Is UV Dwell Time additive?

[obiquiet]

I've read Carl Strohmeyer's UV article carefully (Thanks, Carl!). I have this question:

"If a UV unit has a dwell time of X, and the water in the tank cycles through it N times per hour, does is that the equivalent of an dwell time of X * N?"

In other words, does sufficient UV light to kill have to hit the organism all at once, or can it add up over some small time interval?

Thanks!
-ObiQuiet

P.S. I think the article has a small typo error:
"These are minimums, higher turnover is better and a low watt per gph is best. For pond algae control; 40 gph per watt (SHOULD BE WATTS per GPH) will work and a turn over rate as low as once per three hours will usually work."

[fishfever]

I'm sure Carl will have a good answer for you when he returns from his business trip. I'm not sure I know what you mean by dwell time, unless that is the time unwanted organisms are exposed to the UV lamp as they pass by it (on a single pass by the lamp). My understanding is that as long as you are at the minimum turnover rate (or better) then you need at least a certain number of watts to be effective for that flow rate; anything more is okay but is probably a waste of money. So if looked at in this context then it seems to be a minimum average exposure rather than a "dwell" if I am understanding this right. If dwell is important then it would seem you would want lower flow rates and long exposure tubes/UV lamps... but if too low a flow rate then you may not achieve the minimum tank turnover. I'll be interested to see what Carl has to say about this dwell time because I don't recall reading about it.

[Carl]

Jan 1, 2010 15:50:05 GMT -5 obiquiet said:

In other words, does sufficient UV light to kill have to hit the organism all at once, or can it add up over some small time interval?

The short answer (I will probably catch up to threads over the next few days) is to how you stated it in the above quote.

The answer is basically all at once, otherwise there is not the time for the UVC to damage the cell structure where the pathogen or whatever you are aiming to destroy is fatally damaged.
In green algae the exposure only need to "clump" the algae which is then filtered out or falls to the bottom of the pond.

Some UVs have double passes (such as the TMC 110 watt Pond Advantage), and the very short interruption in exposure time does not interfere with adequate UVC exposure.

I will admit that I have not done any true scientific studies, but I have made many observations using double pass UVs and also placing UVs in line in such a way that there is substantial interruption in this UVC exposure and the difference was noteworthy and hence my answer.

Carl

[obiquiet]

Thank you for the replies!

The reason I ask is that none of the sources I've read about properly sizing a UV for an aquarium seem to take into account that the water is cycled many times per hour, day after day.

They focus on the judging the effectiveness of one single pass through the unit. So, I was curious about how this works in a closed system over time.

It sounds like your empirical experience indicates that the single-pass evaluation is the right one, and my guess that re-circulation can make up for lower power or too-short exposure time is incorrect.

-ObiQuiet

[Carl]

I hope this helps, but your question does raise a question that needs further more scientific study IMO.

Carl

[fishfever]

Carl, does this imply that two smaller UV's connected back to back with a short line might work better than one larger one? For example two 5W UV's versus a single 10W UV? This would give almost twice the exposure time (assuming the interconnecting line was as short as practical) versus the single unit. Again I realize you said it needed further study but if exposure time was found to be a critical factor it would seem interconnected units (and/or having the tube wrap back around the UV lamp more than once) would be superior.