Natural Swimming Ponds

The outdoor swimming pool has always had to work hard to to look good in a natural setting, particularly in climates similar to the UK; afterall, there are not that many sunny days that inspire relaxing around the pool, as you would in the south of Spain for example. For this reason, the concept of the swimming pond really seems to be taking hold in temperate climates. The swimming pond is a body of water to swim in but that's where the similarity with the conventional swimming pool ends. This is a pond, and as such is a creation that uses nature and if designed well, enhances and settles into the garden landscape. This is a natural environment, an oasis for micro-organisms, plants and insects that all form part of the ecosystem, resulting in a crystal clear pool that when the sun does shine, is there to be enjoyed by us as well.

From a brief look at history, we have been swimming in rivers, lakes and ponds since the beginning of time and the big difference today is that we want to enjoy the water without catching something unpleasant.

The concept of using natural systems to clean the water for swimming is quite recent, dating back to the mid 70's in Austria where the green shoots of the environmental movement wanted a better swimming experience. Since then, many swimming ponds have been built on the continent, not only for private use but large public ponds as well. They say the water is good enough to drink once it is returned to the pond. In the UK the history is much shorter. The first private pond was constructed in 2001 but now 100's of pools have been built with a number of specialist companies currently designing and building in the market.

How does it work?

The concept is relatively simple; the idea is to create a swimming zone within the excavated area of the pond. This zone will be deep and large enough for the client's swimming needs. Around the swimming zone is a planted regeneration zone. A retaining wall between these two areas keeps them apart. The retaining wall tends to finish about 300mm below the water surface so that water can flow freely from the swimming zone to the regeneration zone. From a visual perspective, this allows the swimming zone to merge into the overall feel of the pond.

The pond is lined and the regeneration zone outside the swimming area filled with suitable aggregate which allows bacteria plenty of surface area to cling too. These bacteria perform the task of cleaning the water. A variety of bacteria decompose any organic matter, releasing nitrates and phosphates. Above the aggregates, plants not only feed on these nitrates and phosphates, but also transform the pool in to a natural pond nestling in its environment. Plants range from those that prefer deeper water such as lilies to those living in the margins and bog areas. Perforated pipes beneath the regeneration zone allow cleaned water to be drawn by a pump and returned to the swimming zone. Other components in the plumbing system includes skimmers for removing leaves and large debris, filters for finer particles and UV filters for added security.

The bacteria that do the work in the regeneration zone are aerobic so need oxygen. The plants help channel oxygen via their root systems but additional aeration is always a plus and can be built into a feature such as a waterfall or fountain. As a rule of thumb, the filter area should be as large as the swimming zone, in other words, 50% of the total area. Filter efficiency will vary depending on the aggregate used for the medium. The aggregate must not increase the pH above 7.5 so any limestone materials are out of the question. Gravels can be used but materials such as expanded clay and pumice offer much greater surface area for bacteria to live and multiply with the added benefit of being pH neutral. Where space is critical, or the design does not allow for a regeneration zone of the required proportions then biofilters are available. These are boxes filled with a high surface area material into which you introduce the bacteria. The water then passes though the filter and is cleaned. Don't forget though, if you want the look of a natural pond, then you need plenty of space devoted to the regeneration zone to achieve the desired effect.

In my next article we shall look deeper into the design and construction of a natural swimming pond. In the meantime, if you are interested, then please visit Mountain Pools where further details are available.

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