On the other hand, at least some of the separated sludge in the settling tank 5, which has taken up phosphorus in the form of orthophosphate as a result of the biological treatment, is introduced into the solubilizing tank 6 and subjected to a solubilizing treatment by blowing in ozone gas. That is to say, the sludge is oxidatively degraded to BOD components and solubilized with ozone.
As well as pure ozone, ozone-containing air or ozonized air, for example, can be used for the ozone treatment gas in the solubilizing tank 6.
As a result of various studies, it has been recognized that the rate of release of phosphorus is slow with the apparatus for the biological removal of phosphorus shown in FIG. 5 because there is virtually no substrate in the sludge anaerobic tank. Furthermore, because the sludge in the phosphorusreleasing tank is concentrated and the whole amount is solubilized, phosphorus removal and the reduction in sludge volume are even and the extent of the two actions are not interrelated and are difficult to set. Furthermore, in the apparatus for the biological removal of phosphorus shown in FIG. 6, it has been confirmed that the water which is introduced into the MAP reaction column contains a large amount of sludge so that the MAP production reaction is impeded.