Abstract: An automated method for determining the cleaning operation of a water treatment or water distribution apparatus or system to which one or more substances are added, the method comprising the steps of: (a) conducting one or more measurements of a property of the or an added substance; (b) comparing the property measurement s) from step (a) with at least one reference value for that property; and (c) determining when the comparison in step (b) is within, above or below a pre-determined range or level for that property. The determination in step (c) can consider whether the cleaning operation has been successfully carried out. The present invention automates and therefore optimizes the cleaning operation of the water treatment or distribution apparatus or system such that expensive periods of non-operation during sanitization or cleaning are minimized both in terms of frequency and duration.