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## Constructed wetlands

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{{topic header| default.png | Constructed wetlands}}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.<ref name ="Kivaisi">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</ref>. However, they require large areas of land, consistent maintenance, and technical operational knowledge.
<ref name = "Kivaisi"/>

==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.<ref name ="Kadlec Knight">Kadlec, H.R., Knight, R.L.(231996)Treatment Wetlands, Lewis, Boca Raton, New York, London, Tokyo</ref>Dr. Kathe Seidel at the Max Plankck Planck 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.<ref name = "Verhoeven">Verhoeven, J. T. A., Beltman, B., Bobbink, R., & Whigham, D. F.(32006) . Wetlands and natural resource management. New York: Springer.</ref>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) <ref name = "Verhoeven"/> 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 wastewaterargicultural runoff, stormwater retention, acid coal mine drainage, metal ore mine drainage, dairy pasture runoff, animal waste, refineries, mining paper and industrial wastewaterspulp processing, agricultural wastewatersshrimp aquaculture, landfill leachate, stormwatersugar factories, metallurgic industries, domestic wastewater, and runoffagricultural wastewaters.(3)(9) <ref name = "Verhoeven"/> <ref name = "Kivaisi"/> 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==
:'''Soils'''The soil is important to the overall function of a constructed wetland because it supports rooted vegetation, helps to evenly distribute/collect flow at inlet/outlet, provides surface area for microbial growth, and, for subsurface flow wetlands, is an important part of the treatment process. <ref>Westlake, D., Kvet, J., & Szczepankski, A . (1998). The production ecology of wetlands. Cambridge, UK: University Press.</ref> <ref name="Mitsch"/> Surface-flow (FWS ) wetland can be designed with three different zones perpendicular soils are generally less important in removing pollutants but are more similar to natural wetlands. Typically for FWS wetlands, silt clay or loam soils are preferable. For SSF wetlands, high permeability is preferred; the path of flowmaterial should be sand or gravel. <ref name="Mitsch"/>The first zone is shallow inlet and outlet of a CW system contains some soils and heavily vegetated rocks that act as distribution medium to remove suspended solids evenly distribute and BODcollect the influent and effluent. The second zone distribution medium is deeper usually coarse drainfield rock. :'''Vegetation'''Wetland vegetation largely consists of macrophytes, or aquatic plants that grow in or near water. In contrast with natural wetlands, vegetation in CW must be able to survive in waters with open high concentrations of pollutants. Relatively few plants can thrive in these high-nutrient, high-BOD wastewaters. <ref>Reddy, K. R., & DeLaune, R. D. (2008). Biogeochemistry of wetlands, science and applications. London: CRC Press.</ref> Additionally, the vegetation should meet the following criteria: application of locally available macrophyte species; strong root systems and ability to be replanted; large biomass and stem densities to achieve maximum movement of water and nutrient removal; maximum surface area for necessary microbe growth; and efficient oxygen transport into root zone to promote reactions. <ref name = "UNhabitat"/> Cattails, bulrushes, and reed grasses are among the most commonly used CW plants. Macrophytes can be free-floating, emergent, or submerged. SSF wetlands are limited to emergent macrophytes, whereas FWS wetlands often use a combination of free-floating, emergent, and submerged macrophytes. <ref name="Mitsch"/> It is important to allow aeration choose appropriate plants for this environment; however, it has been found that there is little relationship between removal percentages and nitrificationplant species. <ref name = "Solano"/> <ref name = "Greenway">Greenway, M., Woolley, A., Constructed wetlands in Queensland: Performance efficiency and nutrient bioaccumulation, Ecological Engineering, Volume 12, Issues 1–2, January 1999, Pages 39-55, ISSN 0925-8574, 10.1016/S0925-8574(98)00053-6.