All wetland treatment systems in the United States are preceded by some form of preliminary treatment, ranging from the equivalent of primary to tertiary levels from advanced wastewater treatment systems. The level of preapplication treatment required depends on the functional intent of the wetland component, on the level of public exposure expected, and on the need to protect habitat values. The minimal preliminary treatment for municipal wastewaters would be the equivalent of primary, accomplished with septic tanks or Imhoff tanks for small systems or a pond unit with a deep zone for sludge accumulation for larger systems. It is considered prudent to provide the equivalent of secondary treatment prior to allowing public access to the wetland components or developing specific habitats to encourage birds and other wildlife. This level of treatment could be accomplished in a first-stage wetland unit where public access is restricted and habitat values are minimized. Tertiary treatment with nutrient removal may be necessary prior to discharge to natural wetlands where preservation of the existing habitat and ecosystem is desired. Common preliminary features in stormwater wetlands are a trash rack and a forebay to allow the settling and removal of large objects carried with the stormwater runoff. Wetlands designed for mine drainage treatment may require a preliminary unit for pH or alkalinity adjustment (Brodie et al, 1993).
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