14. The apparatus of claim 13, further comprising a fermentation tank, into which the sludge which has been solubilized in the solubilizing device is introduced and subjected to a fermentation treatment, and the sludge which has been subjected to the fermentation treatment in said fermentation tank is returned to the first anaerobic tank.
15. The apparatus of claim 14, further comprising means for supplying some of the sludge which has been subjected to the fermentation treatment in the fermentation tank to the second anaerobic tank.
16. The apparatus of claim 15, further comprising means for supplying at least some of the sludge separated in said solid/liquid separating device directly to the first anaerobic tank.
17. An apparatus for the biological removal of phosphorus from raw water, the apparatus comprising: a first anaerobic tank into which the raw water and activated sludge are introduced to form a mixed liquor, an anoxic tank in which bound oxygen is applied to the mixed liquor from said first anaerobic tank, an aerobic tank in which the mixed liquor from said oxygen-free tank is aerated, mixed liquor returning means which returns some of the mixed liquor from said aerobic tank to the oxygen-free tank, first solid/liquid separating means in which the mixed liquor flowing out of said aerobic tank is subjected to solid/liquid separation, means whereby some of the solid/liquid separated sludge from said first solid/liquid separating means is returned as the activated sludge to the first anaerobic tank, a second anaerobic tank in which the remaining solid/ liquid separated sludge from the first solid/liquid separating means is maintained anaerobically and the phosphorus is released, second solid/liquid separating means in which the sludge admixed liquid from said second anaerobic tank is subjected to solid/liquid separation,solubilizing means in which the sludge separated in said second solid/liquid separating means is solubilized, solubilized sludge returning means with which at least some of the sludge which has been solubilized in said solubilizing means is returned to the first anaerobic tank, means for supplying at least some of the sludge which has been solubilized in the solubilizing means to said second anaerobic tank, and phosphorus-removing means for removing phosphorus from water which has been separated in the second solid/liquid separating means.