Mo. Rev. Stat. § 91.593
All cities in the state under special charter which have or may hereafter have a population of not less than three thousand and not more than ten thousand inhabitants, which now own and operate or may hereafter own and operate a waterworks system shall have the right to purchase, install and maintain pipes, mains, machinery, buildings, easements, rights-of-way and real estate necessarily connected therewith outside the limits of any such city including the right of condemnation and the right to purchase and maintain any such facilities now existing from any person, firm, corporation or governmental unit authorized to sell the same, whenever in the opinion of its board of public works any such facilities are reasonably necessary for the purpose of connecting with another water supply system capable of furnishing emergency water to such city or whenever in the opinion of the board there is sufficient need and demand from persons outside the limits of such city for water from the water supply system thereof to justify, make profitable and advantageous the purchase, installation and maintenance of such facilities as a part of the water supply system of such city.
§ 91.593, RSMo