Mich. Comp. Laws § 123.745
Any 1 or more municipalities or other public corporations, either within or without the county, shall have authority to contract for the purchase of water or sewage or refuse services or transportation from a county operating under this act. Any charges specified in any such contract shall be subject to increase by such county at any time, if necessary, in order to provide funds to meet the obligations of the project involved. The county operating under this act may enter into contract with any public or private corporation, for the purchase by such county from or for the sale by the county to the corporation of water or sewage or refuse services, and for the right to transport sewage through the sewers of the county or of the corporation or to use the facilities of the other. Any contract authorized herein shall be for a period of not exceeding 50 years.
MCL 123.745