The bonds authorized by section 91.596 shall be signed by each member of the board, attested by the secretary under the seal of the board. Said bonds shall run for a period not to exceed twenty years and draw interest at a rate to be designated by the board of public works, interest to be payable semiannually and to be evidenced by the coupons attached. The bonds and coupons shall be negotiable in form; the coupons may be signed only by the lithographed signatures of the president of the board and the secretary thereof.