UV exposure time

[Biffer]

Just had a theoretical question about UV sterlizition flow rates...

My understanding is that 20 gph per watt is the general rule for a good kill rate for a UV sterlizer. I remember reading this in an article by Bob Fenner some time ago, but I don't know if this is still considered accurate.

What happens if the flow rate is taken down even further, to say 5 gph per watt? Is there some point where the flow is TOO slow for the sterilzer to function properly? Does the UV lightbulb overheat? (I'm not sure when this would become a concern) Does it denature a portion of the chemicals in the water because of prolonged UV exposure?

The reason I ask is because I am setting up a continuous plankton culture, with a periodic dosing of the plankton out of the culture vessel replaced by fresh(but aged) SW. The new SW will be coming from a dosing system that I plan on keeping as sterile as possible to keep the phtyo culture from becoming contaminated (as long as possible - I know it isn't feasible in a basement setting to keep it from crashing eventually).

Part of the sterilization process will be a UV sterilizer, and the new water will also be dosed with nutrients for the phyto. Will the UV sterlizer break down organic nutrients if the exposure time is too great?

 

[wooden_reefer]

20 gph per watt is too high I believe. I use 5-10 gph per watt, upper end of this range for the twisting kind.

First, do not think that flow thru the UV is always in the form of a "plug", ie of flat velocity profile. When the flow is slow, it is laminar, and water that is nearer to the side of the annular ring will almost "cling" to it. This is the actual true residence time of the flow, for adequate kill. In laminar flow, the pathogens near the wall of the annular crossection of the UV will be killed. This is how an undersized UV, based mistakenly on plug flow calculation, can be effective.

It is certainly possible for flow to be too slow. A bacteria can only die once so slower than laminar flow rate is less effective. Size the UV for 1x -2x with 5-10 gph per watt, for a range of effectiveness.

 

[wooden_reefer]

Actually, even at bulk flow rate somewhat higher than that for totally laminar flow, there will still be a major laminar region close to the walls, where the flow fartherest from the wall will be turbulent flow.

 

[tmz]

Biffer, Never heard 20g per watt. That's quite low. That would be about a 4 watt sterilizer for your 75.
What do you want to kill with it? The lethality depends on the size of the targeted organism relative to the radiation it receives which is a function of the wattage of the bulb and the dwell time. For example , according to the manufacturer's data an 18w turbo twist uv sterlizer will control bacteria at a flow rate of 500gph, algae at 240 and the larger organisms such as parasites at 110gph.

It will only effect what passes through it. So at 20 w per gallon that 18w sterlizer would go on a 360 g tank and turn the water over only once every 3.5 hours( @ 110gph) to be effective against the parasites that went through it. A more reasonable turnover is about
5x per hour.
So to be effective against the parasites that went through it , a 75 g tank would need a sterilizer with a flow rate of 375 g per hour to accomplish 5 passes per hour. To accomodate that flow rate the bulb would have to be 375/110 = 3.4 x as large as th 18w or 61 watts.For bacteria lethality you would need about 1/5 of that or about 15w for a 75 gallon.

Uvs are most effective in single pass operations for parasite , algae and bacterial control. In a recirculating system the organisms that don't go through it will equilibrate(multiply faster) thwarting eradication efforts . The best you can hope for is a temporary reduction in density and virulence ,if that.
Uvs are highly overrated for disease control ,in my opinion. I've used several of them in the recent past and have seen no difference since discontinuing thier use. I do still run a 39watt sterilizer on my 29 g quarantine tank at certain times.

They are pricey and bulb replacements are also costly.

 

[billsreef]

Are you reusing old tank water for the algae, or mixing fresh SW? If your mixing fresh SW, just mix it fresh right in the culture container and you won't need to UV or ozone sterilize the culture water. If your reusing water, than you want to filter and sterilize, add the nutrient media after sterilization.

 

[wooden_reefer]

UV is basically for bacteria, virusus, and algae.

If you want UV for ick etc, you may size your UV quite differently, much larger. I have no experience with UV being effective against ick, even 8W in a 20 gal QT did not work well.

I find 1-2x 5-10 gph per watt enough to greatly reduce bacterial infection.

Most manufacturers tend to think that a recommendation of low gph doesn't sell. Most people believe in high flow, IMO.

If you use the plug flow calculation, most UV will be undersized, however. Air flow thru a UV is likely in plug flow, but not water, which has much higher viscosity.

In terms of effectiveness, it is better to have a diatom filter for ick and UV for bacteria. Quarantine makes diatom filter unnecessary however, but UV against bacteria is always helpful for fish.

 

[wooden_reefer]

"Uvs are most effective in single pass operations for parasite , algae and bacterial control."

Perhaps "single pass" is the only way.

What is single pass? This idea is frequently misunderstood.

Water inside a UV flows at very different rate when the bulk (mixed bucket) flow is low. When the flow is laminar, or has a large laminar region, part of the water, that which is near the wall of the conduit, flows much more slowly. I believe this is the little discussed crux.

UV that is undersized based on plug flow calculation is still effective against bacteria if only for prevention.