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# Constructed wetlands

 See also the Constructed wetlands category. for subtopics, how-tos, project pages, designs, organization pages and more.

name 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. [1]

## 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).

## Theory

A FWS wetland can be designed with three different zones perpendicular to the path of flow. The first zone is shallow and heavily vegetated to remove suspended solids and BOD. The second zone is deeper with open water to allow aeration and nitrification. The third zone is also shallow and vegetated to allow denitrification.

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.

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

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 ratio 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.

$\, h_{L} = s*L$
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. Zone 1 has an HRT of 1-2 days, Zone 2 has an HRT of 2-3 days, and Zone 3 has an HRT of 1 day.

$\, HRT = V/Q$

## Construction of Free Water Surface Wetland

This is basic guide of the major construction phases to building a FWS wetland.

Basin excavation

A suitable site must be chosen; this site should be flat or no more than 1% grade. The site must be cleared of preexisting vegetation and debris. Once cleared, the earthwork can begin. Based on the calculated dimensions, begin to dig the basin. Zones 1 and 3 are designed for a 6-cm water depth, and Zone 2 is designed for a 1-m water depth. The cut and fill can be calculated so that the soil removed from Zone 2 can be used to raise Zones 1 and 3. Once the earth has been moved, the surface must be compacted. Additionally, brick or earthen berms must be built around the perimeter of the site. The height of the berms should be taller than the calculated water depth in case of precipitation or additional flows.

Installation of basin liner

If the soils are permeable, a liner must be installed. If a plastic liner is chosen and is being placed on a rocky bed, 2-5 cm of sand can be spread over the site basin to protect the liner. After this, the liner should be carefully laid over the basin, including the berms. Another layer of sand should be spread over the liner to protect the liner from gravel.

Substrate filling

Inlet and outlet structures

Planting vegetation

## Re-Design

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