Exergy analysis of the urban water cycle: a case study of Brasília, Brazil

Abstract

"Urban water supply and sewage systems must operate in a way that guarantees service to the population, optimizing the consumption of resources, environmental impacts, and energy depletion. Seeking to contribute to this goal, this thesis studied the applicability and reliability of exergy analysis as a diagnostic tool in the urban water cycle systems of Brasília, Federal District, Brazil. A bibliometric review conducted in the Scopus and Web of Science databases revealed a growing interest in applying the concept of exergy to the exergy-urban water nexus in water cycle systems and formed the basis for the study's methodology. Using operational data from both systems provided by CAESB, the steady-state processes were modeled, and mass, energy, and exergy balances were performed. In the city's water supply system, 2,461 m³ s⁻¹ of raw water are collected, 17.6% are lost in the distribution of treated water, and 72.1% return to the environment, closing the water balance. The electricity demand is 10.68 MW, with an energy intensity of 4.34 kWh m⁻³ and a specific energy of 3.6 W inhabitant⁻¹. Raw water pumping dominates the thermodynamic profile, where approximately 60% of the total exergy is destroyed and efficiencies range between 59.8% and 76.1%. At the water treatment plant, the exergy efficiency is 7.6%, with 53.6% of the incoming exergy related to chemical reagents, of which 82.9% is destroyed by internal irreversibilities. In treated water distribution, the exergy efficiency of pumping is 70.7%, and losses represent 43%, equivalent to 260 kW of useful pumping exergy. In the sewage system, wastewater treatment is the most dissipative stage of the cycle due to biochemical oxidation. The overall exergy efficiency of the Brasília’s UWCremained around 69–70% throughout the operational chain. Analyses indicated that strategies such as modernization of old pumps, reduction of treated water losses, and recovery of biogas generated in wastewater treatment plants (WWTPs) can significantly improve the operational performance of the both systems and the exergy efficiency of the cycle. The exergy analysis method, combined with the evaluation of process emissions, proved effective in identifying bottlenecks, guiding operational improvements, and promoting more rational use of resources. Finally, the study offers a replicable quantitative methodology capable of identifying critical points, supporting optimization actions, and contributing to the reduction of environmental impacts on the urban water cycle.