Ok we’re going to use our imagination. Imagine that tiny specs of dirt, hardly visible to the human eye are as big as a tennis ball. Personally I think of them as a little ball of mud. Now, because UV can only shine on the outside shell of the ball, everything inside is hidden from being exposed to the UV light. Only bacteria on the surface will be treated.
Back to real life sized dirt and the same thing happens, bacteria can take refuge inside all kinds of particles in the water. Alongside this, if bacteria are close enough to one of these particles it can be hidden in its shadow during the treatment process. A very lucky bacteria I’ll grant you but one we take into consideration none the less. We get around this by placing pre-filters in series before UV treatment. Due to the absence of these particles in the supply when it reaches the UV reactor, the bacteria present has no place to dodge UV exposure. This means that all of the supply passing through the unit will receive at least the minimum dosage the unit was installed to provide.
Filters on an average supply will need changing every 2-6 months depending on the area you’re in. The easiest way to spot your filter needs changing is when you notice a drop in water pressure, this means your filter is clogged up, a lot like your vacuum cleaner.
Now why wouldn’t you want a pre-filter?