All revenues from water and sewer operations and from any other public utilities operated by the city shall be put into a separate fund to be used for current operating expenses of these utilities, replacement and extension of facilities, and payment of interest and principal on bonds issued for these purposes. If these revenues are not sufficient, the deficit must be made up from the general funds of the city. If these revenues exceed the amounts needed for the above purposes, the surplus may be used for the general governmental purposes of the city. (Acts 1951, Ch. 539, § 106)