70 ILCS 3705/20

Any district organized under this Act may contract to supply water to any city, village or incorporated town owning and operating a waterworks system whether said city, village or incorporated town is located within the corporate limits of the district or not, and any such district may also supply water for public and domestic use in the area surrounding the limits of said district; provided, the supplying of such water is merely incidental to the maintenance and operation of its waterworks properties for the use and benefit of the inhabitants of such district.