Inhibitory effects of long-chain fatty acids on VFA degradation and β-oxidation

The inhibitory effects of major long-chain fatty acids (LCFA), which have 16 or 18 carbons, not only on acetate degradation, but also on propionate degradation and β-oxidation were examined in anaerobic serum bottle tests at 35°C with the acclimated granular sludges. A modified Gompertz equation described cumulative methane production to assess the rates of VFA degradation and β-oxidation, which were applied to a simplified noncompetitive model and a simplified substrate inhibition model, respectively. The specific methane production rates on acetate decreased as LCFA concentration increased, which was in good agreement with the noncompetitive inhibition model. Unsaturated oleate (C18:1) and linoleate (C18:2) were more inhibitory than saturated stearate (C18:0) and palmitate (C16:0) on acetate degradation. LCFA inhibition on propionate degradation was similar to that for acetate; however, propionate degradation was less inhibited than acetate degradation. β-oxidation was the rate-limiting step in LCFA degradation in most cases. As LCFA concentration increased, β-oxidation rate reached the maximum value, and then decreased, which confirmed the substrate inhibition of LCFA. Oleate, the most abundant LCFA in wastewater, could be degraded more quickly than saturated LCFA containing the same or even less carbon in spite of relatively high toxicity on acetate degradation.