PURPOSE: To efficiently remove phosphorus contained in activated sludge by adding calcium ion or aluminum ion to activated sludge introduced into a sludge concentrating storage tank and allowing the same to react with phosphate ion in sludge to produce insoluble material, and thereafter removing the insoluble material.

CONSTITUTION: In an intermittent aeration tank 8, a blower 15 is intermittently operated by setting a timer 20 and thereby BOD and T-N are removed. Furthermore, activated sludge is introduced into a precipitation tank 9. Liquid-solid separation is performed and supernatant is discharged as treated water. At this time, the timer 20 of the blower 15 is set and an excess sludge drawing pump 11 having been interlocked is driven. Return sludge in the precipitation tank is rapidly transferred to a sludge concentrating storage tank 14. In the midway of transfer of return sludge, aluminum sulfate stored in an aluminum sulfate tank is added by a quantitative pump 18 and mixed with return sludge and the mixture is stored in the sludge concentrating storage tank 14.