Algae staining on pool walls

[hnkput]

Hi All,

Water looks good - sparkly and clear. We're in Fresno (100+ deg. F. summer days). I've been lucky and have never had cloudy water or an algae bloom since the pool was installed (2010). But after the first year, I've had persistent staining of algae on the pool wall surfaces that are in the shade, even when FC > 2.5 ppm (which at CYA = 35 ppm, I presume should not allow algae growth). The algae brushes off easily enough, but within 2-3 days, it comes back.

I was concerned that the issue was too high a CYA (from reading this forum), so this spring I drained the pool and switched from tabs to bleach. I add bleach in the AM to reach FC = 3.6-4.0 ppm, and I lose about 1 to 1.2 ppm during the day. I see no loss of FC overnight. I maintain pH between 7.2-7.6. I can keep algae at bay if I raise FC to ca. 5 ppm, falling to 4 ppm during the day, but would rather not keep FC so high.

I make my own tests using the Taylor complete/FAS-DPD kit.

Does anyone else have this problem? Thanks for your help.

Today at 10:00 AM:
FC = 3.0 ppm
CC = 0 ppm
pH = 7.5
TA = 95 ppm
CH = 220 ppm
CYA = 35 ppm
Borates = not used

 

[JohnT]

Welcome to TFP!

Algae growing in the shaded area sounds like mustard algae. Check out pool-school/mustard_algae for some info about it.

 

[hnkput]

Thanks for the rapid reply. The algae doesn't clump but it is indeed slightly yellow, and has all the other characteristics noted. Let's say I successfully get rid of it. Can I then go back down to running at 7.5% CYA for a min FC? The 6.1 ppm FC min. in the Chlorine/CYA chart is > the 5 ppm max for safe swimming. I could drain & lower CYA, but with CYA <30 I lost 2 ppm of FC/day, which just seemed like too high a loss rate.

By the way - this is a great site. I've learned a lot and am glad I stumbled across it.

 

[danacc]

You can safely swim as long as FC is below shock level for your CYA level. Chlorine levels above 5 ppm are safe for swimming as long as there is CYA in the water. The CYA buffers the chlorine, making it much less harsh.

With a CYA of 35 ppm, you do not want to let FC drop below 3, ever.

 

[hnkput]

I've discovered a non-linearity in the FAS-DPD test. The calculator on this site, and the dosage recommendations in the test kits both indicated that to get to ca. 24 ppm FC, I would need to add about 2.75 to 3 gal of 10% NaOH. But that dosage only got me to 16 (from about 2-3 ppm starting point). So that's about 5 ppm/gal. That's not only less than what I expected (based on a history of using NaOH to increase FC), but the next gallon only earned me 3 ppm (16 to 19 ppm) and the next only raised FC by 2 ppm. For laughs (well, really as a test), I tossed in another gallon, and that only increased FC by about 1.5 ppm. I'm guessing that I am actually way over the target of 24 ppm FC but that the FAS-DPD just doesn't read it properly (at least by the standard methods of adding two scoops of R-0870 to 10 ml of pool water, then adding R-0871). So I'm probably at 36-40+ ppm FC but just can't read it.

Has anyone found a similar problem? Of course, I've added a lot of NaOH, and so pH must be very high, which reduced the effectiveness of the FC. I'll calculate how much HCl I'll need to make this high FC more effective, but that's for another post. Many thanks for the help.

 

[hnkput]

Error in prior post. I'm not adding NaOH. I'm adding Na hypochlorite, or NaClO. I don't know how strong a base NaClO is.

 

[duraleigh]

The chances of the FAS/DPD test being incorrect in the manner you describe are almost nil. There is a good chance your FC is not of the percentage that is posted on the bottle or that your pool is larger than you think.

 

[Smykowski]

Tell us about your bleach. That's the most likely problem.

 

[hnkput]

I use "Kemteck" bleach (is it OK to post brand names?). The ingredients are "Sodium hypochlorite 10%, Other ingredients 90%". I suppose the "other ingredients" are mostly H2O. I don't believe that the problem is NaClO concentration. That doesn't explain the systematic non-linearity: each successive gallon I added induced a smaller increase in FC. If it were [NaClO], then it means of the 6 gallons I've added, I somehow accidentally selected all six in the precise order of decreasing potency. The probability of that is 1/6! or 1/720. Too unlikely. One possibility is that the "other ingredients" have some active ingredient that push back on the equilibrium:

NaClO + H2O = HClO + Na+ + OH-

Is that what you're thinking? Or there is something intrinsic to the equilibrium as it reacts with other components in the water (pH is now super high). But I don't know how. But surely Kemtek would not formulate a bleach that would interfere with itself at high concentrations...?

Thanks again for the replies.

 

[hnkput]

Did my last question/observations stump the forum? Regardless, I'm highly appreciative of the help. I think the diagnosis (mustard algae) was correct, as was the suggested treatment. And this helps address a problem that while minor, I've nonetheless been fighting for a long time. At present, the stains are gone (I've never had measurable overnight loss of FC, and CC is still 0, so I think the shock treatment is done). The FC is now at 9 ppm, and I just adjusted the pH down from 7.8 to 7.3. I'll let the FC drift back to down to swimmable levels, but this time operate at a minimum FC of 3 ppm.

Thanks for all your help.

 

[hnkput]

Update: It occurred to me that I should use the 0.2 ppm increment test rather than the 0.5. That more precise test yields FC = 8.2 ppm and CC = 0.2.

 

[chem geek]

The FC level will drop in proportion to its amount (i.e. it drops by a percentage over time). So if added during the day, it will drop a lot in absolute amount from breakdown from the UV in sunlight. If at night, it will drop if you have a chlorine demand, say from algae. You'd need to test the chlorine level not too long after you add it, but if you test too soon then it won't be thoroughly mixed so you can artificially read too high or too low depending on where you measure in the pool.

 

[linen]

Update: It occurred to me that I should use the 0.2 ppm increment test rather than the 0.5. That more precise test yields FC = 8.2 ppm and CC = 0.2.

There is really no need to test to this precision, but it doesn't hurt anything either (except having to buy reagents more often).

As chem geek said, you don't want to shoot for too low of an FC or you run the good chance of dipping below the minimum at some point. Additionally, since you have just finished off a MA outbreak, you don't want to be back to that again. It wouldn't be unusual at 35 ppm cya to loose ~60% FC during the day (assuming you get a decent amount of sun), so I would shoot for a minimum of ~4 ppm, so that would mean your would need to target ~ 10 ppm (if you want to/can only dose once per day).