Kinetics of acetate, propionate and butyrate removal in the treatment of a semi‐synthetic landfill leachate on anaerobic filter

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Abstract
The kinetics of acetate, propionate, and butyrate removal was studied in conditions of leachate treatment in a plug flow anaerobic fixed-film reactor made of a sequence of seven perfectly mixed compartments. An original experimental procedure was followed under sequential feeding conditions so as to maintain the Bacteriol biomass in a quasi-steady state all along the study. With an appropriate computer program based on the least squares method, the apparent kinetic parameters of VFA removal were calculated within concentration ranges below the levels of salt inhibition. The models proposed are based on simple theoretical considerations. For acetate and n-butyrate removal, the best fits were given by the Michaelis-Menten equation with respectively: V = 0.49 ± 0.06 g CH3 COOH g−1 biomass h−1and 0.18 ± 0.02 g n-CH3CH2CH2COOH g−1 biomass h−1 and: Ks = 21.2 ± 0.9 g CH3COOH L−1 liquid phase and 8.2 ± 0.9 g n-CH3CH2CH2COOH L−1 liquid phase, Iso-butyrate was produced during n-butyrate catabolism and the apparent removal rate of (n + iso)-butyrate considered as a whole was also described by the Michaelis-Menten equation with V = 0.14 ± 0.02 g(n + iso)-butyrate g−1 biomass h−1 and Ks = 9.0 ± 1.2 g (n + iso) butyrate L−1 liquid phase. On the other hand in the case of propionate, the best fit was obtained with a first-order equation with Kspec = (0.88 ± 0.05) 10−2 L liquid phase g−1 biomass h−1. These constants were subsequently used to predict the removal of mixtures of the three major VFAs under study, at various feed concentrations. Three sets of concentrations were tested, and the experimental data were compared to the simulations. This study, together with other experimental observations previously reported, tends to show that under sequential feeding conditions the classical assumption of butyrate β-oxidation should be rejected. Butyrate seems to be anaerobically decarboxylated, but propionate thus formed inside the biofilm is degraded as soon as its formation proceeds. It was therefore considered that butyrate degradation produces, through propionate intermediate, 1 mole acetate per mole butyrate removed. When propionate or butyrate concentrations were high, the same phenomenon was noted, to a much lower extent, for the degradation of acetate formed inside the biofilm.