Purification of industrial waste waters by flotation

In a process for clarifying and removing suspended and dissolved foreign particles from wastewater wherein the wastewater is subjected to flocculating conditions, the improvement which comprises first coagulating the foregin particles by mixing into the wastewater from about 10 to about 1000 ppm of a multivalent metal salt selected from the group consisting of ferric sulfate, ferric chloride, aluminum sulfate, alum and lime, next incorporating into the waste-water microbubbles having a diameter of about 10 to 500 microns and in a number in excess of about 10.sup.5 per liter of wastewater treated so as to interact with the coagulated foreign particles to form embryo flocs of said microbubbles and coagulated foreign particles, which embryo flocs and substantially all of the wastewater remain unseparated from each other, subsequently mixing said unseparated embryo flocs and water with a polyelectrolyte polymer flocculant so as to combine a plurality of embryo flocs into a final floc which floats to the surface of the wastewater, said polyelectrolyte being added in an amount of about 0.1 to 100 ppm based on the amount of wastewater being treated and only after said step of incorporating microbubbles to form embryo flocs has ceased and only within a period of from approximately 2 seconds to about 30 minutes after initiating said step of incorporating microbubbles, and removing from the wastewater the final floc that is floating at the surface of the wastewater.