Selected BMPs: Constructed Treatment wetland

Definition: Constructed treatment wetlands are artificial shallow water-filled basins that have been planted with emergent plant vegetation. Constructed treatment wetlands are designed to achieve specific stormwater water -quality objectives before the water is discharged.

Purpose: Constructed treatment wetlands can be an efficient method for removing a wide variety of pollutants, such as suspended solids, nutrients (nitrogen and phosphorus), heavy metals, toxic organic pollutants, and petroleum compounds. wetlands also are an effective means of reducing peak runoff rates and stabilizing flow to adjacent natural wetlands and streams.

Application:Constructed treatment wetlands require an area sufficiently large for impounding stormwater in shallow basins. In sloping terrain, wetland cells can be arranged in series on terraces. Treatment wetlands can be completely artificial. Alternatively, for limited applications, existing natural wetlands can be used to accept pretreated runoff from watersheds that are adversely affected by urbanization or agricultural land use. Constructed wetlands are appropriately located at the lower parts of sites.

Recommended Design Criteria:

Requirements for Regulatory Compliance

Stormwater wetlands that are constructed entirely outside of the Waters of the United States (33 CFR Part 328), and are explicitly designed for stormwater management, are not subject to the provisions of Sections 401 and 404 of the Clean Water Act. However, when the stormwater wetlands are abandoned, they may be regulated as wetlands.

Existing wetlands may be used as part of a stormwater management system if the wetland can be considered an "urban or degraded nontidal wetland" and no feasible alternatives exist (U.S. Army Corps of Engineers (USCOE), 1997). When the specific conditions established by USEPA are satisfied, construction in an existing wetland for creating or enhancing stormwater management functions is permitted under the Clean Water Act, Section 404(b)(1). In addition, the provisions of 25 Pennsylvania Chapter 105 must be considered before initiating any project that will influence existing wetlands.

Wetlands that 1) will have a contributing drainage area exceeding 100 acres, 2) will have embankments higher than 15 feet, measured from the downstream toe, or 3) will impound more than 50 acre-feet of runoff during the high-water condition, may be regulated as dams PADEP. The designer should consult 25 Pennsylvania Chapter 105 to determine which provisions may apply to a specific project.

Performance-Based Guidelines

Pretreatment Requirements

Most raw stormwater must be pretreated to maximize the treatment effectiveness and ancillary wildlife benefits of treatment wetlands. In particular, highly variable water levels and high hydraulic loading rates (HLRs) are not conducive to wetland plant survival and treatment efficiency. In many instances, stormwater flows should be attenuated and equalized in a preliminary DRY POND, or forebay, to optimize pollutant assimilation in the wetland. Pretreatment in stormwater pond systems or conveyance facilities such as grass swales also may be important for reducing high sediment and pollutant loads before they are discharged to the treatment wetland. Approaches for implementing pretreatment are presented in the BMP descriptions for WET POND and GRASS SWALE.

Area Requirements

The design of constructed treatment wetlands requires knowledge of pollutant influent concentrations, base flow and stormwater flow rate characteristics, and effluent goals. Constructed treatment wetlands typically are designed to achieve a specific HLR, computed as the design inflow rate divided by the surface area of the wetland. For stormwater wetlands, the water quality design storm should be used as the basis for computing the design inflow rate (see Section 5.3 of the Handbook). A conservative approach is to use the time-weighted average of the highest inflow rates occurring during a 1-hour time period.

The North American wetland Treatment System Database (USEPA, 1993) and other similar data, are a basis for designing most wetland treatment systems. Kadlec and Knight (1996) have developed area-based, first-order wetland design models to predict treatment area requirements. HLRs computed using this method will vary widely, because of site-specific differences in runoff quality, discharge requirements, seasonal low temperatures, and stormwater flow characteristics.

Depth Requirements

The water balance for the facility, including infiltration, percolation and evapotranspiration losses must be calculated to determine the expected range of pool levels. wetland plants are dependent upon saturated soil conditions for varying time periods. The plants typically incorporated in constructed wetlands will require some standing water in deep zones during all but the driest periods. Shallow water zones can be dry at the surface for longer periods, but not exceeding 1 month. The permeability of the wetland base may need to be reduced by introducing a clay layer to maintain the required hydroperiod for wetland plant communities.

The wetland vegetation depends greatly on water depth. Shallow areas, typically 1 foot in depth, enable emergent plant vegetation to grow, whereas submerged plants prefer deeper water. Large unvegetated open-water areas near the wetland outlet should be avoided so the potential planktonic algae growing is reduced. As a "rule-of-thumb," the shallow water zone should comprise at least 80 percent of the total wetland area.

