Constructed Wetlands

Background
Constructed wetlands are engineered, man-made ecosystems specifically designed to treat wastewater, mine drainage, and other waters by optimizing the biological, physical, and chemical processes that occur in natural wetland systems. Constructed wetlands can provide effective, economical, and environmentally-sound treatment of wastewater as well as serve as wildlife habitats.

Constructed wetland systems are grouped into three main types: free-water surface (FWS), subsurface flow systems (SFS), or aquatic plant systems (APS). FWS systems, or soil substrate systems, consist of aquatic plants rooted in a soil substrate within a constructed earthen basin that may or may not be lined depending on soil permeability and groundwater protection requirements [1]. FWS systems are designed to accept preliminary-treated, low-velocity wastewater, in plug flow, over the top of the soil media or at a depth between 1 and 18 inches. SFS are typically gravel substrate systems that are similar to FWS systems, however, aquatic vegetation is planted in gravel or crushed stone and wastewater flows approximately 6 inches below the surface of the media. The aggregate typically has a depth between 12 and 24 inches. No visible surface flow is evident in SFS [1]. APS also are similar to FWS systems, but the water is located in deeper ponds and aquatic floating aquatic plants or submerged plants are used [2].

Applicability
Constructed wetlands may be used to treat municipal wastewater, agricultural runoff, mine drainage, and other effluents. Biochemical oxygen demand (BOD) and total suspended solids (TSS) are effectively reduced by these man-made wetland systems.

Limitations
Technical guidance for designing and operating constructed wetlands may be limited due to the lack of long-term operational data. Potential seasonal variability and impact on wildlife may negatively impact system operation and securing of permits, respectively [3]. Relatively large parcels of land are required and water consumption is high due to large evapotranspiration rates.

Performance
Results of previous investigations, listed by the type of effluent or stream treated, are summarized in Tables 1 to 6.

Table 1. Performance data - unspecified waste streams [4].

Type of System	Free-water	Subsurface	Combination
BOD Removal Efficiency, %	53 - 89	80 - 82	58 - 96
TSS Removal Efficiency, %	76 - 88	90 - 92	77 - 94

BOD: biochemical oxygen demand
TSS: total suspended solids

Table 2. Performance data - swine/farm pond wastewater [5].

Parameter*	Removal Efficiency, %
BOD	90.4
TSS	91.4
Fecal Coliform	99.4
Fecal Streptococci	98.4
Total Phosphorus	75.9
Total Kjeldahl Nitrogen	91.4
Ammonium Nitrogen	93.6

* Free-water system (FWS)
BOD: biochemical oxygen demand
TSS: total suspended solids

Table 3. Performance data - river water for domestic water supply [6].

Parameter*	Average Efficiency, %
BOD	70
COD	70
Total Coliforms	99
Fecal Coliforms	99
Ammonium-nitrogen	60
Iron	80
Phosphorus	65
Aluminum	85
Nitrate	95
Color	90
Turbidity	95

* aquatic plant system (APS) with filtering soil beds
BOD: biochemical oxygen demand
COD: chemical oxygen demand

Table 4. Performance data - septage [7].

Parameter*	Percent Removed, %
COD	97
Ammonia nitrogen	72
Total Kjeldahl nitrogen	92
Phosphate	84
Total solids	99+

* aquatic plant system (APS) and free-water system (FWS)
COD: chemical oxygen demand

Table 5. Performance data - landfill leachate [8].

Parameter*	Percent Removed, %
TSS	97
TDS	63
COD	90
TOC	87
Cu	52
Pb	94
Hg	0
Ni	88
Zn	62

* free-water system (FWS)
TSS: total suspended solids
TDS: total dissolved solids
COD: chemical oxygen demand
TOC: total organic carbon

Table 6. Performance data - pulp mill effluent [9].

Parameter*	Percent Reduction, % (1989 - 1990)	Percent Reduction, % (1990)
BOD	7	6
TSS	81	33
Total Phosphorus	53	-32
Total Kjeldahl Nitrogen	14	32
Ammonium Nitrogen	25	18
Nitrate Nitrogen	80	64

* free-water system (FWS)
BOD: biochemical oxygen demand
TSS: total suspended solids

Data Requirements
Flow rates of wastewater or effluent streams to be treated by wetlands and their levels of contamination must be determined. Parameters that are monitored at conventional waste treatment sites, such as BOD, TSS, bacteria, nutrients, and vectors, also need to be monitored.

The overall size of the wetland system is determined primarily by the types and levels of contaminants present. Components that make up the wetland treatment systems are determined by: physical layout (area requirements, water depth, number of cells, cell shape, flow velocity, wastewater retention time, and substrate); flora (algae, bacteria, mosses, plants, planting practices); weather (stormwater runoff); and maintenance requirements (dredging, sediment buildup, plant growth control, and pest control) [2].

Cost
When net present worth of costs of wetland wastewater treatment systems are compared to conventional wastewater treatment plants, the cost of wetland systems are lower than that of equivalent conventional systems at flows less than 5 million gallons per day (MGD), which is comparable to a system serving a community of about 50,000 people [4].

Present costs of constructed wetland wastewater treatment facilities are plotted in the figure below. The results are based on a secondary effluent treatment level, discount rate of 10%, net project life of 20 years, average capital cost of $1.60 per gallon, and average operations and maintenance cost of $0.40 per gallon [4]

Status of Technology
Constructed wetlands are currently being demonstrated at numerous sites across the United States.

References
1. Freeman, R.J. Jr, 1993, Constructed Wetlands Experience in the Southeast, in Constructed Wetlands for Water Quality and Improvement, Chapter 6, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

2. Witthar, S.R., 1993, Wetland Water Treatment Systems, in Constructed Wetlands for Water Quality and Improvement, Chapter 14, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

3. Bastian, R.K. and D.A. Hammer, 1993, The Use of Constructed Wetlands for Wastewater Treatment and Recycling, in Constructed Wetlands for Water Quality and Improvement, Chapter 5, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

4. Cueto, A.J., 1993, Development of Criteria for the Design and Construction of Engineered Aquatic Treatment Units in Texas, in Constructed Wetlands for Water Quality and Improvement, Chapter 9, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

5. Hammer, D.A., B.P. Pullin, T.A. McCaskey, J. Eason, and V.W.E. Payne, 1993, Treating Livestock Wastewaters with Constructed Wetlands, in Constructed Wetlands for Water Quality and Improvement, Chapter 35, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

6. Manfrinato, E.S., E.S. Filho, and E. Salati, 1993, Water Supply System Utilizing the Edaphic-Phytodepuration Technique, in Constructed Wetlands for Water Quality and Improvement, Chapter 34, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

7. Ogden, M.H., 1993, The Treatment of Septage Using Natural Systems, in Constructed Wetlands for Water Quality and Improvement, Chapter 58, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

8. Johnson, K.D., C.D. Martin, G.A. Moshiri, and W.C. McCrory, 1999, Performance of a Constructed Wetland Leachate Treatment System at the Chunchula Landfill, Mobile County, Alabama, in Constructed Wetlands for the Treatment of Landfill Leachates, Chapter 5, G. Mulamoottil, E.A. McBean, and F. Rovers, eds., CRC Press, Boca Raton, FL.

9. Tettleton, R.P., F.G. Howell, and R.P. Reaves, 1993, Performance of a Constructed Marsh in the Tertiary Treatment of Bleach Kraft Pulp Mill Effluent: Results of a 2-Year Pilot Project, in Constructed Wetlands for Water Quality and Improvement, Chapter 46, G.A. Moshiri, ed., CRC Press, Boca Raton, FL.

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