</ref> <ref name = "Belmont">Marco A. Belmont, Eliseo Cantellano, Steve Thompson, Mark Williamson, Abel Sánchez, Chris D. Metcalfe, Treatment of domestic wastewater in a pilot-scale natural treatment system in central Mexico, Ecological Engineering, Volume 23, Issues 4–5, 30 December 2004, Pages 299-311, ISSN 0925-8574, 10.1016/j.ecoleng.2004.11.003. </ref>The third zone actual effect of plants in SSF wetlands has been debated. <ref name = "Dan">Truong Hoang Dan, Le Nhat Quang, Nguyen Huu Chiem, Hans Brix, Treatment of high-strength wastewater in tropical constructed wetlands planted with Sesbania sesban: Horizontal subsurface flow versus vertical downflow, Ecological Engineering, Volume 37, Issue 5, May 2011, Pages 711-720, ISSN 0925-8574, 10.1016/j.ecoleng.2010.07.030.</ref> Generally, wetland plants provide improvements, although small, in BOD and pathogen removal. However, they enhance nutrient removal, although mostly through indirect means. Unless nutrient loadings are very low, net removal by direct plant uptake is also shallow generally a small proportion of total removal. Plants primarily affect treatment performance by enhancing nutrient processes such as nitrification and vegetated denitrification by transferring oxygen to allow denitrificationsoils and supplying of organic matter. <ref name = "Tanner">Tanner, C. C. (2001). Plants as ecosystem engineers in subsurface-flow treatment wetlands. Wetland Systems for Water Pollution Control 2000, 44(11), 9-17.</ref>
How ===Key components===:'''Inlet'''The inlet releases and distributes the influent wastewater at the wetland entrance. Inlet structures for FWS or HF SSF wetlands include perforated or slotted PVC pipe or open trenches perpendicular to Size the direction of the flow, and the influent is released onto the distribution medium for further dispersion and velocity reduction, creating uniform flow throughout the width of the wetland cell. In VF SSF wetlands, a Free Water Surface Wetland using Kadlec grid of pipes or trenches is laid over the bed, and influent is released down into the substrate. The medium will assist in the spreading of the water throughout the bed, but it is important for the inlet grid to be as uniformly distributed as possible. Pipe sizes, orifice diameters, and spacing are determined by the design flow rate. <ref name = "UNhabitat"/>:'''Outlet'''The outlet allows the exit of the effluent and Knight model helps to control the water depth. In FWS or HF SSF wetlands, most systems have a perpendicular perforated or slotted pipe enclosed in drainfield rock. A sump can be positioned downstream of the outlet to control the water level. In VF SSF systems, the collection system can be a grid network of pipes in drainfield rock. The sump of this system can allow the soil bed to completely drain. <ref name = "UNhabitat"/> The outlet can release the effluent to a soil infiltration system or a to surface water body. <ref>Tanaka, N., Ng, W. J., & Jinadasa, K. B. S. N. (232011). Wetlands for Tropical Applications: Wastewater Treatment by Constructed Wetlands. Imperial College Press.</ref>:'''Liner'''The liner, at the base of the system, keeps the wastewater in and the groundwater out of the system. If the soil is clayey and impermeable, a liner may not be needed. However, if the intrinsic permeability of the soil is greater than 10-6 m/s, the wetlands must be lined. There are a few options for lining the system. A 30-mil PVC liner is the most common and the most reliable choice. 10-20 mil liners can be found in the developing world. <ref name ="Milhelcic"/> Geosynthetic clay liners are not recommended because they may crack. <ref name = "Gustafson"/> Another option is to decrease the soil permeability by mixing Portland cement or bentonite with the soil and compacting on-site <ref name = "UNhabitat"/>.:'''Berm'''The berms, on either side of the system, help to contain the wastewater within the system. Further, these berms are important because they are designed in an effort to prevent flooding of dangerous wastewater. The berms usually contain about 0.6 to 0.9 meters of freeboard above the surface of the water. On either side of the berms, there is a grassed slope that sits on top of a sturdy soil like clay. On the top of the berm, there is often times a gravel path that is about three meters wide. The ratio for the grassed slopes should be greater than 3:1. Within, the berm, the PVC liner is usually tucked in to prevent any wastewater from leaking out of the constructed wetlands.
