Design of acidogenic reactors for the anaerobic treatment of the organic fraction of solid food waste

Volatile Fatty Acids (VFA) production by anaerobic fermentation of organic solid wastes was studied at laboratory scale. The influence of initial substrate concentration was evaluated on VFA production. Completely mixed reactors (0.9 l) were used at mesophilic temperature (35 degrees C). Food wastes had 43.8% Total Solids content. Three dilutions of substrate (1/25, 1/10 and 1/5) corresponding to 1.75%, 4.38% and 8.76% of Total Solids and five values of Organic Loading Rates: 2, 5, 10, 12.5 and 25 kg COD/m(3) d were studied. It was found that substrate 1/10 led to 14 g VFA/l at a loading rate of 12.5 kg COD/m(3) d and an hydraulic retention time of 3.7 d. The main VFA produced were especially acetate and butyrate. Substrate diluted 1/5 led to 26.1 g VFA/l at a loading of 5 kg COD/m(3) d and an hydraulic retention time of 15.1 d, but biomass production was not optimal. In a second study, a cascade of three reactors was used. An effluent with 42 g VFA/l was obtained at steady-state conditions at a loading of 12.5 kg of COD/m(3) d and an hydraulic retention time of 12.5 d. The distribution of VFA was the following: 36% of propionate, 34% of acetate and 22.5% of butyrate.