PURPOSE: To decrease the amount of addition of coagulant and reduce the size of the dehydrating device by mixing the separating liquid after the digestion treatment and dehydration of the waste water containing organic matter and the excess sludge of aerobic treatment and dehydrating the mixture after freeze-thaw treatment.

CONSTITUTION: In waste water treatment of the waste water containing organic matter such as human waste, etc., the digestion sludge from a digesting tank 3 is led to a centrifugal dehydrator 15, where it is dehydrated to dehydrated digestion sludge 17. The separating liquid of this time is sent to a sluge mixing tank 9, where it is mixed with the excess sludge from a precipitating tank 7 and is then transferred into a reaction tank 10. Here, inorganic salt coagulant 11 is added to cause coagulation and reaction, after which the sludge is dehudrated in a freeze-thaw type sludge dehydrating unit 16. Since the digestion sludge is dehydrated in this way, the amount of addition of coagulant can be 3 to 5% of the solids of unrecovered digestion sludge and excess sludge. Since the amount of the sludge to be treated decreases, the dehydration device may be made smaller in size.