How does a natural pool work?

Chlorinating the water is one way to deliver water that is safe and free of diseases, however, chlorine is not great for human health and over history, people have found lots of other ways to keep their water clean. The Romans used settling tanks and zig zag aeration channels, the Victorians engineered coastal pools where the water changed at high tide and Sanskrit texts from thousands of years ago mention sand filtration.

Below is a brief introduction to how modern natural pools work to keep the water clean.

It’s all about the water

The key to water purity in a natural pool is keeping down the level of nutrients, especially phosphorous and nitrogen. If the concentration of these nutrients rises, it will feed the growth of algae and duckweed – not what you want in a swimming pool.

A natural pool is a self-contained ecosystem which does not need a flow of fresh water through it. In fact, allowing the water level in the pool to rise and recede with the rainfall can be a good thing, providing a habitat for specialised species to thrive.

For once we can be grateful for all the rain we get in Calderdale – around a metre a year. That means it is a practical proposition to fill the pond with low-nutrient rainwater.

And the micro-organisms

Starting off with clean water is important but keeping it clean is important too and that is where the micro-organisms come in. Most natural swimming pools draw water over a bed of gravel which filters the water. Micro-organisms grow on the surface of the gravel and scavenge nutrients from the passing water, including bits of dirt from human bodies.

And the plants & animals

Usually natural swimming pools are surrounded by plants – for instance reeds, water lilies and a wide variety of other aquatic plants. As well as looking pretty, suppressing the growth of algae and providing a habitat for beneficial insects, these help sequester nutrients and aerate the water.

diagram of a natural pool showing a central deep water swimming area surrounded by a shelving regeneration area

Regeneration zone

The area of a natural swimming pool where the water is cleaned by the plants and micro-organisms is called the regeneration zone. It often surrounds the swimming zone but it can be located at some distance from it. The regeneration zone needs to be equal or bigger in area than the swimming zone and is usually shallower.

Diagram showing that swimming zone water is cleaned by drawing it through a gravel bed where plants are growing

Swim zone

The main swimming zone is usually over 2 metres deep but shallower areas can be included for non-swimmers and children. The water is cleaned in the regeneration zone and circulated through the swimming area

There are lots of different ways to arrange the regeneration zone - water can move across it upwards, downwards or sideways. Equally there are lots of different ways to move the water through the regeneration zone.

picture of water being sprayed over a gravel bed

Water is pumped over a gravel bed in a public pool in Germany

diagram showing bubble pumps drawing water through a gravel bed where plants are growing

Water can be drawn through a gravel bed by air bubbles. This method uses less energy than pumping water.

There are literally thousands of natural pools spread across the globe and hundreds of public ones in Europe and the USA. This means that there are industry standards for issues like water quality, volumes of water per swimmer and such like as well as established companies who know how to build such pools.

Natural Pool complex in Austria

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