When serial bonds have been issued, the net revenue derived from the operation of such systems shall be applied to pay the interest on and the principal of such serial bonds as the same become due, but no sinking fund shall be required in respect of serial bonds. The board shall, if necessary, levy an annual tax upon all property in such sanitary district, subject to local taxation, to pay such principal and interest as shall annually become due for payment. In the event net revenue exceeds the annual principal and interest due thereon, the board may use such net revenue for extensions and additions to the system, purchase and retirement of outstanding bonds of the district, or for any other lawful purpose consistent therewith.