Each year, contaminants in millions of galls of domestic wastewater must be removed before the water can be safely returned to the environment. The requisite level of wastewater treatment depends upon the desired level of contaminant reduction or removal. During primary treatment, physical operations such as sedimentation remove floating and settleable solids. Secondary treatment provides additional purification through the reduction of organic content and removal of nonsettleable solids. However, many of the contaminants now found in wastewater, such as nitrogen and phosphorus, are not affected by these conventional treatment processes. Advanced, or tertiary, treatment processes- processes that require costly technology and use large amounts of energy- are needed to remove these "resistant" contaminants.
Interest in the use of wetlands for the advanced treatment of domestic wastewater has grown within recent years. Wetlands are known to effectively remove nitrogen, phosphorus, and suspended sediments from wastewater. Their use eliminates the high capital cost of treatment plant construction and operation, and greatly reduces energy consumption. Significantly, the application of nutrient-rich wastewater can be used to enhance degraded wetlands or even create completely new wetland systems. But wetland treatment facilities have not been carefully monitored for any extended period of time and there is little data on the long-term impacts of repeated discharges of nutrient-rich wastewater into a wetland system.