Utslipp av drivhusgassene N2O og CH4 fra sandfiltre og konstruerte våtmarker i sør-Norge / Emission of the greenhouse gases n2O and Ch4 from sand filters and constructed wetlands in south-east norway

This article reports about flux measurements of CH4 and N2O from two types of constructed wetlands in south-eastern Norway, a free surface water (FSW) wetland polishing chemically treated municipal wastewater and a meso scale sub-surface flow (SSF) wetland receiving wastewater from a single domestic household. Fluxes of both gases had a large temporal and spatial variation. The average flux of CH4 was found to be 160 og 110 mg CH4-C m–2 d–1 in the FSW and SSF wetlands, respectively, while the average flux of N2O was 8,3 og 75 mg N2O-N m–2 d–1, respectively. The measured fluxes of methane are in the same range as fluxes measured in other types of constructed wetlands, and are also similar to fluxes from natural wetlands in the subartic and boreal zones. Regarding fluxes of N2O, constructed wetlands treating wastewater have in general higher emission rates than European forested ecosystems, but seem to have rates comparable to most European agricultural ecosystems. The climatic impact of constructed wetlands is considered to be low, and the positive effects such as improved water quality are considered to be more important than the negative effects from the greenhouse gas emissions.