Greenhouse gas emissions from surface flow and subsurface flow constructed wetlands treating dairy wastewater.

Agricultural wastewater treatment is important for protecting water quality in rural ecosystems, and constructed wetlands are an effective treatment option. During treatment, however, some C and N are converted to CH₄ and N₂O, respectively, which are potent greenhouse gases (GHGs). The objective of this study was to assess CH₄, N₂O, and CO₂ emissions from surface flow (SF) and subsurface flow (SSF) constructed wetlands. Six constructed wetlands (three SF and three SSF; 6.6 m² each) were loaded with dairy wastewater in Truro, Nova Scotia, Canada. From August 2005 through September 2006, GHG fluxes were measured continuously using transparent steady-state chambers that encompassed the entire wetlands. Flux densities of all gases were significantly (p < 0.01) different between SF and SSF wetlands and changed significantly with time. Overall, SF wetlands had significantly (p < 0.01) higher emissions of CH₄ and N₂O than SSF wetlands and therefore had 180% higher total GHG emissions. The ratio of N₂O to CH₄ emissions (CO₂–equivalent) was nearly 1:1 in both wetland types. Emissions of CH₄–C as a percentage of C removal varied seasonally from 0.2 to 27% and were 2 to 3× higher in SF than SSF wetlands. The ratio of N₂O–N emitted to N removed was between 0.1 and 1.6%, and the difference between wetland types was inconsistent. Thus, N₂O emissions had a similar contribution to N removal in both wetland types, but SSF wetlands emitted less CH₄ while removing more C from the wastewater than SF wetlands.