A natural swimming pool or natural swimming pond (NSP) is a system consisting of a constructed body of water, where the water is contained by an isolating membrane or membranes, in which no chemicals or devices that disinfect or sterilize water are used, and all clarifying and purifying of the water is achieved through biological filters and plants rooted hydroponically in the system.
This definition is based on the FLL (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V - the Landscaping and Landscape Development Research Society) publication "Recommendations for the planning, construction and maintenance of private swimming and natural pools" 2006 Edition.
It is called a "natural swimming pool" because the filtration systems used have biological equivalents in the natural world. In fact if there is not an example of the type of filtration being used in the natural world then it is not seen by the IOB (International Organization for natural Bathing waters) to be naturally filtered.
Systems such as UV, ozone and copper/silver ion disinfection techniques are examples of non natural methods
The first NSPs were built in the early 1980s in Austria, where they are known as Schwimmteiche. The first was built by DI Werner Gamerith in his private garden. Some of the first companies to use the idea of NSPs commercially were the Austrian firm, Biotop Landschaftsgestaltung, and the German firm, BioNova. In 1990 the first public NSP was built by BioNova, followed by another one in 1991 built by Biotop in Unzmarkt (Styria, Austria)
The market for NSPs spread into Germany in the late 1980s. In 1998, the first public NSP in Germany was designed and built by Gerhard Brandlmeier and Rainer Grafinger of BioNova.
The residential market for NSPs spread throughout Europe into Switzerland, the United Kingdom, France, Italy, Spain, Belgium, Holland, Hungary, and the Czech Republic. In November 2005, British landscape designer Michael Littlewood wrote the first major English-language book on Natural Swimming Pools.
The American market was introduced to natural swimming pools when The New York Times published an article Today, there are over 20,000 NSPs in Europe, 100 of which are actually public pools in Germany. Bad Maria Einsiedel, designed by BioNova, marked the 100th public swimming pool and was opened to the public in Munich on June 7, 2008. In the Summer of 2010 the first public natural pool in Sweden was built by BioNova in the city of Sigtuna. In July 2011 the first Indoor Natural Swimming Pool was built by Clear Water Revival  in Cornwall, UK.
Design and construction
The NSP is divided into two areas:
- The swimming zone is the area intended for swimming that resembles a conventional swimming pool or pond.
- The regeneration zone consists of a lined overflow pool filled with specific filtration substrate (gravel, sand or lavastone) and flora. Plants used for water regeneration can be supplemented with decorative flowering species to create an attractive water garden environment. The biological processes that clean the water take place in this zone.
The swimming zone should be physically separated from the regeneration and should reach a depth of 2 m (6 ft 6 in) in swimming ponds. The regeneration zone and swimming zone must be equal in area for sufficient purification. The swimming portion of the pool can look like a conventional swimming pool or a natural pond. The regeneration zone can be placed adjacent to the swimming area or in a remote location depending on the space available. In modern natural swimming pools there is no minimum depth for the swimming zone and the regeneration zone can now be reduced greatly and in some cases is non existent. Clear Water Revival was the first company to build indoor natural swimming pools in the UK. In these pools a regeneration zone can be used outside the building or a natural filtration chamber can be built without a planted area indoors.
In an NSP, swimming pool water flows via gravity from the swimming pool into distribution shafts. The water then passes through a biological fine filter/bioreactor before it is pumped into the regeneration zone. Plants in the regeneration zone are planted in the substrate, and these plants compete for nutrients that would feed algae. The water is cleaned biologically by the metabolic processes found in the substrate in which the plants are rooted. Thus, microorganisms and the water plants ensure effective, continuous cleaning. No processes beyond these, such as UV sterilization, are found in true European style NSPs.
Because of their reliance on natural environmental factors, each NSP system is built with consideration to the region and climate where it is installed. Construction elements such as the biological filter and the combination of contaminant ridding plants vary with each pool.
Natural Kit Pools
Natural Swimming Pools have historically been available only in bespoke designs and therefore cheap alternatives have been hard to come by. However in 2011 a natural kit pool was developed in the UK based on a number of set design features mainly using wooden wall panelling to maintain the natural look and feel of a traditional natural pool.
Typically, a chemically treated pool can discharge up to 3 times its volume of water into the sewer per year. As there is no use of chemicals in the water of NSPs  and their water is maintained as "living" (i.e. the NSP is not drained and refilled), there is no need for this waste of water being discharged into the sewer. Also, there is considerably reduced energy consumption for the mechanical operation of many NSP systems.
Additionally, amphibious and aquatic creatures like frogs, salamanders, and snails often make their homes in the regeneration zones of NSPs. While some pool owners might be made uncomfortable by this idea, others find comfort knowing that their swimming pool environment is clean and balanced enough to support life as it is well known that these types of creatures indicate a healthy ecosystem. The equipment used in NSPs takes the possibility of this wildlife into consideration, as skimmers utilize fine sieves that prevent small creatures from being sucked through the filtration process.
