65 ILCS 5/8-4-17

Whenever refunding revenue bonds are issued under this Division 4, sufficient revenues received from the operation of the municipally-owned utility or facility shall be set aside as collected and be deposited in a separate fund, which shall be used only in paying the cost of operation and maintenance of the utility or facility, providing an adequate depreciation fund, and paying the principal of and interest upon the revenue bonds issued that are payable by their terms only from such revenues.