Any municipality or county may pledge the net revenues of any waterworks system maintained and operated by such municipality for the purpose of supplying such municipality or county and its inhabitants with water to secure the payment of any bonds issued by such municipality or county, whether such bonds shall have been issued for such waterworks system or for any other purpose.

The term “net revenues,” as used in this section, shall include the entire revenues derived from such waterworks system after deducting therefrom the cost of operation and administration and such portion of such revenues as shall be required to satisfy any pledge of such revenues theretofore lawfully made.