The revenues from the rates and charges for services and facilities furnished pursuant to this article may be used for the following purposes:

1. (a) To pay the operating and maintenance expenses of the facilities.
2. (b) To provide a reasonable surplus for improvements, extensions, and enlargements.
3. (c) To pay the interest on any bonded debt for the facilities.
4. (d) To provide a sinking or other fund for the payment of the principal of such bonded debt as it becomes due.