Emission of greenhouse gases from constructed wetlands for wastewater treatment and from riparian buffer zones

Abstract

We measured N2O, CH4 and CO2 fluxes in horizontal and vertical flow constructed wetlands (CW) and in a riparian alder stand in southern Estonia using the closed chamber method in the period from October 2001 to November 2003. The average rates of N2O, CH4 and CO2 emission from the riparian gray alder stand were from −0.4 to 58μg N2O-N m−2h−1 and 0.1–265μg CH4-C m−2h−1, 55–61mg CO2-C m−2h−1, respectively. The average N2O-N emission from the microsites above the inflow pipes of horizontal subsurface flow (HSSF) CWs was 6.4–31μg N2O-N m−2h−1, whereas the outflow microsites emitted 2.4–8μg N2O-N m−2h−1. In vertical subsurface flow (VSSF) beds the same value was 35.6–44.7μg N2O-N m−2h−1. The average CH4 emission from the inflow and outflow microsites in the HSSF CWs differed significantly ranging from 640 to 9715 and from 30 to 770μg CH4-C m−2h−1, respectively. The average CO2 emission was somewhat higher in VSSF beds (140–291mg CO2-C m−2h−1) and at inflow microsites of HSSF beds (61–140mg CO2-C m−2h−1). The global warming potential (GWP) from N2O and CH4 was comparatively high in both types of CWs (4.8±9.8 and 6.8±16.2 t CO2 eq ha−1 a−1 in the HSSF CW 6.5±13.0 and 5.3±24.7 t CO2 eq ha−1 a−1 in the hybrid CW, respectively). The GWP of riparian alder forest from both N2O and CH4 was relatively low (0.4±1.0 and 0.1±0.30 t CO2 eq ha−1 a−1, respectively), whereas the CO2-C flux was remarkable (3.5±3.7 tha−1 a−1). The global influence of CWs is not significant. Even if all the global domestic wastewater were treated by wetlands, their share in the trace gas emission budget would be less than 1%.