For the purpose of financing the cost and expense of constructing or acquiring any waterworks or constructing extensions to or improving existing waterworks, any city of the second class may issue non-debt revenue bonds secured solely by a pledge in whole or in part of the annual rentals or charges imposed upon consumers of water. Said bonds shall not pledge the credit, not¹ create any debt nor be a charge against the general revenues nor be a lien against any property of the city, but shall be a lien upon and payable solely from the annual rentals or charges imposed upon consumers.