Abstract: In a first stage, a flow of wastewater is adjusted to near zero streaming potential by injecting a mineral acid into the wastewater. The thus adjusted flow enters a basin where it is subjected to a decreasing gradient of small bubbles to form and separate off a first buoyant floc from the flow of wastewater. The flow of wastewater then passes to a second stage. In the second stage, a metal coagulant is injected into the wastewater, and the wastewater and coagulant enter a coagulation cell into which dense quantities of bubbles are supplied. Buoyant composites of particulates, coagulant and bubbles are formed in the cell. These composites leave the cell together with the wastewater flow and enter a second flotation basin where a second buoyant floc is separated off.