On or before the first day of September of each year, the board shall certify to the county commission of the county within which the district is located a rate of levy so fixed by the board as provided by law, with directions that at the time and in the manner required by law for levy of taxes for county purposes such county commission shall levy a tax at the rate so fixed and determined upon the assessed valuation of all the taxable tangible property within the district, in addition to such other taxes as may be levied by such county commission.