Mo. Rev. Stat. § 250.030

Any such city, town or village desiring to operate and maintain a combined waterworks and sewerage system shall adopt an ordinance declaring that its waterworks whether then existing or to be acquired or constructed and its sewerage system then existing or to be acquired or constructed shall thenceforth be operated and maintained as a combined waterworks and sewerage system and may provide that such combined system shall include all future improvements or extensions, whether to the waterworks or to the sewerage system or to both.