PURPOSE: To raise the dehydrating effect for solids and the cleaning degree of waste water by a method in which solids separated from an original water in a separation zone are once dehydrated, coagulated in a coagulation accelerating zone, and then again dehydrated.

CONSTITUTION: When original water and a coagulant are supplied into an original water inlet pipe 120, the water is trapped in a rotary drum 101, during which suspended particles in the water are precipitated to form aolids in precipitating chambers 122W126 and also supernatant water flows out of a drain hole 105. The solids S go onto the periphery of a conical body 110, guided by a guide plate 117, falls down through a sludge discharge hole 106 from the tip of the conical body 110 into the rotary drum 201 in the coagulation accelerating zone 2, and then enters a groove between scraping blades 206. The solids S in the groove are lifted, transferred onto a radial partition plate 214, dropped onto a conical body 208, and then discharged through a sludge discharge port 209. The separated water is flowed out of the hole 214' of the radial partition plate 214.