The city may operate its own garbage or yard refuse service or contract with one or more contract agents to provide such service. At any time the city operates its own garbage or yard refuse service or there is in force a contract entered into by the city with another for the collection of garbage, wet garbage, rubbish, refuse, or yard refuse in the city, it shall be unlawful for any person other than the contract agent, except as otherwise provided in this chapter to collect or transport the same within the. city. (Ord. No. 1038, § 3 (part).)