Thermophilic anaerobic co-digestion of olive mill wastewater with olive mill solid wastes in a tubular digester

Abstract

Abstract This study investigates for the first time the thermophilic (55 °C) anaerobic co-digestion of olive mill wastewater (OMW) with olive mill solid wastes (OMSW) in laboratory scale semi-continuous tubular digesters. Each digester was fed with an influent composed of OMW and OMSW over a range of hydraulic retention times (HRTs) of 12, 24 and 36 days and at organic loading rates (OLRs) between 1.19 and 10.84 g COD/(l day). The TCOD concentration of OMW were 43, 67 and 130 g COD/l and the amount of the dry OMSW co-digested with OMW was 56 g TS/l. The results indicated that the best methane productivity and SCOD removal efficiency of 46 l/((l OMW) day) and 69%, respectively, were achieved at an OLR of 3.62 g COD/(l day) corresponding to an OMW of 130 g COD/l digested at an HRT of 36 days. Furthermore, the best net energy production from digesters operated at thermophilic temperature was 427 kJ/day higher than from those operated at mesophilic temperature for the same conditions of feed concentration and HRT. In contrast, an HRT of 12 days caused a sharp increase of both total volatile fatty acids (TVFA) and chemical oxygen demand (COD) contents in the effluents and a cessation in methane production for the three influents substrate concentrations studied.