Abstract: Contaminated water is caused to flow up through a chamber while aerated to produce air bubbles of a size mainly in the range of 0.03 mm to 1 mm by means of air diffusors disposed at the bottom of the chamber. Organic and inorganic impurities in the water is, by a combination of water flow, flotation and physical adsorption caused by the air bubbles, brought to the surface of the water in the chamber, where a foam containing dissolved impurities is formed and instantly removed from the chamber while a mixture of water particles and microscopic air bubbles is allowed to flow over a partition wall into an adjacent basin having a considerably larger volume than that of the chamber. In the basin a flotation process will take place causing a slurry to form on the surface of the water in the basin, which is removed, preferably at the end of the basin opposite the chamber.