1==Theory==A FWS wetland can be designed with three different zones perpendicular to the path of flow. Determine the limiting effluent requirements for 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. This method of alternating vegetation and open water significantly improves nutrient removal. <ref>Ibekwe, A. M., Lyon, S. R., Leddy, M., & Jacobson‐Meyers, M. (2007). Impact of plant density and microbial composition on water quality from a free water surface constructed wetland. Journal of applied microbiology, nitrogen102(4), or pathogens921-936.</ref>
===How to Size a Free Water Surface Wetland using Kadlec and Knight model <ref name ="Kadlec Knight"/> <ref name ="Milhelcic"/>===* 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.
:$\, A = LW = \frac{0.0365Q}{k_{t}}ln\frac{Ci-C*}{Ce-C*}$
:C* = background natural concentration of BOD, nitrogen, or pathogens (mg/L),(mg/L),(coliforms/100mL)
'''Table 1: Constant values for calculating the surface area of free surface water CWs''' <ref name ="Milhelcic"/>
{| class="wikitable"
|-
==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. <ref name ="Milhelcic"/>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. <ref name = "UNhabitat"/> 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. <ref name ="Milhelcic"/>

:'''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. <ref name ="Milhelcic"/>

:'''Planting vegetation'''
After soils are in place, macrophytes can be planted using rhizome cuttings. <ref name = "UNhabitat"/>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 m<sup>2</sup> 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. <ref name = "UNhabitat"/>

:'''Start-up'''
Before the CW can be used, it is best if the plants are well-developed before they encounter the wastewater effluent, so that they have a strong foundation and greater stress tolerance. <ref> Vymazal, J. (Ed.). (2010). Water and Nutrient Management in Natural and Constructed Wetlands. Springer.</ref> Also, the water level should be appropriate for developing plants. Too much water will prevent oxygen from reaching the plant roots. A few centimeters of water should be in the basin at all times. <ref name = "Purdue">Purdue University. (1998). Individual residence wastewater wetland construction in Indiana. Retrieved from https://engineering.purdue.edu/~frankenb/NU-prowd/buildcw.htm</ref> The water level can be raised gradually to the design operating level. A well-constructed FWS wetland will take around six weeks before wastewater should be routed into it, and the vegetation will be fully developed around the second growing season. Right after construction is the point at which the most maintenance is required. Large areas where plants fail to grow should be re-planted, and intended free surface areas should be kept clear through harvesting. Once the wetland has reached equilibrium, the only real maintenance tasks required are water level and quality monitoring, erosion control, and berm maintenance. In the established system, vegetation should cover a bit more than 50% of the surface.
This is basic guide of the major construction phases to building ==Operation and Maintenance=='''Constructed wetlands, once they are operational, should require minimal but regular attention and maintenance. For a FWS wetland. , the operator must:''' <ref name = "UNhabitat"/>
Basin excavation:Adjust water levels and flow uniformity - Check for any changes in water level. Reasons could include leaks, clogging of inlet or outlet, overflow, increase or decrease in flow to system, or storm water.
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 :Clean and debris. Once cleared, the earthwork can begin. Based on the calculated dimensions, begin to dig the basin. Zones 1 inspect inlet and 3 are designed for a 6outlet -cm water depth, and Zone 2 Debris or sediment 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 likely 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 clog these structures or additional flows.drainfield rock
Installation of basin liner:Maintain plant communities - Harvest plants, remove weeds, and replant in areas where plants have died. If this is a system-wide problem, adjust water levels, reduce pollutant loads, and check for animal or insect attack.
