Hmm! I don't think that the FDA would ever allow that in a food processing plant. But what the FDA will do and what a clever individual can do are two different things.
First clean and agitate the tank with detergent. Use a circulating pump and a spray wand like is used on RV black water tanks to get into all corners. Dump, rinse and do it again. Then go over every weld joint with a torch to heat the metal up to above boiling temp. Have an assistant follow up every so often with a drop of water to make sure it sizzles a few seconds after the torch passes. The welds are where bacteria can hide.
Then sanitize with a bleach solution. Bleach will attack aluminum so don't leave it there long, maybe a couple of hours. Whoops, it is SS so maybe overnight will be ok. Use a pretty strong solution, maybe a cup to 5 gallons of water. Dump the bleach solution and rinse thoroughly.
Then after all of that, I would drink from it, but only if it held 80+ proof booze ;-).