The funds received from collection of charges as provided for hereunder, along with such other amounts as are collected for tap-in charges or from any operation of the water system, shall be paid into the Water Fund. Subject to the provisions of any ordinance or indenture of mortgage authorizing the issuance of and securing mortgage revenue bonds for the system, monies in the fund shall be used for the payment of the cost and expense of operation, maintenance, repair, and management of the system. Any surplus in the fund over and above the requirements herein mentioned may be used for payment of debt charges on bonds issued for construction of the system and for enlargements of and replacements to the system and parts thereof.