The rates and charges to be collected by the district shall be so fixed as to yield an amount sufficient to do each of the following:
(a) Pay the operating expenses of the district.
(b) Provide for repairs and depreciation of works owned or operated by the district.
(c) Pay the interest on any bonded debt.
(d) So far as possible, provide a fund for the payment of the principal of the bonded debt as it becomes due.
It is intended by this section to require the district to pay the interest and principal of its bonded debt from the revenues of the district, except that this requirement is not intended to apply to a bonded debt for sewage disposal purposes.