In no case shall the aggregate amount of all bonds issued exceed the benefits assessed. The bonds shall not be issued for a greater amount than the aggregate amount of assessments for the payment of which they are issued, nor for a longer period of maturity than twenty years. The bonds shall bear interest at a rate determined by the board, notwithstanding chapter 74A, payable semiannually, on June 1 and December 1 of each year. The interest on unpaid assessments shall be at a rate determined by the board.