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OWDP Home > Online Resources > Treatment Systems : Constructed Wetlands |
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Natural wetlands, marshes, swamps, and bogs play an important role in protecting water quality. Constructed or artificial wetland systems mimic the treatment that occurs in natural wetlands by relying on plants and a combination of naturally occurring biological, chemical and physical processes to remove pollutants from the water. Because constructed wetland systems are designed specifically for wastewater treatment, they typically work more efficiently than natural wetlands. Some constructed wetland system designs can closely resemble natural wetlands enough to provide additional habitat areas for many birds, animals and insects that thrive in wetland environments.
Constructed wetlands can treat wastewater from a variety of sources. One of the more common uses is to provide additional or advanced treatment of wastewater from homes, businesses and even communities. Wetlands treat wastewater that has already had most of the solid materials removed from it through some type of primary or secondary treatment.
Homes, businesses, farms, schools and other individual wastewater sources in rural areas sometimes can add a constructed wetland to a septic system or other onsite system to replace or assist a soil absorption field. Some onsite systems can be specifically designed from the start to use a constructed wetland in addition to a soil absorption field on properties with site constraints, such as tight or saturated soils.
Wetlands are good at handling intermittent periods of both light and heavy wastewater flows. Therefore, they often work well with wastewater treatment systems that serve hotels, campsites, resorts and recreational areas.
In environmentally sensitive areas, constructed wetlands can be used with onsite systems to improve the quality of the effluent before it is returned to the environment. They are also used on farms as an inexpensive way to provide extra treatment to animal wastes and by certain industries such as pulp and paper mills. Constructed wetlands are common in mining regions and are used to treat mine drainage.
Wetlands are not practical for treating industrial wastewater that includes pesticides, herbicides or large amounts of ammonia. Additionally wetland plants may accumulate high concentrations metals from some wastewater sources. This may affect the habitat value of the wetland.
There are two main types of constructed wetlands used for wastewater treatment: (1) surface flow wetlands (also called free-water surface wetlands) and (2) subsurface flow wetlands.
Surface flow systems most resemble natural wetlands both in the way they look and the way they provide treatment. Both designs can be used to treat wastewater from individual and community sources, but surface flow wetlands are usually more economical for treating large volumes of wastewater.
Wetlands are areas on land where the ground maintains saturated conditions for much of the year. Surface flow wetlands stay saturated enough to maintain a shallow level of water and wastewater (4 to 18 inches deep) above the soil.
Wetland plants also are present in surface flow system, and natural forces such as wind, sun, rain and temperature affect the plants, water and the treatment processes in these systems.
As soon as wastewater enters a surface flow wetland cell, natural processes immediately begin to break down and remove the waste materials in the water. Before the wastewater has moved very far in the wetland small suspended waste materials are physically strained out be submerged plants, plant stems, and plant litter in the wetland. The roots, stems, leaves, and litter of wetland plants also provide a multitude of small surfaces where wastes can become trapped and waste-consuming bacterial can attach themselves.
Bacteria provide the majority of wastewater treatment. Aerobic bacteria thrive in wetlands wherever oxygen is present, especially near the surface. Wind, rain, wastewater and anything else that agitates the water surface can add oxygen to the system. Anaerobic bacteria thrive where there is little or not oxygen. In surface flow cells, oxygen is scarce in the lower substrate and soil.
When the bacterial consume waste particles in the water they convert them into other substances, such a methane, carbon dioxide and new cellular material. Some of these substances are used as food by plants and other bacteria.
For any of the processes in wetlands to work, the wastewater must remain in the system long enough for treatment to occur and for viruses in the wastewater to die-off naturally. The hydraulic residence time for wastewater in surface flow systems is based on wastewater strength, the level of desired treatment and climatic factors.
Surface Flow Wetlands
One of the most important factors affecting treatment is temperature. Biological treatment processes tend to speed up in warm weather and slow down in cold weather. In cold climates, systems must be large enough to accommodate the longer hydraulic residence times needed for treatment.
Plants help treatment processes in several ways. The wetland plants filter wastes, regulate flow and provide surface area for bacterial and treatment. Floating plants, such as water lilies and emergent plants, such as cattails, shade the water surface and control algae growth. Even in winter when plants are dormant, they often are contributing to treatment
Plants also contribute to treatment by taking up nutrients, metals, and other substances and retaining them. Many of these substances can accumulate again in the wetland when the plants die and decompose. Therefore harvesting vegetation is a good idea.
