Fluid dynamics & UV light function

[D&RW]

Hi again everyone,

Laying in bed waiting to fall asleep last night. thinking about the algae in our pond and wondering whether the UV light is as effective as it should be. My question is both general about flow and effectiveness of UV lights, but also specific about fluid dynamics.

It's a 57 watt Aqua UV. Recommended flow is, if I remember right, about 3200. Pump flow rated 4700, as measured actually closer to 4500. Head at the UV light given joints and length of pipe is about 2-3 ft. Overall I've computed that the final flow with combined head over the falls is 3700-3800gph. I'd have to look at the chart to know the flow at 2-3 ft of head, but it'd be between 3800 and 4500.

Seems to be some differences of opinion about optimum flow through a UV light, but frankly Aqua's claim that the 57 watt can handle 3200 gph seems high. I mean they should know, but the effectiveness of the light last year wasn't as good as I'd expected.

Total piping run is in 2 in pipe and about 20ft to the falls, 15 ft to the light. I plumbed the light in 2" PVC top leg of a "Y" configuration; bottom leg is open (with a valve) & they rejoin via a second "Y" and a 5 ft. run to the filter/falls.

The tricky plumbing part was that the outlets on the light are on the sides. So if you can visualize this, the 2 in. main pipe from the pump comes to the Y. The top leg starts with a gate valve, then a 45 degree up to the light connected with a rubber fernwall joint. Same thing on the downstream side of the light. Where's Rube Goldberg when you really need him. see photos

Current plumbing has gate valve in both Y legs. The gate valve for the UV light leg is BEFORE the light.

First year, I just partially closed the light's gate valve thinking that should slow the flow (no way really to measure). Now, thinking about it again with lots of algae and murky water and not being a physics whiz, I'm feeling a bit unsure.

When you close a garden hose valve at the upstream end, it slows the flow, but when I try to visualize the UV system it seems that a valve downstream of the light would "hold" the water in the light longer. Is that one of those optical/mental illusions/confusions? Last year the UV light seemed to work ok, but wasn't as effective as I'd expected. Water clarity improved, but wasn't the crystal clarity I hear others describe.

Does it matter? It will be a pretty big hassle to replumb this system, but I'm ready to do it if it'll be move effective. I just don't know.

It's been a hundred years since I took physics. I'm hoping someone has enough experience and physics knowledge to answer this easily, but if not no worries. I might be able to find a physics for dummies or fluid dynamics for dummies type book. Geeze, they even have a Quantum Physics for dummies!

Last foto is attempting to show how murky the water is. You can see two pots and one of the white goldfish in the foreground, probably about 2 ft. down.

 

[Otter]

When you close a garden hose valve at the upstream end, it slows the flow, but when I try to visualize the UV system it seems that a valve downstream of the light would "hold" the water in the light longer. Is that one of those optical/mental illusions/confusions?

I have only a rudimentary understanding of fluid dynamics, but I think it's enough to help. Flow resistance is much like electrical resistance. You add resistances in series, and it doesn't matter which one comes first. Think about that garden hose again. Closing the valve at either end will slow the flow of water through the hose.

If you find something like Fluid Dynamics for Dummies, please post about it. I'd like to know more too.

It occurs to me that maybe the problem is that you don't have enough flow through your UV system. If most of the water is going around it, that could account for the clarity problem too.

Finally, why not just use barley straw to inhibit algae and plants to compete with it for nutrients? UV will kill beneficial free-floating organisms too, and it won't do anything for attached algae. I admit I've never tried UV, but it seems to me that UV algae control masks the problem of nitrate and phosphate in the water without actually solving it.

 

[D&RW]

It occurs to me that maybe the problem is that you don't have enough flow through your UV system. If most of the water is going around it, that could account for the clarity problem too.

Finally, why not just use barley straw to inhibit algae and plants to compete with it for nutrients? UV will kill beneficial free-floating organisms too, and it won't do anything for attached algae. I admit I've never tried UV, but it seems to me that UV algae control masks the problem of nitrate and phosphate in the water without actually solving it.

