The City may, if the public interests will be served thereby, contract with any responsible person, co-partnership or corporation for the operation of any utility owned by the City, upon such rentals and conditions as it may deem necessary; but such contract shall be embodied in and let only by an ordinance approved by two-thirds vote of the Council and subject to popular referendum. Such ordinance shall not be an emergency ordinance. In no case shall such contract be for a longer term than ten years.