Yellow/Brown dusty deposits that reappear repeatedly

[chem geek]

jstonemo,

You only get a CC>0 when the chlorine has incomplete oxidation of an organic or ammonia. That is, when chlorine first quickly combines with ammonia or a nitrogenous organic, it forms a chloramine which increases the CC, but with more chlorine this slowly breaks down to nitrogen gas and the CC returns to 0. Sunlight may also help further this breakdown process. Also, every time you add chlorine to your pool, you give it a very high concentration in a local area that also helps shock to breakpoint (this is also, presumably, how SWCG systems work, shocking locally in the salt cell). So in an outdoor pool with sufficient chlorine levels properly maintained, you usually see CC stay at 0. Generally, the CC becomes measurable if the chlorine is overwhelmed, say after a heavy bather load or rain with lots of junk blowing in the pool, but even this would likely be temporary if you continued to manually dose your pool with chlorine.

The fact that your chlorine is getting used up more than normal (especially overnight when the reduction in chlorine from sunlight doesn't occur) shows that the chlorine is indeed combining with something and it's most likely to be this brown spotted algae.

matt4x4,

I am curious as to what your CYA levels were when you needed to use 20-25 ppm of chlorine to zap this algae (and where 15 doesn't work). I'm trying to get a sense for the actual disinfecting chlorine level needed to kill various forms of algae, and this one sounds particularly hearty. Also, if you recall the pH when you killed the algae, that would also be helpful.

Thanks,
Richard

[matt4x4]

Hi Richard,
My CYA is steady at 20-30 ppm, probably started at around 30 in spring and slowly dropped over the course of the season to 20 as rain dilluted my water, I don't use much dichlor or trichlor, only a few pucks in the stairs to keep the chlorine concentration up inside them to avoid algae breakouts in there - very effective.
I didn't add any CYA this year because I opened to it being fine, and this late in the season I won't be bothered, rather keep the solar cover on during the day.
My alk and pH have also been in check all summer, it's been a relatively trouble free season, but every onset of fall I've seen this stuff creeping in, the funny thing about it is it is VERY slow to set in, you can keep your pool at 15PPM and it just slowly increases in mass, say over the course of a week, but you will not get it to go away, even if you don't do anything about it, it only creeps up on you slowly. The first time I saw it, I actually thought it was clay dust and attributed it to fields being tilled up the road, now I'm leaning more towards algae, especially since teh pool turns that tell tale blue once you do kill it off. I know ragweed is in bloom right now, I wonder if this stuff feeds off the pollen or something.
It's definitely not your typical common algae.
Oh, and tehy're not spots, rather like first described, it starts as a fine dust deposit in craters and at folds where the current would drop it, it eventually dusts teh whole floor if left alone, however, it does not "grab" on to the liner, even just creating a slight current above it will disperse it like dust and leave a spotless liner behind.

I just found this post - dealing with the same stuff - 20+ PPM cured it too.....although he's keeping the cl relatively high and still losing it daily - telling me the stuff's still lurking.....
http://www.poolforum.com/pf2/showthread.php?t=5471

And another - all in mid august onwards!

http://www.poolforum.com/pf2/showthread.php?t=5423

[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

[matt4x4]

Richard, My guess is it's algae, it multiplies if left alone to the point where you get the heavy deposits in the craters getting even heavier, and an even film over the rest of the floor, I guess what i mean by "telltale blue" is the cloudy/white, but mixed with the background of the blue liner, it looks blue to me.
I will recheck my CYA tonite since i just dealt with this crap last week and it won't have changed from tehn, but I'm guessing it's somewhere closer to 20 than 30.

Even with DE in my filter, some (a lot) will get filtered out and looks green upon backwash, however, it comes back just as quick, and like i stated before 15PPM only slows it down, 20-25 has an effect. Dirt won't oxydize, so i know it's not clay dust - nor would my pool collect that much of it so quickly with a cover on it. And backwashing would be brown, not green.

Now, here's another observation my neighbour and i had the other evening - the air was just THICK with microscopic flying bugs, and I mean thick! No, it was not the beer we were drinking!
We couldn't see them if having the sky as the backdrop, however, get the trees as backdrop and you could see these things EVERYWHERE - now, maybe they're not the cause, but I could see that they may be the food keeping this stuff going.

Also, this year, I added salt to my pool, I thought salt was supposed to help deter algae as well, obviously not this stuff....

[Rangeball]

Hmm...

I walked out to check my pool this noon. I have this stuff in the corners of my deep end. Water looks great, is holding CL, but I still have this stuff.

I plan to vacuum it to waste tonight, then shock if it comes back. I'm not totally convinced it's algae. It could just be massive amounts of pollen/dust, but I don't know.

If I get a chance, I'll try to get a sample and run the test Richard mentioned.

[matt4x4]

after 3 years of dealing with this crap, and this year being the first where I was able to make it go away fast, I am convinced it's algae - unfortunately, I have none left in my pool to experiment with (well, maybe unfortunately is NOT the word I should be using...)
I have found several posts regarding the same stuff, all started mid august or later, this is a seasonal thing and because it takes chlorine (lots of it) to eliminate it, then it must be something organic. Since it multiplies, it's gotta be algae. I will be researching this further since it seems to be happening relatively frequently.

[matt4x4]

Ok, I found some algae with similar if not identical properties - called Brown Algae or Silica Algae - caused commonly through excess silicates and nitrates, inadequate light (like covering your pool because you're not using it much due to cooler weather).
Fed by things such as fertilizer - hmmm - fall, farmers fertilizing the fields before turning them over, me, living in the country? Anybody else in the country?

Anyways, that's all i have right now!