Constructed Wetlands Factsheet

By National Small Flows Clearinghouse

What are constructed wetlands?

Constructed wetlands are designed and built similar to natural wetlands to treat wastewater. They consist of a shallow depression in the ground with a level bottom. The flow is controlled in constructed wetlands so the water is spread evenly among the wetland plants. In natural wetlands, 90% of the water may flow through small channels. Controlling the flow allows natural processes to occur and clean the wastewater more efficiently.

Why build wetlands?

Constructed wetlands provide simple and effective wastewater treatment. They can be used to treat domestic, agricultural, industrial and mining wastewaters. Their construction costs are much less (50 to 90%) than conventional systems and their operating costs are very low. Constructed wetlands are also pleasant to look at, attract desired wildlife, and provide environmental education opportunities.

How do constructed wetlands work?

Wastewater flows through a pipe from a septic tank or other type of primary wastewater treatment system into the constructed wetland. Wastewater can either flow on top of the existing soil (surface) or through a porous medium such as gravel (subsurface). Flow is distributed evenly across the width of the wetland cell. A waterproof liner is used on the sides and bottom of the cell to prevent leaks and assure adequate water for the wetland plants. This cell is planted with wetland plants such as cattails and bulrushes. Roots and stems of the plants form a dense mat. Here chemical, biological, and physical processes occur to treat the wastewater. Water levels are con trolled in both surface and subsurface systems. In subsurface systems, the normal water level is kept 1 inch below a gravel surface which improves treatment and controls mosquitoes. A second cell may be added for more treatment.

How is wastewater treated in constructed wetlands?

As wastewaters flow through the system, suspended solids and trace metals settle and are filtered. Plants and organic material also absorb trace metals. Organisms that live in water, on rocks, in soil, and on stems and roots of wetland plants use these organic materials and nutrients as food. Plants provide much of the oxygen needed by the organisms to live and grow. Plant roots keep the rocks or soil loose so that water can flow through easily.

Why not use constructed wetlands to treat all wastewater?

The lack of standard information for engineers is currently limiting the design and construction of effective low cost systems. Constructed wetlands may not effectively treat some types of complex pollutants. These systems also need more land than conventional systems. High land costs and lack of suitable land can make construction of large systems impractical. Sites which are relatively flat, have deep soils, and a low groundwater water table are needed for small scale stems. Mosquito can also be a problem, but this can be prevented or controlled with proper system design and management.

Are constructed wetlands for you?

Constructed wetlands will answer many, but not every, wastewater need. They may be used by small towns, developers, small businesses, individual homes with failed septic systems, farms operations, and some types of industry. They meet secondary treatment limits and can be designed for advanced treatment. Their construction and operating costs are very low, and they do not need operators who are highly trained.

Small towns and rural areas often have problems meeting todays strict water quality standards. Many lack central wastewater treatment systems. Constructed wetlands can now provide them with a good wastewater treatment alternative. Smaller versions can also be built to meet the needs of homes with failed septic systems. Constructed wetlands can help us improve water quality as we learn more and apply them effectively to treat various types of wastewater

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