The governing body of the city shall establish rates and charges for water and for the use of the sewage disposal system. The amount of such rates and charges shall be reasonable and sufficient to pay the cost of operation, repairs, maintenance, extension and enlargement of the water and sewage system and improvements thereof and new construction and the payment of any bonds and the interest thereon as may be issued for such water and sewage system. No revenue shall be used for the payment of bonds payable primarily by assessments against property in sewer districts. Such revenue may be used to pay revenue bonds or general obligation bonds payable by the city at large issued either for the waterworks system or sewage disposal system before the systems were combined or for the water and sewage system after they have been combined. The city is authorized to discontinue water service for any failure to pay the rates or charges fixed for either water service or the use of the sewage disposal system or both when due.