Assessment of the exergy and power density potential in the effluent flow of wastewater treatment plants

In this paper, historical data from the seven wastewater treatment plants in the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) is analyzed for its potential of waste heat recovery. The potential recovered waste heat is calculated monthly and yearly from effluent flow data and average regional minimum and maximum temperatures over a 10-year period between the years of 2011–2020. Based on the data analyzed, between 79.98 and 381.02 GWh/year could be recovered from all wastewater treatment plants in the MWRDGC combined. While the temperature differences between the effluent flow and the ambient temperature ranged between 0.3 and 19.2 °C, the primary factor in work energy potential was found to be the effluent mass flow rates, which ranged between 24,800 and 36,600 kg/s. Additionally, by viewing the work energy potential as the thermal source to either a theoretical Carnot heat engine or an endoreversible heat engine, a corresponding theoretical mechanical power output can be calculated. Despite Carnot and endoreversible thermal efficiencies of less than 5%, a power density between 0.004 and 2.84 W/m² was calculated for every month except for June – August and was found to be comparable to hydropower generation systems.