Where has all the chlorine gone - long time passing?

[Rwhipple]

I am at my wits end. My pool apparently got a case of algae this Spring. It was cloudy, but is now nice and clear. The PH is fine and Alkinity is a bit high, but that is normal for my pool.

The problem is that I cannot keep chlorine in the pool (22K gal inground). Over the past weekend I have put in over 18 pounds of shock and two gallons of liquid chlorine. I have 10 3" tabs of chlorine in the dispenser, which is set to high flow and seems to be working.

What happens is that the shock causes a white mass of foam to form on top. The chlorine (I even tried 10 pounds of heavy shock one time) flashes off in just a few hours. This leaves the pool water clear, but with no measurable chlorine in it. (My measure is total chlorine).

Question - is there any way to get ahead of this thing and get the chlorine to stay for several days or weeks like normal? I have never had this problem before in over 15 years of having my pool.

[chem geek]

Did your Cyanuric Acid (CYA aka stabilizer or conditioner) level drop significantly, perhaps even to 0? If so, then first see if you are only losing chlorine during the day or if it also drops soon after adding it at night as well. If you are only losing it during the day and your CYA level is 0, then add CYA to the water because your chlorine is getting broken down by sunlight.

If your chlorine loss occurs at night as well or if you have some CYA in the water but the CYA level has dropped since before the algae outbreak, then bacteria in your pool may have converted some of the CYA into ammonia creating a huge chlorine demand. Technical details about this are described in this thread and my personal experience with this problem is described in this thread.

You should just keep adding chlorinating liquid (or bleach) frequently until it starts to hold. You can use a bucket test if you want to estimate how much it will take.

What was the "shock" you were using? Was it Cal-Hypo or something else like Dichlor or Trichlor powder or lithium hypochlorite?

[Where has all the chlorine gone????]

Yes, my stabilizer (CYA) level is between 20-30. I am losing chlorine both during the day and at night.I am not smelling any ammonia odor.

I have been using many bags (up to 9) of the following types of shock
One type tried at one time - not mixing shock types.

Calcium Hypochlorite
Sodium Dichloro -s-Trizinetrione
Potassium peroxymonosulfate

I have put in over 12 gallons of liquid shock over the past week.
My chlorinator is full of 4" tabs
The water is the correct PH, alkininity is a bit high, but that is normal for my pool
I have put in a fresh quart of high powered algecide on top of my normal algecide added on opening plus maintence levels.
Used 2 liters of Phosfree

Here is what happens when I add shock. Water gets white and a white mat forms on the surface indicating oxidation bubbles in the water. This clears, and no chlorine is present in a few hours.

The water has stayed clear as a bell, but the pool just keeps eating chlorine as fast as I can add it. Do I just keep doing this or is there some other trick to overpower it and get some stability with the chlorine? This is costing me a fortune!

[chem geek]

You wouldn't likely smell the ammonia. When you add the chlorine, test soon afterwards (say after 5 minutes) and see if you measure Combined Chlorine (CC). Also, you can get an ammonia test kit from a fish/pet/aquarium store and calculate the ammonia level which will be the ammonia ppm minus 1/5th the CC level, but measure the CC before you add chlorine (i.e after some hours).

If you read the thread I linked to where it happened to me, you will see that it took a lot of chlorine to get rid of the ammonia and partially oxidized CYA. A record of what I did is in this post.

The least expensive approach would be to use chlorinating liquid or bleach though Cal-Hypo might be reasonable if your CH level is low. I would NOT use Dichlor or Trichlor since you don't want to raise your CYA level too much -- that will only slow down how long it takes for the ammonia to get oxidized by chlorine. That means removing the Trichlor pucks from your chlorinator or shutting off that chlorinator.

If you want to estimate how much chlorine it will take, you can do a bucket test where 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC so you can see how much you have to cumulatively add before the FC holds over an hour or so.

To prevent this in the future, you want to close your pool as late as possible when the pool water temp is 50ºF or below and open it as soon as possible before the temp rises above 50ºF. Also you can close by shocking with chlorine and then add PolyQuat 60 algaecide -- again, when the water has gotten cold and you are still circulating it (i.e. shock and add algaecide before you turn off the pump for the winter). If you are in an area that doesn't freeze so you don't close your pool (i.e. drain below skimmer level, plug returns, etc.), then just maintain chlorine in the pool through the winter -- very little will be needed if you have an opaque pool cover.

[Guest]

OK Thanks. So my best course of action is to just keep adding liquid chlorine or bleach until it starts to hold. I can also take out the chlorinating tablets in my chlorine dispenser since they are adding CYA.

I can do that, but it seems like it is an awful expensive way to be filling my pool. I have already put in 12 gallons of liquid chlorine, but I have a stock of 8 more gallons available plus a couple gallons of bleach. Do you have any recommendation on the pattern of addition. Should I do one gallon an hour or put in 4 gallons at one time, or maybe 2 gallons a day.

This is really a lesson I shall not soon forget. Thanks very much for the advice. I will keep you posted on this.

[chem geek]

You can add it as frequently as you want so long as the FC is still dropping quickly. Look at the link I gave you to on my experience where you can see the fairly rapid chlorine addition at first that I then trailed off as the chlorine drop rate slowed down.

[Guest]

Thanks, you have been so helpful. I do not know what I would do if it was not for the information and insight you are providing. I have one more question.

Now that I have taken the Chlorine tablets out of the dispenser, and I am using only liquid chlorine, there is no additional source of CYA, so this should stop the creation of additional ammonia - right? Does this mean (I hope) that the chlorine I have already added has not gone to waste. Just because it does not stay in the water more than a minute or two, I am still gaining on the problem every time I add chlorine - right? (I added 4 gallons today) Sooner or later, if I keep adding chlorine, I will have consumed all the ammonia and the chlorine level will stay. Then I go back to my normal routine.

If I am understanding your thesis, this is how it will play out.

Tomorrow I am going to try the bucket test you suggested to get an estimate of how many tanker cars of liquid chlorine to order.

[chem geek]

Yes, what you wrote is correct. You want to hit it hard enough and quickly enough to try and kill off the bacteria if they are still consuming CYA. Most of the chlorine is oxidizing the ammonia, but at some point that will get all oxidized (as well as intermediate partially oxidized CYA chemicals) and the FC will begin to hold.