A constructed wetland provides an additional treatment stage, necessary when a conventional system alone is not adequate to overcome the limitations of a site. These artificial wetlands typically consist of one or more trenches referred to as cells. Each cell has a lining which can either be impermeable or permeable, depending on the system, and contains vegetation anchored in a medium such as gravel. When planning to use such a system, proper care needs to be taken to ensure that it is designed to accommodate the specific characteristics of the site. Bulrushes, cattails, reeds, rushes, and sedges are types of vegetation used in constructed wetlands.
Irises are very attractive wetland plants that perform well in constructed wetlands.
The medium and vegetation present in the cells provide several functions for the system. The principal function of the plant roots is to transpire oxygen and thus aerate the water. Aerobic conditions allow for a larger variety of microorganisms to attach themselves to the surface of the roots and medium. These microbes are the primary source of treatment, feeding on the waste products in the water. The medium and plant roots help polish the water by trapping tiny particles.