Black algae in grouting of tiled pool

[Zeerob]

Hi,
My pool is 5x10m, 89 m3 = 23.500 gal. It has 2x2 cm tiles. Since about a year, black algae has started to grow. Mostly in the edges of the floor (where the floor and the wall meet). But now it is spreading to the rest of the floor. See pictures. It is almost impossible to brush off. This is because:

• The pool is 2 m deep and it is in the edges.
• It is in the grouting which is a bit below the tiles, so the brush hardly touches it.
• It is usually windy here, so because of the waves you cannot see the bottom well, so, you can not see what you are doing.

I might be able to brush it off while diving, part by part. But then I cannot combine that with high FC levels (SLAM).
So, I am now thinking of emptying the pool, using a power washer to clean it off, and then brush it with bleach several times. Most of the algae is in the deep part of the pool, so, I could put some water there with bleach, after the power washing, to keep it exposed to chlorine for a longer time.
I have read here that I am supposed to brush it and then keep it in reasonable high FC for a longer time. But washing it with pure bleach a few times should do the job as well, or even better I suppose?
I have tried to expose it to a very high FC by building a 1x1m PVC 'cover' that I would put on the floor and inject bleach below. It did work to some extend, the algae in the middle of the device would disappear (mostly) after about 15-30 minutes. But it did not work well enough, and doing the round edges like that would be very difficult.
So, please let me know what you think.
I am in Portugal, near the ocean. I have a solar heating that works very well, so my water has been warm for a long period.
Thanks a lot!!

 

[duraleigh]

High chlorine and continued exposure and brushing with a stainless steel brush should clear it up......in time.

Do you have CYA in your pool? Can you post any test results?

Draining and scrubbing is always an option but I think may be more work than you anticipate and black algae needs PROLONGED exposure to chlorine to kill it all.

 

[Zeerob]

I had a CYA of 20. CA was 165, FC was 0.5, CC was 0.1.
FC was low because I had not added bleach because I was going to drain anyway.
I have drained the pool and have seen that the black algae has eaten very much into the white grouting. On some places the grouting was completely gone, and on many places the grouting was weakened so that the pressure washer did blow it out.
I have pressure washed every cm of the pool. That took 2 days. Where there was no algae, the grouting was strong enough to withstand the power washer. So, I have tried to blowout all eaten-in grouting, and will have to repair that.
I have sprayed the walls with diluted bleach. I intend to put some water in the pool so that the floor and most of the spots are covered. I will bring that water to shock level (++) and wait for a week to kill all traces of algae left.

 

[setsailsoon]

Zeer,

If this ever happens again I've had good luck on my brother's pool using 3" pucks rubbing the edge into the grout directly. It's a lot of work with just a snorkel but is easy if you have dive gear or know a diver. The black just bleaches out right in front of your eyes. On bad spots I've had to use a small wire brush. Use rubber gloves of course. Brother doesn't use TFP method so he has more frequent algae problems (uses pucks and drains).

Chris

 

[ajw22]

I had a CYA of 20. CA was 165, FC was 0.5, CC was 0.1.
FC was low because I had not added bleach because I was going to drain anyway.
I have drained the pool and have seen that the black algae has eaten very much into the white grouting. On some places the grouting was completely gone, and on many places the grouting was weakened so that the pressure washer did blow it out.

Note that algae does not eat the grout.

The loss of the grout was likely caused by your water chemistry. If CH of 165 was normal for you that was too low. You need CH in the 300-400 level. Your water chemistry was likely too aggressive.

Please review ABCs of Pool Water Chemistry and Recommended Levels - Trouble Free Pool Tiles with grout should use plaster pool guidelines. You should monitor your CSI using PoolMath

 

[Zeerob]

@Chris:
Thanks, I had read that, and it seems like a good idea. But in this case grouting was very damaged, so I had to drain anyway.

 

[Zeerob]

@ajw22:
Thanks. But the grouting was really eaten in by the black algae. There was a 100% relationship between where the grouting was damaged/weak and where algae was. At places where algae existed for a longer time, the grouting was gone and at newer spots, there would only be a shallow dent in the grouting. And I read everywhere that black algae grows into grouting and plaster.
But I will look at the CH. It may have gotten lower because I use muriatic acid to lower PH. The PH needs lowering because of all the added liquid chlorine.

 

[ajw22]

@ajw22:
Thanks. But the grouting was really eaten in by the black algae. There was a 100% relationship between where the grouting was damaged/weak and where algae was. At places where algae existed for a longer time, the grouting was gone and at newer spots, there would only be a shallow dent in the grouting. And I read everywhere that black algae grows into grouting and plaster.
But I will look at the CH. It may have gotten lower because I use muriatic acid to lower PH. The PH needs lowering because of all the added liquid chlorine.

Sorry, but you have your cause and effect mixed up. The black algea will take hold in the weak grout areas. It does not cause the weak grout. The water chemistry causes the weak grout that the black algae finds small cracks to grow in.

Muriatic acid does not affect CH in water. It affects pH and TA.

 

[Zeerob]

Sorry, but you have your cause and effect mixed up.

Well, I do not think so. The black algae did clearly spread like a virus, from one place to the next near it. I do not think that weak grout will spread like that. There might be a tiny spot of black algae with a tiny hole in the grout below it and the grout around it full strength. I have power washed the whole floor cm by cm, and there were no weak spots that blew out without algae.
And I think it is well known that black algae grows into plaster/grouting. The grouting of my pool is not very hard / strong.

Yes, I mixed up CH and TA as far as the effect of muriatic acid. Thanks for that.
I thought MA does remove calcium because it can be used to remove calcium scaling.

 

[ajw22]

Yup, black algae will find the weak spots in your grout.

However the black algae did not cause the weak grout. It was either a poor grout job when applied or poor water chemistry that weakened the grout.

 

[duraleigh]

+1 what ajw22 said. Likely water chemistry but the black algae simply found easier access into the damaged grout rather than grout being damaged by black algae.

 

[Zeerob]

Well, from the way it spread I can tell it was the algae that caused the weak grouting and not the other way around.
Algae can move from one spot to the spot next to it, or slowly climb a wall at one place. Weak grouting does not spread like that. Apart from a very few spots there was nothing above 40 cm above the floor.

 

[Zeerob]

So, I have repaired the grouting and refilled the pool. The removal of weak grouting was most of the work. The actual re-grouting was not so bad. It all looks perfect now, and I hope the algae will not return.