From Conventional to Sustainable: Decadal Evolution of Wastewater Treatment and Its Environmental Impacts in a Fast-Growing City

This study analyzes wastewater treatment plants (WWTPs) in Querétaro, Mexico, over a decade (2013–2022), highlighting the interdependencies between pollutant removal, energy consumption, and greenhouse gas (GHG) emissions. Despite being conventional systems, the WWTPs achieved high pollutant removal efficiencies (92–97% BOD5, 50–60% TN), preventing 146.1 MtCO2e over ten years. However, this efficiency came with some drawbacks: a 10% reduction in pollutants increased electricity consumption by 7%. CO2 emissions from grid-fed operations increased by 130% between 2021 and 2022, emphasizing the carbon intensity of improving water quality. In 2022, the system emitted 0.002 tCO2e/m3 of treated water, due to indirect N2O and CH4 emissions from untreated flows and electricity consumption. With only 70% of wastewater treated and minimal energy recovery, the existing infrastructure offers environmental benefits but operates near its efficiency limits in the face of increasing demand. The transition to energy-neutral models, through biogas cogeneration, solar integration, and advanced nutrient removal, is crucial for achieving urban water systems resilient to climate change. This case study provides valuable insights for cities seeking to balance water security, sustainability, and decarbonization in rapidly developing regions.