Abstract: Ozonization of the bath water taken from the reservoir (1) occurs according to the invention utilizing only 1% of ozone while using from 1 to 1.2 g/h of ozone in 700 l/h of (untreated) air from a convenient-to-handle ozonizer (7) of this capacity per 100 m.sup.3 of natural water. Ozonization occurs without ozonization reactors and ozone destroyers under pressure directly in the conduit (2) of the natural water stream Q. For this purpose. the ozone air is whirled with a partial stream (q) of the natural water in a flow spiral (9) to form an aerosol-like mixing condition and is blasted into the main stream of natural water. The high phase interfaces thus achieved effect rapid ozonization with quantitative ozone consumption. After the uncomplicated conversion of the baths with original pure chlorination operation to the ozonization-chlorination technique according to the invention, the chemical costs can be reduced by up to 70% with a simultaneous improvement in the water quality.