Any such contract shall be authorized by resolution or resolutions duly adopted by the governing body of the city. Such resolutions and contract shall provide that the cost of the plant shall be paid solely out of the net revenue thereof, to be derived from special rates and charges imposed and collected for the service thereof to users of the municipal waterworks system, after payment of the reasonable and current expenses of operation and maintenance of such plant. The governing body may provide for the payment of such cost by the issuance of revenue bonds in accordance with the provisions of chapter 40-35, which bonds may be sold as provided in said chapter or may be delivered to the contractor in payment of the contract price, or it may be provided in said resolutions and contract that such price shall be payable in stated installments over a period not exceeding ten years, with interest at a rate not exceeding eight percent per annum, payable annually or semiannually. Said payments may be evidenced by certificates executed by the executive and recording officers, and such certificates may have interest coupons attached.