Whenever a public agency shall have executed a contract under this act and the payments thereunder are to be made either wholly or partly from the revenues of the public agency’s systems, or any part thereof, or a combination of such systems, the duty is hereby imposed on the public agency to establish and maintain and from time to time to adjust the rate or fees charged by the public agency for the services of such systems, so that the revenues therefrom, together with any taxes and special assessments levied in support thereof, will be sufficient at all times to pay:
The expense of operating and maintaining such systems, including all of the public agency’s obligations to the county authority, its successors or assigns under such contract; and
All of the public agency’s obligations under and in connection with bonds theretofore issued, or which may be issued thereafter and secured by the revenues of such systems. Any such contract may require the use of consulting engineers and financial experts to advise the public agency whether and when such rates and fees are to be adjusted.