From the New York Times: From Europe, a No-Chlorine Backyard Pool.
Natural swimming pools (or swimming ponds, as they are called in Europe, where the concept originated 20 years ago) are self-cleaning pools that combine swimming areas and water gardens. Materials and designs vary — the pools can be lined with rubber or reinforced polyethylene, as in the case of Total Habitat’s, and may look rustic or modern — but all natural pools rely on "regeneration" zones, areas given over to aquatic plants that act as organic cleansers.
The pools have skimmers and pumps that circulate the water through the regeneration zone and draw it across a wall of rocks, loose gravel or tiles, to which friendly bacteria attach, serving as an additional biological filter. Unlike artificial ponds, which tend to be as murky with groundwater runoff and sediment from soil erosion as the natural ponds they’re modeled on, in a natural pool the water is clear enough to see through to the bottom. [continue]
Here’s more on the idea from The Ecologist’s Natural Swimming Pools article:
Superficially, a natural swimming pool looks like a normal garden pond, but it actually consists of two zones: a deep, central swimming area, and a shallower ‘regeneration’ area where plants, specially chosen to purify the water and harbour wildlife, grow.
Ask most swimming-pool contractors to build a backyard pool and chances are they’ll roll out a long list of materials, including reinforcing steel, cement, fibreglass, an energy-intensive filtration system and an arsenal of chemicals. In contrast, a natural swimming pool cleanses the water by organic means, using the purifying properties of plants. A small filter extracts surface debris such as leaves, and a pump keeps the water circulating suffi ciently through the planted area. Building materials, including gravel, stone and clay, are chosen to enhance and blend the pool into the natural landscape.
Because about half of the pool’s space is devoted to aquatic plants, it is as much a garden feature as a pool and can be admired year round without any need to cover it in tarpaulin at the end of summer. It changes constantly with the seasons and matures over the years, allowing the plants to grow into their habitat. [continue]