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:Check for odor -5 cm of sand can Odor may be spread over the site basin to protect the liner. After this, the liner existent in any wetland but should be carefully laid over the basin, including the bermsminimal. Another layer of sand should be spread over the liner to protect Strong odor possibly could mean problems related with anaerobic conditions in the liner from gravelsystem.
Substrate filling:Maintain berms - Repair erosion and cracks in the berms
Inlet and outlet structures'''Yearly maintenance tasks:'''
Planting :Harvest, trim, and replant vegetationwhere necessary
==Operation and Maintenance====Evaluation ====Impacts====Dissemination==:Check sludge levels of primary treatment
==Re-Design==:Thoroughly flush and clean inlet, outlet, and distribution medium
==Evaluation ==
Constructed wetlands for wastewater treatment are most commonly evaluated by measuring the percent removal of key wastewater pollutants: biological oxygen demand (BOD), total suspended solids (TSS), pathogens such as E. coli, nitrogen, and phosphorus. The performance of wetlands depends on different factors, the most important being the hydraulic loading rate and the influent characteristics. Removal rates are generally high for BOD, TSS, and pathogens – at 80-99% in most cases. For phosphorus and nitrogen, the rates are lower and more variable. <ref name = "Verhoeven2">Jos T.A Verhoeven, Arthur F.M Meuleman, Wetlands for wastewater treatment: Opportunities and limitations, Ecological Engineering, Volume 12, Issues 1–2, January 1999, Pages 5-12, ISSN 0925-8574, 10.1016/S0925-8574(98)00050-0.</ref>
The different wetland systems vary in performance. FWS and SSF systems are compared in Table 2. In HF subsurface wetlands, oxygen has difficulty reaching the saturated distribution media and therefore has low nitrification. In contrast, VF subsurface wetlands have low denitrification. Different types of CW can be combined in sequence to better treat wastewater. <ref>J. Vymazal, The use of constructed wetlands with horizontal sub-surface flow for various types of wastewater, Ecological Engineering, Volume 35, Issue 1, 8 January 2009, Pages 1-17, ISSN 0925-8574, 10.1016/j.ecoleng.2008.08.016.</ref> Another important factor of treatment is seasonal differences. The removal of parameters such as BOD, suspended solids, and pathogens can significantly decrease during winter. <ref name = "Solano"/> However, an insulating layer can be added to SSF wetlands to almost completely reduce the negative effects of low temperature to treatment processes. <ref>Shubiao Wu, David Austin, Lin Liu, Renjie Dong, Performance of integrated household constructed wetland for domestic wastewater treatment in rural areas, Ecological Engineering, Volume 37, Issue 6, June 2011, Pages 948-954, ISSN 0925-8574, 10.1016/j.ecoleng.2011.02.002.</ref>
'''Table 2: Removal of BOD, TSS, N, and P in 170 FWS and 1329 SSF Wetlands in 19 Countries''' <ref name ="Milhelcic"/>
{| class="wikitable"
|}
==Impacts==
Constructed wetlands are in limited use in wastewater treatment in developing countries. <ref name = "Kivaisi"/> They have many challenges in conjunction with being a new, unfamiliar technology. They require a large amount of land, knowledge of local aquatic plant species, preexisting primary wastewater treatment, and operational knowledge of wetlands. The land requirements are deceptively large compared to other treatment methods. An approximate figure for surface area is that one cubic foot of CW is required for every gallon per day of influent. For an average single-bedroom, one-person house, this amounts to a 120 square foot system. <ref>Jenkins, J. (2005). The humanure handbook: A guide to composting human manure. (3rd ed.). Grove City, PA: Joseph Jenkins Inc.</ref> Another difficulty to implementing this technology is the fact that this is a secondary treatment method. In developing countries, the main wastewater treatment goal is the control of pathogens to prevent transmission of waterborne diseases and eutrophication of surface waters. <ref name = "Canter">Canter, L. W., Malina, J. F., Reid, G. W., Li, K. G., & Lewis, S. (1982). Wastewater disposal and treatment. Appropriate Methods of Treating Water and Wastewater in Developing Countries. Ann Arbor Science, Ann Arbor MI. 1982. p 207-270.</ref> However, many communities are unable to reach that goal due to lack of resources and knowledge. If these communities are still practicing open or pit defecation, it will be difficult to convince them to adopt a constructed wetland.
