Peak germicidal effect occurs at 254nm (UV-C). A 55-gallon drum seems like a lot of water to purify this way. I mean, the distance from the drum wall to the center of the drum may be too far to penetrate effectively. Even with my Steripen, with its relatively big lamp, you're still instructed to stir it around inside the 16 or 32 oz container to make sure all the water gets sterilized.

A modification to this DIY project would be to create a smaller UV sterilizing tank, maybe even just gallon-sized, which you fill when you actually need to use some water.