Rates for water fixed precedent to the issuance of bonds shall not be reduced until all of the bonds have been fully paid, and, whenever necessary, may be increased in amounts sufficient to provide for the payment of the bonds, both principal and interest, and to provide proper funds for the depreciation account and operation and maintenance charges.
     If any surplus accumulates in the operating and maintenance fund in excess of the cost of maintaining and operating the waterworks during the remainder of the then current fiscal year, and during the next ensuing fiscal year, that excess may be transferred by the corporate authorities either to the depreciation account or to the bond and interest redemption account as the corporate authorities may designate. If any surplus accumulates in the depreciation account over and above that which the corporate authorities find may be necessary for the probable replacements which may be needed during the remainder of the then present fiscal year and the next ensuing fiscal year, that excess may be transferred to the bond and interest redemption account. If any surplus exists in the bond and interest redemption account that surplus shall be applied insofar as possible in the purchase or retirement of outstanding revenue bonds payable from that account, and for that purpose the corporate authorities are hereby authorized to purchase bonds not due in the open market at not more than the fair market value thereof.