Subsurface flow (SSF) wetlands consist of shallow basins or channels with a seepage barrier and inlet and outlet structures. The bed is filled with porous media and vegetation is planted in the media. The water flow is horizontal in the SSF wetland and is designed to be maintained below the upper surface of the media, hence the title subsurface flow. In the United States, the most common medium is gravel, but sand and soil have been used in Europe. The media depth and the water depth in these wetlands have ranged from 1 ft (0.3 m) to 3 ft (0.9 m) in operational systems in the United States. The design flow for most of these systems in the United States is less than 50,000 gpd (189 m3/d). The largest system in the United States (Crowley, LA) has a design flow of 3.5 mgd (13,000 m3/d) (Reed et al., 1995). A schematic of a typical SSF wetland is shown in Figure 7.1.
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