We have had this problem with many new jugs . I believe I have the answer now .
To a half filled jug of warm water, add a desert spoon of "Jif " , an abrasive white creamy cleaner. Leave for 10mins or so . Scrub inside of jug thoroughly with dish wash brush or similar , including underside of lid .
Discard contents, and refill and boil, discard several times . .
If water does taste better, but not good enough, repeat Jif process. .
I wish I knew about this process years ago ...