The board of commissioners of the district issuing bonds pursuant to this chapter 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 thereof; and shall revise such rates, fees, tolls or charges from time to time whenever necessary to insure the economic operation of such system or systems. The rates, fees, tolls or charges prescribed shall be, as nearly as possible, such as will always produce revenue at least sufficient to:
Provide for all expenses of operation and maintenance of the system or systems, including reserves therefor;
Pay when due all bonds and interest thereon for the payment of which such revenues are or shall have been pledged, charged or otherwise encumbered, including reserves therefor; and
Provide funds for reasonable expansions, extensions and improvements of services.