The commissioners of the district issuing bonds pursuant to this act shall prescribe and collect reasonable rates, fees, tolls or charges for the services, facilities and commodities furnished by such works, and shall revise such rates, fees, tolls or charges from time to time, to provide that all such works shall be and always remain self-supporting. The rates, fees, tolls or charges prescribed shall be such as will produce revenue at least sufficient, (a) to pay when due all bonds and interest thereon for the payment of which such revenue is or shall have been pledged, charged or otherwise encumbered including reserves therefor, and (b) to provide for all expenses of operation and maintenance of such works, including reserves therefor.