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 sanitary sewer system, shall be paid into the Sewer 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 the payment of debt charges on bonds issued for construction of the system and for enlargements of and replacements to the system and parts thereof.