The invention relates to a method and an installation for treating liquids, especially for purifying contaminated water. Said method consists of the following steps: the raw water is pre-purified by adding a foam-forming reaction agent; the pre-purified water undergoes a main purification process by foaming the water with ionising radicals contained in the exhaust from the subsequent steps, in a known perforated cylinder washer, the foam formed is eliminated; the de-foamed water is ionised; and the impurities in the purified water are sedimented. The purified water is then additionally purified, the mud consisting of impurities from the pre-purification and the main purification process is physically separated, and the transformation of raw water into purified water is carried out continuously.