644.551. All bonds herein authorized to be issued shall be paid at maturity and all interest accruing thereon shall be paid when it falls due by the state treasurer, at a place designated in the bonds and coupons attached. Thirty days before any of the bonds mature and any of the interest thereon falls due, it shall be the duty of the board of fund commissioners to draw its requisition for the amount necessary to pay such interest on the bonds and the principal of maturing bonds and the necessary expenses to be incurred in transmitting such moneys. Whereupon the commissioner of administration shall certify the amount to the state auditor, and the state auditor shall issue his warrant upon the state treasurer therefor in favor of the president of the board of fund commissioners, payable out of the water pollution control bond and interest fund; and the warrant so drawn shall be delivered to the state treasurer who shall transmit the amount of money therein specified to the paying agent named in the bonds with instructions to place such money to the credit of the board of fund commissioners for the payment of interest or principal of such bonds. Whenever in the opinion of the board of fund commissioners it is advisable to do so, and there are sufficient funds therefor, the board may redeem any of the bonds before maturity if the holders thereof agree thereto, and may also purchase any of the bonds in the open market whenever funds are available and in the opinion of the board it is to the advantage of the state to do so; but, in the event any of the bonds are redeemed before maturity, the purchase price shall not exceed the face value of said bonds plus accrued interest not previously paid.