Methane emissions from sewer covers: A case study of Calgary's wastewater collection system

Sewer covers present an interface for biogenic methane (CH₄) gas produced in the wastewater collection and transportation system to escape to the atmosphere. However, the contribution of these emissions to municipal CH₄ footprints is not well understood. In this study, direct methane emission measurements were made at 118 sewer covers in Calgary, Alberta, Canada to quantify and characterize the emissions from the city's wastewater collection network. Emission rates ranged from 0.00 to 0.241 g CH₄ h^-1. The annual cumulative wastewater collection emissions are estimated to be 3.92 t CH₄ (3.38 to 4.47 t CH₄, 95% confidence interval). This is 1.40% of all wastewater treatment-related emissions in Calgary, based on a gridded inventory and 0.005% of Calgary's total CH₄ footprint, according to a city-scale satellite estimate. Sewer covers measured near locations in the wastewater network that are known to be more favorable environments for CH₄ production (n = 27) were found more likely to emit CH₄ and had statistically higher emission rates compared to the sewer covers that were not (n = 91). Overall, this study indicates that wastewater collection emissions are small for Calgary relative to other biogenic and thermogenic sources in the city; however, this result might not be indicative of the municipal emission patterns from other cities.Implications: This research is one of only a handful of urban case studies that has quantified CH₄ emissions from sewer covers using a direct measurement method. The findings of this study indicate that sampling strategy and upscale modelling of sewer cover emissions depend on knowledge of city-specific wastewater infrastructure, which is useful for guiding future work in this limited field.