Comparing Greenhouse Gas Fluxes from Passive Urban Stormwater Management to Conventional Wastewater Treatment

Abstract

Green infrastructure (GI) is a popular tool to improve city resilience to climate change and improve water quality. However, because these systems receive pulses of nutrient rich stormwater (SW), they may be contributing excess greenhouse gas (GHG) emissions. This study quantifies the amount of CO2-eq emitted per L of stormwater treated for both GI basins and conventional wastewater treatment plants (WWTP). GHG and stormwater monitoring were conducted at two stormwater basins in Ithaca, NY: one site was often saturated (anaerobic basin), and the other was quick draining (aerobic basin). In opposition to our hypothesis, rain events did not produce larger CH4 or N2O fluxes at the basins. Compared to literature values of field-scale monitoring at WWTPs (0.22 g CO2-eqL−1), the anaerobic basin produced similar CO2-eq (0.23 g CO2-eqL−1), but the rates were 55 times lower in the aerobic basin (0.004 g CO2-eqL−1). Using EPA calculations, SW sent to a WWTP would produce seven times more CO2 than if it were sent to an aerobic basin. Conversely, SW sent to a WWTP instead of an anaerobic basin would lead to an approximately eight times decrease in CO2. This shows that some GI (aerobic treatments) may be a benefit to reduce GHG emissions instead of the alternative WWTP. However, anaerobic GI may actually increase GHG emissions over the traditional WWTP. These results are especially relevant to combined sewer systems, where decisions must be made to upgrade decaying infrastructure and/or invest in GI. From a GHG perspective, installing aerobic GI practices would likely reduce the GHG emissions from cities.