Only revenue producing utilities shall be acquired, owned, or operated by a district. So far as possible the board shall fix such charges for commodities or service furnished by any revenue producing utility as will pay all of the expenses of the government of the district, or such portion as the board determines justly apportionable to such utility. The expenses to be paid include:
(a) Salaries, office expenses, and other necessary disbursements.
(b) The operating expenses of the utility.
(c) The interest on any indebtedness incurred for the acquisition, construction, and completion of the utility.
(d) Provisions for a sinking or other appropriate fund for the payment of the principal of such debt as it becomes due.
(e) Provisions for an appropriate fund for repairs, replacements, and betterments.
It is the intention of this section that a district pay all of such charges and expenditures and the interest and principal of its debt from the revenues derived by the district from the operation of its public utilities, and that each public utility owned and operated by a district shall be self-sustaining.