UV sterilizer sizing question

[esb]

I am in the process of planning a new 120 gallon reef tank (48" x 24" x 24"). I am going to include a UV sanitizer and had some questions about sizing it. It will have a dedicated sump and 2 return pumps. From what I've been able to learn a UV sterilizer can operate on two different classes of organisms, each with their own requirements in terms of UV intensity and how often the tank's water needs to be cycled through the sterilizer. For single-celled organisms (bacteria, algae), there is typically a lower intensity, but the water needs to be cycled through more often due to the organisms' short life cycle. Larger parasites need a higher intensity, but don't need the water to be cycled through the sterilizer as frequently.

Typically, a system is set up to treat either one or the other due to the different requirements. In my case with two large enough pumps and the UV sterilizer on one, I could have two different modes to change the speeds of both pumps.

However, it seems to me that it may be possible to size a UV sterilizer such that it met both requirements. I've been looking at the Aqua UV models and their specs include the following:

And from the same doc, it looks like the 30,000-45,000 uw/cm^2 columns are appropriate for single-celled organisms and the 75,000-90,000 column are appropriate for larger parasites.

In my case, I could get a single 57 watt unit and run it ~1000 gph. That would cover the 75k-90k columns in terms of intensity and would cover the recommended gph rates for the 30k-45k columns for my size tank (which seems to really match a recommendation of 6-9x tank volume per hour).

Am I understanding the actual requirements here correctly? Can I actually continuously cover both classes of organisms with a single UV sterilizer and flow rate?

And a related question... Are there common situations where you want to make sure to turn the UV sterilizer off (other than when the pump isn't running)? When initially cycling? When dosing with a bacterial product? Or ???

 

[MarineandReef Jaron]

I have used UVs for years. Without reading the chart I would say a 40 watt is what I would use on a 120.

That being said the 57 watt AquaUltraviolet units are a whole lot smaller than the 40 watt units so they fit much better in the stand and they have more UVC so I would encourage you to go with a 57-watt unit if it is in your budget. My favorite way to mount them is horizontally on the side of the aquarium stand. This will let you easily pull out the bulb and sleeve while the unit is in place. Aqua UV does not include the mounting clips for these units so make sure that you purchase them if you want to mount the unit on a stand side or back.

As far as how well a 57 watt unit would kill Parasites and Bacteria. I am no microbiologist just a fish guy but it should do a pretty good job. You will likely immediately notice a clarity improvement. Disease prevention is harder to quantify. If your fish don't get sick maybe it was because of the UV or maybe not. If they do get sick,, maybe it would have been worse without the UV or maybe not. I can tell you that many many people in the public aquarium industry, and aquaculture industry spend absolutely ridiculous amounts of money on UVs and they wouldn't do it if they didn't believe that they worked.

You should not run your UV dry or without flowing water ever. With a small 57-watt unit most likely nothing will happen but in larger units, the heat from the UV will either melt the PVC body or worse boil the water inside the unit and then explode creating a PVC pipe bomb. This is really scary but I have only seen it happen on 400-watt plus units. If you use an aquarium controller it is best to add a line of code that turns off the UV whenever the feed pump is off. I would also suggest plumbing the UV so that even with the feed pump off water stays in the unit. This will offer some thermal buffer if the pump fails. Don't be afraid. On a unit this small most likely nothing will happen if it runs dry but it is still best practice to only run the lamp when water is flowing.

Some additives and medications will include instructions to turn off UV when dosing or a specific period of time after using. This is either because they have living bacteria the UV will kill or in the case of many medications the UV will break down some of the compounds in the medication and render them ineffective. Just read the label on what you dose. (Normal additives like amino acids, CA, Alk Mag, Kalkwasser, and so on have no issue with a UV sterilizer.}