Provided are a method and system with which it is possible to economically inactivate a raw water tank and wastewater accumulated therein. In a method which inactivates wastewater (A) which is accumulated in a raw water tank (1): a switch valve (15) is disposed which selectively connects, to either an outflow path (12) or the raw water tank (1), an exhaust port of a heating device (5) which takes in the wastewater (A) and, while heating the wastewater (A) to a prescribed temperature (θ), retains the wastewater (A) at the prescribed temperature (θ) for a prescribed time (D1) when connected to the outflow path (12) by the switch valve (12), the heating device (5) is heated to a high sterilization temperature (θ1) with which a prescribed sterilization level can be obtained at a prescribed retention time (D1), and the inputted wastewater (A) is inactivated and when connected to the raw water tank (1) by the switch valve (15), with the backflow of outputted wastewater (F) of the heating device (5), the raw water tank (1) is heated to a low sterilization temperature (θ2) and is retained at the low sterilization temperature (θ2) for a time (D2) at which the prescribed sterilization level can be obtained or longer, inactivating the wastewater in the raw water tank (1). It would be preferable to heat the interior overall of the raw water tank (1), including the vapor part thereof, at the low sterilization temperature (θ2) or higher, with the backflow of the outputted wastewater (F) of the heating device (5).Linvention porte sur un procédé et sur un système, avec lesquels il est possible de désactiver économiquement un réservoir deau brute et des eaux usées accumulées à lintérieur de celui-ci. Dans un procédé qui désactive des eaux usées (A) qui sont accumulées dans un réservoir deau brute (1) : une vanne de commutation (15) est disposée, laquelle effectue une connexion sélective soit vers une trajectoire découlement de sortie (12) soit vers le réservoir deau brute (1), un or
