70 ILCS 3010/6a
So long as any revenue bonds of the sanitary district are outstanding, sufficient revenue derived from the operation of such a sewerage system shall be set aside as collected, and deposited in a special fund of the sanitary district, and this revenue shall be used only for the purpose of paying the cost of operating and maintaining the sewage system, providing an adequate depreciation fund, and paying the principal of and interest on the bonds issued by the sanitary district under this Act; any revenue not required to be so deposited may be used for the purpose of paying the principal and interest on any other bonds or indebtedness issued or incurred by the district or for paying for the construction, acquisition, improvement, extension, operation or improvement of the sewerage system, or for any other corporate purpose of the sanitary district.
70 ILCS 3010/6a