I get the white drums from the local packing plant. They held an industrial strength Clorox bleach. I believe it is around 12%.

A simple rince is all that's needed.

The contents of the drum will always leech into the plastic of the drum no matter how well it is washed. Why not buy the drums that contained something that you are going to put into the drums anyway? So I have a little bleach leeching into my water. I look at it as time release purification.