IN THIS SYSTEM WASTE WATER, PARTICULARLY REFINERY WASTE WATER, IS TREATED IN A SERIES OF LAGOONS IN WHICH THRIVE MICROORGANISMS THAT FEED ON CONTAMINANTS IN THE WASTE WATER. THE SYSTEM IS CHARACTERIZED BY MOBILE AERATOR APPARATUS MOVING ACROSS THE LAGOONS'' SURFACE. THESE APPARATUS INTRODUCE AIR INTO THE LAGOONS AND CHURN SURFACE WATER, AND THEY HAVE MEANS WHICH DIRECT AT LEAST SOME OF THE CHURNING SURFACE WATER TOWARDS THE LAGOONS'' BOTTOMS. THE UNDERWATER TURBULENCE SCOURS THE LAGOONS'' BOTTOMS AND PROVIDES A WAY OF CONTROLLING SLUDGE DEPOSITS.