65 ILCS 5/11-134-5

During the term of such a contract, the entire revenue received by the city from the operation of its waterworks system shall be deposited in a separate fund designated as the water fund of the city of ..... This fund shall be used only in paying, first, the cost of maintenance and operation of the waterworks system, and then the obligations, in whatever form, of the city that are payable by their terms from that revenue. All charges or payments required to be paid by the city under such a contract for the filtration and treatment of its water supply shall be deemed to be part of the cost of maintenance and operation of its waterworks system.