A county may issue revenue bonds for the purpose of owning, operating, constructing, and equipping a sewerage disposal system and plant or plants or improving or extending an existing system. Such revenue bonds shall not impose any general liability upon the county but shall be secured only by the revenue as provided from such sewerage disposal system and plant or plants. Such revenue bonds shall be sold for not less than par value and bear interest at a rate set by the county board. The amount of such revenue bonds, either issued or outstanding, shall not be included in computing the maximum amount of bonds which the county may be authorized to issue under any statute of this state.