The council may, if the public interest 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 four-fifths of the council and subject to popular referendum. In no case shall such contract be for a longer term than ten years. The contractor shall be subject as far as possible to the rules as to rates and service, and as to council control, laid down for the holders of franchise in this charter.