An indenture may include a clause relating to the bonds issued thereunder requiring the board to fix, prescribe and collect fees, rates, tolls, rentals or other charges in connection with the services and facilities furnished from the improvements acquired, constructed or purchased from part or all of the proceeds of the bonds, sufficient to pay the principal of and interest on the bonds as they become due and payable, together with all expenses of operation, maintenance and repair of the improvements, and with such additional sums as may be required for any sinking fund, reserve fund or other special fund provided for the further security of such bonds or as a depreciation charge or other charge in connection with such improvement.