UV is only applicable as for ...

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UV is only applicable as for all disinfection where the turbidity is less than 1 NTU. Also although it can be done on water with a low UV transmissivity it  will require more power etc and bigger units and ultimately may be outside of the UV system verification conditions. 

The biggest thing with UV is that not all biology is equally affected, bacteria are destroyed by low levels and the same level is great for rendering oocysts none infective but much higher doses are required to have a high reduction of certain viruses. However these are quickly destroyed by low levels of chlorine so a residual chlorine dose for distribution sorts that out.