65 ILCS 5/11-141-18

All revenue derived from the operation of such a sewerage system, improvement, or extension shall be set aside as collected, and deposited in a special fund of the municipality. It shall be used only for the purpose of paying the cost of operating and maintaining the sewerage system, improvement, or extension, providing an adequate depreciation fund, and paying the principal and interest on the bonds issued by the municipality under Sections 11-141-12 through 11-141-18 for the purpose of constructing or acquiring the system, improvement, or extension.