An aerated treatment of water is performed using a serially coupled reactor and flotation tank and is performed using so much activated sludge in the water that small bubbles from an aerator are entrained in the activated sludge, leading some of or all the activated sludge to flotate in the flotation tank. A reactor (1) and a flotation tank (36) for performing the process are equipped with at least one aerator (19, 20, 21, 22). A number of preferred processes and designs elaborate on the invention, including how the invention may be incorporated into existing wastewater treatment facilities and exploited there.