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# Difference between revisions of "Constructed wetlands"

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

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. [2] Dr. Kathe Seidel at the Max 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. [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 argicultural runoff, stormwater retention, acid coal mine drainage, metal ore mine drainage, dairy pasture runoff, animal waste, refineries, paper and pulp processing, shrimp aquaculture, landfill leachate, sugar factories, metallurgic industries, domestic wastewater, and agricultural wastewaters. [3] [1] In developing communities, they can be used to treat greywater or used as a secondary treatment for domestic sewage.

## Design

Constructed wetland treatment systems generally fall into two categories: subsurface ﬂow systems and free water surface ﬂow system. Free water surface (FWS) systems are planted basins with slowly flowing shallow surface water, and the treatment processes occur through interactions between the vegetation, naturally-occurring microbes, and contaminants [4]. Subsurface ﬂow (SSF) systems are designed with horizontal or vertical subsurface ﬂow through a permeable medium (typically sand, gravel or crushed rock). Both subsurface types of horizontal (HF) or vertical flow (VF) involve a flat bed of permeable soil covered with macrophytes. For HF systems, the influent enters in the bed subsurface at the beginning of the wetland cell and flows through horizontally using pressure and gravity forces. For VF systems, wastewater is fed from the top and then gradually percolates down through the bed and is collected by a drainage network at the base. [5] SSF system allows for filtration with some plant uptake of contaminants. Advantages to a sub-surface wetland include a minimized risk of odors or insect vectors. In addition, the SSF media provides a greater surface area for contaminant filtration and treatment and allows for greater thermal protection in colder environments. Advantages to FWS wetlands include: decreased construction costs, decreased risk of clogging, decreased time for initial plant development, and an increase in aerobic conditions which are often desired for nutrient uptake. [6] [7]

### Design Considerations

The most important considerations on the design of a constructed wetland include hydrology, basin morphology, chemical loadings, soils, and vegetation. [8]

Hydrology

Hydrology is one of the most important variables in the wetland design. Some of the main parameters used to describe the hydrologic conditions of treatment wetlands include depth, hydraulic loading rate, and retention time. [9] Hydraulic loading rate is the rate of wastewater per unit area per time, and retention time is the amount of time for water to make it through the system. Efficiency of the system is shown to increase with longer retention times and lower hydraulic loading rates. [10] The design of subsurface flow wetlands should allow controlled flooding to 15 cm to foster desirable plant growth and to control weeds. The depth of the system is important not just in treatment efficiency but also in safety. The water in SSF systems must be high enough to reach the plant root system and low enough to not submerge the system, creating a mosquito breeding ground. [11]

Basin morphology

Basin morphology refers to the shape and slope creating certain flow conditions in the wetland. Flow conditions can be calculated so that the entire wetland is effective in nutrient removal. In order for a proper detention time, there should not be a shortcut path possible for contaminants to flow through. [8] Recommended length:width aspect ratio for FWS wetlands is 5:1 to 10:1. The bed bottom grade should be >3%. Recommended aspect ratio for SSF wetlands is in the range of 0.25:1 to 1:1. The bed bottom grade should be >0.5%. [12]

When water flows into a wetland, it brings chemicals that may be beneficial or detrimental to the functioning of that wetland. The influent of the CW has normally undergone primary treatment. This entails a removal of large solid waste, a settling of heavy suspended solids, and an equalization of wastewater flow and quality. The most common primary treatment for small-scale CWs worldwide is the septic tank. [13] The wastewater influent still has high concentrations of nutrients such as nitrogen and phosphorus, biological oxygen demand (BOD), and suspended solids. [8] The system can be designed to remove a certain percent of these contaminants.

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. [14] [8] Surface-flow (FWS) wetland 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 material should be sand or gravel. [8] The inlet and outlet of a CW system contains some soils and rocks that act as distribution medium to evenly distribute and collect the influent and effluent. The distribution medium is 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 high concentrations of pollutants. Relatively few plants can thrive in these high-nutrient, high-BOD wastewaters. [15] 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. [13] 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. [8] It is important to choose appropriate plants for this environment; however, it has been found that there is little relationship between removal percentages and plant species. [10] [16] [7] The actual effect of plants in SSF wetlands has been debated. [4] 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 generally a small proportion of total removal. Plants primarily affect treatment performance by enhancing nutrient processes such as nitrification and denitrification by transferring oxygen to soils and supplying of organic matter. [17]

### 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 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 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. [13]

Outlet

The outlet allows the exit of the effluent and 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. [13] The outlet can release the effluent to a soil infiltration system or a to surface water body. [18]

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. [12] Geosynthetic clay liners are not recommended because they may crack. [11] Another option is to decrease the soil permeability by mixing Portland cement or bentonite with the soil and compacting on-site [13].

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.

## 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. This method of alternating vegetation and open water significantly improves nutrient removal. [19]

### How to Size a Free Water Surface Wetland using Kadlec and Knight model [2] [12]

• 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)

Table 1: Constant values for calculating the surface area of free surface water CWs [12]

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. [12]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. [13] 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. [12]

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. [12]

Planting vegetation

After soils are in place, macrophytes can be planted using rhizome cuttings. [13]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. [13]

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. [20] 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. [21] 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 Cite error: Closing </ref> missing for <ref> tag

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. [22] Another important factor of treatment is seasonal differences. The removal of parameters such as BOD, suspended solids, and pathogens can significantly decrease during winter. [10] However, an insulating layer can be added to SSF wetlands to almost completely reduce the negative effects of low temperature to treatment processes. [23]

Table 2: Removal of BOD, TSS, N, and P in 170 FWS and 1329 SSF Wetlands in 19 Countries [12]

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

## Impacts

Constructed wetlands are in limited use in wastewater treatment in developing countries. [1] 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. [24] 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. [25] 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. [26] Positive impacts include production of biomass from the harvesting of macrophytes, especially water hyacinths, [1] and less environmental impacts compared to other treatment methods, especially for the VF subsurface wetland. [27]

## 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.

[28]


## 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 [29] 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. [30]

## References

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2. Kadlec, H.R., Knight, R.L. (1996) Treatment Wetlands, Lewis, Boca Raton, New York, London, Tokyo
3. Verhoeven, J. T. A., Beltman, B., Bobbink, R., & Whigham, D. F. (2006). Wetlands and natural resource management. New York: Springer.
4. 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.
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18. Tanaka, N., Ng, W. J., & Jinadasa, K. B. S. N. (2011). Wetlands for Tropical Applications: Wastewater Treatment by Constructed Wetlands. Imperial College Press.
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20. Vymazal, J. (Ed.). (2010). Water and Nutrient Management in Natural and Constructed Wetlands. Springer.
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23. 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.
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