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

## TL;DR

This study measures methane and CO2 emissions from a conventional wastewater treatment plant in Mexico, offering insights for developing countries.

## Contribution

The study provides direct, unit-process-level measurements of methane emissions in a conventional WWTP in a developing country.

## Key findings

- The primary settler accounted for 72.3% of methane emissions.
- Methanotrophy in the aerated reactor oxidized 91–98% of methane from the primary settler.
- CO2 emissions were primarily from the aerated reactor at 97.4 g CO2 per cubic meter of treated water.

## Abstract

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from plants in developing countries, where outdated technologies,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.