The term constructed wetland refers to a specially designed and constructed area containing wetland plants through which wastewater is passed to provide treatment. There are two basic types of constructed wetlands. One is a surface flow wetland where there is an open water surface and the second is a subsurface flow wetland where water is not visible nor directly exposed to the atmosphere. Both types are shallow excavations, lined to prevent infiltration and filled with gravel or stone. The gravel or stone supports the plant roots and provides surfaces on which microorganisms grow. Both the plants and microbes are involved in the wastewater treatment process. Constructed wetlands are sized to retain wastewater for several days as it flows through. The exact time depends upon the strength of the wastewater being applied and climatic conditions. Many constructed wetlands consists of two or three cells in series The wetland plants used are varied from inlet to outlet with the plants near the inlet being shallow rooted and capable of dealing with higher wastewater strengths; deeper rooted plants are located near the outlet end.

During Agriculture & Natural Resources Week, a special program was held on constructed wetland applications in cold climates. Speakers were invited from Indiana, Wisconsin and Minnesota to discuss experiences and present data from constructed wetlands for domestic wastewaters, milking center wastewater and agricultural processing waste. In addition, speakers from Michigan discussed applications of constructed wetlands for individual home wastewater treatment and larger domestic wastewater facilities.

Even in winter, constructed wetland treatment results in an effluent that is greatly reduced in pollutant concentration compared to effluent from a septic tank where the waste has simply undergone primary treatment. Some of the concentration reduction in wetlands is due to dilution by precipitation but significant biological treatment occurs as well. The data presented at the workshop showed that most of the oxygen demanding organic matter is removed as wastewater flows through constructed wetland. Pathogenic bacteria are reduced along with concentrations of both nitrogen and phosphorus. Data presented show a nitrogen concentration reduction in the range of 40-60%. Phosphorus reduction varied from a low of 7% for one milking center wastewater system to over 75% for some domestic wastewater systems.

Clearly, constructed wetlands have the potential to improve the quality of dilute wastewaters but probably not to the point where they could be surface-discharged. These wastewaters will still need to be land applied either in subsurface soil absorption systems, in the case of domestic effluents, or on agricultural land in the case of agricultural effluents. The improved effluent quality will allow higher soil application rates either in subsurface trenches or on the surface of agricultural lands. For subsurface systems, it will reduce or eliminate soil-clogging resulting in systems of much longer life.