Anaerobic co-digestion Anaerobic digestion Article Chemical industry Cost benefit analysis Environmental problems Hydraulic retention time Mediterranean countries Methane Municipal sewage sludge Municipal wastewater treatment Olive mill wastewaters Olive oil Petroleum industry Phenols Sewage sludge Sludge digestion Treatment and disposal Wastewater disposal Wastewater treatment adsorption anaerobic digestion biogas cost benefit analysis filtration hydraulic retention time methane municipal wastewater olive mill wastewater olive mill wastewater; sewage sludge; anaerobic digestion; polyphenolic compounds; municipal wastewater; cost–benefit analysis