Mich. Comp. Laws § 123.234

Sec. 4. For the purpose of constructing, acquiring, extending or improving a system of sewers or sewage disposal, 2 or more political subdivisions may borrow money and issue revenue bonds in the name, and by authority, of said political subdivisions jointly, but payable solely from the revenues of such system. Such political subdivisions shall provide by an ordinance, adopted by their respective governing bodies, for the issue of such revenue bonds in the name of such political subdivisions jointly, for the execution thereof by suitable officers of said political subdivisions and for the sale of said bonds at a joint meeting of the governing bodies thereof.