As explained in this article, you need a mechanical filter to go along with the uv light. The uv light only kills the algae, and the filter removes the dead debris from the pond.
And also explained there is that generally the best place the have the uv light is right AFTER the filter:
Generally, the better, and most common, placement is right after the filter. In fact, a lot of the biological filters (that have a uv light built in) place this uv light after the filter itself. There are a couple of good reasons for this:
- The uv light works more efficiently when the water flowing along it is clearer. By filtering out big particles and debris helps, the light can penetrate deeper into the water, and reach more algae.
- Any small rocks being sucked up by the pump and transported to the uv light can harm the quartz sleeve around the bulb. Having a filter before the light prevents damage to the quartz sleeve.
- A filter before the light traps more sludge, thus slowing down the build up of sludge in the uv light casing.
- Ultraviolet light also kills ‘good’ bacteria. Usually this is not something to worry about, because the bacteria tend to live along the sides of the pool and on the bottom. But if some are free-floating, it helps having the filter before the uv light: the good bacteria might stick to the filter material (the pad) itself.
However, there’s more to say about that
In some cases it might be a good idea to have the filter AFTER the uv light.
In a setting where the uv light is the last stage of the inline filtering process, the killed algae clump together and are then released into the pond. They might sink to the bottom, maybe in a spot where no pump can reach them. What’s not so good about this is that all this biomatter ends up being food for more microorganisms like algae.
Whether or not this is a problem depends on the specific situation of your pond: for example, if you have rocks on the bottom that biomatter will have a hard time being sucked up by the pump, as opposed to a naked plastic liner where any debris is much more likely to be able to swept up.
So, depending on your situation, it can be a good idea to have a filter AFTER the uv light as well as before. That is, of course, if your budget permits this.
That said, if you had to choose between a filter before or after the light, I’d definitely suggest having a filter before the uv light. The pros outweigh the cons.
There’s one type of situation that comes to mind where you cannot do this, where you have to have the filter after the uv. This is when you have a biological filter that spills right off a waterfall.
photo “bio falls”
You can’t have the uv clarifier/sterilizer after these types of “bio-falls”, so in such cases place it before the waterfall.
But in the case when you don’t have such a bio-waterfall (and if your budget permits) it might be worth to place another mechanical filter before the uv clarifier as well.