The Council shall, subject to the applicable provisions of the general laws of the state, have authority to borrow money in anticipation of the payment of special assessments made for the purpose of defraying the cost of any public improvement, or in anticipation of the payment of any combination of such special assessments, and to issue bonds therefor. Such special assessment bonds may be an obligation of the special assessment district or districts or may be both an obligation of the special assessment district or districts and a general obligation of the city. All collections on each special assessment roll or combination of rolls shall be set apart in a separate fund for the payment of the principal and interest of the bonds issued in anticipation of the payment of such special assessments, and shall be used for no other purpose.