Each subsurface flow wetland cell is filled with a treatment media, such as rock or gravel, which is placed on top of the soil or lining on the cell bottom. The depth of the media layer is usually about one to two feet. The wastewater flows just below the media surface and remains unexposed to the atmosphere while it saturates the layers below. The saturated media and soil, together with the wetland plants roots, create conditions below the surface of the system that are conducive to treatment.
Treatment in the subsurface flow system is more efficient than in the surface flow wetland because the media provides a greater number of small surfaces, pores and crevices where treatment can occur. Waste-consuming bacterial attach themselves to the various surfaces, and waste materials in the water become trapped in the pores and crevices on the media and in the spaces between media. Chemical treatment also takes place as certain waste particles contact and react with the media.
Sub-surface Flow Wetlands
Biological treatment in subsurface flow wetlands is mostly anaerobic because the layers of media and soil remain saturated and unexposed to the atmosphere. Commonly used plants are cattails, bulrushes and reeds. These plants are able to grow extensive roots even in these anaerobic conditions. The area where the roots grow is called the root zone and usually includes the upper six to twelve inches of media. If cells are alternated or allowed to rest periodically, or if the water level is regularly cycled, the roots can reach throughout the media layer.
Some wetland plant roots contribute oxygen to the cells, which allow some aerobic treatment to take place in the root zone. The plants further contribute to wastewater treatment by providing additional surfaces where bacterial can reside and where waste materials can become trapped. Plants also take up and store some of the metals and nutrients in the wastewater.
The size and configuration of surface flow systems are based on estimates and strengths of the influent. Designers must also consider climatic factors, such as temperatures, evapotranspiration rates, and precipitation amounts to predict and maintain the level of water in the system. Designs typically include multiple cells each providing the same level of treatment that may be operated simultaneously or independently.
Most surface flow wetland cells are self-contained rectangular-shaped basins surrounded by banks on all sides. The inlet and outlet are located on opposite sides. Exceptions include certain systems in arid climates designed for no discharge, which do not have an outlet, and systems used for advanced treatment that open directly to natural wetland areas.
The bottoms of surface flow wetlands cells should be somewhat free of bumps and ridges and have a slight downgrade (approximately 0.5 percent) to assist the flow of wastewater through the cell by gravity. Cells usually must be excavated by backhoe or by hand.
The cell bottom must also be self-contained to prevent wastewater from seeping into the groundwater below and the surrounding environment. It is usually necessary to line the bottom of the cells with clay, bentonite, or a synthetic liner. Soil or other material must be placed on top of the lining to form a substrate that will support the growth of wetland plants.
Surface flow systems can treat wastewater much more efficiently than natural wetlands. The reason for this is that the rate and pattern in which wastewater flows through the system is controlled by the design.
In natural wetlands, wastewater tends to flow through relatively narrow, well-established channels and may never even be exposed to a large portion of wetland area. To prevent this short-circuiting of the wastewater flow, and to make the best use of every inch of cell space, the wastewater in constructed wetlands should be evenly distributed across the width of each cell. Wastewater enters surface flow cells by means of perforated distribution pipes, gated pipes, or a series of weirs at the inlet. At the outlet, most systems have control valves and other devices to help operators adjust the water level.
Even the rectangular shape of the cells and the amount and placement of wetland plants is designed to optimize wastewater flow and treatment. The ratio of the cells’ length to its width (the aspect ratio) usually ranges from 2:1 to 4:1, but may be higher depending on the site and other factors.
Systems designs must estimate the amount of head loss wetland vegetation is likely to cause in flow rates. The placement of the plants can be planned and arranged as well. For example, some surface flow cells are designed to have areas of open water as well as areas of dense vegetation to allow wind and sunlight to reach parts of the cell to influence flow and treatment.
Most systems are designed for the wastewater to flow once through the system. However, systems can be designed to re-circulate all or a portion of the effluent to treat the wastewater more than once.
Like surface flow systems, subsurface flow systems consist of one or more rectangular cells, which may be operated in parallel or in series. It is common for subsurface systems to be designed with multiple cells operated in parallel to allow the cells to be alternated and rested during maintenance. The bottoms of the cells may be slightly loped (up to 0.5 percent) to assist the flow of wastewater through the system. A natural clay or synthetic lining may be necessary for certain sites with high groundwater or permeable soils. Wetlands should be sited away from downspouts and natural drainage areas to avoid excess water from entering the system.
