The board of commissioners of any district shall prescribe and collect reasonable rates, fees, tolls, or charges for the services, facilities and commodities of its system or systems, shall prescribe penalties for the nonpayment of the rates, fees, tolls, or charges, and shall revise such rates, fees, tolls or charges from time to time whenever necessary to ensure that such system or systems shall be and always remain self-supporting. The rates, fees, tolls or charges prescribed shall be such as will always produce revenue at least sufficient to:

(1) Provide for all expenses of operation and maintenance of the system or systems, including reserves for the expenses; and
(2) Pay when due all bonds and interest on the bonds, for the payment of which such revenues are or shall have been pledged, charged or otherwise encumbered, including reserves for the bonds and interest.