constructed wetland constructed wetland 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). constructed wetland in an ecological settlement in flintenbreite near lübeck, germany similar to natural wetlands, constructed wetlands also act as a biofilter and/or can remove a range of pollutants (such as organic matter, nutrients, pathogens, heavy metals) from the water. constructed wetlands are designed to remove water pollutants such as suspended solids, organic matter and nutrients (nitrogen and phosphorus). all types of pathogens (i.e., bacteria, viruses, protozoans and helminths) are expected to be removed to some extent in a constructed wetland. subsurface wetlands provide greater pathogen removal than surface wetlands. there are two main types of constructed wetlands: subsurface flow and surface flow. the planted vegetation plays an important role in contaminant removal. the filter bed, consisting usually of sand and gravel, has an equally important role to play. some constructed wetlands may also serve as a habitat for native and migratory wildlife, although that is not their main purpose. subsurface flow constructed wetlands are designed to have either horizontal flow or vertical flow of water through the gravel and sand bed. vertical flow systems have a smaller space requirement than horizontal flow systems. terminology many terms are used to denote constructed wetlands, such as reed beds, soil infiltration beds, treatment wetlands, engineered wetlands, man-made or artificial wetlands. 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 effluent from a constructed wetland for greywater treatment at an ecological housing estate in hamburg-allermoehe, germany constructed wetland for domestic wastewater treatment in bayawan city, the 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. aerobic and anaerobic micro-organisms facilitate decomposition of organic matter. microbial nitrification and subsequent denitrification releases nitrogen as gas to the atmosphere. phosphorus is coprecipitated with iron, aluminium, and calcium compounds located in the root-bed medium. 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. it is a two-step process, consisting of nitrification followed by denitrification. the nitrogen cycle is completed as follows: ammonia in the wastewater is converted to ammonium ions; the aerobic bacterium nitrosomonas sp. oxidizes ammonium to nitrite; the bacterium nitrobacter sp. then converts nitrite to nitrate. under anaerobic conditions, nitrate is reduced to relatively harmless nitrogen gas that enters the atmosphere. nitrification further information: nitrification nitrification is the biological conversion of organic and inorganic nitrogenous compounds from a reduced state to a more oxidized state, based on the action of two different bacteria types. nitrification is strictly an aerobic process in which the end product is nitrate (no−3). the process of nitrification oxidizes ammonium (from the wastewater) to nitrite (no−2), and then nitrite is oxidized to nitrate (no−3). denitrification further information: denitrification denitrification is the biochemical reduction of oxidized nitrogen anions, nitrate and nitrite to produce the gaseous products nitric oxide (no), nitrous oxide (n2o) and nitrogen gas (n2), with concomitant oxidation of organic matter. the end product, n2, and to a lesser extent the intermediary by product, n2o, are gases that re-enter the atmosphere. ammonia removal from mine water constructed wetlands have been used to remove ammonia and other nitrogenous compounds from contaminated mine water, 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. biomass plants incorporation aquatic vegetation may play an important role in phosphorus removal and, if harvested, extend the life of a system by postponing phosphorus saturation of the sediments. 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). all types of pathogens are expected to be removed in a constructed wetland; however, greater pathogen removal is expected to occur in a subsurface wetland. in a free water surface flow wetland one can expect 1 to 2 log10 reduction of pathogens; however, bacteria and virus removal may be less than 1 log10 reduction in systems that are heavily planted with vegetation. this is because constructed wetlands typically include vegetation which assists in removing other pollutants such as nitrogen and phosphorus. therefore, the importance of sunlight exposure in removing viruses and bacteria is minimized in these systems. 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. in subsurface flow wetlands, the expected removal of pathogens is reported to be 1 to 3 log10 for bacteria, 1 to 2 log10 for viruses, 2 log10 for protozoa, and 2 log10 for helminths. the log10 removal efficiencies reported here can also be understood in terms of the common way of reporting removal efficiencies as percentages: 1 log10 removal is equivalent to a removal efficiency of 90%; 2 log10 = 99%; 3 log10 = 99.9%; 4 log10 = 99.99% and so on. types and design considerations process flow diagram for a sewage treatment plant with subsurface flow constructed wetlands. constructed wetland systems can be surface flow systems with only free-floating macrophytes, floating-leaved macrophytes, or submerged macrophytes; however, typical free water surface systems are usually constructed with emergent macrophytes. 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. the main three broad types of constructed wetlands include: subsurface flow constructed wetland – this wetland can be either with vertical flow (the effluent moves vertically, from the planted layer down through the substrate and out) or with horizontal flow (the effluent moves horizontally, parallel to the surface) surface flow constructed wetland (this wetland has horizontal flow) floating treatment wetland 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. in most cases, the bottom is lined with either a polymer geomembrane, concrete or clay (when there is appropriate clay type) in order to protect the water table and surrounding grounds. 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). constructed wetlands can be used after a septic tank for primary treatment (or other types of systems) in order to separate