Enhanced Biogas Production in Pilot Digesters Treating a Mixture of Olive Mill Wastewater and Agro-industrial or Agro-livestock By-Products in Greece

The selection of appropriate co-substrates is very important to the feasibility of an anaerobic co-digestion process. A proper choice of co-substrate compositions leads to system balance and increased methane generation. To valorize agricultural wastes and byproducts in southern Greece, anaerobic co-digestion of four feedstocks (olive mill wastewater—OMW, poultry manure—PM, liquid pig manure—LPM, and cheese whey—CW) was studied to produce biogas for renewable energy generation. Pilot- scale continuous co-digestion approaches were adopted to carry out the investigation under mesophilic temperatures (35 ± 2 °C) for 30 days. The feedstocks were mixed at different percentages according to their availabilities in southern Greece with a total solids (TS) ratio of approximately 10% and an OLR of 2.2 kg VS m−3 day−1. The main agro-industrial feedstock of this study is OMW. Two types of influent feedstock were utilized: a mixture of 30% v/v OMW and 70% PM and LPM, and a mixture of 40% v/v of OMW and 60% PM and CW. Therefore, optimization of biogas production from OMW was attempted by co-digesting with PM and LPM or CW. The aim of this study was to investigate the effect of different liquid feedstocks in order to have a total solids (TS) ratio of approximately 10%. Reduction in the volatile solids ranged between 50 and 57%. The average removal of dissolved chemical oxygen demand (COD) was 50% and 58% for the two examined scenarios, while the biogas production rate at the steady state reached 0.7 ± 0.4 and 1.2 ± 0.3 L/Lreactor/day respectively.