Any city of the third class owning and operating an electric light plant, sewer system or waterworks plant whose water supply may be insufficient to operate said utilities is by this act authorized to increase its bonded indebtedness above fifteen percent but not exceeding twenty percent of the assessed valuation of all taxable property within said city as shown by the assessment books for the year next preceding the year in which improvements shall be authorized, for the purpose of enlarging, repairing, extending and improving its waterworks system.