One of my tenants gives them away for free when he has too many to store. He makes sauces and gets the ingrediants in large blue plastic barrels, they usually stink very badly though. Even though they are food safe unless you clean them out very well it would leave a bad taste in the water if stored for a while.
I picked up a couple of the blue food grade barrels a few years ago. One had some kind of pickle or brine residue. The other had a strong coffee smell (I can't stand coffee, drinking or smelling).
I first put in a couple of gallons of water and some bleach. I rolled them around a bit for a day or two. I drained/rinsed them then put in a few gallons of water with some baking soda. Same deal, roll them around a bit and let soak for a few days. Drain and rinse thoroughly. Huge improvement. The spicy barrel was nice and fresh. The coffee barrel was 99% improved. I filled them up and treated with bleach for storage.
This summer I drained them. Both had a slight smell of bleach left (which is good). One still had a subtle coffee smell. I plan on running the water from this through a Brita charcoal water filter to take the edge off.
Good luck, TR