Wetlands are defined for this book as ecosystems where the water surface is at or near the ground surface for long enough each year to maintain saturated soil conditions and related vegetation. The major wetland types with potential for water quality improvement are swamps that are dominated by trees, bogs that are characterized by mosses and peat, and marshes that contain grasses and emergent macrophytes. The majority of wetlands used for wastewater treatment are in the marsh category, but a few examples of the other two types also exist. The capability of these ecosystems to improve water quality has been recognized for at least 30 years. The use of engineered wetland systems for wastewater treatment has emerged during this period at an accelerating pace. The engineering involved may range from installation of simple inlet and outlet structures in a natural wetland to the design and construction of a completely new wetland where one did not exist before. The design goals of these systems may range from an exclusive commitment for treatment functions to systems that provide advanced treatment or polishing combined with enhanced wildlife habitat and public recreational opportunities. The size of these systems ranges from small on-site units designed to treat the septic tank effluent from a single-family dwelling to 40,000-ac (16,200 ha) wetlands in South Florida for the treatment of phosphorus in agricultural stormwater drainage. These wetland systems are land intensive but offer a very effective biological treatment response in a passive manner so that mechanical equipment, energy, and skilled operator attention are minimized. Where suitable land is available at a reasonable cost, wetland systems can be a most cost-effective treatment alternative, while also providing enhanced habitat and recreational values.
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