For the purpose of paying for such main sewer herein provided for, the municipal authorities of cities of the second class may issue bonds and sell and dispose of the same in any sum not exceeding forty thousand dollars, in addition to the authority now vested in them by law for the creation of indebtedness of such city; that is to say, they may issue said bonds without regard to the other indebtedness of said city, for the purpose of building said main intercepting sewer. The bonds thus to be issued shall be issued as provided by law.