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 CO2C 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−2h−1, respectively). In vertical subsurface flow (VSSF) CWs the CH4Cemission was significantly lower than in horizontal subsurface flow (HSSF) CWs (median values 3.0, 6.4,and 4.0 mg m−2h−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−2h−1correspondingly).The highest value of emission factor (EF) of CH4((CH4C/inflow TOCin) * 100%) was found for FWSCWs (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 FWS,and 0.018% for VSSF CWs.We found a significant correlation between TOCin and CH4C emission and between the TNinandN2O 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 thepoint 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 GH Gemissions.201

Municipal effluents contain heavy metals in particular copper, lead, nickel, zinc and mercury. The concern regarding the impact of these contaminants in the aquatic environment is reflected in the latest criteria revisions to water-related legislation, such as the Clean Water Act in the US and the Water Framework Directive in Europe. These revisions may lower metal concentrations considered acceptable in final effluents. The study of metal behaviour within the wastewater treatment process is crucial to develop and optimise systems to achieve acceptable metal concentrations in effluent discharges. To comply with more stringent environmental quality standards, utilities may require the implementation of additional treatment technologies. This review evaluates the performance of conventional municipal wastewater treatment processes, that is, primary sedimentation and biological treatment via the activated sludge and trickling filter processes. The mechanisms and parameters influencing metal removal are discussed to evaluate how an understanding of metal behaviour within wastewater treatment processes may allow optimisation and enhancement of conventional treatment processes, or inform the selection of new technologies to enhance trace metal removal. The performance of advanced treatment technologies capable of removing metals from municipal wastewater under real-world conditions has also been evaluated.