Integrated Mass and Energy Balance Modelling for Energy Recovery from Wastewater Sludge Through Anaerobic Digestion Within a Circular Economy Framework

The transition towards circular economy models is driving the transformation of wastewater treatment plants (WWTPs) from energy-intensive facilities into resource recovery systems capable of generating renewable energy. In this context, this study developed an integrated mass and energy balance methodology to assess sludge production, anaerobic digestion performance, biomethane recovery, and electricity generation in a full-scale urban WWTP. The proposed framework integrates the water treatment line, sludge processing line, and energy recovery system, combining primary and secondary sludge management with biogas upgrading and combined heat and power (CHP) generation. Representative operating parameters from the scientific literature were applied to a facility treating 204,000 m3 d−1 and serving approximately 425,000 population equivalents. The results showed that primary sludge accounted for approximately 70% of the volatile solids fed to the anaerobic digester. Methane production was estimated at 1.12 × 103 kg CH4 d−1, corresponding to a biogas production of 2.40 × 103 m3 d−1. Under two alternative valorisation scenarios, the maximum recovered biomethane flow was 1.48 × 103 m3 d−1, whereas the maximum annual electricity generation potential through CHP was 1.9 × 106 kWh. These findings highlight the potential of integrated sludge valorisation strategies to enhance renewable energy recovery and support the transition of WWTPs towards energy-efficient and low-carbon resource recovery facilities.