Kinetics of LCFA inhibition on acetoclastic methanogenesis, propionate degradation and beta-oxidation

Kinetics of long-chain fatty acids (LCFAs) inhibition on acetoclastic methanogenesis, propionate degradation and beta-oxidation were studied with granular sludge under mesophilic batch conditions. Mathematical expressions used for reaction rates were as shown below: [formula: see text]. The simulated results revealed that the methane production rates from acetate decreased with an increase in both concentration and the number of double bonds of LCFAs. The concentrations of oleate (C18:1), linoleate (C18:2), palmitate (C16:0), and stearate (C18:0) were 0.54 mM, 0.11 mM 1.62 mM, and 2.58 mM, respectively, at which the methane production rates from acetate dropped 10%, and 3.10 mM, 0.72 mM, 5.71 mM, and 5.37 mM, respectively, at which the rates dropped 50%. The inhibitory effects of LCFAs on propionate degradation showed a similar tendency with acetoclastic methanogenesis; however, were less severe. The concentrations of oleate, linoleate, palmitate, and stearate were 1.02 mM, 0.18 mM, 2.34 mM, and 1.92 mM, respectively, at which the propionate degradation rates dropped 10%, and 4.38 mM, 1.17 mM, 5.88 mM, and 5.18 mM, respectively, at which the rates dropped 50%. The observed maximum beta-oxidation rates of oleate, linoleate, palmitate, and stearate were 0.21 mmol (gVSS)(-1) d(-1), 0.09 mmol (gVSS)(-1) d(-1), 0.12 mmol (gVSS)(-1) d(-1), and 0.08 mmol (gVSS)(-1) d(-1), respectively. The lag-phase times in beta-oxidation were also dependent on LCFA concentrations. The concentrations of oleate, linoleate, palmitate, and stearate, at which the lag-phase times became 5 days, were 5.93 mM, 2.24 mM, 4.02 mM, and 2.81 mM, respectively.