Any profits derived by any of the boards of public utilities created under this subchapter, after there has been set aside from the earnings a sum sufficient to pay all outstanding indebtedness of the plants or sewerage systems under the control of the board and a sum sufficient to provide for expenses, extensions, and enlargements found necessary, or which may be reasonably anticipated, shall be used by the board to retire any outstanding bonds or interest thereon issued by any of the boards of improvement of the district constructing the plants under its control. In case there are no such outstanding bonds or interest or when all of such outstanding bonds and interest thereon have been paid, such profits shall be paid to the treasurer of the city or town wherein the board is created. These funds are to be used by the city or town council to defray any expense or pay any debt of the city or town.