Bonds which are issued and secured by a mortgage on the utility, as provided in section 14-365.02, shall not be a general obligation of the city, but shall be paid only out of the revenue received from the service charges, as provided in section 14-365.03, or from a sale of the property and the franchise, referred to in section 14-365.02, to operate the system, under a foreclosure proceeding. If a service rate or charge is charged, such portion of such rate or charge as may be deemed sufficient shall be set aside as a sinking fund for the payment of the interest on such bonds and the principal of such bonds at maturity.