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Constructed wetlands

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Constructed wetlands (CW), or artificial wetlands, are engineered wetland ecosystems that have been designed and constructed to use natural wetland processes for the removal of pollutants. These systems mimic marshes with aquatic plants, soil, and associated microorganisms but take advantage of a controlled environment to treat wastewater. Wetlands have shown the ability to meet this goal in an aesthetic, sustainable, and economical manner[1]. However, they require large areas of land, consistent maintenance, and technical operational knowledge.

History

Natural wetlands have been used as wastewater discharge sites since the beginning of sewage collection. Once their ability to treat water was discovered, as early as the 1950s, early research efforts to use and assess constructed wetlands were begun.(23)Dr. Kathe Seidel at the Max Plankck Institute in Plon, Germany, tested the ability of bulrushes to treat wastewater. Her discoveries led to the first subsurface CW for municipal wastewater treatment in 1974 in the community of Liebenburg-Othfresen, Germany.(3) The first free water surface CW was implemented in The Netherlands in 1967. This system had a star-shaped layout and was called a "planted sewage farm".(3) During the later 20th century, the popularity of CWs grew in Europe and North America. CWs have traditionally been used to treat sewage but, since the late 1980s, have been used to treat a variety of wastewater types such as domestic wastewater, mining and industrial wastewaters, agricultural wastewaters, landfill leachate, stormwater, and runoff.(3)(9) In developing communities, they can be used to treat greywater or used as a secondary treatment for domestic sewage. The main wastewater treatment goal in developing countries is protection of public health through control of pathogens in order to prevent transmission of waterborne diseases and eutrophication of surface waters(22).

Design

Theory

How to Size a Free Water Surface Wetland using Kadlec and Knight model (23)

1. Determine the limiting effluent requirements for BOD, nitrogen, or pathogens.

2. Calculate the surface area for BOD, nitrogen, or pathogens using the following equation. The largest surface area will be the control.

[math]\, A = LW = \frac{0.0365Q}{k_{t}}ln\frac{Ci-C*}{Ce-C*}[/math]
[math]\, k_{T} = k_{20}\theta^{T-20}[/math]


A = wetland area required (hectares)
Q = volumetric flow rate (m3/day)
kt = rate constant for BOD, nitrogen, or pathogen removal at a specific temperature T (m/day)
Ci = influent concentration of BOD, nitrogen, or pathogens (mg/L),(mg/L),(coliforms/100mL)
Ce = effluent target concentration of BOD, nitrogen, or pathogens (mg/L),(mg/L),(coliforms/100mL)
C* = background natural concentration of BOD, nitrogen, or pathogens (mg/L),(mg/L),(coliforms/100mL)


Parameter k20 /theta C*
BOD (mg/L) 34 1 3.5+0.053Ci
Total Nitrogen (mg/L) 22 1.05 1.5
Fecal coliform (coliforms/100mL) 75 1 300

3. Select the L:W aspect ration based on site constraints. Calculate surface dimensions. 4. Check the head loss to ensure that it is smaller than the elevation difference between the inflow and outflow points. This amount allows continuous flow.

[math]\, h_{L} = s*L[/math]
hL = head loss (m)
s = hydraulic gradient slope (dimensionless)
L = wetland length (m)

5. Design zones 1 through 3 based on hydraulic retention time, volume, flow rate, and calculated length and width.

Construction

Operation and Maintenance

Evaluation

Impacts

Dissemination

Re-Design

Constituent Free-Water Surface Subsurface Flow
BOD 93% 93%
TSS 91% 72%
Nitrogen 88% 94%
Phosphorus 53% 65%



Suggested projects and requested content

Interwiki links

References

  1. Kivaisi, A. K. (2001). The potential for constructed wetlands for wastewater treatment and reuse in developing countries: a review. Ecological Engineering, 16(4), 545–560. doi: http://dx.doi.org/10.1016/S0j925-8574(00)00113-0

External links

  • Constructed wetlands by Bruce Lesikar (Extension Agricultural Engineering Specialist, the Texas A&M University System)