Black Algae questions and help (with pics and video)

[QPSUtah]

All, looking for some feedback here. I’m a new owner of a pool maintenance company so still coming up to speed on everything. I do have a couple fairly experienced techs, but they don't seem to adhere to TFP methods much. Have a client with what appears to be black algae. Have read up on it in these forums including the official black algae post. But still have some questions or would like confirmation.

For reference, you can see pics of the pool and video of me scrubbing a couple spots with a chlorine tab here:

https://imgur.com/a/HWk66uI

Some of it seems to brush away quite easily. Other spots seem stuck on. Tried the smear test but couldn’t really get any on my fingernail to then smear. Just kind of rubbed away in the water. But the couple spots I tried near surface, appeared to smear underwater.

For addl context, we only service this client every other week and their salt cell stopped working some time ago and despite our admonishments never moved on replacing his salt cell, so the pool would essentially sit without chlorine for days and days, if not two weeks. Also, owner is 45 min away so challenging to get out there to monitor once a day or quickly check in on things, nevermind multiple times a day.

We did shock it heavily late afternoon yesterday, and then I returned mid-day today and took measurements (before adding addl chlorine). Measurements today were

Ph 7.7
Chlorine 1.4
TA 120
CH 550
CYA 35 or so
Salt 4400

Questions are

1. Does black algae stain after removal? There seems to be some areas where there isn't a growth, but still some remnants of what may have been there before.
2. Assuming we give this 72 hours of keeping the pool well chlorinated with regular brushing, if there is minimal improvement, does it then make sense to drain and acid wash?
3. Given client already likely needs to drain 1/4 of his pool to get to a more ideal salt level, is going straight to drain and acid wash perhaps the best route? Seems that while the community generally eschews acid washing, it seems to be a pretty common practice.
4. Re salt cells, seems like 4200-4400 is still acceptable if not ideal for salt cells, correct?

Thanks in advance?

 

[PoolStored]

I would do a bit more investigation. Some of that looks like black algae (round spots). Some look like iron stains (the ones with "halos" around them) like these:


Get some vitamin C tabs and rub them over a few spots and see if they clear at all. If they are iron, it is likely rebar ties that are rusting because they were too near the surface.

Acid washing will clear the stains, it will also shorten the life of the plaster.

With black algae on the sides and bottom, the best method is to use a power washer (lower pressure the better, and wider tip, starting far away from the spots and moving in until it starts to blow the algae away). Then get a pressure spray pump, fill with the strongest liquid chlorine you can find and spray the areas with the LC with the pump off.

You will then need to run your FC at 20% of your CYA for several weeks. Not sure how you will accomplish that without a salt generator.

Either way, as a result of having BA, after the few weeks of FC at 20%, you probably need to run FC at 15% of CYA for life of this plaster.

As to the salt, depends on the chlorinator. Might be 2700-3400, might be 3000-4000, you need the manual. Some turn off with high salt warning, some do not and just alarm, you need to see the manual.

 

[QPSUtah]

So do you suggest moving straight to emptying the pool and pressure washing for the black algae? Without acid. Or continue to give it a few days with elevated chlorine to see if it goes away first?

The client has finally purchased a new salt cell so that will be available shortly. I just don’t want to ruin the new salt cell if the salt level is too high.

The other complication, we’re nearing the end of our season, particularly at the elevation of the client. So client may not be willing to run equipment for several weeks post washing. They’ll want to be winterizing it n the next couple of weeks.

 

[PoolStored]

I'm saying draining and acid washing is extreme.

I'm saying some of the stains do not look like BA and should be tested with vitamin C pills in a sock to see if any are iron stains from rebar ties near the surface.

I would not drain. I would pressure wash with the pool filled. Use lower pressure, wider tips and slowly get closer until the algae is blown away, so as to not damage the plaster.

Then use 12.5% liquid chlorine in a pump sprayer, pump off and spray all the areas allowing the chlorine to sit on the spots (walls will be harder).

Then need to run your FC at 20% of your CYA for several weeks.

Either way, you probably need to run FC at 15% of CYA for life of this plaster after the 20% is completed.

 

[QPSUtah]

Thanks. When you mentioned pressure wash, I assumed you meant with a drained pool. And yes, I realize some of the marking is staining, not black algae.

 

[QPSUtah]

As an update, after several days of elevated chlorine and daily brushing they seem to be going away. Will stay the course.