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.