Enhanced Biomethane Production from Olive Mill Wastewater via Co-Digestion with Cow Dung, Fruits, Vegetable, and Fish Wastes: An Experimental and Kinetic Study

Olive mill wastewater (OMW) is the main effluent resulting in huge amounts from olive oil manufacturing. This effluent is mostly composed of organic matter and polyphenolic compounds, known for their antimicrobial activity and compromise their biological treatment. This work investigates the impact of the co-digestion of olive mill wastewater with fruits and vegetable waste (FVW), fish waste (FW), and cow dung (CD) under mesophilic conditions at two different inoculum-to-substrate ratios. The effect on biomethane yield, volatile solids reduction, and polyphenol removal efficiency were evaluated. Moreover, kinetic modeling was applied to describe biomethane production. The co-digestion of OMW with CD at an I/S ratio of 2:1, and a mixture consisting of 33% OMW, 33% FVW, and 33% FW at I/S ratio of 3:1 achieved biomethane yields of 155.00 NmLCH ₄ gVS ^− 1 and 132.20 NmLCH ₄ gVS ^− 1 , respectively after 49-day retention time at 37°C whereas the mono-digestion of OMW was completely inhibited. These treatments demonstrated strong performance in terms of volatile solids and polyphenol removal, achieving rates of 76%, 81%, and 95% and 84%, respectively. Similarly, the logistic function model provided a good fit for predicting biomethane production, with high R² values of 0.9941 and 0.9930, respectively.