Chlorine Demand - Par Pool & Spa has a good handle on how to treat this growing issue.  If you haven't dealt with it yet, you will.

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Question:
I've been having the "disappearing chlorine" problem and I am trying to achieve breakpoint chlorination. 

I have an 8500 gallon pool and I added appx 2 lbs of non-chlorine based shock, then 6 lbs of chlorine based shock - that was Tuesday night (and added two tablets in the chlorinator). 

48 hours later I used one of those new test kits that tests for both free and combined chlorine and here's what I got: 

1st try - 20ppm FAC, 1.5ppm combined

2nd try (20 minutes later) 17.5ppm FAC, 1.0 combined

Do you think I have achieved it?  If not, what else do I need to do?

Response:
If you're holding onto 17 ppm FAC, the 1.0ppm CC shouldn't make a big difference.  Retest after 2 days to verify that the chlorine is still holding.  It should be least 10.0 ppm FAC.  If it drops down to 3.0 or less, I would recommend shocking with another 10 to 12 lbs. of shock.

Also, what is the pH?  If the pH is low (under 6.8 especially), you will burn through chlorine quickly.

Follow Up Question:
Thanks for the advice - I just tested again this morning, 4 days later, and I think I might have "jumped the grand canyon"... 

I've got 9.5 FAC and 0.5 CC, my PH is around 7.2, maybe a tad higher (hard to tell for sure with the color coding).  Do you agree that I have achieved breakpoint chlorination?

If so, is it safe to swim with chlorine at these levels?  How about using the attached hot tub?  I'm thinking it might be too high for the hot tub, but maybe not the pool?  What would you say is the safe limit of FAC to swim? (My 3 year old is getting a little antsy)

Thanks again for the advice.  By the way, I read a lot of different articles about chlorine demand and breakpoint chlorination in my search to fix the problem, and I must say that your website was the most informative, and easiest to understand.

Follow up response:

Congratulations!!  It all sounds good.  The chlorine is a little high.  Typically we want to wait for the chlorine level to drop to under 3.0 ppm, but...many water parks & commercial pools often have chlorine levels as high or higher.  If your daughter has sensitive skin, you may want to keep her out.  But a few minutes in the pool with a good shower afterward should be fine.

In the hot tub, you're going to get a good "gas off" of chlorine between the warmer water & the jet action.  It should come down relatively quickly.

You can also turn down or off your automatic chlorinator or remove the chlorine tabs or sticks from the skimmer for a day or two also.  Put them back in when the chlorine level has gone to under 3.0 ppm.


Take a look at our sister site, www.spacareonline.com for more spa & hot tub info.

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Chlorine Demands can also be attributed to Bio-film problems. Here's some information.