At the time 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 within 30 years from the time of contracting the same: Provided that any such tax levied to pay the interest on bonds and to discharge the principal thereof for bonds heretofore issued prior to November 6, 1956, or for Refunding Bonds thereafter issued to refund said bonds, shall be levied and extended only upon property within the territorial limits of such sanitary districts as said territorial limits existed on November 6, 1956.
70 ILCS 2605/10