Anaerobic digestion of OMW : intermittent feeding strategy and LCFA oxidation profile

Abstract

An intermittent feeding strategy was applied to the anaerobic treatment of raw olive mill wastewater (OMW). Two reactors were operated under influent concentrations of 5 to 50 g COD L-1. Two and one batch (feed-less) periods were applied to reactor R1 and R2, respectively, operating in continuous thenceforth. It was demonstrated that the intermittent feeding of OMW improved the mineralization of accumulated Long Chain Fatty Acids (LCFA) inside the reactor. Nevertheless, LCFA accumulated again when the organic loading rate was increased from 2 to 3 and 5 kg COD m-3 d-1. The profiles of LCFA, obtained with OMW digestion, were different from previous studies with synthetic effluents. At the beginning of reactors operation, oleate was the main LCFA compound (~50%) followed by palmitate. Afterwards, a shift in the LCFA pattern accumulation was noticed for both reactors. At periods with higher OMW concentrations (30-50 g COD L-1, 3-5 kg COD m-3 d-1) palmitate was the main LCFA accumulated with 69% at R1 and 54% at R2. For real oily wastewaters, a periodically batch period could be a practical solution to maintain low values of LCFA inside the reactor. The addition of a nitrogen source was essential to enhance the methane yield.