Choosing the treatment media is one the most important design considerations for subsurface flow wetlands. Designers must carefully consider the type, size, uniformity, porosity and hydraulic conductivity of the media material. These characteristics affect the flow of wastewater in the system and the system performance.
Gravel and rock are the most common media used in subsurface flow wetlands. Whatever type or size of media is chosen, the most important concern is that it be as uniform in size as possible. When different size media are placed together the finer materials settle into the open spaces leaving little room for the wastewater flow.
To prevent system clogging the treatment media should be sorted and measured to ensure its uniformity. It should also be washed to remove any fines. A method used to analyze the size and uniformity of treatment media includes sorting them through a series of mechanical sieves of diminishing size. These characteristics are expressed as the media’s “effective size” and “uniformity coefficient”. The effective size of medium-size gravel is about 32 millimeters and the effective size of coarse gravel is about 128 millimeters.
Certain characteristics of the media determine the rate and pattern of the flow and the efficiency of the treatment. Gravel in the small to medium size range tends to work better than coarse gravel because of the following:
· Medium sized gravel offers a greater number of surfaces where biological treatment can take place than course gravel.
· The smaller spaces between medium sized gravel tend to provide better support for plant growth than the large spaces between coarse gravel.
· Medium sized gravel is more likely than coarse media to promote the slow, even non-turbulent flow of wastewater through the system.
· Medium sized gravel is not as likely to become clogged by the accumulation of solids as fine gravel or sand.
Once the type and size of the media is chosen, its porosity and hydraulic conductivity should be verified in a field or laboratory test. These characteristics are used together with information about the wastewater and the site to design the system.
Most subsurface flow wetlands are designed so that wastewater travels through the length of the cell one time to receive treatment. Typical retention times range from two to six days.
The inlet and outlet areas of subsurface flow systems are sloped more dramatically than the rest of the treatment bed and are filled with coarse gravel or rock to prevent clogging in these areas. Wastewater enters the system either above the gravel at the inlet or below it through perforated pipe or weirs. It is important that the wastewater is distributed as evenly as possible as it flows into the cell to prevent short-circuiting.
Subsurface flow cells are usually designed with aspect ratios of 3:1 or less. Wider cells tend to be more cost-effective because long narrow cells must be deeper and require more treatment media. In addition, the wastewater is less likely to back up in wider cells if too much water enters the system or it the rate of flow changes.
Formulas such as Darcy’s law or Ergun’s equation can be used to estimate the flow rate through the cells. A combination of approaches and built-in safety factors must be used to ensure system performance. The design can also incorporate a controlled outlet by using and adjustable pipe which can be swiveled up or down to allow the cell to drain to a desired level.
Surface flow wetlands have few operation and maintenance requirements, but maintenance must be performed properly to ensure system performance. Operation may entail alternating cells or adjusting water levels and harvesting vegetation. Some systems may have banks and berms that need to be maintained, and inlet and outlet structures that should b cleaned periodically. Mosquitoes and burrowing animals present problems in some systems. Different control methods are available, including natural solutions, such as trapping and relocating animals, introducing fish that eat insect larvae, and building bat houses.
Surface flow wetlands have few operation and maintenance requirements, but maintenance must be performed properly to ensure system performance. Operation may entail alternating cells or adjusting water levels and harvesting vegetation. Some systems may have banks and berms that need to be maintained, and inlet and outlet structures that should be cleaned periodically
Some Advantages and Disadvantages are listed below.
Advantages:
· Constructed wetlands are typically inexpensive to build and maintain.
· They require little or no energy to operate.
· They can provide effective tertiary treatment.
· They can provide additional wildlife habitat.
· They can be aesthetically pleasing additions to homes and neighborhoods.
· They are viewed as an environmentally friendly technology and are generally well received by the public.
Disadvantages:
· Constructed wetlands require more land area than many other treatment options.
· Surface flow wetlands can attract mosquitoes and other pests.
· Wetlands are not appropriate for treating some wastewater with high concentrations of certain pollutants.
· The performance of wetlands may vary based on usage and climatic conditions.
· There may be a prolonged initial start-up period before vegetation is adequately established.