We found this water filter at Skiddaw House. The frequency of UV is of the order of 10^16 giving a photon energy of the order of 10^-17. Since ionisation energies for carbon, hydrogen and oxygen are 10^-18, the UV can ionise the atoms in living organisms. It can damage the DNA in micro-organisms, killing them. Flow rate needs to be slow enough to allow all the water to be adequately treated but fast enough to stop it overheating.
http://filterbutler.com/blog/what-does-a-uv-water-filter-do-and-how-do-they-work/
