The local unit or units of government may enter into a contract with a person to receive, treat, transfer, and process in the manner provided in this part, the sewage, night soil, garbage, and refuse of the local unit or units of government. The contract may authorize the person to charge the owners of the premises served a service rate determined by the local unit or units of government to be just and reasonable, or the local unit or units of government may contract to pay a flat rate for the service, paid out of their general fund or funds, or assess the owners of the property served a reasonable charge to be collected as provided in this part and paid into a fund to be used to defray the contract charges.