Yellow/Brown dusty deposits that reappear repeatedly

[chem geek]

Well, I had the CL levels at 23 ppm overnight last night and I added Alum to the filter to act as a filter aid. I woke up this morning and checked the pool. It is almost totally clear and clean. The CL level dropped to 19.5 ppm overnight, so something was being metabolized. In fact, after adding the bleach last night to get the levels above 20 ppm, I was in the yard and could smell the chlorine smell half way across the yard.

Man, this stuff is incredibly hearty. You had earlier quoted a CYA level less than 30 since it was getting cloudy at 30 (you said maybe 25, but it could be even lower than that). An FC of 20 with a CYA of 20 is a disinfecting chlorine (HOCl) level of 2.1 while at a CYA of 30 it is 0.9 so this is well above Ben's shock levels in his table and is higher than my own guess at 0.5 being sufficient to kill algae. It would seem that the easier-to-kill green algae seems to get killed over about a week at disinfecting chlorine levels of around 0.2-0.3 (so 0.5 would be faster), but this brown stuff needs somewhere between 1-2 ppm of disinfecting chlorine to kill. Yikes! Those people with high CYA in their pools are going to have a heck of a time killing this stuff. At 70 ppm CYA, it might take 46-58 ppm FC to kill this stuff.

As for the chlorine smell, yes you probably were smelling chloramines as intermediate by-products since these are formed rapidly. Then they are more slowly broken down (releasing nitrogen gas) and the smell goes away. This just shows that this brown stuff contains either ammonia or nitrogenous organics.

Richard

[chem geek]

I do not understand the chemistry of vinyl liner fading so do not know exactly what level of chlorine will start to fade a liner. It has been written elsewhere on this forum that low pH does more to damage and fade a liner than chlorine, but obviously very high levels of chlorine can bleach dye out of most anything (as several pieces of my clothing with spots on them will prove -- I never wear anything good outside anymore when I dose with chlorine, no matter how careful I am).

Now that said, your CYA level of 40 will tie up most of the chlorine and in theory this chlorine tied up (combined with) CYA should not affect your liner, but I can't be certain of that.

I also do not know definitively precisely what level of disinfecting chlorine is needed to kill this mustard/yellow algae. It has also been reported elsewhere on this forum that using a polyquat algicide will help kill this stuff though generally the philosophy of this forum is to use chlorine whenever possible and save the extra chemicals only when needed (and this may be one of those times).

Refer to this chart and notice that to get 1-2 ppm of disinfecting chlorine (at a pH of 7.5) that appears to be needed to kill this algae, you would need (at 40 ppm CYA) somewhere around 25-35 ppm FC. Keep in mind that this will only give you the equivalent of 2-4 ppm FC if you were not using CYA, but again I do not know if this would damage or fade a liner (my guess is that it wouldn't if only exposed over a week or so -- a year, well maybe).

If you want to play it safe, you could try the polyquat algicide to see if that helps, especially if you are concerned with your vinyl fading. I personally don't think you'll run into a problem with the 25-35 FC level, but then again I don't have a vinyl pool nor your situation. If you do decide to go the "chlorine-only" route, start out at 25 and slowly work your pool up to 35 only if needed and please let us know what happens.

Sorry I'm not more definite about this.

Richard

P.S.
Your web-site is really nice!

[matt4x4]

Since I had to up my chlorine every august to 30-35 ppm when this stuff set in, I can tell you from experience that for the week or so that you need to keep the high chlorine settings, your liner will not fade, you probably get more fade from one month worth of sun.