How does poor circulation allow algae to grow?

[nostaljake]

I've seen in lots of threads where people say that poor circulation can lead to localized algae growth due to lower FC in those areas.

How does this happen? I thought Chlorine mixes pretty quickly, as shown in the dye video in Pool School.

I'm not doubting that poor circulation is a problem. Indeed, in my pool I get algae growth in a couple spots despite my FC being plenty high, and it always happens in the same spots which uncoincidentally have poor circulation.

So what's the mechanism here? Is FC really lower in areas with poor circulation? If so, how does that happen?

The only thing that makes sense to me is that, in poorly circulated areas, the rate of chlorine loss from the sun is outpacing the rate of new chlorine entering. In my pool, the algae always grows near the surface first, so this would make sense. But only if chlorine actually disperses VERY slowly in still water.

 

[PoolStored]

Proper FC/CYA levels and daily brushing to get the chlorinated water into areas that have poor circulation will keep it away.

 

[Jimrahbe]

Jake,

When a pool owners does not pay any attention to his FC/CYA, then the pool will get algae, poor circulation or not...

This algae will first start in any area that has poor circulation, but the cause is always the poor FC/CYA, not the poor circulation.

This is how myths begin..

Thanks,

Jim R.

 

[nostaljake]

Jim, is that really true, though? There are lots of threads where people have proper FC/CYA and they still get some algae. And in those threads people always bring up circulation as a potential problem. Here are a few I found in just a couple minutes of googling: thread 1, thread 2, thread 3

Regardless, this doesn't answer my question as to WHY areas with poor circulation get algae first.

For what it's worth, my CYA is currently 20-30 (raising it as we speak) and my FC has been above 5 all week. Today I noticed some light algae growth in the usual spots. I took another FC reading and it was 5.5.

 

[PoolStored]

Here are the direct quotes from the threads you noted.

OK, so I get it. I probably did not quite get it all last time. And yes, places where there are poor circulation. I brush the pool here and there but not as regularly as I should.

^^Admitted they didn't complete the SLAM with an OCLT.

Brush, dose chlorine, pool looks clean for a few days, but then the algae patches return

Brushed once and expected that to be that for a few days.

We have been trying to keep a constant FC level of at least 5 PPM,

Current numbers are:
FC 3

^^^These two statements were in the same post. We Try, but then report a minimum number, implying they are trying, but likely let it get below minimums.

I stand by my prior statement...

Proper FC/CYA levels and daily brushing to get the chlorinated water into areas that have poor circulation will keep it away.

 

[nostaljake]

@PoolStored I'm not arguing that brushing isn't important. But it's only important BECAUSE of circulation, isn't it? As you mention, the point of brushing is to get the chlorine into areas with poor circulation.

I'm just trying to understand why that matters. Why is circulation/brushing necessary if chlorine mixes quickly in water? I'm not arguing that it ISN'T necessary. I'm trying to understand why -- how can chlorine levels be significantly lower in certain parts of the pool if chlorine mixes quickly like it shows in the pool school video?

 

[PoolStored]

But it's only important BECAUSE of circulation, isn't it?

Yes and no. Brushing cleans the pool and breaks the biofilm of any algae that might get in the pool, and move chlorinated water around.

Why is circulation/brushing necessary if chlorine mixes quickly in water? I'm not arguing that it ISN'T necessary. I'm trying to understand why.

Turn your pool pump off. Pour chlorine in the pool, it is denser than water and will pool on the bottom of the pool. That is why we have pumps and a circulation system. To mix chemicals, to heat, filter skim etc.

If you have an area with low circulation, and that area consumes FC and is a lower FC than other areas, algae can take hold and grow.

You are assuming the chlorine magically mixes and stays evenly distributed.

Another example. When you open the pool in the spring after all the snow and rain, the rain and snow water will be stratified on top of the pool water. Try it in the spring when you pull the cover. Take water from the surface and test it next year. I did it for fun this year, FC was 0. Pumped for 24 hours and my FC was 2.5.

 

[Bperry]

@PoolStored I'm not arguing that brushing isn't important. But it's only important BECAUSE of circulation, isn't it? As you mention, the point of brushing is to get the chlorine into areas with poor circulation.

