Notwithstanding any other provision of law to the contrary, in a county of the first classification with a charter form of government and having a population of more than six hundred thousand but less than nine hundred thousand inhabitants, any person who is a water service customer of any municipality located in whole or in part in such county may continue to receive water service from such municipality even in the event that a public water supply district shall claim the exclusive right to provide water service to such person.
§ 91.055, RSMo