Greenhouse gas emissions and nitrogen removal from a subalpine zone treatment wetland: Implications of mass transfer effects and dosing

Abstract

Today, there is increased focus on greenhouse gas emissions (GHGs) from wastewater treatment, as industrialization and global population growth threaten to exponentiate current emission trends. This study focuses on the effects of intermittent dosing on nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) emissions from a two-stage, low temperature vertical flow treatment wetland (VFTW) treating 3 °C ski resort wastewater. The system has a partially saturated first stage and unsaturated second stage with recycle to optimize for total nitrogen removal. Following two automated closed-loop GHG sampling campaigns across 16 locations in the TW, emission profiles were found to vary significantly by location. Smaller influent wastewater doses resulted in higher proportion of dissolved N2O and indications of carbon limitations across the first stage of the system due to incomplete denitrification. Small and large doses resulted in spikes of CH4 and N2O emissions, suggesting mass transfer effects temporarily increased emissions. CH4 was generated almost entirely in the primary sedimentation tank, with an observed 70 % drop in emissions from hours 1 to 8 of the dosing schedule. Additionally, heavy snowpack and ice lens formations resulted in preferential gas flow in the first stage. These results suggest that estimates of GHG emissions from TWs may be falsely elevated due to mass transfer and environmental conditions that hinder transport. The results indicate a need to continually inventory GHGs due to the complexity of wastewater systems, and that specific wastewater application methods may help to mitigate emissions.