The council 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 contracts shall be embodied in and let only by an ordinance approved by 4 members 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 10 years.