The theory of it being in the filter is to sterilize the microbes as they are trapped on the media. Even outside the filter UV
sterilization is very effective method of controlling disease and microbe outbreaks and at the very least, creating a natural environment. Afterall, the ocean has the sun.
In a repeated, constant flow the UV
can handle the microbial load of the tank. Of course the slower the flow, the better.
Think of it this way, a human can potentially consume an entire years worth of radiation consumption in an hr., if exposed to a source that was emitting that amount consistently. How long do you think it would take a microscopic particle to reach it's max exposure if the source was amplified by, say something as small as 10x, if only exposed for a second. Now if these particles are trapped on a filter and have constant exposure, how effective would that be.
sterilization is used in air filtration, eg; hospitals use it to prevent bacteria/viruses from traveling room to room. Food packing plants use it to control bacteria, especially on chicken. That food is moving on a conveyor belt and may only have a 1-2sec exposure. And of course, water purification plants use UV
sterilization was used on a global scale to control bacteria and virus when hurricane Katrina hit N.O. and when the S.A.R.S. virus hit China.
Check out one of the leaders in the industry @ rgf.com if you want to learn more.