Biogas production from anaerobic co-digestion of agroindustrial wastewaters under mesophilic conditions in a two-stage process

Co-digestion of organic wastes is a technology that is increasingly being applied for simultaneous treatment of several solid and liquid organic wastes, in which the content of nutrients can thereby be balanced, and the negative effect of toxic compounds on the digestion process may be decreased giving an increased gas yield from the biomass. Moreover, co-digestion may contribute to a more efficient use of anaerobic digestion (AD) reactors and cost-sharing by processing multiple waste streams in a single facility. In this study, efficient biodegradation of a mixture containing olive mill wastewater (OMW), liquid cow manure (LCM) and cheese whey (CW) using a two-stage anaerobic process was examined. Two continuously stirred tank reactors (CSTRs) were used under mesophilic conditions (35°C) in order to enhance acidogenesis and methanogenesis. The overall process was designed with a hydraulic retention time (HRT) of 19 days. The acidogenic reactor was initially fed with an influent mixture composed of 55% OMW, 40% CW and 5% LCM. After the 87th day of operation, the mixture was changed to 90% of CW and 10% of LCM based on the actual availability of wastes produced by the respective agroindustries. The average removal of dissolved chemical oxygen demand (COD) was 75.5% and 85.2% for the two examined scenarios at OLR of 5.5 ± 0.36 and 4.5 ± 0.30 g total COD/Lreactor/d, while the methane production rate at the steady state reached 1.35 ± 0.11 and 1.33 ± 0.15 L CH 4/Lreactor/day respectively.