Gaseous Fluxes from Subsurface Flow Constructed Wetlands for Wastewater Treatment

Abstract

We measured nitrous oxide (N₂O), dinitrogen (N₂), and methane (CH₄) fluxes in two constructed wetlands (CW) in Estonia using the closed chamber method and the He-O method in the period from October 2000 to March 2003. Emission rates of N₂O-N, N₂-N and CH₄-C from both CWs varied significantly on a both spatial and temporal scale, ranging from 1 to 2,600, 170 to 130,000, and –1.7 to 87,200 μ g m^{− 2} h^{− 1} respectively. The average flux of N₂O from the microsites in the Kodijärve horizontal subsurface flow (HSSF) CW and Kõo hybrid CW ranged from 27 to 370 and from 72 to 500 μ g N₂O-N m⁻² h⁻¹, respectively, whereas the average dinitrogen flux from the microsites in the HSSF CW in Kodijärve was 2–3 magnitudes higher than the N₂O flux, ranging from 19,500 to 33,300 μ g N₂-N m^{− 2} h^{− 1}. The average methane emissions from the microsites in the Kodijärve HSSF CW and the Kõo hybrid CW ranged from 31 to 12,100 and from 950 to 5,750 μ g CH₄-C m^{− 2} h⁻¹, respectively. The highest emission values for all three gases were observed in the warm period. There was a significant relationship between emission rates and water table depth: CH₄ and N₂ emission increased and N₂O emission decreased when the water table did rise. Although the emission of N₂O and CH₄ from CWs was found to be relatively high, their global warming potential (GWP) in the time horizon of 100 years is not significant, ranging from 4.5 to 16.3 tonnes of CO₂ equivalents per ha per year in Kodijärve and from 12.1 to 17.3 t CO₂ equivalents ha^{− 1} yr⁻¹ in Kõo.

Key Words:

• Constructed wetland
• Denitrification
• Dinitrogen
• Global Warming Potential
• Methane
• Nitrification
• Nitrous oxide.

Notes

^#—average of left and right beds.