Constructed wetlands (see “Constructed Wetlands,” EPA and factsheet from the Center for Clean Water Technology) are engineered systems for wastewater treatment, reuse or disposal that use natural biological treatment technologies that incorporate wetland vegetation, soils and associated microorganisms to remove contaminants. They are shallow water bodies or gravel (or engineered media) filled basins vegetated with plants adapted to continuous or periodic inundation. They range in size from small to large and provide natural treatment by settling solids, degrading organic wastes, and assimilating nitrogen and phosphorus through natural biological transformations. Water quality is improved through system sizing, uniform depth, and flow distribution to maximize contact between water and the wetland sediments and biological communities.

They are typically used as a secondary treatment for blackwater, greywater, and stormwater after primary pretreatment. They are relatively simple to implement on small scales, and can be modified to increase the degree of wastewater treatment, including providing tertiary treatment through enhanced nitrogen and phosphorus removal. Constructed wetlands function as wastewater polishing for cluster systems, satellite systems, or centralized systems, and as such, can serve populations of fewer than 50 all the way up to city-sized populations. They can also be used to address overflow during high flow conditions; they also have the added benefit of being ecologically important and aesthetically beautiful.

Constructed wetlands are found worldwide and are designed to meet a variety of objectives (e.g., mangrove microcosms in Hong Kong) and tertiary treatment through an engineered wetland in New Delhi). In general, there are two basic types of constructed wetlands:

• Subsurface Flow Systems, also called root-zone systems, rock-weed filters, and vegetated submerged bed systems, that are media-based using soil, sand, gravel and/or crushed rock. Essentially, wastewater flows through natural wetland filters. As designed and operated, these systems provide limited opportunity for benefits other than water quality improvement.
• Free Water Surface Systems simulate natural wetlands in which water flows over the soil. In these systems, breakdown and decomposition of solids is facilitated by the algae and bacteria that live in the oxygenated water, while larger plants also growing in the system take up the abundance of nutrients that are produced during decomposition. These systems are often designed to maximize wetland habitat value and reuse opportunities, while providing water quality improvement (e.g., Green Cay Treatment Wetlands).

Constructed wetlands can be used to both create and restore wetlands. Many free water surface systems also function as wildlife refuges and/or parks. These systems provide an area for public education and recreation in the form of birding, hiking, camping, hunting, and more. (See the image below for some free water surface systems in the U.S.) It is an elegant eco-friendly solution that converts the problem of wastewater management into a natural benefit for the environment and its inhabitants (see EPA case studies). They are typically decentralized solutions suitable for both small communities and as upgrades to large treatment facilities.