Abstract: An apparatus for purification of waste water in a tank or a pool has a housing rotatably journalled on a raft and provided with pump means to force air and water therethrough. The housing contains first discs having central holes and a second discs having peripherical apertures at the wall of the housing. Each disc forms a baffle for air and water flows through the housing, thereby imparting a tortuous path to the air and water passing through the housing. The discs are corrugated and are arranged in a stack with the corrugations of adjacent discs oriented at right angles. The corrugations strengthen the discs and form support points at which the discs contact each other, and also define channels between the discs, through which channels of water and air can flow. A spiral pump rotating with the inlet end of the rotor forces metered volumes of water and air through the rotor.