All revenues derived from the operation of any combined system under the provisions of this article shall be set aside as collected and used only for the purpose of paying the cost of repairing, maintaining and operating such system, providing an adequate reserve fund, an adequate depreciation fund, and paying the principal of and interest upon the revenue bonds issued by the municipality under the provisions of this article. The ordinance pursuant to which any bonds are issued shall pledge the revenues derived from the combined system to the purposes aforesaid and shall definitely fix and determine the amount of revenues which shall be necessary and set apart in a special fund for the bond requirements. The amounts so set apart into said special fund for the bond requirements shall be remitted to the West Virginia Municipal Bond Commission to be retained and paid out by said commission consistent with the provisions of this article and the ordinance pursuant to which the bonds have been issued: Provided, That payments of principal of and interest on any bonds owned by the United States of America or any agency or department thereof may be made by the municipality directly to the United States of America or said agency or department thereof.