Constructed wetland

thumb|Constructed wetland in an ecological settlement in Flintenbreite near [[Lübeck|Lübeck, Germany]]

A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development. Constructed wetlands are engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater. The design of the constructed wetland has to be adjusted according to the type of wastewater to be treated. Constructed wetlands have been used in both centralized and decentralized wastewater systems. Primary treatment is recommended when there is a large amount of suspended solids or soluble organic matter (measured as biochemical oxygen demand and chemical oxygen demand). A biofilter has some similarities with a constructed wetland, but is usually without plants.

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.

Overview

thumb|Effluent from a constructed wetland for [[greywater treatment at an ecological housing estate in Hamburg-Allermöhe, Germany]]

thumb|Constructed wetland for domestic wastewater treatment in Bayawan City, [[Negros Oriental province, Philippines ]]

A constructed wetland is an engineered sequence of water bodies designed to treat wastewater or storm water runoff.

Vegetation in a wetland provides a substrate (roots, stems, and leaves) upon which microorganisms can grow as they break down organic materials. This community of microorganisms is known as the periphyton. The periphyton and natural chemical processes are responsible for approximately 90 percent of pollutant removal and waste breakdown. The plants remove about seven to ten percent of pollutants, and act as a carbon source for the microbes when they decay. Different species of aquatic plants have different rates of heavy metal uptake, a consideration for plant selection in a constructed wetland used for water treatment. Constructed wetlands are of two basic types: subsurface flow and surface flow wetlands.

Constructed wetlands are one example of nature-based solutions and of phytoremediation.

Constructed wetland systems are highly controlled environments that intend to mimic the occurrences of soil, flora, and microorganisms in natural wetlands to aid in treating wastewater. They are constructed with flow regimes, micro-biotic composition, and suitable plants in order to produce the most efficient treatment process.

Uses

Constructed wetlands can be used to treat raw sewage, storm water, agricultural and industrial effluent. Constructed wetlands mimic the functions of natural wetlands to capture stormwater, reduce nutrient loads, and create diverse wildlife habitat. Constructed wetlands are used for wastewater treatment or for greywater treatment.

Many regulatory agencies list treatment wetlands as one of their recommended "best management practices" for controlling urban runoff.

Removal of contaminants

Physical, chemical, and biological processes combine in wetlands to remove contaminants from wastewater. An understanding of these processes is fundamental not only to designing wetland systems but to understanding the fate of chemicals once they enter the wetland. Theoretically, wastewater treatment within a constructed wetland occurs as it passes through the wetland medium and the plant rhizosphere. A thin film around each root hair is aerobic due to the leakage of oxygen from the rhizomes, roots, and rootlets. Suspended solids filter out as they settle in the water column in surface flow wetlands or are physically filtered out by the medium within subsurface flow wetlands. Harmful bacteria, fungi, and viruses are reduced by filtration and adsorption by biofilms on the gravel or sand media in subsurface flow and vertical flow systems.

Nitrogen removal

The dominant forms of nitrogen in wetlands that are of importance to wastewater treatment include organic nitrogen, ammonia, ammonium, nitrate and nitrite. Total nitrogen refers to all nitrogen species. Wastewater nitrogen removal is important because of ammonia's toxicity to fish if discharged into watercourses. Excessive nitrates in drinking water is thought to cause methemoglobinemia in infants, which decreases the blood's oxygen transport ability. Moreover, excess input of N from point and non-point sources to surface water promotes eutrophication in rivers, lakes, estuaries, and coastal oceans which causes several problems in aquatic ecosystems e.g. toxic algal blooms, oxygen depletion in water, fish mortality, loss of aquatic biodiversity.

Ammonia removal occurs in constructed wetlands – if they are designed to achieve biological nutrient removal – in a similar ways as in sewage treatment plants, except that no external, energy-intensive addition of air (oxygen) is needed. Nitrification is strictly an aerobic process in which the end product is nitrate (). The process of nitrification oxidizes ammonium (from the wastewater) to nitrite (), and then nitrite is oxidized to nitrate ().

Denitrification

Denitrification is the biochemical reduction of oxidized nitrogen anions, nitrate and nitrite to produce the gaseous products nitric oxide (NO), nitrous oxide () and nitrogen gas (), with concomitant oxidation of organic matter. including cyanide and nitrate.

Phosphorus removal

Phosphorus occurs naturally in both organic and inorganic forms. The analytical measure of biologically available orthophosphates is referred to as soluble reactive phosphorus (SR-P). Dissolved organic phosphorus and insoluble forms of organic and inorganic phosphorus are generally not biologically available until transformed into soluble inorganic forms.

In freshwater aquatic ecosystems, phosphorus is typically the major limiting nutrient. Under undisturbed natural conditions, phosphorus is in short supply. The natural scarcity of phosphorus is demonstrated by the explosive growth of algae in water receiving heavy discharges of phosphorus-rich wastes. Because phosphorus does not have an atmospheric component, unlike nitrogen, the phosphorus cycle can be characterized as closed. The removal and storage of phosphorus from wastewater can only occur within the constructed wetland itself. Phosphorus may be sequestered within a wetland system by:

The binding of phosphorus in organic matter as a result of incorporation into living biomass,
Precipitation of insoluble phosphates with ferric iron, calcium, and aluminium found in wetland soils. Plants create a unique environment at the biofilm's attachment surface. Certain plants transport oxygen which is released at the biofilm/root interface, adding oxygen to the wetland system. Plants also increase soil or other root-bed medium hydraulic conductivity. 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. When roots decay they leave behind ports and channels known as macropores which are effective in channeling water through the soil.

