Myth Buster - UV Sterilizer Dwell Time

kendragon · Jul 6, 2022

I've seen many times over the years the suggestion that one should slow the flow going through the UV for better results. Analysis shows UV dosage is higher with slower flow but this does not take into account the total volume of the tank/pond or conditions.

I want to share a recent encounter on a pond that had a poorly designed system resulting in ugly green water. The pond is approximately 3500g that is heavily stocked with kois. It is facing west in full sunlight most of the day. The owner was directed by a LFS to install a UV sterilizer to combat green water. Sounds pretty easy. A pond guy came out and installed a 57W on the existing pump. That did not fix the issue so they added plants. After almost three years of unsuccessful attempts, the owner was ready to drain the pond.

This is were I come in. A friend gave them my name to them because I'm the crazy fish guy. First, the existing pump was way undersized. It was pumping less than 1000gph. Not enough water movement to swipe the particles to the filter. Second, the UV is also undersized. My analysis shows the pond requires 80 to 120 watts of UV.
To take it in steps, I installed an additional 4000gph pump with a large leaf basket to prevent clogging and way more efficient than the existing pool pump. This gave the fish more oxygen and water circulation. Water actually got greener and thicker. So thick you can cut it with a knife. The following week, I installed a 80 W UV which consist of two 40W lamps. The unit forces the water to travel through the full length of two bulbs instead of the short travel of a compact 57W.

Behold, the water is clear after a week and continues to get clearer. Walls of the pond is also getting deep green due to exposure to sunlight.

So my myth buster is this:
Effectiveness of the UV is not just dwell time. If you don't move enough water through the UV, it will not keep up with the production of green water or pathogens and slowing the flow rate will only make things worse. Move the entire water volume at least once per hour then size the UV accordingly for the proper UV dosage.

AFTER

Fishman Dave · Jul 6, 2022

Fully agree, one large pump works fine, but that flow should be split to pass through multiple smaller uv to give way better results than one large one. The running cost is the same although the bulb replacement cost is a little higher but the results are usually much improved. Great that you have seen the same too and solved their issue.

duanes · Jul 6, 2022

If your using UV just to prevent algae (green water) , it takes much less detention time, than if you are trying to scramble the organelles of parasites like bacteria and protozoans.

Hatredy · Jul 7, 2022

Not my words. reference https://aquariumscience.org/index.php/14-1-uv-sterilizers/

There is an excellent human analogy. Sunburn is caused by exposure of the skin to UV rays. The experts will correctly tell one that you can not get a sunburn from being under a sun lamp for ten minutes, i.e. “the residence time is too short”. But the human analogy of an aquarium being turned over six times per hour is being under a sunlamp for ten minutes once an hour for 24 hours a day, seven days a week. That sort of regimen will give a human severe sunburn every time. By the same token, that sort of regimen in the aquarium kills most waterborne organisms.

kendragon · Jul 8, 2022

duanes said:
If your using UV just to prevent algae (green water) , it takes much less detention time, than if you are trying to scramble the organelles of parasites like bacteria and protozoans.

Yes, floating algae requires less dosage than most pathogens. UV dosage varies depending on the target.

kendragon · Jul 8, 2022

Hatredy said:
Not my words. reference https://aquariumscience.org/index.php/14-1-uv-sterilizers/

There is an excellent human analogy. Sunburn is caused by exposure of the skin to UV rays. The experts will correctly tell one that you can not get a sunburn from being under a sun lamp for ten minutes, i.e. “the residence time is too short”. But the human analogy of an aquarium being turned over six times per hour is being under a sunlamp for ten minutes once an hour for 24 hours a day, seven days a week. That sort of regimen will give a human severe sunburn every time. By the same token, that sort of regimen in the aquarium kills most waterborne organisms.

The situation that I'm trying to illustrate is the fact that the UV is too small and so dwell time is pointless because you can't get enough of the water into the UV to be effective. Floating algae production far exceeds the kill rate.

Human analogy:
Only a small room has sunlight for tanning (UV sterilizer). You can't service all the hotel guess seeking a sun tans (floating algae). Speeding them through will not help and slowing them down will only leave a lobby full of pale people (green water).