55 ILCS 5/5-15019
Whenever revenue bonds are issued under this Division the income and revenue derived from the operation of the waterworks properties or sewage facilities, or waste management facilities, as the case may be, shall be used only to pay the cost of operation and maintenance (including insurance) of the waterworks properties or sewage facilities, or waste management facilities, or a combination thereof, as the case may be, to pay principal of and interest on any revenue bonds issued hereunder, 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, or sewage facilities, or a combination thereof, or waste management facilities, as the case may be, 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 authorizing the issuance of the bonds, and to maintain such other reserves and sinking funds as may be deemed necessary or desirable by the county for the payment of the bonds or the extension or improvement of the waterworks properties, or sewage facilities, or a combination thereof, or waste management facilities, as the case may be.
55 ILCS 5/5-15019