One negative impact of CWs, especially FWS CWs, is the creation of a habitat for mosquitos. This problem can be mitigated with careful wetland design or incorporating anti-mosquito devices such as the mosquito fish. <ref>Knight, R. L., Walton, W. E., O’Meara, G. F., Reisen, W. K., & Wass, R. (2003). Strategies for effective mosquito control in constructed treatment wetlands. Ecological Engineering, 21(4), 211-232.</ref> Positive impacts include production of biomass from the harvesting of macrophytes, especially water hyacinths, <ref name = "Kivaisi"/> and less environmental impacts compared to other treatment methods, especially for the VF subsurface wetland. <ref>Fuchs, V. (2009). Nitrogen removal and sustainability of vertical flow constructed wetlands for small scale wastewater treatment. Houghton, MI: Michigan Technological University.</ref>
==Case Studies==
Houghton Lake, MI is a good example of a natural wetland altered for improving the quality of wastewater. In 1978 a wetland was added on to the wastewater treatment plant to better protect the large lake. The an average discharge is around 120 million gallons a year, with the wastewater being introduced into the wetland throughout the length of a 1,600 foot discharge pipe. The wetland is slightly sloped and water exits the wetland via nautual streams, with some minor backflow.
Impressively, the wetland has indicated consummption of over 90 % of the nitrogen and phosphorus from the treatment plant effluent.
Some changes have been noted in the wetland since the introduction of wastewaster, as sedimentation in the wetland has increased over 10 cm. Cattail and duckweed have taken over as dominant vegetation in the wetland, due to higher levels of nutrients in the effluent from the treatment facility.
Another example of a constructed wetland for wastewater treatment is the Lakeland wastewater treatment plant in Polk Co, FL. The treatment plant accepts 10.8 million gallons of wastewater daily. When effluent discharge into a nearby lake was determined to have a detrimental effect on the water quality, a wetland was created for the wastewaster treatment. Around 1,400 acres of wetlands were constructed for the treatment process.
The wetland significantly reduces the amount of nitrogen and phosphorus present in the wastewater, and provides habitat for an abundance of species.
Restoration processes have increased biodiversity within the wetland, which was predominatly covered in cattail and willow vegetation.
<ref name = "EPA">United States Environmental Protection Agency (1993). Constructed Wetlands for Wastewater Treatment and Wildlife Habitat
17 Case Studies. September 1993. EPA832-R-93-005.</ref>
==Dissemination==
Many groups are promoting constructed wetlands around the world. North America and Europe have been using CWs for decades, and now other areas are exploring them as well. CWs are researched at many universities and are used for many wastewater applications. The U.S. Environmental Protection Agency has created design manuals for the construction of treatment wetlands <ref>EPA. US Environmental Protection Agency, Office of Research and Development. (2000). Constructed wetlands treatment of municipal wastewaters (EPA/625/R-99/010). Retrieved from website: http://water.epa.gov/type/wetlands/restore/upload/constructed-wetlands-design-manual.pdf</ref> CW are not only being promoted by the government; individuals interested in green technology and sustainability can take classes where they learn to design and build their own home constructed wetland. <ref>YesterMorrow. (2012). Constructed wetlands for wastewater treatment. Retrieved from http://www.yestermorrow.org/workshops/detail/constructed-wetlands-for-wastewater-treatment</ref>
==Suggested projects and requested contentSee also== *How to determine the necessary size and design of a constructed wetland? (Start a how-to: [[How to design and build a constructed wetland]]. == Interwiki links == * [[Wikipedia:Constructed wetlandsTreatment pond]]
==References==