In addition to the treatment objectives, consideration must be given to the ability of the vegetation to accommodate the range of inundation depths anticipated in the wetland. Some wetland systems have failed because of transient hydraulic problems. To prevent the disruption of the wetland plant communities and to improve the pollutant-removal efficiency, wetlands may be constructed in combination with stormwater detention facilities. In this way, the peak flow rate to the wetlands during rainfall events can be reduced and flows equalized.

As an alternative, so-called extended detention wetlands can be designed by providing adequate wetland area and embankment freeboard to impound the design storm. If the base elevation of the supplemental detention area is approximately equal to the normal pool elevation, a new growth zone is created in which water is ephemerally ponded adjacent to the margins of the permanent pool.

Figure 1. Schematic cross section of constructed treatment wetland.

Hydraulic Requirements

Stormwater wetlands are likely to have periods without inflow between storm events. However, a regular supply of inflow water is preferred for the biological health of the system. Flow equalization ponds upstream of treatment wetlands should be designed with outlet structures that detain flow over the longest practical interval to help maintain a steady flow to the wetland. If the inflow of water is not reliable, episodes of stagnant water will interfere with the treatment function of the wetland and increase the likelihood of mosquitoes and nuisance odors. A method for manually adjusting the normal water level should be provided. Being able to adjust the water level is important for assisting plant growth in the early phases of development and for optimizing performance in response to seasonal variations in inflow rate. The outlet design must be resistant to fouling by floating or submerged plant material and accessible to operators.

Even distribution of flow is vital for achieving the treatment function of wetlands. Narrow, deep water zones should be at the inlet and outlet to balance head conditions and evenly distribute flow. Inlets also may incorporate pipe manifolds to enhance flow distribution. Deep water zones, oriented transverse to the direction of flow, and internal berms, oriented parallel to flow, also can be used to minimize the potential for short-circuiting of flow.

Embankments must be designed to accommodate:

1. High water events associated with large rainfall events
2. Head loss through the system under varying operating conditions

Figure 2. Conceptual layout of constructed treatment wetland.

Head loss is an especially important consideration for stormwater treatment wetlands where a large range in flow rates are encountered. Mathematical descriptions are often adaptations of open-channel flow formulae. The formulae are discussed in detail in a number of texts (for example, French, 1985) and empirical constants from treatment wetlands are available (Kadlec and Knight, 1996). The general approach uses equations for mass, energy, and momentum conservation coupled with an equation for frictional resistance. Examples of the equations are in the "Specifications and Methodology" section. All wetlands must have a high-level outlet to pass large runoffs from storm events.

Plant Requirements

High pollutant-removal efficiencies are dependent on a dense cover of emergent plant vegetation. Actual plant species do not appear to be as important as plant growth habit. In particular, species should be used that have high colonization and growth rates, can establish large surface areas that continue through the winter dormant season, have high potential for treating pollutants, and are very robust in continuously flooded environments. Noninvasive native species should be emphasized. Examples include bulrush (Scirpus sp.), arrowhead (Sagittaria latifolia), soft rush (Junus effusus), and pickerelweed (Pontederia cordata). Other plant species can be incorporated in constructed treatment wetlands to enhance ecosystem diversity and to create greater wildlife value. A comprehensive list of wetland plant species adapted for incorporation in constructed treatment wetlands is provided in Appendix H.

Operation and Maintenance: The designer should understand the biological requirements of the plants and manage water levels to provide for their needs. Optimum conditions are not always required. The plants’ environment is most critical during seed germination and early establishment of plants.

wetland plants can be drowned by excessive water depth. Usually, initial growth is best with transplanted plants in wet, but well-aerated soil. Leaving the majority of the growing plants exposed, and occasionally inundating, will enable the plants to obtain oxygen and grow fastest. On the other hand, frequent soil saturation is important for wetland plant survival.

Plant cover needs to be assessed periodically and documented. Dramatic shifts can occur as plant succession proceeds. The plant community reflects management and can indicate improvement or problems. For example, submergent aquatic plants such as pondweed (Potamogeton pectinatos) require that light penetrate into the water column. The disappearance of these plants indicates problems with water clarity.

Dikes, embankments, and hydraulic control structures should be inspected regularly and immediately after any unusual flow event. wetlands also should be checked after periods of rapid ice break-up. Any damage, erosion, or blockage should be corrected as soon as possible.

Unlike wet or dry stormwater ponds, sediment is rarely removed from constructed treatment wetlands. Sediment removal disturbs stable vegetation cover and disrupts flow paths through the wetland. The embankment height of constructed treatment wetlands should be designed to accommodate the gradual accumulation of sediment over the lifetime of the facility. Likewise, outlets should be designed to compensate for sediment accumulation by allowing the normal pool elevation to be adjusted to higher levels.

