All revenue derived from the operation of such a sewerage system or drainage system, improvement or extension shall be set aside as collected, and deposited in a special fund of the sanitary district. It shall be used only for the purpose of paying the cost of operating and maintaining the sewerage system or drainage system, improvement, or extension, providing an adequate depreciation fund, and paying the principal and interest on the bonds issued by the sanitary district, under Sections 10.1 to 10.7 inclusive, of this Amendatory Act of 1955, for the purpose of constructing or acquiring the system, improvement, or extension.
70 ILCS 2605/10.7