Butyric acid proved to be non-interactive.) Production of methane was found to respond almost instantaneously to changes in the inlet conditions of the UASB system. This indicated that methane is not primarily growth associated, as has often been assumed, but is related to changes in the culture's maintenance energy requirements. Reactor operation was found to be stable even when the concentration of each VFA in the feed was simultaneously changed by 50%. Even at very high organic throughput rates (35 kg COD/day m(3)-reactor) conversions of 82% were observed.