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. Examples Austria The total number of constructed wetlands in Austria is 5,450 (in 2015). 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. See also Decentralized wastewater system Ecological engineering Ecological sanitation Floodplain restoration Integrated constructed wetland Passive treatment system Sanitation Vegetative treatment system Water-sensitive urban design Wetland classification Wetlands Construídos (a company in Brazil) 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 Artificial landforms Environmental engineering Environmental terminology Sewerage infrastructure Stormwater management Sustainable design Wetlands Ecological restoration Water and the environment Chinese inventions