70 ILCS 3715/13
Whenever revenue bonds are issued under this Act the income and revenue derived from the operation of the water supply or other waterworks properties of the authority shall be used only to pay the cost of operation and maintenance of the waterworks properties, to pay principal of and interest on any revenue bonds issued hereunder and to provide an adequate depreciation fund, which fund is hereby defined to be for such replacements as may be necessary from time to time for the continued, effective and efficient operation of the waterworks properties of such authority, which such fund shall not be allowed to accumulate beyond a reasonable amount necessary for that purpose, the terms and provisions of which shall be incorporated in the ordinance or resolution authorizing the issuance of the bonds.
However, the board of trustees shall have power to agree and covenant in the ordinance or resolution authorizing such revenue bonds that the cost of operation and maintenance of such waterworks properties shall be defrayed from the tax authorized by Section 6 of this Act to the extent that the proceeds of such tax will suffice to meet such cost of operation and maintenance.
70 ILCS 3715/13