The present invention concerns a method and a system for purifying water from a basin, in particular a swimming pool (1), by circulating a water flow (7) through a filtration system by pumping means (10), whereby at least a sub-flow (28) of the water filtrated in first filtration means (9) and then forwarded to secod filtration means (29) of the filtration system and being subjected to a photochemical treatment in a UV radiation system (40) before returned to the basin, wherein the water flow passing through the first filtration means (9) is filtered in membrane filter means (29), where at least a sub-flow (39) of the water is passed through a membrane filtrated means (29) for e.g. nanofiltration and that at least a partial flow (39) of this membrane filtrated water is irradiated with UV electromagnetic radiation. A system according to the invention is relatively compact due to the avoidance of sand filters. It is by the invention realized that the system and method may be used for purifying different types of water, i.e. not only bath water from swimming pools but also waste water e.g. from domestic households, contaminated water, etc. Moreover, an efficient removal of THM, AOX, EDC, and PO.sub.4 as well as micro organisms with only a minimum of molecular chlorine in the bath water is achieved.