Natural Swimming Pools



Status Report (spring 2015): We finally built our first natural pool last spring, and now planning our next! The first is a large size pool incorporating a 50' lap lane, an 8' deep swim zone, a diving platform, a sloping beach entry, and a state of the art filtration system, surrounded by a large and diverse water garden providing biological filtration. We had a few unexpected issues in our prototype, but nothing we couldn't solve, and currently the water is clear and clean, the systems up and functioning, and the clients and swimmers are ecstatic. We have looked in detail at plans from some of the major European companies providing natural pool systems, and feel we offer a very comparable system at a more economical price that the large companies can offer. If you are interested in a natural pool we are ready to work with you!

All the pictures included in the body of the article are of our first pool. At the end of the article we provide additional inspirational examples (not ours) from around the world.

Finally, thanks for all the interest expressed by readers, near and far. Due to all the back-and-forth travel involved such a complex job, we are currently only taking on natural pool installations in Ulster (and possibly Dutchess) County. For the adventerous clients further afield who have a good relationship with their local contractor, we may be able to act as consultant, providing the details needed for building your own natural pool.

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You may have never heard of these, but the principles and technology have been well established and tested in Europe and the UK over the past two decades. For about the same cost per square foot as a normal swimming pool (including the plant zones), a natural swimming pool features chlorine and chemical-free water which is cleaned and purified by a biological filter which also creates an attractive and natural backdrop.


Several components work together to keep the water in a natural pool clean. A skimmer system creates a gentle surface flow and removes most debris from the surface before it has a chance to sink. A sieve filter removes large debris, and the skimmed water is further cleaned by a fine particle filter after it is drawn through the pump. Water is then gently circulated through a layered gravel bed, on top of which plants grow. As the water filters through the gravel, nutrients are bound by a biofilm of beneficial bacteria, and transformed into a form which can be absorbed by the plants. After passing through this regeneration zone, the water crosses a barrier and returns to the swimming area which itself is an important part of the biological filter.


In the swimming area live billions of zooplankton, which are microscope animals, invisible to the eye, which consume phytoplankton. Phytoplankton, otherwise known as green and blue-green algae are the nemesis of any pool or pond owner and are responsible for the "pea soup" green water in poorly functioning ponds. These zooplankton are harmless to humans and billions of these filter feeders living in the deeper water means the water is constantly being filtered in place at a very efficient rate. Due to the importance of zooplankton, the pumping system is paired with a skimmer and run at a very low flow rate only about 25% of the time. This removes surface debris while leaving the zooplankton unharmed in the deeper water and decreasing energy use considerably compared to a traditional pool or pond. Besides being harmless to humans, zooplankton are very effective in controlling and removing harmful bacteria and pathogens from the water. With thousands of natural swimming pools built and in-use in the past two decades in Europe, there have been zero reports of ill health effects. You can compare this to the increasing research on the ill-effects of chlorine on short term and long term health, and decide for yourself which type of pool is a greater health risk.


While this system has been engineered to a high degree, it is still a natural ecosystem, and subject to fluctuation. In the early spring, before the bacteria, biofilms, zooplankton, and plants become established and active, the phytoplankton has a window of opportunity to thrive, and a spring "greening" of the water is possible. This is well before the waters are near being at a swimmable temperature. It doesn't take the zooplankton long to catch up, however, and as soon as water temperatures begin to rise the zooplankton population explodes and the water is quite clear by swimming season. At this point, many nutrients have been bound up in bodies of the zooplankton. As their food source disappears they begin to die and nutrients are re-released into the water, which has the potential to cause a second mid-summer bloom of phytoplankton or string algae. However, at this point the plants are in full growth and are removing nutrients from the water as fast as they are added. Having the proper size plant zone means the algae are outcompeted by the plants. Deeper water plants are especially important in removing phosphates from the water, which is the main limiting nutrient for algal growth, and can be trimmed several times in the summer to remove phosphates from the system.


As an additional control option, we offer a phosphate removal filter in the form of a fluidized bed reactor filled with special phosphate binding granules, which are replaced when they become saturated. As mentioned, no phosphate equals no algae, and the binding granules are especially effective. Replacing the granules adds cost, but on systems where the natural fluctuations described above are unacceptable, or where a large planting area is not feasible, a system can be designed to provide consistently clear water. We do not endorse the use of UV sterilizers, salt water systems, or copper-based systems, as these essentially create sterilized water, killing off an important part of the biological filter, and in the case of salt (which produces chlorine) and copper, leaving toxic residues in the water.

It should be noted that there are some aesthetic differences between a natural swimming pool and a chemically treated pool. Because the water is not sterilized, a brownish to greenish patina of biofilm will develop on the walls and gravel, and normally the water will maintain a very slight brownish to greenish tinge due to a small and healthy level of phytoplankton and natural tannins. Both of these conditions are part of a healthy functioning ecosystem and desirable for the role they play in keeping the system in balance. This is very similar to what you would experience from swimming in a clean lake. If you are comfortable with that then a natural pool should be no problem for you, but if you feel more comfortable with bleached white concrete, a traditional pool may be more to your liking.

Besides seasonal fluctuations, there is also a period of about 2 years required for the plant filter to become fully established and functional. For the first year especially, the system will rely on the pump and filters more than the plants to remove nutrients and algae, which can mean nearly continuous pump operation in the middle of summer. Some of the slack can also be taken up by additional annual floating-filter plants such as water hyacinth. Once plants are more established in the second and third year, pump operation can be reduced to about 25% of the time.


To achieve sufficient filtration, the area of the plant zone should equal at least 50% of the area of the swimming zone. In the most common design style, the plant zone is placed around the periphery of the swimming zone. There is no reason, however, why the plant zone couldn't be placed in a different configuration or separated from the pool entirely. It is possible to have a completely normal looking pool where the water is pumped to a separate plant zone for filtration. In fact, an existing concrete pool could be converted to a natural pool in this way.

To explore the possibilities, give us a call. We would be happy to create a customized natural pool, of any size or style, which meets your needs.

Below are some additional picures of natural pools (not ours) from around the world for your further inspiration.

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FernCreek Design * Olivebridge, NY 12461
(845) 657 - 0324 * info@ferncreekDB.com
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Serving the Catskills, Shawangunks, and Hudson Valley including Ulster County, Dutchess County, Kingston, New Paltz, Woodstock, Saugerties, Phonecia, Wawarsing, Accord, Marbletown, Naponach, High Falls, Rosendale, Stone Ridge, Tillson, Gardiner, Hurley, Shokan, Rhinebeck, Rondout, Port Ewen, Hillside, Lincoln Park, Connelly, Eddyville, Bloomington, Esopus, Pine Bush, Boiceville, Olivebridge, Krumville, and surroundng areas.