70 ILCS 3705/21
Whenever revenue bonds are issued under Section 16 of this Act the income and revenue derived from the operation of the waterworks properties of the district 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 district, 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.
70 ILCS 3705/21