The proceeds from the sale of any bonds issued under authority of this chapter shall be applied only for the purpose for which the bonds were issued; provided, however, that any premium and accrued interest received in any such sale shall be applied to the payment of the principal of or the interest on the bonds sold; and provided, further, that if for any reason any portion of the proceeds shall not be needed for the purpose for which the bonds were issued, such unneeded portion of the proceeds shall be applied to the payment of the principal of or interest on the bonds. The cost of acquiring any pollution control facilities shall be deemed to include the following: The actual cost of the construction of any part of any pollution control facilities which may be constructed, including architect’s and engineers’ fees; the purchase price of any land necessary therefor; the purchase price of any part of any pollution control facilities that may be acquired by purchase; all expenses in connection with the authorization, sale and issuance of the bonds to finance such acquisition; and the interest on the bonds for a reasonable time prior to construction, during construction, and for not exceeding one year after completion of the construction.