PURPOSE: To attain a high removing effect of phosphorus and to prevent a generation of a hardly concentrating and dehydrating sludge by adding the slurry of particles consisting of ferric hydroxide and/or aluminum hydroxide to a phosphorus-containing raw water to subject the slurry to a biological treatment and subjecting an effluent water to solid-liq. separation treatment to obtain a treated water.

CONSTITUTION: The slurry (a) prepared by suspending particles consisting of ferric hydroxide and/or aluminum hydroxide good in settling and concentrating properties is added to sewage 1 at a liq. injecting point 10 of a raw water supply pipe 20 and flows into an aeration tank 2 of an activated sludge process together with the sewage 1. Then, when the activated sludge and the slurry (a) are brought into contact with the sewage 1 while being fluidized and suspended by an air discharged from a diffusing member 5, BOD in the sewage 1 is subjected to the biological treatment and removed, and phosphorus in the sewage is sufficiently removed by the slurry (a). Then, a mixture of the activated sludge and a flock formed from the slurry (a) are settled and separated in a sedimentation basin 3 and the sewage is separated into a clarified treated water 4 highly removed in phosphorus, BOD and SS and a settled sludge 17.