Black Algae Killed by pucks

[chem geek]

In addition to using Trichlor pucks (which are highly acidic AND highly concentrated in chlorine) for getting rid of black algae, the other takeaway is a confirmation of what we've said all along about SWG systems. The superchlorination that appears to allow for lower Free Chlorine (FC) levels in keeping away free-floating green algae does not apply to algae stuck in biofilms on pool surfaces, such as black algae. So keeping the minimum chlorine level in Ben's Best Guess CYA chart is important. At 20 ppm CYA and 2 ppm FC that's at the minimum in Ben's chart, but at 40 ppm, you would need to have the FC at a minimum of 3, but probably closer to 4 ppm. This is the tradeoff between higher CYA protecting chlorine from sunlight vs. needing a higher FC level to keep away algae.

As for shocking, perhaps it needs an even higher level than that for green algae. There isn't a lot of experience with black algae on this forum -- mostly green algae and some yellow/mustard algae. Trichlor is the fastest way (and has been discussed as a definitive way to determine if a stain is organic or algae vs. a metal stain), but others have shocked and scraped with success so it's probably a matter of the degree of shocking. At any rate, you had success using Trichlor and that's what is most important. Now, at the 40 ppm CYA level you need to target an FC level of 4 ppm to keep away this algae.

Richard

[CTIGuy]

Richard

Do you think the white crust, etched Spas tiles and black algae had anthing to do with adding too much CYA at one time? I added 8lbs one day and another 8lbs the next. I was going to add another but stopped when I noticed the white crust and black algae.

If so do you have any suggestions as to how to add large ammounts of CYA without getting the damage or black algae?

On Cl level the Install and Ops guide from the Mfg of the CL Generator http://www.jandy-downloads.com/pdfs/H05675-RevE.pdf on page 18 says "Free Chlorine 1.0 - 3.0 PPM. Above 3.0 PPM may cause corrosion of
pool metals." Is this just more bad information from the Mfg? On the same page above "The AquaPure is warranted to operate properly
only if these conditions are met." Seems like a catch22?

[chem geek]

It is the ratio of the Free Chlorine (FC) and Cyanuric Acid (CYA) levels that determines the rate of corrosion from the chlorine. The SWG industry's own studies showed that (here). The SWG manufacturers have always listed 1-3 ppm FC when that is clearly not correct. Most SWG users are able to run at 3 ppm FC with 60-80 ppm CYA, but not all. This does seem to keep away the free-floating green algae due to superchlorination in the cell (we think), but it does not good for algae stuck on pool surfaces.

I would not worry about corrosion of metal at the higher FC level so long as you have CYA in the pool. At least this particular vendor is disclosing the increased corrosion rates that can occur due to higher salt levels, but really the 3 ppm FC limit doesn't take into account CYA levels. Indoor pools without CYA will have a real corrosion problem even at 3 ppm FC as a couple of users found out with their stainless steel, but having CYA in the pool significantly reduces such corrosion rates.

So though I think your pool will be fine at Ben's chlorine/CYA table levels, I'm not sure what to suggest given their warranty requirements. It is a catch-22.

Richard