Revenue bonds issued hereunder shall be payable solely from the revenue derived from the operation of the waterworks properties maintained and operated by said district. These bonds shall not in any event constitute an indebtedness of the district within the meaning of any constitutional or statutory limitation. It shall be plainly stated on the face of each bond that the bond has been issued under this Act and that it does not constitute an indebtedness of the district within the meaning of any constitutional or statutory limitation.
70 ILCS 2405/16.6