All special assessments provided by sections 31-727 to 31-762 and all connection charges collected shall, when levied, constitute a sinking fund for the purpose of paying the cost of the improvements herein provided for with allowable interest thereon and shall be solely and strictly applied to such purpose to the extent required; but any excess thereof may be by the board or the administrator, after fully discharging the purposes for which levied, transferred to such other fund or funds as the board of trustees or the administrator may deem advisable.