For the purpose of providing for a sewage disposal plant and sewerage system, or improving or extending such existing system, any city or village may also enter into a contract with any corporation organized under or authorized by the laws of this state to engage in such business, to receive and treat in the manner provided in sections 18-501 to 18-510, the sewage of such system, and to construct, and provide the facilities and services as provided in section 18-501. Such contract may also authorize the corporation to charge the owners of the premises served such a service rate therefor as the governing body of such city or village may determine to be just and reasonable, or the city or village may contract to pay such corporation a flat rate for such service, and pay therefor out of its general fund or the proceeds of any tax levy applicable to the purposes of such contract, or assess the owners of the property served a reasonable charge for such service to be collected as provided in section 18-503 and paid into a fund to be used to defray such contract charges.