A schematic representation of the phosphorus-removing apparatus of Japanese Unexamined Laid Open Patent Application Hll-188383 is shown in FIG. 5, where the raw water 31 is introduced into the anaerobic tank 33 o f the biological phosphorous-removal activated sludge treatment process 32 along with return sludge 38 from the settling tank 35 and ozone-oxidized sludge 49 which is discharged from the ozone oxidation tank 48, and BOD elimination and absorption of phosphorus by the phosphorus-removing bacteria are carried out in the anaerobic tank 33 and the aerobic tank 34 which is connected thereto in the process 32. Some of the sludge in the same process is sent to the settling tank 35 and the supernatant w ater in the settling tank 35 is taken out as treated water 36, and most of the settled sludge 37 which has settled out is recycled to the anaerobic tank 33 as the returned sludge 38.
A part of the sludge is divided off from the aforementioned settled sludge 37 and is poured into the sludge anaerobic tank 40 as the branched or divided sludge 39 and phosphorus is released from the phosphorus-containing sludge on being left to stand here under anaerobic conditions. The sludge in the sludge anaerobic tank 40 is then subjected to solid/liquid separation by means of a membrane separating tank 41, and then the separated sludge 43 obtained by solid/liquid separation is introduced into an ozone-oxidation tank 48 and subjected to oxidative degradation with ozone, and soluble organic material is dissolved out. The ozone-oxidized sludge 49 produced in this ozone- oxidation tank 48 is returned to the anaerobic tank 33 of the biological phosphorous removal activated sludge treatment process 32 and then flows into the aerobic tank 34, and the ozone-oxidized sludge 49 (o f which the biodegradability by microorganisms has been improved) is degraded to carbon dioxide gas and water microbiologically.