Cleaning an RO/DI Water Reservoir

[WLDOR]

Hey all! My first "proper" post on this forum (other than a quick "Hi" in the intro thread)

I'm picking up a 55G food-grade barrel to act as a reservoir for storing RO/DI water (I intend to stand it on a 2ft-cubed frame, with a lower tap to gravity-drain into mixing buckets, and have a float-valve at the top to regulate the amount of water coming in from the 100GPD Aquatek RO/DI unit.

I'm buying the barrel from a place in Whitby, ON: http://www.envirosponsible.com/rain_barrels.html

My question: These barrels have residue brine left in them - I'd appreciate any input re: the best way to clean off this residue before I use the stored water for WC's on my 50G reef (20G sump). Thanks in advance for any help you can offer!