Low amine epoxies can be 'rated' for potable water by the NSF/FDA. The marine "coatings" companies such as Interlux Commercial Division, etc. have been supplying such low amine epoxies for the aviation, railroad, commercial marine etc. industries for years.
To apply you usually have to cut open sealed tanks to be sure that the 'lining' is 100% and without 'holidays' or voids, roll-on the epoxy, make-up a sister flange thats larger than the 'cut' and stud the tank closure using silicone, etc. rubber strips.

Small quantities of potable water grade low amine epoxies for 'lining' can be obtained from www.nsp-specialty.com/prodcase.html --- the product is called NSP-120 and is available in quarts. I usually use it to repair aluminum water tanks that have 'burned through' due the action of chlorine vs. the aluminum. If there is any weak spot and ALWAYS across any weld zones, I first lay down WEST etc. plus cloth to 'repair the weakness', then roll-on the NSP-120, allow to cure then scrub the hell out the surface, then shock sanitize. Because of the small tank geometry you will only get 'close' to the recommended emissions of leachables; but, definitely magnitudes better than raw styrene or intense amine contamination .... so if you do this 'lining' that would be the only time that I'd recommend installing a suitable charcoal filter on the effluent water spiggot.