Quantifying greenhouse gas emissions from wastewater treatment plants: A critical review

Greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs) are increasingly recognized as significant contributors to anthropogenic climate change, primarily through the release of methane (CH₄), nitrous oxide (N₂O), and carbon dioxide (CO₂). Current research on GHG quantification in WWTPs predominantly relies on estimated emission factors. However, this introduces substantial uncertainties in emission estimates due to limited in situ measurements and variability in quantification methods. Here we review advances in GHG measurement techniques, integrating literature data with our in situ studies. We show that unit-based methods, such as flux chambers and optical gas imaging, pinpoint emission hotspots in individual processes, while plant-integrated approaches-like tracer gas dispersion, mobile laboratories and aerial surveys-deliver comprehensive plant-scale estimates. These techniques reveal wide variability in emissions, with CH₄ rates spanning 0.04-427 kg h^-1 and N₂O up to 22.1 kg h^-1, but most studies are short-term, gas-specific and neglect fossil CO₂, which can inflate IPCC inventories by up to 22.8 % upon inclusion. Technology- and plant-specific emission factors, calibrated via on-site data, markedly enhance accuracy by accounting for local factors like treatment processes and influent composition. We call for national emission inventories via long-term, multi-gas measurements, guiding targeted mitigation strategies and transforming WWTPs toward carbon-neutral, climate-smart infrastructures.