Bio pool aka natural swimming pool or pond is the chemical-free alternative to evil chlorine and complicated mechanical filtering systems, designed to use living plants and micro-organisms as bio-filters. The principles and technology have been well established and tested in UK, Germany and Austria over the past two decades.
The basic philosophy here is the constant circulation of the water from the swimming area, which is structurally distinct to the shallow wetland or regeneration zone with the friendly micro-organisms and selected aquatic plants. To achieve sufficient filtration, the area of the plant zone should equal at least 50% of the swimming area.
A skimmer creates a gentle surface flow and collects large debris, and to assure biological safety a UV steriliser finishes off the cleaning process. (diagram source)
The cost of a natural swimming pool tends to be the same with a conventional pool, however the maintenance and running costs are considerably less.
Check out a video introduction to natural swimming pools