Mo. Rev. Stat. § 650.295
Any person certified by the department of natural resources as a certified backflow prevention assembly tester shall be eligible to be registered or licensed by any county, city, town, village or other political subdivision of this state to test and repair a backflow prevention assembly pursuant to the practice of his or her trade within that political subdivision as long as he or she maintains state certification as a backflow prevention assembly tester. However, political subdivisions may set additional testing standards for individuals who are seeking to be certified as backflow prevention assembly testers. Notwithstanding any other provision of law to the contrary, agencies of the state or its political subdivisions shall only require carbonated beverage dispensers to conform to the backflow protection requirements established in the National Sanitation Foundation standard eighteen, and the dispensers shall be so listed by an independent testing laboratory.
§ 650.295, RSMo