Abstract: Water to be treated is introduced into a basin having a closed bottom and an outer wall. A stationary diffusor is located at the bottom at a position spaced from the outer wall and produces air bubbles of varying size acting to lift water impurities toward the surface of the water and to circulate the water in the basin as a first stream flowing vertically upwardly from the diffusor, horizontally outwardly toward the outer wall, vertically downwardly toward the bottom and horizontally along the bottom back to the diffusor. A second stream is caused to circulate circumferentially in the basin. The first and second circulating streams cooperate to cause foam and slurry formed on the surface to concentrate in an outer peripheral flow path from which the foam and slurry are removed.