Algae is back

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R

robl45

Well-known member
Oct 27, 2007
918
Parkland, FL
some of you may remember me battling algae for quite a bit and finally buying the TF Test kit. Well my pool was good for about 2 months and now I have algae starting to grow again. SWG has been going the whole time so not sure why it started to grow again. I did get lax on the PH and it did creep up to like 8.5 or so.

So latest tests.

FC 5.0
CC 0
PH 8.5
CYA 40
CH 470
TA 120
 
If your measured numbers are correct, then the saturation index was over +1.0 with a significant chance of scaling and in particular the SWG cell may have become less efficient with too much scale.

CYA acts as a chlorine (hypochlorous acid) buffer so moderates the effect of pH on its effectiveness. At a pH of 8.5, a pool with 5 ppm FC and 40 ppm CYA has a chlorine equivalent to a pool with a pH of 7.5 and 4 ppm FC (with the same 40 ppm CYA) so at least in theory the pH alone may not be the problem (unless there is something new to understand about algae growth in these conditions). Nevertheless, you don't want the pH to be that high and should add acid to lower it.

How certain are you of the numbers you are measuring? Are you using a TF100 or Taylor K-2006 test kit? How did you measure a pH above 8.0 or 8.2 since these test kits only go up to that value? Also, what kind of algae is it? Is it free-floating green algae or is it yellow/mustard algae that looks like yellow/orange dirt and grows in shady areas?

Richard
 
I guess it was about 8.2 then. It was at the top of the scale. In florida where I am, if you don't touch the ph, it just goes up and up and up. I didn't have acid for awhile and was busy and one thing led to another and it went without acid for awhile. I just recently put the acid back in.

Its greenish algae right on the walls. I plan to put in bleach and kill the algae off as before, but its disconcerting that the algae came back. If its just because the PH got too high, I can fix that. But if it isn't, I'd like to figure out what caused it.

My FC was around 5ppm the last time I measured it. Usually it would hover around 3ppm with the normal setting of the SWG. I had turned it up alittle when I noticed the Algae.
 
If it was maxed out at 8.2, then it could have been above. I doubt it was above 8.5 since then you'd be at equilibrium with carbon dioxide in the air vs. the water so no more would outgas. The SWG produces hydrogen gas bubbles and these outgas carbon dioxide (and a small amount of chlorine) and that causes the pH to rise.

At a pH of 8.2-8.5 and with the other numbers you had and an FC of 3.0, then that's a hypochlorous acid concentration of 0.025 which is below the minimum required for manually dosed pools which is around 0.03. Usually with an SWG pool, you can get away with a somewhat lower chlorine level, but that is only for killing free-floating algae and in your case it sounds like you've got algae stuck on walls so obviously they don't make their way to the SWG cell to get superchlorinated. At least things are now starting to make more sense. I'll bet you don't regularly (weekly or so) brush your pool. Regular brushing probably let's those with SWG pools get away with lower chlorine levels since free-floating algae would get super-chlorinated in the SWG cell.

So your pool probably has decent nutrients for algae so going below the Minimum is a problem for your pool. Though the minimum at 40 ppm CYA is 2.9 ppm FC, that's at a pH of 7.5. I would say that you should not only keep a watch on your pH, but also target a somewhat higher FC of around 4 ppm FC at your 40 ppm CYA level. That should keep the algae from growing even if the pH rose somewhat.

If you ever get a water test to find out the phosphate level of your pool, that would be good information to let us know -- not that you need to do anything about it, but since your pool is behaving near the "edge" of the minimum FC level, it would be nice to know roughly what phosphate level that is. We currently think the level where the minimum FC starts to not work is around the neighborhood of 3000 ppb so info from your pool would help validate this theory of algae growth.

Richard
 
okay, I should mention, the SWG I use is contained in a robotic pool cleaner such as an aquabot. Its a prototype, but the real production unit I understand will be in stores like Leslies and pinch a penny in the next couple of months or so. So it basically chlorinates through the robot as it moves around the pool, since the robot cannot get in the jacuzzi, it would make sense that the algae is most prevalent over there. The jacuzzi has a spillover so the water is going through there.

I am doing the test with the TF test kit. I have added more acid and will be working this weekend to get some bleach or liquid chlorine in the pool to get rid of the algae.
 
Nothing to add other than you can't just 'set it and forget it' when it comes to pool care.

Just a few minutes a day several times a week will keep you on top of things.

In the end, you give up a lot more time trying to fix problems that could have been avoided.

Not picking on you, we're all guilty of being laxed from time to time - just a reminder.
 

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Why not just follow the normal steps? Sure, it may take a couple of days, but it will also have the advantage of getting you in the habit of checking your pool on a regular basis so you can prevent this from occurring again.

Instead of looking for a quick fix, work on being consistent with your pool care.
 
With a plaster pool you can shock to a higher levels than is normally recommended, but I wouldn't go up to 150 ppm or anything like that. At very high FC levels, particularly with salt in the pool, there can start to be fairly rapid corrosion of metal touching the water. If CYA is really and truly zero you should not go over FC of 5. If CYA reads zero but there has been CYA in the pool before then 15 to 20. With measurable CYA levels then double the normal shock level for the CYA level should be alright for a few of days.

You said your CYA level was 40, so I wouldn't shock to anything higher than 30.
 
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