[elunned]

The UV is used to kill very very small single-cell suspended algea. The UV radiation damages the algea so that it clumps into particles that can be removed either in settlemt chambers or by filter matting. Healthy green algea passes through mechanical filtration where larger solids are removed.

For maximum efficiency, the UV light should be located after the solids settlement - that way suspended solids don't shield the algea and reduce the effectiveness of the UV radiation. I'm not sure on the timetable for how long it takes for the algea to clump after UV exposure. Taking the conservative approach, I would tend to place the UV after the last chamber, just to reduce the chance of fouling the biomedia with dead algea. Make sense?