Whenever revenue bonds are issued under this Division 139, sufficient revenue derived from the operation of such a combined waterworks and sewerage system shall be deposited in a separate fund, designated as the waterworks and sewerage fund of the municipality. It shall be used only (1) to pay the cost of maintenance and operation of the combined system, (2) to provide an adequate depreciation fund, and (3) to pay the principal of and interest upon the revenue bonds of the municipality issued under this Division 139.
65 ILCS 5/11-139-9