Greenhouse gas emission in constructed wetlands for wastewater treatment: A review

A literature analysis of 158 papers published in international peer-reviewed journals indexed by the Thomson Reuters Web of Knowledge from 1994 to 2013 showed that CO2-C emission was significantly lower in free water surface (FWS) constructed wetlands (CW) than in subsurface flow (SF) CWs (median values from 95.8 to 137.0 mg m(-2) h(-1), respectively). In vertical subsurface flow (VSSF) CWs the CH4-C emission was significantly lower than in horizontal subsurface flow (HSSF) CWs (median values 3.0, 6.4, and 4.0 mgm(-2) h-1, respectively). There were no significant differences in N2O-N emission in various CW types (median for FWS, VSSF and HSSF CWs: 0.09, 0.12, and 0.13 mg m(-2) h(-1) correspondingly). The highest value of emission factor (EF) of CH4 ((CH4-C/inflow TOCin)*100%) was found for FWS CWs (median 18.0%), followed by HSSF CWs (3.8%), and VSSF CWs (1.28%). Median values of N2O EFs ((N2O N/inflow TNin)*100%) differed significantly in all three CW types: 0.34% for HSSF, 0.11% for PATS, and 0.018% for VSSF CWs. We found a significant correlation between TOCin and CH4-C emission and between the TNin and N2O-N emission values for all of the types of CWs we studied. Hybrid CWs (e.g., the subsequent combination of VSSF, HSSF and FWS CWs) are beneficial from the point of view of both water purification and minimization of greenhouse gas (GHG) emissions. Likewise, intermittent loading in VSSF CWs and macrophyte harvesting in HSSF and FWS CWs can mitigate GHG emissions. (C) 2013 Elsevier B.V. All rights reserved.