In the ordinance for the issuance of bonds the governing body shall provide that the revenues derived from the operation of the works shall be used only for:
(a)
The payment or providing for payment, including payments into any reserve or sinking funds, as the same falls due, of the principal of and the interest on the bonds;
(b)
The management, maintenance, operation and repair costs of the works.
After provision has been made for the payment of the foregoing, any surplus remaining may be used as follows: (a) for the purchase in the open market of the outstanding unmatured bonds of the district; (b) for extensions, or for the enlargement, replacement or betterment of the works; (c) for any lawful purpose of the district.