The mayor and city council of any city of the metropolitan class, in addition to other sources of revenue available to the city, may by ordinance set up appropriate rental or use charges to be collected from users of its sewerage system and provide methods of collection of such charges, except that users shall include in part any users outside of such city where the sewer is directly or indirectly connected to the sewerage system of such city and users within any sanitary and improvement district now existing or hereafter organized under the laws of this state when the sewerage system, or any part thereof, of the sanitary and improvement district directly or indirectly connects to any part of the sewerage system of the city. Such charges shall be charged to each property served by the sewerage system, shall be a lien upon the property served, and may be collected either from the owner or the person, firm, or corporation using the service. All money raised from such charges shall be used for maintenance or operation of the existing sewerage system, for payment of principal and interest on bonds issued, as is provided for in section 14-365.06, or to create a reserve fund for the payment of future maintenance, operation, or construction of a new sewerage system for or additions to the sewerage system of the city. Any funds raised from such charges shall be placed in a separate fund and not be used for any other purpose or diverted to any other fund.