The commission shall prescribe and from time to time when necessary revise a schedule of such fees, rents and charges which shall comply with the terms of any contract of the commission with the holders of bonds of the commission made pursuant to §§ 21-269 to 21-275 and in any event shall be such that the revenues of the commission will at all times be adequate to pay all expenses of operation and maintenance of the sewage disposal system of the commission, necessary to preserve the system and to assure its operation as a going concern, including reserves, insurance, extensions, and replacements, and to pay punctually the principal of and interest on any bonds or other indebtedness of the commission and to maintain adequate reserves or sinking funds therefor. The schedule shall be so prescribed and from time to time revised by the commission after public hearing which shall be held by the commission upon such public notice as the commission may determine to be reasonable.