Considerations: As Figure 3 shows, wetlands are effective sedimentation devices and provide conditions that facilitate the chemical and biological processes that cleanse water. Pollutants are taken up and transformed by plants and microbes, buried in sediment, or released in the wetland’s discharge.

Figure 3. wetland microbes, plants, and soil transform and take up pollutants in the wastewater.

Plants improve water quality by slowing water flow, settling solids, taking up wastewater pollutants, and supplying reduced carbon and attachment area for microbes (bacteria and fungi). Of these functions, the most important are physical; dense stands of vegetation create the quiescent conditions that facilitate the physical, chemical, and biological processes that cleanse water. Most herbaceous wetland plants die annually. Because the dead plant material requires months to years to decompose, a dense layer of plant litter accumulates. Like the living vegetation, the litter creates a substrate that supports bacterial growth and physically traps solids.

The most important microbial processes are decomposition of organic compounds, ammonification (conversion of organic nitrogen to ammonia), nitrification (conversion of ammonia to nitrite and nitrate), and denitrification (release of nitrogen to the atmosphere).

Microorganisms, adhering to vegetation, roots, and sediment in the wetland can convert significant quantities of nitrate directly to nitrogen gas. Large amounts of nitrogen and phosphorus also can be incorporated in new soil and in the extra biomass of the wetland vegetation. For these reasons, maintaining the health of the vegetative community is critical.

Long-term data from wetland treatment systems indicate that treatment performance for parameters such as 5-day biochemical oxygen demand (BOD₅), total suspended solids (TSS), and total nitrogen (TN) typically does not deteriorate with age. However, the dissolved oxygen (DO) concentration in wetland effluent may be below 1.0 mg/L. Higher DO concentrations can be achieved in effluent by incorporating turbulent or cascading discharge zones.

Site conditions that may increase the cost of constructed treatment wetlands included high land costs, sloping topography, highly permeable soil, and low depth to bedrock. A liner may be required in some constructed treatment wetlands to reduce percolation and conserve water. Wetland topsoil must be suitable for healthy plant growth. Where the existing site soil is unsuitable for growth (such as clayey or rocky soil), it is beneficial to apply a rooting zone of about 12 inches of loamy or sandy soil. Embankments must be designed with adequate freeboard to accommodate the accretion of sediment over the design life of the facility.

References:

Nursery Information and Referral Assistance

Pennsylvania Landscape Nurseryman’s Association, 1707 S. Cameron St.,1924 N. Second St., Harrisburg, Pa. 17104
[800-898-3411]

Morris Arboretum, 9414 Meadowbrook Ave., Philadelphia, Pa. [215-247-5777]

Design Guides

Kadlec, R. H. and R. L. Knight. Treatment wetlands. Boca Raton, Florida: Lewis Publishers. 1996.

USEPA, North American Treatment wetland Database, Electronic database created by R. Knight, R. Ruble, R. Kadlec, and S. Reed, 1993 (Copies available from USEPA; 513-569-7630).

Wren, C.C., C.A. Bishop, D.L. Stewart, and G.C. Barrett. Wildlife and contaminants in constructed wetlands and stormwater ponds: current state of knowledge and protocols for monitoring contaminant levels and effects in wildlife. Canadian Wildlife Service, Environmental Conservation Branch, Ontario Region. Technical Report Series Number 269. 1997.

Davis, Luise. A Handbook of Constructed wetlands, A Guide to Creating Wetlands for Stormwater in the Mid-Atlantic Region (Volume V). National Resources Conservation Service and U.S. EPA–Region II, in cooperation with Pennsylvania Department of Environmental Resources. 1996.

Schueler, T. R. Design of Stormwater wetland Systems. Metropolitan Washington Council of Governments. Anacosta Restoration Team, Department of Environmental Programs. 1992.

U.S. Army Corps of Engineers. Guidelines for Reconciling Stormwater Management and Natural Resources Protection Issues. Federal-State Interagency Stormwater/wetlands Workgroup, USEPA–Region III. 1997.

Cooper, P. F. and Findlater, B. C. (eds.). Constructed wetland in Water Pollution Control. Oxford. 1990.

Reed, S. C., E. J. Middlebrooks, and R. W. Crites. Natural Systems for Waste Management and Treatment. New York: McGraw-Hill. 1988.

Water Pollution Control Federation. Natural Systems for Wastewater Treatment Manual of Practice. FD-16. S. Reed (ed.). Washington, D.C. 270 pp. 1990.

Tchobanoglous, G. and F. L. Burton. Wastewater Engineering Treatment, Disposal, and Reuse. Third Edition. New York: McGraw-Hill. 1991.

