Pre-filters are very strongly recommended for installation with UV Systems. The reason for this is that whilst the UV system is very highly efficient at destroying micro-organisms, if the water contains particulates, it is possible that they may 'hide' within these and not be subject to the ultra violet light. Removing these particulates beforehand will ensure that this cannot occur.
We recommend a 5 micron pre-filter, size dependent up on flow rate required. In heavily contaminated water it will be best practice to install a 20 micron filter before the 5 micron to prevent it becoming overwhelmed quickly by contaminants.