PURPOSE: To easily and rapidly obtain odorless sludge which is free from pathogenic fungi and parasite eggs and may be fertilized without requiring a high cost by using an existing purification device by treating the sludge generated in a sewer treatment with a peracid, then completely rotting the sludge in a septic device.

CONSTITUTION: The sludge generated in the sewer treatment is put into, for example, a prethickening device where the peracid (e.g.: peracetic acid) of 1 to 3C in the amt. of 500 to 6000 mg per 1 liter sludge is added to the sludge and the sludge is treated for 10 minutes to 2 hours. The sludge is adjusted to pH of 7 to 9 with CaO or Ca(OH)2 at need and is then transferred to the septic device where the sludge is completely rotten for 8 to 20 days.