Hi Otter,

Not enough flow? Hum, I wonder. When I bought the UV I had no idea how to plumb it. I couldn't afford the 80 Watt which is claimed to handle 4000 + gph. And the 57, if plumbed directly, would mean that I'd have to reduce the flow over the waterfall to at least the maximum rate of the UV light.

So, I decided on the Y. Gad, these seem like dumb questions, but if you have an open Y with similar plumbing wouldn't each leg be 1/2 the flow but at the same rate in each. That's what I figured. So I figured if I put a valve and slowed it on one leg, the other leg would compensate so overall flow wouldn't be reduced. Obviously, I know very little about fluid dynamics and need to do some more research.

I can't imagine there is too little flow, but you might be right. I've got to do more digging. Meanwhile, for now I've closed the red valve on the UV light leg by 3/8th because I think maybe you're right, it doesn't matter which side the valve is on.

As far Barley Straw, used it last year and will again this year. Plus, last year we had probably 65-75% plant coverage with lilies, arrowhead, water hyacinth. I agree these are way important plus so pretty.

There is some disagreement about UV and it limiting or killing beneficials. I think the consensus thinking is that most beneficials tend to locate and not circulate ergo are not harmed. However, one is advised to turn off the UV for 2-3 days after adding Microbe Lift while they get established. As for the type of algae on the sides, it's said to be beneficial so not a problem.

If I find a good resource that's simple enough to digest, I'll let you know.

Thanks Otter

 

[Otter]

Gad, these seem like dumb questions, but if you have an open Y with similar plumbing wouldn't each leg be 1/2 the flow but at the same rate in each. That's what I figured.

Yep. And in my book, the only dumb questions are the ones you need to ask but don't.

So I figured if I put a valve and slowed it on one leg, the other leg would compensate so overall flow wouldn't be reduced.

This is true for practical purposes provided that the friction head of the tubing is low compared to the total head, which it will be if you didn't economize too much on the tubing to begin with.

But it's difficult to gauge just how much water is flowing through each branch of an unbalanced Y, and it's hard to judge the effect of closing one of the valves a bit. It's also hard to say how much the filter itself resists the flow.

What I'm getting at is that you may have diverted too much water to the non-UV side. If an average cell of algae takes the safe side of the Y several times before getting killed in the UV side, it might have time to reproduce before its luck runs out.

Unless I missed something, your light should be able to handle 60-80% of the full flow rate of your system with algal death to spare. If Aqua was telling the truth, all you need to do is bleed off 600 gph or so from the UV side. If the numbers came from marketing instead of engineering, maybe two or three times that. But you definitely want at least half the flow going through the UV side. Hence I suggest you open the UV side all the way, and partially close the valve on the safe side. My fluid dynamics is not strong enough to give you a firm recommendation on how much. Turbulence complicates this considerably, and I know only enough to know that I don't know enough. Where's that dummy's guide when you need it? I'd probably start with the valve halfway closed if I believed the UV manufacturer's specs, and maybe a quarter if I didn't.

 

[koiguy1969]

if your gonna use barley,use the extract, its already biodegraded, and contains the humic acids that turn to peroxide which is what does the alge control. without the 6 to 8 week wait for bails to break down and work efficiently. pellets work faster too, but will turn your water brown for a while.

 

[D&RW]

Thanks Koi guy. I didn't know that. Extract it is.

Otter,

you raise the important question, i.e. rate of algal growth vs. kill rate of the light. If, like you say, the little bugger makes it around 3-4 times before it goes through the UV light, reproduction might exceed kill capacity.

i'm going to write Aqua. AND, for education sake, find a book or two, or article or two, on fluid dynamics. I remember concepts/factors like turbulence, friction, etc. but not enough--heck i never took that level of physics / engineering anyway, so never knew that much to begin with.

thanks again. i'll fill you in when i hear from aqua tech folks, IF i can get to them.

rw

 

[D&RW]

ok, i found Aqua's website and wrote them. During this I realized something you already knew, i.e. you were talking about turbulence.

