the solids from the liquid effluent. some constructed wetland designs however do not use upfront primary treatment. subsurface flow schematic of a vertical subsurface flow constructed wetland: effluent flows through pipes on the subsurface of the ground through the root zone to the ground. schematic of the horizontal subsurface flow constructed wetland: effluent flows horizontally through the bed. vertical flow type of constructed wetlands (subsurface flow) in subsurface flow constructed wetlands the flow of wastewater occurs between the roots of the plants and there is no water surfacing (it is kept below gravel). as a result, the system is more efficient, does not attract mosquitoes, is less odorous and less sensitive to winter conditions. also, less area is needed to purify water. a downside to the system are the intakes, which can clog or bioclog easily, although some larger sized gravel will often solve this problem. subsurface flow wetlands can be further classified as horizontal flow or vertical flow constructed wetlands. in the vertical flow constructed wetland, the effluent moves vertically from the planted layer down through the substrate and out (requiring air pumps to aerate 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. due to the increased efficiency a vertical flow subsurface constructed wetland requires only about 3 square metres (32 sq ft) of space per person equivalent, down to 1.5 square metres in hot climates. the "french system" combines primary and secondary treatment of raw wastewater. the effluent passes various filter beds whose grain size is getting progressively smaller (from gravel to sand). applications subsurface flow wetlands can treat a variety of different wastewaters, such as household wastewater, agricultural, paper mill wastewater, mining runoff, tannery or meat processing wastes, storm water. 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. design considerations depending on the type of constructed wetlands, the wastewater passes through a gravel and more rarely sand medium on which plants are rooted. a gravel medium (generally limestone or volcanic rock lavastone) can be used as well (the use of lavastone will allow for a surface reduction of about 20% over limestone) is mainly deployed in horizontal flow systems though it does not work as efficiently as sand (but sand will clog more readily). constructed subsurface flow wetlands are meant as secondary treatment systems which means that the effluent needs to first pass a primary treatment which effectively removes solids. such a primary treatment can consist of sand and grit removal, grease trap, compost filter, septic tank, imhoff tank, anaerobic baffled reactor or upflow anaerobic sludge blanket (uasb) reactor. the following treatment is based on different biological and physical processes like filtration, adsorption or nitrification. most important is the biological filtration through a biofilm of aerobic or facultative bacteria. coarse sand in the filter bed provides a surfaces for microbial growth and supports the adsorption and filtration processes. for those microorganisms the oxygen supply needs to be sufficient. especially in warm and dry climates the effects of evapotranspiration and precipitation are significant. in cases of water loss, a vertical flow constructed wetland is preferable to a horizontal because of an unsaturated upper layer and a shorter retention time, although vertical flow systems are more dependent on an external energy source. evapotranspiration (as is rainfall) is taken into account in designing a horizontal flow system. the effluent can have a yellowish or brownish colour if domestic wastewater or blackwater is treated. treated greywater usually does not tend to have a colour. concerning pathogen levels, treated greywater meets the standards of pathogen levels for safe discharge to surface water. treated domestic wastewater might need a tertiary treatment, depending on the intended reuse application. plantings of reedbeds are popular in european constructed subsurface flow wetlands, although at least twenty other plant species are usable. many fast growing timer plants can be used, as well for example as musa spp., juncus spp., cattails (typha spp.) and sedges. operation and maintenance overloading peaks should not cause performance problems while continuous overloading lead to a loss of treatment capacity through too much suspended solids, sludge or fats. 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. comparisons with other types subsurface wetlands are less hospitable to mosquitoes compared to surface flow wetlands, as there is no water exposed to the surface. mosquitos can be a problem in surface flow constructed wetlands. subsurface flow systems have the advantage of requiring less land area for water treatment than surface flow. however, surface flow wetlands can be more suitable for wildlife habitat. for urban applications the area requirement of a subsurface flow constructed wetland might be a limiting factor compared to conventional municipal wastewater treatment plants. high rate aerobic treatment processes like activated sludge plants, trickling filters, rotating discs, submerged aerated filters or membrane bioreactor plants require less space. the advantage of subsurface flow constructed wetlands compared to those technologies is their operational robustness which is particularly important in developing countries. the fact that constructed wetlands do not produce secondary sludge (sewage sludge) is another advantage as there is no need for sewage sludge treatment. however, primary sludge from primary settling tanks does get produced and needs to be removed and treated. costs the costs of subsurface flow constructed wetlands mainly depend on the costs of sand with which the bed has to be filled. another factor is the cost of land. surface flow schematic of a free-water surface constructed wetland: it aims to replicate the naturally occurring processes, where particles settle, pathogens are destroyed, and organisms and plants utilize the nutrients. surface flow wetlands, also known as free water surface constructed wetlands, can be used for tertiary treatment or polishing of effluent from wastewater treatment plants. 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. the soil layer below the water is anaerobic but the roots of the plants release oxygen around them, this allows complex biological and chemical reactions. 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. newly planted constructed wetland same constructed wetland, two years later hybrid systems a combination of different types of constructed wetlands is possible to use the specific advantages of each system. integrated constructed wetland an integrated constructed wetland is an unlined free surface flow constructed wetland with emergent vegetated areas and local soil material. its objectives is not only to treat wastewater from farmyards and other wastewater sources, but also to integrate the wetland infrastructure into the landscape and enhancing its biological diversity. integrated constructed wetland facilitates may be more robust treatment systems compared to other constructed wetlands. this is due to the greater biological complexity and generally relatively larger land area use and associated longer hydraulic residence time of integrated constructed wetland compared to conventional 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. other design aspects the design of a constructed wetland can greatly effect the surrounding environment. a wide range of skills and knowledge is needed in the construction and can easily be detrimental to the site if not done correctly. 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. ecological landscape architects are also qualified to create wetland restoration designs in coordination with wetland scientists that increase the community value and appreciation of a project through well designed access, interpretation, and views of the project. 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