Clean production of power and heat for waste water treatment plant by integrating sewage sludge anaerobic digester and solid oxide fuel cell

•Anaerobic digester and solid oxide fuel cell was integrated to generate power and heat for WWTP.•4E analysis (energy, exergy, economic, environmental) utilized for system modeling.•Multi-objective (payback period and exergy efficiency) method was applied for optimizing.•System energy and exergy efficiency increased about 6.86% and 7.55% respectively with 1.74 year pp.•SOFC net electrical power output and electrical power sold to the grid increased by 7.76% and 35.56%