Metals removal

Constructed wetlands have been used extensively for the removal of dissolved metals and metalloids. Although these contaminants are prevalent in mine drainage, they are also found in stormwater, landfill leachate and other sources (e.g., leachate or FDG washwater at coal-fired power plants), for which treatment wetlands have been constructed for mines.

Mine water—Acid drainage removal

Constructed wetlands can also be used for treatment of acid mine drainage from coal mines.

Pathogen removal

Constructed wetlands are not designed for pathogen removal, but have been designed to remove other water quality constituents such as suspended solids, organic matter (biochemical oxygen demand and chemical oxygen demand) and nutrients (nitrogen and phosphorus). 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.]]

[[File:Tilley et al 2014 Schematic of the Horizontal Subsurface Flow Constructed Wetland.jpg|thumb|Schematic of the Horizontal Subsurface Flow Constructed Wetland: Effluent flows horizontally through the bed. In the horizontal flow constructed wetland the effluent moves horizontally via gravity, parallel to the surface, with no surface water thus avoiding mosquito breeding. Vertical flow constructed wetlands are considered to be more efficient with less area required compared to horizontal flow constructed wetlands. 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. Treated domestic wastewater might need a tertiary treatment, depending on the intended reuse application. They are also suitable to treat stormwater drainage.

Surface flow constructed wetlands always have horizontal flow of wastewater across the roots of the plants, rather than vertical flow. 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.

Surface flow wetlands have a similar appearance to ponds for wastewater treatment (such as "waste stabilization ponds") but are in the technical literature not classified as ponds.

Pathogens are destroyed by natural decay, predation from higher organisms, sedimentation and UV irradiation since the water is exposed to direct sunlight.

Surface flow wetlands can be supported by a wide variety of soil types including bay mud and other silty clays.

Plants such as water hyacinth (Eichhornia crassipes) and Pontederia spp. are used worldwide (although Typha and Phragmites are highly invasive).

However, surface flow constructed wetlands may encourage mosquito breeding. 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.

File:Newly planted constructed wetland.jpg|Newly planted constructed wetland

File:Mature Constructed Wetland.jpg|Same constructed wetland, two years later

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Hybrid systems

A combination of different types of constructed wetlands is possible to use the specific advantages of each system.

Integrated constructed wetland facilitates may be more robust treatment systems compared to other constructed wetlands.

Integrated constructed wetlands are used in Ireland, the UK and the United States since about 2007. Farm constructed wetlands, which are a subtype of integrated constructed wetlands, are promoted by the Scottish Environment Protection Agency and the Northern Ireland Environment Agency since 2008. Landscape architecture has a long history of engagement with the aesthetic dimension of wetlands. Landscape architects also guide through the laws and regulations associated with constructing a wetland.

Plants and other organisms

Plants

Typhas and Phragmites are the main species used in constructed wetland due to their effectiveness, even though they can be invasive outside their native range.

In North America, cattails (Typha latifolia) are common in constructed wetlands because of their widespread abundance, ability to grow at different water depths, ease of transport and transplantation, and broad tolerance of water composition (including pH, salinity, dissolved oxygen and contaminant concentrations). Elsewhere, Common Reed (Phragmites australis) are common (both in blackwater treatment but also in greywater treatment systems to purify wastewater).

Plants are usually indigenous in that location for ecological reasons and optimum workings.

File:Newly planted constructed wetland - details (3110372910).jpg|Newly planted constructed wetland for blackwater treatment (Lima, Peru)

File:Constructed wetland (5547294390).jpg|The large roots of this uprooted plant growing in a constructed wetlands indicate a healthy plant (Lima, Peru)

File:Flowform.jpg|A hybrid system using Flowforms in a treatment pond, in Norway.

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Animals

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".

Costs

Since constructed wetlands are self-sustaining their lifetime costs are significantly lower than those of conventional treatment systems. Often their capital costs are also lower compared to conventional treatment systems. They do take up significant space, and are therefore not preferred where real estate costs are high.

History

Primary clarifier effluent was discharged directly to natural wetlands for decades before environmental regulations discouraged the practice. Subsurface flow constructed wetlands with sand filter beds have their origin in China and are now used in Asia in small cities. Due to legal requirements (nitrification), only vertical flow constructed wetlands are implemented in Austria as they achieve better nitrification performance than horizontal flow constructed wetlands. Only about 100 of these constructed wetlands have a design size of 50 population equivalents or more. The remaining 5,350 treatment plants are smaller than that.

In 2023, Clearwater County implemented the first biofilter-based wetland wastewater treatment system in the province. The system uses 2 types of subsurface constructed wetlands to conduct both aerobic and anaerobic wastewater treatment.

References

External links

Constructed Wetlands – US Environmental Protection Agency — Handbook, studies and related resources
Publications on constructed wetlands in the library of the Sustainable Sanitation Alliance