The board shall fix the rates, fees and all other charges to be made for all projects, services or facilities furnished, acquired, constructed or completed under this chapter for the use thereof by any persons or public or private agencies utilizing such projects, services or facilities. Subject to such contractual obligations as may be entered into by the board and the holders of the revenue bonds issued under this chapter, the board is authorized to change such rates, charges and fees from time to time as conditions warrant. All rates, fees and charges shall be at all times fixed to yield annual revenue equal to annual operating and maintenance expenses including insurance costs and all redemption payments and interest charges on the revenue bonds at any time issued and outstanding in connection with the project of which such facilities are a part as the same become due. The bond redemption and interest payments shall constitute a first, direct and exclusive charge and lien on all such fees, and charges and other revenues and interest thereon and sinking funds created therefrom received from the use and operation of the project for the acquisition, construction or completion of which such revenue bonds were issued, and all such fees and charges and other revenues, together with the interest thereon shall constitute a trust fund for the security and payment of such bonds and shall not be used or pledged for any other purpose so long as such bonds, or any of them, are outstanding and unpaid. The board may provide that the rates, fees and charges established are minimum rates, fees and charges and subject to increase or decrease in accordance only with the terms of the indenture under which the revenue bonds were issued.