The existing waterworks system in its entirety, together with all additions, improvements and extensions thereto that may hereafter be made and the existing sewerage system in its entirety, together with all additions, improvements and extensions thereto that may hereafter be made of the city, be and the same are hereby declared to be a combined system, and said combined waterworks and sewerage system shall be maintained and operated as a single utility, and a charge or rate shall be established for the use of such combined system, which shall be reasonable and commensurate with the services performed by said combined system, and shall be sufficient to maintain, operate, provide an adequate depreciation fund and pay the principal and interest on any revenue bonds which may be issued, which by their terms are made payable from the revenues of such combined system. (1972 Code)