Abstract: A method of clarifying proteinaceous waste water is provided involving feeding the water through a treatment zone where it is subjected to various treatment steps and from which it issues as a clarified product. Solid impurities are coagulated in said zone and 20-50% by mass of the waste water feed is recirculated as clarified product which is aerated to provide a foam containing less than 10% by volume of liquid, and which is mixed with the feed in or upstream of the treatment zone. Foam rises and floats to the surface of the liquid in the treatment zone from which it, with solids entrapped therein, is removed.