PURPOSE: To improve the dehydrating property of sludge by mixing initial seed sludge with the concd. org. sludge to be treated and adjusting the pH thereof to initial pH then aerating the sludge, discharging part of the sludge, adding the same amt. of the concd. org. sludge thereto and repeating the operation after the adjustment of the pH.

CONSTITUTION: An alkaline agent is added to the org. sludge which is settled and concentrated by a biological treatment after dilution to adjust the initial pH to 8W10. The sludge is aerated while the pH thereof is kept as it is without adjustment. Part of the sludge is discharged and the same amt. of the concd. org. sludge is charged thereto. The discharged sludge is returned after dehydration and the operation after the adjustment of the pH is repeated to prepare the initial seed crystal. Such initial seed crystal is mixed with the concd. org. sludge to be treated and the operations consisting of adding the alkaline agent thereto to adjust the initial pH to 8W10, aerating the sludge without adjusting the pH, discharging part of the sludge, charging the same amt. of the concd. org. sludge thereto, adjusting the initial pH and aerating the sludge and removing part of the treated sludge are repeated. As a result, the dehydrating property of the settled and concd. org. sludge is improved.