I was thinking in terms of a true Y, but my plumbing is not a Y. one leg goes straight and the other bends up. That is bound to reduce the flow to the light some. how much is anybody's guess

Be interesting to see what they say. One main question still is, if it were a true Y, would the flow be reduced to 1/2 the rate through each leg or could it be 1/2 the volume with same rate.

rw

 

[Otter]

One main question still is, if it were a true Y, would the flow be reduced to 1/2 the rate through each leg or could it be 1/2 the volume with same rate.

I'm not sure what you mean by 1/2 the volume with same rate, but with a true Y, the flow rate through each leg would be half the total. Now, if you want to be exact, the total flow rate would also change slightly compared to a straight pipe. Going through two tubes instead of one would reduce total resistance infinitesimally, and going through the Y fittings would increase resistance slightly. But for practical purposes, it's 1/2 through each leg.

I was thinking in terms of a true Y, but my plumbing is not a Y. one leg goes straight and the other bends up.

Because my browser wouldn't load the large images, I confess just glanced at the thumbnails before. Now that I've DLed the photos and looked more carefully, though, I think that with both valves open you'd have much more water going through the straight pipe than the UV clarifier. The UV leg has tons of turbulence compared to the straight leg, and the inertia of the water will make it tend to take the main branch at that type of Y anyway. This could easily account for the poor performance last season. For now, I'd close the black valve in the straight leg most of the way and leave the red one wide open.

 

[D&RW]

This is SO interesting. I think you are probably right, but it's counterintuitive or at least it is for me. GAD I wish I could measure the flow in each. But I'm going to go with what you are suggesting until I hear from Aqua. They did write back and asked how many gallons in the pond. We'll see what they say. I'll see if I can send them some pictures.

 

[D&RW]

So far, Aqua's people don't understand the issue. Their replies haven't been helpful. Will keep trying

 

[Otter]

Good luck. Most of the time when I get a non-responsive reply from some company's tech support, it's because the person I'm corresponding with doesn't know either. They don't want to admit that, so they "misunderstand" the question. Hopefully in this case it really is a misunderstanding, though, and the pictures will help.

This is SO interesting. I think you are probably right, but it's counterintuitive or at least it is for me.

Which part is counterintuitive to you? Or to put it another way, what does your intuition tell you?

 

[DrCase]

I dont think you have a flow prob,, The uv will kill the green water but your filter has to get rid of it...The uv does help..
I had a uv on my old filter,,,When i changed to the 55 gal drum filter ,The water cleared up and i was able to take out the uv

 

[D&RW]

i agree Otter. Aqua probably will recommend buy the bigger light or a smaller pump--plumb directly. it's probably beyond their folks knowledge & experience.

maybe intuition isn't the right word. Saying first analysis is better-up to now mine has been to decrease flow.

going to recalculate head as best I can with what "data" i can find for length of pipe, 45 & 90 degrees, uv lights etc. article i read last night said folks underestimate.

As i try to visualize some of the water going up the 45, through another 45 and into the light I think you and Doc are right--the flow is probably not too fast with a wide open valve and may be too slow.

Doc, my filter is a medium EasyPro filter-falls with 4 layers of matting, two are standard that came with it and two are Matalla (fine and medium). Then on top I have two bags of media ribbon for additional bacteria colonization.

It's amazing you were able to get rid of your light. Where do you put your 55 gallon drum? I'd have trouble hiding one.

 

[D&RW]

Looks like Otter is right, at least based on recalculation of head. Flow is probably too little through the light for the pond size. I'm turning off the main line, running it all through the light and see what happens.

 

[Otter]

Nothing beats experimentation. But do you now estimate the total flow within the recommended range of the light?