The proceeds derived from the sale of any bonds issued under this article shall be deposited in a special fund, separate and distinct from all other funds, and applied solely to the cost of the improvements for which were levied the assessments in anticipation of which the bonds were issued, except that the premium, if any, shall be placed in the fund established for the payment of the principal of the bonds and the accrued interest, if any, shall be used to discharge in part the first interest to become due on such bonds. Should any surplus remain after paying for the cost of the improvements, it shall be deposited in the fund established for the payment of such principal and interest.