204.440.  Charges to be at rate recommended by advisory board, exception. — The board of trustees shall impose, charge and collect a reasonable charge from the sewer districts and municipalities, based upon sewage discharge as shown by metering such flows, the volume of water used by the residential, commercial, and industrial establishments' customers within the corporate limits of such district or municipality, or other equitable measure.  Such charges shall be fixed at such rate or rates as are recommended to the board of trustees by the advisory board; except that such rates shall ensure that the rates fixed will provide sufficient revenues for the operation and maintenance of the system and the payment of principal and interest on all outstanding revenue bonds as provided in sections 204.250 to 204.470.