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CONSTRUCTED WETLANDS FOR MUNICIPAL
WASTEWATER TREATMENT

By STEVEN L. BOTCH and ROBERTA J. LIGHT

May 10,1994
Institute of Local Government Affairs
A Joint Project of the Office of the Comptroller, State of Illinois and Southern Illinois University

The past 10 or 15 years have witnessed a major shift in the funding of federally-mandated programs. As federal and state funds for these programs have declined or disappeared, municipalities have frequently borne the brunt of this funding shift due to their relative position in the federal system. One positive outcome of the resulting challenge to municipalities has been increased efficiency and innovation in the development of both new technology and financing arrangements.

Such innovation can be seen in the response to clean water legislation, and is illustrated by the communities of Benton and Hardin in Kentucky and in Granger, Iowa. These small communities, ranging in size from 500 to 4,200, have installed constructed wetlands for treatment of their municipal wastewater. This method of treating wastewater is both cost effective and environmentally sound. The results demonstrated by these examples are particularly relevant for Illinois, which has more than 1,000 communities with populations below 5,000. The approach is not necessarily limited to very small towns, however; Columbia, Missouri, is presently developing constructed wetlands to meet the wastewater treatment needs of its 62,000 residents.

A wetland is a transitional area between terrestrial and aquatic ecosystems populated by hydrophyte plant species, those which grow only in water or very wet earth. A constructed wetland is a manmade or engineered ecosystem that is designed with a saturated substrate, the soil level to which the plant is attached and from which it derives its nutriment. The saturated soil is then planted with emergent and submergent hydrophytes that emulate a natural wetland. The ideal choice of plants includes bulrush, cattails, and reeds due to their widespread natural distribution, hardiness, pollution uptake qualities, and extensive root systems,

The Tennessee Valley Authority (TVA) recommends use of constructed wetlands for their simplicity and efficiency. Dr. Donald Hammer of the TVA lists the following five main components for this type of treatment system:

1. substrates with various rates of hydraulic conductivity;

2. plants adapted to water-saturated anaerobic substrates;

3. a water column (water flowing in or above the surface of the substrate);

4. presence of both invertebrates and vertebrates;

5. an aerobic and anaerobic microbial population (Hammer, 1989:14)

The two primary types of constructed wetlands are freewater surface and subsurface flow systems. Surface flow systems use shallow ponds with either floating or emergent plants to treat wastewater as it flows over the roots. Floating plants such as water hyacinth are very effective in treatment but their temperature sensitivity makes them unsuitable for Illinois.

The subsurface flow system employs a 2-foot permeable medium of rock, sand, or soil in a sloped bed with an impermeable liner. Wastewater enters at the influent end, and flows laterally through the roots and rhizomes of the aquatic plants that have been placed there, emerging as significantly treated effluent at the discharge end.

Microbial degradation occurs along with filtration, sorption, and precipitation processes, as the plants stimulate both decomposition of organic matter and growth of microorganisms. Since microbial populations are ubiquitous to most wastewater, there is no need for restocking. The submerged portions of the plants also provide the substrate for microbial growth.

The Benton, Kentucky (population 4,200) system was begun in 1986, utilizing both the surface and the subsurface design. The actual construction cost was $250,000. Initially, Benton had two lagoons in series, the first 16 acres and the second 10 acres. The second lagoon was divided into three equal cells, the first two of

June 1994 / Illinois Municipal Review / Page 11


which would each handle 25% of the flow. The third cell was of the subsurface type, and designed to handle 50% of the flow. Each cell used a different plant/substrate combination including cattails, bulrush, and an ornamental mixture, for comparison. Some corrections and replantings were required as the system became fully operational.

A performance study of the Benton cells' individual effluent was done from May to October of 1989. The monitoring data was summarized in a 1990 TVA Report, which also included data from the Hardin facility. Table I is a summary of the findings from this report.

Table I

Benton Cell #1 Cell #2 Cell #3
Vegetation Cattail & Ornamentals Woolgrass & Duckweed Softstem Bulrush
BOD 10 mg/l 17 mg/l 7 mg/l
TSS 11 mg/l 14 mg/l 5 mg/l
NH3-N 8 mg/l 7 mg/l 8.5 mg/l
Coliforms 700/100 ml 130/100 ml 100/100 ml

Hardin, Kentucky, uses a subsurface type constructed wetland to meet the water treatment needs of its 540 residents and a small package plant. This facility was designed with two equal sized gravel marsh cells, each having a surface area of approximately three quarters of an acre. The actual construction cost was $234,000.

The Hardin cells were planted in Fall of 1988, the first cell with common reed, and the second with soft- stem bulrush. Both species required replanting at a later time, and the replanting has been successful. Volunteer species have also established themselves, and there is presently a dense mass of vegetation in both cells. A summary of the pollution uptake data can be seen in Table II.

Table II

Hardin Cell #1 Cell #2
Vegetation Common Reed Softstem Bulrush
BOD 10 mg/l 4 mg/l
TSS 20 mg/l 10 mg/l
NH3-N 11 mg/l 6.5 mg/l
Coliforms 1200/100 ml 260/100 ml

The award-winning system at Granger, Iowa, was designed by the engineering firm of Veenstra & Kim. This central Iowa community of 700 treats its 500,000 gallons per day effluent with two aerated lagoons of approximately 2.5 acres each. Following this process are two "L" shaped wetland cells with an area of 3.2 acres each. The vegetation used at Granger includes 80% bulrush and 20% cattails, both taken from local species, with no differentiation between cells. Replanting has not been required. The actual cost of this system was $472,000.

The Granger system is unique in that it was designed specifically as a polishing process to reduce the Ammonia Nitrogen in the effluent. Currently, 95% of the ammonia in the effluent is being removed, with the resulting levels as low as 5 mg/1 in the winter, and undetectable in the summer. The Granger marsh efficiency can be seen in the following Table III.

Table III

Granger Cell #1 & 2
Vegetation Bulrush/Cattail
BOD 6 mg/l
TSS 19 mg/l
NH3-N 5 mg/l
Coliforms less than 400/100 ml

Constructed wetlands can be used in an original design, to upgrade an existing facility, or to alleviate nonpoint source pollution problems. They are also potentially useful for treating industrial and toxic discharges, as wetland vegetation has the ability to remove and store toxins. The major constraint associated with this method is the availability of adequate land.

In terms of cost effectiveness, the surface flow system is the preferred method, in spite of a slight treatment advantage enjoyed by the subsurface design. Surface flow systems are simpler, and do not suffer from the tendency to plug up, as subsurface systems do. A polyculture system with a dominance of bulrush is best, and local plant species should be selected.

In conclusion, constructed wetlands offer a low-cost, environmentally sound, and aesthetically pleasing alternative that may be appropriate for many Illinois municipalities. This option should be investigated before a decision is made to employ high-cost, conventional methods for wastewater treatment, particularly where compliance with IEPA standards has been difficult to achieve.

Table IV

Table IV

More detailed information regarding use of constructed wetlands for municipal wastewater treatment is available in a paper entitled "Constructed Wetlands in Municipal Wastewater Treatment: A Cost-Effective Option for Small Cities." A copy of this paper can be obtained from the Institute of Local Government Affairs, Southern Illinois University, 150 Pleasant Hill Road, Carbondale, IL 62901, or by calling 618-453-8222.

Page 12 / Illinois Municipal Review / June 1994


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