All general obligation bonds, issued under the provisions of sections 247.230 to 247.670 shall be payable serially, beginning not more than five years after date they bear, and the last installment thereof shall be payable not more than twenty years after such date. All revenue bonds issued under the provisions of sections 247.230 to 247.670 shall be payable serially, beginning not more than five years after the date they bear, and the last installment thereof shall be payable not more than thirty years after such date. Such bonds shall bear a rate of interest not exceeding six percent per annum, payable semiannually, shall be executed by the president of the board, attested by the secretary of the board, under the seal of the district, and shall be of such denomination as the board of directors may determine.