Greenhouse Gas Mass-Balance in Conventional Activated Sludge Wastewater Treatment: A Case Study in Mexico for Developing Countries

While numerous studies report methane emissions from wastewater treatment plants (WWTPs) in developed countries, few address emissions from plants in developing countries, where outdated technologies, such as the lack of enhanced primary and sludge treatment, are common. Moreover, these studies often rely on indirect calculations rather than direct measurements. Our study fills this gap by providing unit-process-level direct measurements of methane emissions in a conventional WWTP in Mexico, serving as a case study for developing countries. A standard plant was selected and visited on five occasions. It includes a primary settler, an aerated reactor, and a secondary settler, with no sludge treatment in place. Our findings revealed a CH4 emission factor of 0.396 +/- 0.218 g CH4 m-3 of treated water, with the primary settler accounting for 72.3 +/- 15.9% of emissions, and the aerated reactor contributing 27.7 +/- 15.9%. Notably, the emission factors are comparable to those reported for plants with more advanced treatment technologies, suggesting that technological obsolescence may not significantly enhance CH4 emissions. Methanotrophy in the aerated reactor was a key process, oxidizing 91-98% of the CH4 transported from the primary settler. Additionally, a carbon dioxide (CO2) emission factor of 97.4 +/- 34.4 g CO2 m-3 was measured, primarily from the aerated reactor, consistent with the plant's overall treatment efficiency. These findings provide crucial data for understanding greenhouse gas emissions from WWTPs in developing regions and highlight the need for targeted mitigation strategies.