Yellow/Brown dusty deposits that reappear repeatedly

[chem geek]

I've been doing some thinking and looking at the numbers for this hard-to-kill yellow/mustard algae. Though it appeared that high chlorine levels were needed to kill this algae, my saying that it was around 1.0 ppm disinfecting chlorine (HOCl) assumed that the pH was around 7.5. Normally, the CYA acts as a chlorine buffer so higher pH doesn't change this concentration by that much, but this buffering starts to fall apart at the higher HOCl concentrations needed to kill this algae. What this means is that perhaps the actual disinfecting chlorine concentration needed to kill yellow/mustard algae is not 1.0 but is less, perhaps 0.7, but adding a lot of chlorine makes the pH rise which results in less disinfecting chlorine than I was assuming.

For practical purposes, using this chart when you start at around a pH of 7.5 and then add chlorine it is probably OK if you use the 0.3 ppm column (or at least 0.2) for green algae and the 1.0 column for yellow/mustard algae. However, next time this happens to someone, we should try to not only add chlorine to raise FC, but also start out at a lower pH (perhaps 7.2 or even 7.0) or add acid to keep the pH lower since that will allow for lower FC levels to kill the algae (meaning less bottles of chlorine and therefore lower cost).

I suppose this will have to wait until next August to see... Of course, everyone will probably be adding PolyQuat 60 and/or have 50 ppm borates in their pool and no one will get yellow/mustard algae so we can "experiment".

Richard

[matt4x4]

Can SOMEONE PLEASE STICKY this post, it contains very usedful information to a problem that seems to be reocurring frequently!
Also, if the title can be changed to "Yellow/Brown dusty deposits that reappear repeatedly" it may help.

Oh yeah, can it be cross stickied to the ALGAE forum too (because that just makes sense to me) ???? and maybe rated as 5/5 since it's helped many get rid of this crap through persistence even beyond the pool looking sparkling clean...

[Watermom]

OK, Matt. I've done as requested. I am going to lock this particular thread, however, so we don't get people responding to both threads. If anyone wants to add any posts to this thread, please do so in the algae forum.

Thanks.