65 ILCS 5/11-141-6
So long as any revenue bonds of the municipality under the provisions of this Division 141 are outstanding, all revenue derived from the operation of such a sewerage system shall be set aside as collected, and deposited in a special fund of the municipality, and this revenue shall be used only for the purpose of paying the cost of operating and maintaining the sewerage system, providing an adequate depreciation fund, and paying the principal of and interest on the bonds issued by the municipality under the provisions of this Division 141. When no such revenue bonds are outstanding, such revenue shall be used for the purpose of paying the principal of and interest on any other bonds or indebtedness issued or incurred by the municipality for the construction, acquisition, improvement, extension, operation or improvement of the sewerage system, or for paying for the construction, acquisition, improvement, extension, operation or improvement of the sewerage system.
65 ILCS 5/11-141-6