> Steripen uses UV-B which does not penetrate clear plastic or glass.
Correction. Steripen, and all other UV systems for treating water (up to the municipal level) use UV-C, at 254 nm. To quote Wikipedia:
"Ultraviolet disinfection of water consists of a purely physical, chemical-free process. UV-C radiation in particular, with a wavelength in the 240 nm to 280 nanometers range, attacks the vital DNA of the bacteria directly. The radiation initiates a photochemical reaction that destroys the genetic information contained in the DNA. The bacteria lose their reproductive capability and are destroyed. Even parasites such as Cryptosporidia or Giardia, which are extremely resistant to chemical disinfectants, are efficiently reduced. "
UV-B is not reliable. It causes sunburn, but does not really sterilise water.