In the fields of application of ozone plants produced by the German ozone industry, the treatment of pool water is foremost. Especially the precursors of the chlorine-nitrogen compounds (combined chlorine) and the chlorine-carbon compounds (haloforms) which, for reasons of health, are reckoned among the undesirable substances in pool water, can be considerably reduced through the use of ozone during pool water treatment. The efficiency of the treatment processes depends substantially on the reaction period of the ozone with the pool water contaminants, as is shown by tests with new processes having long ozone contact periods.