70 ILCS 3405/20
At the time of or before incurring any indebtedness the board of trustees shall provide for the collection of a direct annual tax sufficient to pay the interest on such debt as it falls due, and also to pay and discharge the principal thereof as the same shall fall due, and at least within 20 years from the time of contracting same; such tax shall be in addition to and in excess of all other taxes heretofore or hereafter authorized to be levied by such district. A certified copy of the ordinance adopted by the board of trustees authorizing the issuance of such bonds and levying a tax to pay the same shall be filed in the office of the county clerk of each of the counties wherein any of the territory of the district is situated which certified copy shall constitute the authority for the county clerk or clerks, in each case, to extend the taxes annually necessary to pay the principal of and interest on such bonds as the same mature.
70 ILCS 3405/20