Constructed wetlands: Difference between revisions

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).

Design

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.

${\displaystyle \,A=LW={\frac {0.0365Q}{k_{t}}}ln{\frac {Ci-C*}{Ce-C*}}}$

${\displaystyle \,k_{T}=k_{20}\theta ^{T-20}}$

A = wetland area required (hectares)

Q = volumetric flow rate (m³/day)

k_t = 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)

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.

${\displaystyle \,h_{L}=s*L}$

h_L = 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. 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.

${\displaystyle \,HRT=V/Q}$

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. However, the root system of the plants must be able to extend down as necessary. 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. An area in the wall should be left for the inlet and outlet pipe to be installed. 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.

Inlet, outlet, and soil placement

Next the inlet and outlet structures are installed in the berms, which are filled to seal the pipes in. The pipes are also cut through the liner. A 0.5-m section of large gravel should be placed to enclose the inlet and outlet pipes. The sump can also be installed at the outlet end of the wetland. The basin should be filled as necessary with sandy/loamy soils. Zones 1 and 3 require more soil for their plants with deeper root systems.

Planting vegetation

After soils are in place, macrophytes can be planted using rhizome cuttings. The rhizomes of chosen plants can be dug up at the beginning of the planting season. Rhizomes with one undamaged internode and two nodes with lateral buds should be cut for use. These cuttings can be planted at a density of 4 per m² at a 45 degree angle so that at least one node is 4 cm buried in the ground. These should be watered so that one end remains above water.

Operation and Maintenance

Evaluation

Impacts

Dissemination

Re-Design

References

Template:Reflist

External links

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