U.S. Environmental Protection Agency. Design Manual. Constructed wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment. EPA/625/1-88/022. 1988.

Hammer, D. E. and R. H. Kadlec. Design Principles for wetland Treatment Systems. U.S. Environmental Protection Agency. Office of Research and Development. EPA-600/2-83-026. 1983.

French, R. H. Open-Channel Hydraulics. New York: McGraw-Hill. 1985.

Specifications and Methodology:

First-Order, Area-Based Constructed wetland Sizing Model for Conceptual Design

(Based on Kadlec and Knight, 1996)

General Model:

J = k (C - C^*) ; where k = k₂₀ q _k^(T-20)

C^* = C^*₂₀ q _c^(T-20)

where: J = removal rate (g/m²/yr)

k = first-order, area-based rate constant (m/yr)

k₂₀ = rate constant at 20°C (m/yr)

C = pollutant concentration (mg/L)

C* = irreducible background concentration (mg/L)

C*₂₀ = irreducible background concentration at 20^o C (mg/L)

T = temperature, °C

q _c = temperature coefficient for background concentration

q_k = temperature coefficient for rate constant

wetland Area (based on modified plug-flow hydraulics):

where: HLR = hydraulic loading rate (m/yr)

A = wetland area at normal pool elevation (m²), excluding habitat islands

Q = design inflow rate (m³/yr)

C₁ = inflow concentration (mg/L)

C₂ = outflow concentration (mg/L)

Model Parameter Values (at 20°C):

Outlet Design

The outlet for the wetland should be designed to achieve two objectives:

1. Enable adjusting the normal operating level of the wetland manually
2. Detain stormwater

wetlands should have both low-level and high-level outlets. High-level outlets such as weir boxes or broadcrested spillways should be sized to pass the 100-year 24-hour storm event (or larger maximum design storm event). The low-level outlet should be readily adjustable to change the normal pool elevation. In many instances, the wetland also will be designed to achieve a specific runoff peak attenuation. Flow routing using the methods described in the WET POND BMP should be used for the design. If forebays or detention ponds are used for equalizing flow, a multipond routing approach will be required.

The outlet device in Figure 4 incorporates the following design features:

• High-level weir box overflow
• Mid-depth opening to exclude floating plant material or bottom debris
• Adjustable V-notch weir
• Easy accessibility for inspection and maintenance

Figure 4. Design of typical outlet control structure.

Infiltration Design and Water Balance

The rate of infiltration through the base of the wetland can be estimated by using Darcy’s law. For most wetlands, the rate of infiltration is relatively constant. wetlands act as storage reservoirs, retaining water during precipitation events and releasing it slowly as outlet flow and infiltration. During summer months when evapotranspiration losses are large, pool levels commonly drop episodically below the design operating level and outflow ceases. However, water infiltrated in the wetland will continue to replenish the water table and will help stabilize base flow to adjacent drainages.

Ideally, wetlands should not completely dewater under conditions of normal precipitation. To identify potential problems, a monthly water balance should be constructed for the wetland. The pool level at the end of each month can be estimated as follows:

PL = PL0 + [BF + (PR x AW) + (PR x AD x RO) - (ET x AW) -(I x A)]/A

Where: PL = Pool depth at the end of month (feet)

PL0 = Pool depth from the previous month (feet)

BF = Total monthly flow into the wetland (acre-feet)

PR = Total monthly precipitation (feet)

AW = Area of wetland (acres)

AD = Area of tributary drainage (acres)

RO = Runoff coefficient

ET = Monthly potential evapotranspiration (feet)

A = Area inundated at depth PL0 (acres)

I = Monthly infiltration (feet)

If PL is greater than the normal pool depth established at the outlet, then outflow will occur during that month. The quantity is not important. In months with a net outflow, the beginning pool depth for the next month will equal the normal pool depth.

Tables or equations for estimating potential evapotranspiration are available from many sources, including Kadlec and Knight (1996). However, for a conceptual design, wetland evapotranspiration can be estimated as 80 percent of the pan evaporation rate.

In most wetlands, the area that is inundated varies with depth. The normal operating pool depth also may be adjusted seasonally to accommodate changes in the water budget. These factors should be accounted for in the calculation.

If the water balance predicts that the wetland will dewater, design modifications can be considered, including:

• Reducing the infiltration rate by adding a clay layer or synthetic liner
• Increasing the drainage area that is tributary to the wetland
• Increasing the normal operating pool level

Limitations on increasing the normal pool level will be imposed by the need for shallow water habitat to support emergent plant vegetation.

Short periods during which the wetland becomes dry may be tolerated in some instances. However, the selection of plants must be tailored to accommodate these adverse conditions and special considerations will be required for the maintenance of the wetland during dry periods.