For the purpose of owning, operating, constructing, and equipping such sewage disposal plant or sewerage system, including any storm sewer system, or improving or extending such existing system, a municipality may issue mortgage bonds therefor. Such mortgage bonds as provided in this section shall not impose any general liability upon the municipality but shall be secured only on the property and revenue, as provided in section 14-365.04, of such utility including a franchise stating the terms upon which, in case of foreclosure, the purchaser may operate the system. Such franchise shall in no case extend for a longer period than twenty years from the date of the sale thereof on foreclosure. Such mortgage bonds shall be sold for not less than par. The amount of such mortgage bonds, either issued or outstanding, shall not be included in computing the maximum amount of bonds which the said municipality may be authorized to issue under its charter or any statute of this state.