70 ILCS 3305/9

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 the debt as it falls due, and also to pay and discharge the principal thereof as it shall fall due and at least within 20 years from the time of contracting the indebtedness. The tax rate shall not exceed .075 per cent of value, as equalized or assessed by the Department of Revenue.