Constructed wetlands utilize emergent plant growth on saturated soils (bulrushes, reeds and cattails) to support physical-chemical actions and biological activity that remove wastewater pollutants. The two most common forms of constructed wetlands involve either open water surfaces or Subsurface Flow Wetland (SFW) through porous media.
The treatment processes include sedimentation and adsorption of TSS and microbial oxidation of colloidal/soluble organics. Reaeration and translocation of oxygen from plant leaves to the root zone (rhizosphere) provide the major sources of oxygen for carbon oxidation and nitrification reactions. The disadvantages of SFS treatment include the cost associated with purchase, transportation and construction of the porous media and the potential for overloading the influent section with organics and suspended matter, causing plugging and flooding problems. Uniformly graded stone gravel is commonly used at a depth of approximately two (2) feet. The open water surface (FWS) wetlands typically consists of one (1) foot of water over one (1) foot of topsoil.
Subsurface flow constructed wetlands are referred to by a number of descriptive terms including, microbial-plant filter and subsurface vegetated beds. As the subsurface media typically contains, graded rock or stone (1 to 4 inch diameter) and washed gravel (1/4 to 3/8 inch size), the term rock filter is also used to describe this constructed wetland. SFW can be used to reduce the contaminants found in primary effluent down to a level that may approach secondary treatment. These SFW’s are also used to process and improve the quality of secondary effluent to an enhanced treatment level (polishing). The uniform distribution of flow through the rock and gravel media, as established by the influent piping is an important design factor. The influent flow must pass slowly and uniformly around the media at depths of one to two feet and travel the length of the wetland from inlet to outlet. The average time of travel from inlet to outlet, known as the detention time, should equal or exceed 24 hours. Longer detention times may be necessary to accomplish the specified degree of treatment. The outlet structure must provide for an adjustable flow depth, from a maximum flooding depth down to a completely drained condition. Flow level control is a critical factor in the establishment and maintenance of emergent plant root systems. Flooding or submergence of the wetlands media is necessary to control weed growth on the media. Water tolerant plants (reeds, rushes, etc.) that grow naturally in the area around the wetlands location seem to provide the most reliable treatment results.
The SFW wetlands should be lined with impervious man-made materials or natural soil, similar to lagoons, when used to process primary effluent. However, SFW’s receiving secondary effluent may not be provided with an impervious liner along their entire length. The effluent end of these SFW’s may be unlined to provide for soil infiltration of treated effluent with minimal risk of ground water contamination. In addition, recirculation of treated effluent back to the influent of the SFW should improve the quality of the effluent attained.
Open Water Surface Wetlands (FWS) are typically used to provide advanced treatment and nutrient removal for secondary or tertiary effluents. The emergent vegetation in a FWS provides an abundant surface area for attached microorganisms that are capable of reducing organic and nutrient levels in such treated wastewaters.
Design standards for constructed wetlands are not well established at this time, but the design standards developed by the Tennessee Valley Authority (TVA) are considered adequate for small flows. The design loading values for secondary treatment of primary effluent have been proposed at 15 to 25 acres per MGD. The design loadings to accomplish nitrification in a FWS have been estimated at 50 to 100 acres per MGD depending on the temperature of the treated flow. Additional operating data should be evaluated and a number of issues resolved, including:
1.) Pretreatment levels necessary to optimize performance.
2.) Proper hydraulic and organic loading rates.
3.) Optimum geometry and configuration (length to width and use of multiple units).
4.) Most efficient inlet and outlet design.
5.) Optimum depths for liquid and media.
6.) Optimum media size and distribution.
7.) Most efficient type and arrangement of plants and optimum plant management (harvesting).
8.) Maintaining effective nitrification (oxidizing ammonia to nitrate).