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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. [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 domestic wastewater, mining and industrial wastewaters, agricultural wastewaters, landfill leachate, stormwater, and runoff. [3] [1] In developing communities, they can be used to treat greywater or used as a secondary treatment for domestic sewage.

Design

The most important considerations on the design of a constructed wetland include hydrology, basin morphology, chemical loadings, soils, and vegetation. [4]

Hydrology

Hydrology is one of the most important variables in the wetland design. Some parameters used to describe the hydrologic conditions of treatment wetlands include depth, hydraulic loading rate, and retention time. [4] The design of subsurface flow wetlands should allow controlled flooding to 15 cm to foster desirable plant growth and to control weeds.[5] The underdrain moves the treated effluent out of the wetland and keeps the effluent level below the surface of the gravel. This prevents the effluent from coming into contact with people and keeps mosquitoes from breeding in the wetland. It also keeps the water level high enough to sustain plant growth. [6] A significant relationship between percentages of removal and the hydraulic application rate (HAR) was observed. So, the highest percentages of removal were obtained in those beds with the lowest HAR (75 mm day−1) and with the longest retention time (3 days). According to that finding, removal efficiency could be improved by increasing the retention time. [7]

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. [4] 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%. [8]

Chemical loadings

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. [5] The wastewater influent still has high concentrations of nutrients such as nitrogen and phosphorus, biological oxygen demand (BOD), and suspended solids. [4] 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. [5] [4] 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. [4] 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. [4] 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. [5] 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. [4] 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. [7] [9] [10] The actual effect of plants in SSF wetlands has been debated. [11] 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. [12]

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. [5]

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. [5] The outlet can release the effluent to a soil infiltration system or a to surface water body. [8]

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. [8] Geosynthetic clay liners are not recommended because they may crack. [6] Another option is to decrease the soil permeability by mixing Portland cement or bentonite with the soil and compacting on-site [5].


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 [2] [8]

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 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.

[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. 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.

[math]\, HRT = V/Q[/math]

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. [8]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. [5] 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. [8]

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. [8]

Planting vegetation

After soils are in place, macrophytes can be planted using rhizome cuttings. [5]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. [5]

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.[5] 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. [13] The water level can be raised gradually to the design operating level.


Operation and Maintenance

Constructed wetlands, once they are operational, should require minimal but regular attention and maintenance. For a FWS wetland, the operator must: [5]

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.
Clean and inspect inlet and outlet - Debris or sediment is likely to clog these structures or drainfield rock
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.
Check for odor - Odor may be existent in any wetland but should be minimal. Strong odor possibly could mean problems related with anaerobic conditions in the system.
Maintain berms - Repair erosion and cracks in the berms

Yearly maintenance tasks:

Harvest, trim, and replant vegetation where necessary
Check sludge levels of primary treatment
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. (14)

Table [8]

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. Some other important considerations

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).


WH wastewater treatment systems produce large amounts of excess biomass given the rapid growth rate of the plant. To sustain an effective treatment system based WH, the management plan must include provision for harvesting and use of the excess plant material. Integration of WH-systems for wastewater treatment into methane/carbon dioxide production projects as means of using excess water hyacinth biomass has proved successful (Hayes et al., 1987). [1] The potential for application of wetland technology in the developing world is enormous. As mentioned earlier, most of the developing countries have warm tropical and subtropical climates that are conducive for higher biological activity and productivity, hence better performance of wetland systems. Tropical and subtropical regions are known to sustain a rich diversity of biota that may be used in wetlands. Although land may be a limiting factor in dense urban areas, constructed wetlands are potentially well suited to smaller communities where municipal land surrounding schools, hospitals, hotels and rural areas is not in short supply. This section looks at efforts made in exploring this potential [1]


Dissemination

Re-Design

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 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
  2. 2.0 2.1 Kadlec, H.R., Knight, R.L. (1996) Treatment Wetlands, Lewis, Boca Raton, New York, London, Tokyo
  3. 3.0 3.1 3.2 Verhoeven, J. T. A., Beltman, B., Bobbink, R., & Whigham, D. F. (2006). Wetlands and natural resource management. New York: Springer.
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Mitsch, W. J., Gosselink, J. G. (2007). Wetlands. (4 ed.). New Jersey: John Wiley & Sons.
  5. 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 UN-HABITAT, 2008. Constructed Wetlands Manual. UN-HABITAT Water for Asian Cities Programme Nepal, Kathmandu.
  6. 6.0 6.1 Gustafson, D., Anderson, J., Christopherson, S., Axler, R. (2002). Constructed wetlands. Retrieved from http://www.extension.umn.edu/distribution/naturalresources/DD7671.html
  7. 7.0 7.1 M.L Solano, P Soriano, M.P Ciria, Constructed Wetlands as a Sustainable Solution for Wastewater Treatment in Small Villages, Biosystems Engineering, Volume 87, Issue 1, January 2004, Pages 109-118, ISSN 1537-5110, 10.1016/j.biosystemseng.2003.10.005.
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 Milhelcic, J. (2009). Field guide to environmental engineering for development workers: Water, sanitation, and indoor air. Reston, VA: American Society of Civil Engineers.
  9. 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.
  10. 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.
  11. 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.
  12. Tanner, C. C. (2001). Plants as ecosystem engineers in subsurface-flow treatment wetlands. Wetland Systems for Water Pollution Control 2000, 44(11), 9-17.
  13. Purdue University. (1998). Individual residence wastewater wetland construction in Indiana. Retrieved from https://engineering.purdue.edu/~frankenb/NU-prowd/buildcw.htm

External links

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