The funds received from the collection of the rates and charges provided in § 51.25 shall be deposited daily with the City Treasurer, who shall keep the same in a separate fund designated as the Sewer Revenue Fund. Subject to the provisions of any ordinance or indenture of mortgage authorizing and securing the issuance of mortgage revenue bonds for the sewerage system, moneys in the Fund shall be used for the payment of the cost and expense of the construction of a sewage treatment plant, operation, maintenance, repair and management of the sewerage system, and for the payment of debt charges on bonds issued for extensions and improvements of the sewerage system, and any surplus in such Fund over and above the requirements before mentioned may be used for the enlargement and replacement of the sewerage system and parts thereof, but shall not be used for the extension of the sewerage system to serve unsewered areas or for any other purpose whatsoever.
(`59 Code, § 927.03)