Exergy analysis in the processes of municipal wastewater treatment plants

The produced sludge in WWTPs can be used as a renewable energy source to supply a part of their energy demands. The condition of the wastewater entering the treatment plant changes after passing through different units in terms of pollution and exergy. In this research, exergy analysis of raw wastewater entering the WWTPs in the south of Tehran has been studied. Six other WWTPs have also been examined in general in terms of exergy. The results show that physical exergy compared to chemical exergy in the WWTPs south of Tehran is insignificant and negligible. Also, a small part of the wastewater exergy in the Screening units and Grit chamber units is removed from the treatment plant cycle and energy is not recyclable. Other results include the destruction of most of the exergy of WWTPs in the aerobic reactor, which is about 30% of the exergy of the inlet wastewater and about 93% of the total exergy destruction. Also about 3700 kW of exergy is removed through the expulsion of dewatered sludge from the treatment plant. In the study of 6 other treatment plants, it was found that the larger the capacity of the treatment plant, the exergy of consumable work will have a smaller share in the exergies entering the treatment plant compared to the exergy of the inlet wastewater. Also, with increasing biogas exergy, the exergy rate of dewatered sludge decreases. In addition, the exergy change of inlet wastewater in treatment plants of 3 and 5 is about 10% while this variation for other treatment plants is more than 40%.