Cleaning, sanitation, and water quality
NSPs rely on a combination of hydraulic design techniques in conjunction with a finefilter. Materials such as leaves, pollen, dust or suntan lotion commonly found floating on the water's surface are removed by a specialized skimmer or overflow channel. These specially designed devices allow optimal skimming of the water surface and reduce silting. Downstream, a bioreactive biological filter removes and retains additional particulate matter down to a particle size of 100 micrometres. Pumps different from those found in conventional pools hydraulically optimize water flow rates and volumes, thereby accelerating the cleaning process even further. This mechanically enhanced natural filtration produces clear, clean water. In fact, when NSPs are installed in Europe, pool owners and builders will ceremoniously wrap up an installation with a ceremonial drink of water collected directly from the pool.
A biofermenta system is the latest form of filtration for NSPs reducing the need for large plantation areas and filtration beds.
While no guidelines for natural pool water quality currently exist in the United States, European and European-style NSP builders follow the EEC bath waters guideline 2006/7/EG concerning the quality of bath waters and their management. Specific health guidelines are also clearly established in the German FLL standards, which are adhered to by NSP builders throughout Europe. On March 4, 2006, new pool water standards were published in the Official Journal of the European Union. These standards, enacted throughout Europe within two years, outline the methods of monitoring pool water quality. Unlike conventional North American swimming pools whose sanitation levels are monitored by testing for proper pool chemical balance, European NSPs have standards for bacterial levels. Peter Petrich, an originator of natural swimming pools in Austria, has said that the risk of swimmers becoming sick is "very low." 
Properly built public NSPs in Europe, some that see up to 3,000 swimmers per day, are able to meet these high sanitation standards. Others accept 1000 visitors /3046m³ per day. Thus, the efficacy of the NSP cleaning process remains in little doubt.
Temperature and feel
Although certain undesirable microorganisms are able to survive at sustained temperatures over 82 °F (28 °C), below this temperature these harmful bacteria are completely out competed by the natural filtering bacteria in certain types of natural swimming pools.
Earth-friendly heat pumps and solar panel warming systems can be incorporated into NSP construction to keep with the eco friendly feel of a natural pool however normal electric or gas heaters work just as well.
The water in NSPs has many desirable characteristics. For example, the red eyes, dried out skin and hair, and bleached bathing suits associated with overly chlorinated water is not an issue with NSPs
Care and maintenance
Each NSP is totally unique, and so each has a different "break-in" period during which the system finds balance. While NSPs can be used for swimming immediately, it takes approximately two to three years until a stable biological equilibrium is reached. Once an NSP has reached equilibrium, it requires considerably less regular maintenance than a conventional swimming pool. Seasonal care and maintenance is still required over the course of the year, though even this differs from care of a conventional swimming pool. For example, ducks and other water fowl in migration have been known to visit NSPs during the winter months.
As winter turns to spring, plants in the regeneration zone must be pruned and tended to avoid over growth, just as in any typical garden. Any dried or dead plants must also be removed.
Migratory water fowl also tend to stop at natural swimming ponds in the late winter and early spring months. While they may add to the bucolic aesthetic of a well-built NSP, steps must be taken to minimize any foreign contaminants that would compromise swim water quality that such animals can introduce to the NSP (e.g. installing filtering plants such as water hyacinth and water lettuce).
On windy, warm, and dry days, water can evaporate at the rate of up to 1 cm per day. If the water level gets too low, water must be refilled in order to protect the plants planted hydroponically in the regeneration zone.
Dead leaves that fall off trees can make their way into the pool and decompose, introducing microorganisms and nutrients that may serve as food for algae growth. In the late autumn months, netting can be stretched over the pool to catch larger leaves before they fall into the NSP.
While NSPs are allowed to freeze over, and many owners enjoy seeing frozen ponds in their backyards, extended freezes should be avoided as carbon dioxide can accumulate underwater, unbalancing the NSP system. Excessive snow cover may also damage the plants in the regeneration zone.
- Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau home page, accessed May 22, 2009.
- Natural Swimming Pools , by Michael Littlewood, Schiffer Publishing, 2005, Page 16
- Natural Swimming Pools , by Michael Littlewood, Schiffer Publishing, 2005, Page 18
- Kurutz, Steven (April 5, 2007). "From Europe, a No-Chlorine Backyard Pool". The New York Times. Retrieved May 22, 2009.
- Natural Swimming Pools, by Michael Littlewood, Schiffer Publishing, 2005, Page 19
- Clear Water Revival - Indoor Natural Swimming Pool Information
- "Amphibian Facts". Hamline University's Center for Global Environmental Education. Retrieved May 22, 2009.
- "Gardening: Wet and wild" (PDF). The Independent. July 6, 2002. Retrieved May 22, 2009.
- "The Greening of the Swimming Pool". New Jersey Countryside Magazine. May/June 2008. Retrieved May 22, 2009.
- NSP Boekenberg measuring 3046m³