Yes, these are the standard opaque blue HDPE barrels with closed heads. They have two bungs in the head, one fine thread, and one coarse buttress thread. All 3 still had the original labels on them and the remnants of their contents smelled correct, so I feel safe in the assumption that they were only single-use before me. In searching for these, I found that Gebo's has some enormous poly potable water tanks (500 gal was the smallest they had) but wasn't interested enough in it to find out their price.
For these, I filled them up half-way with fresh water, sloshed around a bit and dumped out the loose solute. Then I filled and capped them, let them sit for 3 days to loosen up the syrup on the bottom. Dumped half, sloshed vigorously, dumped the rest. Then took a pressure washer and blasted all the inside surfaces thoroughly. Refilled half-way with a mixture of dish-soap and fresh water, then tipped on it's side and rolled around the yard to make suds (and be sure the inside lid got washed too). Dumped and pressure washed again. Refilled half again with plain water, rolled around to rinse everything including lid. Dumped, then filled completely with bleach water (forget the exact ratio, but more than would be used for simple purification), capped and sat for 2-3 weeks. Dumped, rinsed and filled for real with lesser bleach solution for storage - We are on a private well out here, no chlorination in our water supply.
I have 3 barrels currently, with a schedule set up to dump and freshen one every 4 months - no water will be more than 1 year old. I'll take samples from each batch, and adjust the schedule as needed to keep water good. We also have a few 55gal rain barrels around the yard, but it's been so long since it's rained that they're nearly dry (been using them to water plants too)