All incorporated cities located within the boundaries of any such county are hereby excluded from any sewer district formed under the provisions of sections 249.430 to 249.660, unless such city shall petition the county commission to participate in the county district and be accepted by the county commission. Any incorporated city discharging sewage into the sewer mains of any such sewer district shall pay to such sewer district for the use of said sewer mains an annual rental to be determined by a census of the population served, or by measurement of volume of sewage so discharged into said sewer mains, or by a stipulated contract price.