To coagulate and separate waste water which cannot be coagulated and separated by a conventional method by a method wherein, in a method wherein flocculants are added and mixed into suspended waste water to coagulate suspended particles therein, powder of light calcium carbonate is added and mixed into the suspended waste water before adding flocculant.

Starch waste water 1 being suspended waste water is first introduced into a retention tank 2, however, a part of the water 1 is led to a bacteria propagation tank 4 and then introduced into the tank 2 after bacteria have been propagated in the tank 4. The water 1 introduced into the tank 2 is led into a lime mixing tank 9 two days later. In the tank 9, a dense solution, into which slaked lime is dissolved beforehand, is added to the water 1, which is agitated by an agitation vane and introduced into an inorganic flocculant mixing tank 12. And in the tank 12, a dense solution, into which polychlorinated aluminum is dissolved, is added and in a high molecular flocculant mixing tank 15, a dense solution, into which a high molecular flocculant is dissolved, is added to the water 1 and the water 1 is introduced into a coagulating sedimentation tank 18 to settle suspended particles as flocs.