Yellow/Brown dusty deposits that reappear repeatedly

[chem geek]

Thanks for the info. Well, if this stuff is algae, then it's pretty darn powerful stuff! At a pH of 7.5 and CYA of 30, the disinfecting chlorine (HOCl) level is 0.46 which is already pretty darn close to what I was thinking the "kill algae" level would be at 0.5. If it takes 20 ppm, then that is a disinfecting chlorine level of 0.88 (25 ppm is 1.6) so you can see that the disinfecting chlorine level rises up very rapidly with additions in FC because the FC is getting closer to the CYA level (overwhelming it).

If your CYA was 20 (instead of 30), then the 20 ppm FC gives 2.1 ppm disinfecting chlorine while 25 ppm FC gives 3.6 ppm disinfecting chlorine. So you can see how sensitive the numbers are in this range, making it hard to know what the true "kill level" is for this particular form of algae. If you throw in the inaccuracies of the various tests, especially for CYA, then it's really hard to figure this stuff out, but I appreciate the info nevertheless.

I suppose it's still possible that this stuff isn't algae and that the much higher chlorine levels are needed to oxidize it, though if it wasn't algae then I would figure it would just get caught into your filter or pool sweep. You mention the tell-tale blue that the pool turns after killing it off. I didn't know that is what happens with algae. I wonder if there is any other way to tell for certain if this stuff is really algae -- perhaps putting some into a beaker (covered with fine cloth to prevent other stuff from falling in) in partial sun and seeing if it grows would be one way to tell.

Richard