Mich. Comp. Laws § 123.738
A county operating under this act may acquire outside its corporate limits any part of a water supply system which is necessary for the purpose of securing a source of supply and may acquire outside its corporate limits any part of a sewage disposal system or refuse system which is necessary for the purpose of disposing, including treatment or incineration, of its sewage or refuse. A county operating under this act may also acquire any part of a water supply system, a sewage disposal system, a refuse system or make lake improvements, or acquire erosion control systems in an adjoining county or counties upon the consent expressed by contract with or resolution of the governing body of the municipality or municipalities in such adjoining county or counties in which such part of the system or lake improvements is to be located or which is to be served by such part of the system. The exercise by any county of such powers outside its corporate limits shall be subject to all constitutional provisions relating thereto.
MCL 123.738