Constructed Wetland

The Dave Irwin Memorial Boardwalk and Constructed Wetland was opened in memory of former property manager, David Irwin, on July 12, 2007.  Billy Ewton donated materials and labor for a railing and TVA funded new signage for the boardwalk.

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 controlled in both surface and subsurface systems.  In subsurface systems, the normal water level is kept one inch below a gravel surface which improves treatment and controls mosquitoes.

A second cell may be added for more treatment.  It may be left unlined so that treated water seeps into the soil below.  In smaller systems, the second cell can contain a layer of gravel, covered with soil, and then mulch.  It can be planted with a variety of attractive wetland plants such as iris, elephant ear, and arrowhead.  The plants transpire water into the atmosphere.  The wetlands may or may not discharge treated wastewater into surface waters depending upon the design, size, and local site conditions.

How is wastewater treated in constructed wetlands?

As wastewaters flow through the system, suspended solids and trace metals settle and are filtered. Trace metals are also absorbed by plants and organic material.  Organisms which 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 easily 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 table are needed for small scale systems.  Mosquitoes 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 town and rural areas often have problems meeting today�s 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 wastewaters. 

Where can I get more information?

Contact TVA�s Water Quality Department, HB 2S 270C, 311 Broad Street, Chattanooga, Tennessee 37402-2801 or call (615) 751-3164.