Effect of mixture ratio and organic loading rate during anaerobic co-digestion of olive mill wastewater and agro-industrial wastes

Environmental pollution is a serious concern within the Mediterranean area, this situation results mainly from olive mill wastewaters (OMW), the liquid effluents generated during olive oil extraction. Anaerobic digestion of olive mill wastewaters is negatively affected by nitrogen deficiency, low pH, and the presence of phenolic compounds. This study examines the feasibility of OMW co-digestion with rich nitrogen substrates in batch and continuous mode under mesophilic conditions. Four substrates were tested at different ratios (substrate/OMW) in batch mode. Optimal ratio (substrate/OMW) was 80:20 for poultry manure (PM), 20:80 for both cheese whey (CW) and grass, and 50:50 for slaughterhouse wastewater (SWW). Furthermore, the effect of organic loading rate was assessed in continuous mode, using an anaerobic sequencing batch reactor for the mix (SWW/OMW) at 50:50. The reactor was able to degrade 10 g COD/L/day, as the maximal limit for biogas production and process stability. The results proved that co-digestion of SWW with OMW at 50:50 produced the higher biogas volume in batch mode and ensure good process stability, expressed by high alkalinity (2150 eq mg CaCo₃/L) and low rate of volatile fatty acids (VFA) (150 eq mg acetic acid/L). Co-digestion of OMW with SWW is an efficient solution, to solve environmental pollution and recover renewable energy from these two organic wastes.