(a) For so long as any bond or notes are outstanding and unpaid, the rates, fees, and charges for water and electricity charged and collected by the commission shall be fixed so as to provide revenues sufficient to pay all costs of and charges and expenses in connection with the proper operation and maintenance of the waterworks and electric light systems, respectively, and all necessary repairs, replacements, or renewals thereof; to pay when due the principal of, premium, if any, and interest on all bonds payable from such revenues; to create and maintain revenues as may be required by any resolution or trust indenture authorizing or securing bonds or notes; and to pay any and all amounts which the commission may be obligated to pay from such revenues by law or contract.
(b) Any pledge made by the commission pursuant to this subchapter shall be valid and binding from the date the pledge is made. The revenues so pledged and then held or thereafter received by the commission or any fiduciary on its behalf shall immediately be subject to the lien of the pledge without any physical delivery thereof or further act, and the lien of the pledge shall be valid and binding against all parties having claims of any kind in tort, contract, or otherwise against the commission without regard to whether such parties have notice thereof. The resolution, trust indenture, or other instrument by which a pledge is created need not be filed or recorded in any manner.
(c) Any surplus remaining after the expenditures authorized by this subchapter may be made available for other purposes of the city by resolution of the city council.