For the purpose of paying the cost of any sewer, drainage, or water improvements in any district created pursuant to section 16-667, the city council shall have the power and may by ordinance cause bonds of the city to be issued called District Sewer Bonds of District No. ….., District Drainage Bonds of District No. ….., or District Water Bonds of District No. ….., payable in not exceeding twenty years from date and to bear interest payable annually or semiannually with interest coupons attached. All special assessments which may be levied upon properties specially benefited by such work or improvements shall, when collected, be set aside and constitute a sinking fund for the payment of the interest and principal of such bonds. The cost of such sewer, drainage, or water improvements chargeable by special assessment to the private property within such district may be paid by the owners of such property within fifty days from the levy of such special assessments, and thereupon such property shall be exempt from any lien for the special assessment. Such bonds shall not be sold for less than their par value and if any assessment or any part thereof fails or for any reason is invalid, the city council may make such other and further assessments on such lots or lands as may be required to collect from the lots or lands the cost of any such sewer, drainage, or water improvements properly chargeable to the lots or lands as provided in this section. If such assessments or any part thereof fails or for any reason is invalid, the city council may, without further notice, make such other and further assessments on such lots or lands as may be required to collect from the lots or lands the cost of such improvement properly chargeable to the lots or lands as provided in this section. Nothing in this section shall be construed to prevent a city from paying the cost of sewer, drainage, or water improvements from revenue bonds as otherwise provided by law. When revenue bonds are issued to pay the cost of sewer, drainage, or water improvements, the city council may provide that the collections from any related special assessment district shall be allocated to the gross revenue of the appropriate utility system.