65 ILCS 5/11-151-5
If a municipality annexes part, but not all of the territory of a public water district, sanitary sewer district, or both, the corporate authorities of the municipality and of the district may enter contracts providing for the division and allocation of duplicate and overlapping powers, functions and duties between the 2 entities and for the use, management, control, purchase, conveyance, assumption and disposition of the properties, assets, debts, liabilities and obligations of the district. The corporate authorities of a district and such a municipality may also enter agreements providing for the operation by the municipality of the district's utility systems and other properties or for the transfer, conveyance or sale of those systems and properties to the municipality. "Systems and properties" includes those of every kind and character and whether situated within or outside the municipality. An operating contract made under this Section may not extend for a period longer than 30 years and must be subject to amendment, renewal or termination by mutual consent of the contracting parties. No contract under this Section may contain any provision impairing the obligation of any existing contract of such a municipality or district.
65 ILCS 5/11-151-5