Wastewater treatment processes can produce anthropogenic greenhouse gases, including carbon dioxide (CO2 ), methane (CH4 ) and nitrous oxide (N2 O). Wastewater treatment in the U.S. emitted 17.8 Tg CO2 equivalent (CO2 -eq) in 2012, accounting for 0.27% of the U.S. total greenhouse gas emission and 14.3% of the total greenhouse gas emission from waste management and treatment activities (U.S. EPA, 2014). Constructed wetlands are green infrastructure for treatment of various types of wastewater. As ecologically engineered treatment systems, constructed wetlands mimic the appearance of natural wetlands (Figure 1). Constructed wetlands utilize natural processes involving macrophytes, soils or other porous media, and the associated microbial assemblages for water quality improvement. There are mainly three types of constructed wetlands . Free water surface (FWS) wetlands typically consist of cells with aquatic plants, relatively impermeable rooting substrate, and shallow water. Treatment in FWS constructed wetlands occurs as water flows slowly above ground through the leaves and stems of aquatic plants. Horizontal subsurface flow (HSSF) wetlands contain beds of porous media that may have been planted with aquatic plants. Wastewater flows horizontally beneath the surface of the medium beds. Vertical flow (VF) wetlands contain beds of media that may have been planted with aquatic plants. Water is distributed over the ground surface or from the base, and flows downward or upward through the medium beds.
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