UV Sterilizer Flow Rate on a Nano QT Tank

[Tobin VP]

I just purchased an Aqua Ultraviolet Advantage 2000 (8 Watt) UV sterilizer for my 7.5 gallon QT tank.

I'm trying to figure out what the flow rate through the device should be. There appear to be two different recommendations on this in the instructions:

1. System Turn-Over Recommendation. First, the instructions recommend that for saltwater tanks I should be running 3-5 times my system volume through the unit per hour. This seems to ensure out-pacing the reproduction rate of the organisms throughout the whole system. Effectively, you want to turn over the system volume fast enough to beat them. For my case, this leads to 22-37 gph through the sterilizer.

2. Contact Time Recommendation. Second, the table also suggests 214 gph for sterilization (protozoa) and 642 gph for clarification (algae/bacteria). This is targeting a specific amount of time in the unit on a single pass to ensure that the light has enough time to do its job. In my case, if I were to target sterilization at 214 gph ... I would be running at 30X system turnover through the unit - 3x more than even my return.

For systems in the 40-70 gallon range these two recommendations would be close to the same. But in my case with a 7.5 gallon system these two recommendations are an order of magnitude different. So am I right in assuming that each is an upper bound on the flow rate through the unit? And I would therefore go with the slower of the two?

This of course would then assume that there are no undesirable outcomes for running such large contact times (slow flow rates). After doing some research, it appears that arguments about slow rates harming phytoplankton and/or copepod populations are unfounded and these are not a consideration for this tank anyway since it is a QT. The only other concern I can think of is that the slower rate could increase heat build-up in the unit and shorten the life of the bulb. No idea if this could be an issue.

So I'm planning to go with the slower rate of 22-37 gph. Am I thinking about all of this correctly? Am I missing anything here? Have others run very slow flow rates in their UV sterilizer?

 

[taricha]

1. System Turn-Over Recommendation. First, the instructions recommend that for saltwater tanks I should be running 3-5 times my system volume through the unit per hour. This seems to ensure out-pacing the reproduction rate of the organisms throughout the whole system. Effectively, you want to turn over the system volume fast enough to beat them. For my case, this leads to 22-37 gph through the sterilizer.

So I'm planning to go with the slower rate of 22-37 gph. Am I thinking about all of this correctly? Am I missing anything here?

I'd think about the heat too. If you only ran this tiny flow though the sterilizer, then the unit might run hotter than intended?

I'd pobably do the below instead - just run at the normal low GPH and send the water through the sterilizer on more trips than typical.... but others may have done this and have a different take.

2. Contact Time Recommendation. Second, the table also suggests 214 gph for sterilization (protozoa)

 

[Tobin VP]

@taricha Thanks for your comment. The constraint I'm trying to solve for is having a very small AIO QT and not having much room for a pump to achieve the 214 gph.

So I discussed this dilemma by phone with a rep at Aqua Ultraviolet and wanted to follow up with what I learned and what I decided:

First, the rep started by telling me that anything less than 214 gph would result in the UV sterilization being ineffective. I challenged this statement by stating that slower rates would increase contact time and be even more effective. His reasoning was that hobbiest's just think slower would be better but that is not true. He explained further that system turn-over has to be high enough and that some protozoa can multiply very rapidly and if you do not have enough system turnover that the protozoa can grow fast enough that you don't get enough exposure in the unit. (This was my point in #1 above by making sure that a system turnover rate was achieved by not letting the organism's reproduction outpace the sterilization). I agreed with him and then pointed out that the guidance would be true for tanks 40 gallons and above, but that my tank was 7.5 gallons and I could easily meet both requirements - I could be below 214 gph (larger contact time than recommended) and at the same time be way above the 3-5X system turn over per hour which was also a recommendation. He couldn't refute this and just kept telling me that "we have done a great deal of research and testing and that he can only say to follow their recommendation in the table of 214 gph". Ironically, he also said that none of my logic was flawed.

So what do I make of this? Basically, I left the conversation believing that he is not allowed to recommend anything that is not printed in their manuals. And second, I got the impression that while they have products that target the reef tank hobby that a lot of what they do is build UV for large bodies of water and that trying to sterilize a small 7.5 gallon system is something they just don't think about much. Likely most or all of their testing was likely on larger systems and he was just not willing to extrapolate.

I also asked about the unit heating up too much at lower flow rates and he quickly dispelled that as a concern. He indicated that he would be more worried in the two barbed systems that sit horizontal since air could be trapped inside, but the hang-on-back style would not have any air bubbles and it would not be a problem.

I also called BRS and explained all of this logic and my intent to run a slower flow rate than 214 gph and I got a very quick "you'll be fine" response.

So in the end I put in the biggest pump I could fit and am now running this 8 watt UV sterilizer at 70 gph which is 3X more aggressive (greater contact time) than the standard recommendation for protozoa and at the same time getting more than 2X greater turnover than recommended. If anyone else thinks of a flaw in this approach I would love to hear your thoughts.

 

[Tobin VP]

As a corollary to this whole exercise of putting the biggest pump you can in a small 7.5 gallon nano tank for UV sterilization: consider heat!

I ran for a day and got a temperature alarm on my apex. Sure enough ... the addition of the Syncra Silent 1.0 is generating enough heat that temps increased by at least +3 deg F in an ambient 70 deg F room. I typically keep my tank at 78 F. I turned the bulb off for a time and it did not change temp much ... so my impression is that the pump is the major heat source here. Just before putting fans on to cool the tank it had already gotten to nearly 83 F and it was still climbing.

Now that I'm blowing light air across my tank (unfortunately my ATO reservoir won't last as long now) I can keep my tank at 78 F. The fan is extracting the pump heat through evaporation and it looks like I can find an equilibrium with only small adjustments to my now barely needed heater. But there is more: I'm suppose to minimize on/off cycles with the sterilizer to prolong bulb life. So I really shouldn't be turning it on/off at least once per day. But with this small AIO tank the back reservoir (where the pump is located) comprises only about a gallon of volume. This throws a ton of heat into this small volume of water in a very short period of time during return pump off times ... and then once back on I will be dumping water that is much higher temp right into the display area ... not liking that.

After going through this exercise I'm leaning toward just keeping the UV sterilizer on hand to be used in either the Coral QT or Fish QT on an as-needed basis only. By running it all of the time I'm introducing all kinds of issues and my beautiful little Coral QT / frag tank is quickly becoming a bolt-on monstrosity. Next time around ... if I wanted to have a QT/frag tank with all of the bells and whistles ... I would definitely go to at least a 20 gallon system ... maybe even bigger. I'm quickly learning the hard way about all kinds of sizing design constraints.

Asside: Ironically, I did not notice this temp increase right away because within hours of installing my UV sterilizer my Apex memory chip failed and became corrupted (this has been a known problem with Apex controllers and they have changed chip suppliers ... but unfortunately it is out of warranty now). The good news is that even with this memory failure I still got alerts and could see instantaneous value readings ... but nothing was being logged and I didn't notice the upward trending line as a result right away. Ugh ... isn't this hobby fun?