Ahh yes then we get deep into the science of how different light frequencies affect different things, red light for human health rather than blue light and how we benefit from UV and should not block it with sun cream as it blocks a metabolic pathway to produce vitamin D and how that works in our body but I digress. There are different UV lamps, low pressure as domestic units and medium pressure commercial units etc. Yes the low pressure lamps can still damage the DNA of bacteria etc that are chlorine resistant like protazoa cryptosporidium. Some research suggested the damage from low pressure lamps was not severe enough and caused them to multiply rapidly as they were damaged. Medium pressure UV would break them apart. Of course in the natural world dead bacteria are food for other bacteria so you can see why fine filtration and flocculation are very important and likewise the ability to remove them from the filter and not encourage colonisation as in sand and other media.