Constructed wetland
Constructed wetlands are engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater.
Constructed wetlands are designed to remove water pollutants such as suspended solids, organic matter and nutrients (nitrogen and phosphorus).
The term of constructed wetlands can also be used to describe restored and recultivated land that was destroyed in the past through draining and converting into farmland, or mining.
Vegetation in a wetland provides a substrate (roots, stems, and leaves) upon which microorganisms can grow as they break down organic materials.
They are constructed with flow regimes, micro-biotic composition, and suitable plants in order to produce the most efficient treatment process.
Constructed wetlands have been used to remove ammonia and other nitrogenous compounds from contaminated mine water,[12] including cyanide and nitrate.
The natural scarcity of phosphorus is demonstrated by the explosive growth of algae in water receiving heavy discharges of phosphorus-rich wastes.
As roots and rhizomes grow they are thought to disturb and loosen the medium, increasing its porosity, which may allow more effective fluid movement in the rhizosphere.
[3] Removal in a properly designed and operated free water surface flow wetland is reported to be less than 1 to 2 log10 for bacteria, less than 1 to 2 log10 for viruses, 1 to 2 log10 for protozoa, and 1 to 2 log10 for helminths.
[17] Subsurface flow-constructed wetlands with a vertical or a horizontal flow regime are also common and can be integrated into urban areas as they require relatively little space.
[4] The main three broad types of constructed wetlands include:[18][6] The former types are placed in a basin with a substrate to provide a surface area upon which large amounts of waste degrading biofilms form, while the latter relies on a flooded treatment basin upon which aquatic plants are held in flotation till they develop a thick mat of roots and rhizomes upon which biofilms form.
The substrate can be either gravel—generally limestone or pumice/volcanic rock, depending on local availability, sand or a mixture of various sizes of media (for vertical flow constructed wetlands).
[citation needed] Constructed wetlands can be used after a septic tank for primary treatment (or other types of systems) in order to separate the solids from the liquid effluent.
However, they need to be interval-loaded and their design requires more know-how while horizontal flow constructed wetlands can receive wastewater continuously and are easier to build.
[6] The quality of the effluent is determined by the design and should be customized for the intended reuse application (like irrigation or toilet flushing) or the disposal method.
Depending on the type of constructed wetlands, the wastewater passes through a gravel and more rarely sand medium on which plants are rooted.
Subsurface flow wetlands require the following maintenance tasks: regular checking of the pretreatment process, of pumps when they are used, of influent loads and distribution on the filter bed.
For urban applications the area requirement of a subsurface flow constructed wetland might be a limiting factor compared to conventional municipal wastewater treatment plants.
The advantage of subsurface flow constructed wetlands compared to those technologies is their operational robustness which is particularly important in developing countries.
They require a relatively large area to purify water compared to subsurface flow constructed wetlands and may have increased smell and lower performance in winter.
[22] Pathogens are destroyed by natural decay, predation from higher organisms, sedimentation and UV irradiation since the water is exposed to direct sunlight.
[23] Surface flow wetlands can be supported by a wide variety of soil types including bay mud and other silty clays.
They may also have high algae production that lowers the effluent quality and due to open water surface mosquitos and odours, it is more difficult to integrate them in an urban neighbourhood.
A long list of professions ranging from civil engineers to hydrologists to wildlife biologists to landscape architects are needed in this design process.
The landscape architect can utilize a wide range of skills to help accomplish the task of constructing a wetland that may not be thought of by other professions.
Locally grown non-predatory fish can be added to surface flow constructed wetlands to eliminate or reduce pests, such as mosquitos.
Stormwater wetlands provide habitat for amphibians but the pollutants they accumulate can affect the survival of larval stages, potentially making them function as "ecological traps".
Primary clarifier effluent was discharged directly to natural wetlands for decades before environmental regulations discouraged the practice.
[citation needed] Subsurface flow constructed wetlands with sand filter beds have their origin in China and are now used in Asia in small cities.
[32] As part of the remediation efforts to remove contamination from CFB Goose Bay, one of the waste dumps was transformed into an engineered wetland.