I'm just trying to understand why that matters. Why is circulation/brushing necessary if chlorine mixes quickly in water? I'm not arguing that it ISN'T necessary. I'm trying to understand why -- how can chlorine levels be significantly lower in certain parts of the pool if chlorine mixes quickly like it shows in the pool school video?

Honestly I never brush the pool except when I open the pool and dont get algae when I follow the FC/CYA chart. Some pools seem to need more brushing than others.

 

[PoolStored]

chlorine mixes quickly

It does, if you have your pumps running, and or dolphin going around. Where did you get the notion that without circulation, that chlorine mixes quickly?

 

[nostaljake]

Turn your pool pump off. Pour chlorine in the pool, it is denser than water and will pool on the bottom of the pool. That is why we have pumps and a circulation system. To mix chemicals, to heat, filter skim etc.

If you have an area with low circulation, and that area consumes FC and is a lower FC than other areas, algae can take hold and grow.

Right, this sounds a lot like my explanation at the bottom of my OP. I think where I'm confused is about how this is possible if chlorine disperses quickly like in the pool school video. Am I correct that the answer is: it doesn't, if the water isn't circulating well? So chlorine actually mixes very slowly in areas with poor circulation?

 

[PoolStored]

Ha Ha! Yes, it is clear they poured the dye into the return jet stream!

 

[nostaljake]

It does, if you have your pumps running, and or dolphin going around. Where did you get the notion that without circulation, that chlorine mixes quickly?

Haha, beat me to it, thanks.

If I take FC readings from two spots in the pool -- one in a poor circulation area and one in a normal area -- I'm assuming it should show a difference?

 

[nostaljake]

Honestly I never brush the pool except when I open the pool and dont get algae when I follow the FC/CYA chart. Some pools seem to need more brushing than others.

Makes sense to me. If circulation is good throughout your pool and you maintain proper FC/CYA, then I don't see why brushing would be necessary to control algae growth.

I don't mind doing some brushing, but in the long term I definitely want to fix my circulation issues so I don't have to do it quite as much!

 

[PoolStored]

So chlorine actually mixes very slowly in areas with poor circulation?

I'm not sure the "very slowly" is accurate either. You need circulation to mix chemicals properly.

I'm assuming it should show a difference?

Dunno, maybe. Might be just a small difference. I suggest you try it, report back, then we will know! Don't mix it too much when you sample. Use 25mL sample to get the .2 discrimination...

 

[Jimrahbe]

There are lots of threads where people have proper FC/CYA and they still get some algae

Jake,

Here is my take on that idea..

Not everyone tells the truth, or they only tell the truth as they see it. Just because someone claims that they have 'perfect' FC/CYA does not make it true. Many try to keep there FC between the minimum and their target, which is often allows the FC to get too low..

But most, just never got rid of their algae to begin with. Just because they say they completed a SLAM does not mean they even came close.. I see a lot of posts about pool owners having to do multiple SLAMs.. It is not magic, it is science.. if algae comes back, then something was not done right.

Thanks,

Jim R.

 

[nostaljake]

Dunno, maybe. Might be just a small difference. I suggest you try it, report back, then we will know! Don't mix it too much when you sample. Use 25mL sample to get the .2 discrimination...

Just tried this and there was no real difference. I got 6.5 FC in front of a return and 7.0 in a low circulation area near a bit of algae growth.

I still think there's something missing from the explanation here. If it was as simple as poor circulation = lower FC in certain areas, then we'd expect to see lower FC readings when comparing, wouldn't we?

Rather than the dispersion of chlorine, I wonder if the water movement itself could be the issue. Just hypothesizing. But perhaps chlorine interacts with algae at a much higher rate when the water is moving. So the chlorine sort of "bumps" into the algae more and there are more chemical reactions. But if the water is still, there are fewer reactions.

When I get some time, I might experiment with this idea. Put two 5-gallon buckets right next to each other, and add the exact same amount of chlorine to each. In one bucket, add something that vibrates the water a little. Then wait and see how fast algae starts to grow in each bucket.