How High To Take Cl2 To Break Down Gunk

[jereece]

I have a friend who has a 30ft round AG pool. I calculate about 24000gal. She has been using tabs for years and recently noticed water bugs swimming in the pool. Water was slightly milky. I ran some tests using a Taylor kit and got the following numbers.

pH: 7.2
Free Cl2: 0
CYA: 110

I had her add borax to bring the pH up and 5 gallons of 6% bleach that night. By my calculation that would add 12.5ppm chlorine. It took a couple days adding more borax but the pH came up. The next morning after adding the bleach the free chlorine was down to about 0.5. So I had her add 5 more gallons. The same thing happened. So some biological or chemical process is consuming her chlorine over night. I know this has to be overcome, so I have 3 questions.

1. Should she keep adding 5 gallons until the residual can be maintained over night

OR

2. Should she add something like 10 gallons (25ppm) one night to overcome it all at once?

AND

3. Also, should she do anything about her filter? For example should she super shock her filter to kill stuff that may be trapped in there?

As always I appreciate the help.

Jim

[aylad]

In order to kill off the "gunk" in the pool, your friend is going to need to take the Cl up to 25 ppm and HOLD IT THERE until there is no loss when testing at night after sundown and again in the early morning before the sun is on the pool. She will need to test 2-3 times daily, and more if possible, each time adding whatever amount of chlorine it takes to get back up to 25. THe more consistently she's able to maintain 25 ppm, the quicker it will clear up. In her pool, each 1/2 gallon of 6% bleach will raise FC by 1 ppm, so she can use that as a guide when figuring what it will take to go back up to 25.

So the answer is a combination of the above--for right now, add 10 gallons but understand that it's not a one-time addition--she's going to need to keep adding to maintain that high level until it's holding. Don't add anything extra for the filter--the chlorine in the water will take care of the filter debris as well. While keeping it shocked, she needs to brush the pool at least daily, and keep an eye on her filter pressure and backwash the filter as the pressure indicates. Once the pool turns cloudy blue AND THERE IS NO CHLORINE LOSS OVERNIGHT, she can let the chlorine level drift back down, but to no lower than 8 ppm. Unfortunately, when the CYA is that high, she'll have to maintain the Cl between 8-15 ppm all summer to keep the pool clear. I don't know if a partial drain and refill is an option for her, but if it's feasible, I would certainly consider it so that she doesn't have to maintain her Cl that high.

This is going to take a good dose of POPP--pool owner patience and persistence. It won't happen overnight, but then the problem was created over a long period of time, as well. Fortunately, the cure won't take as long as the creation!

You didn't post a CH level--has she been using cal-hypo at all? Could you post that too, because it's a common cause of milky water.

Also, where is the pH level after the Borax additions?


Janet

[waterbear]

How did you get a CYA of 110? The test only goes to 100.

[jereece]

I estimated 110 CYA because it took a little above the 100 mark till I could no longer see the dot.

I think her calcium level was about 140ppm. I believe they are on well water. However they have been using the Trichlor tabs not Calcium Hypochlorite.

Thanks for the validation. I thought it was going to take 20-25ppm to burn off the gunk.

Jim

[waterbear]

the CYA view tube is not marked in a linear fashion. The marking are closer together at higher CYA levels than at the lower ones. This is why the test only reads to 100 ppm. It is too difficult to judge the level when it is higher than that. There is a good possibility that the CYA is much higher than 110 ppm,

You can dilute a water sample with an equal amount of distilled water and test that. Multiply the test results by 2. While you do lose some precision by diluting you will get a better idea of where the CYA is at the present time.

[chem geek]

I estimated 110 CYA because it took a little above the 100 mark till I could no longer see the dot.

I think you meant a little below the 100 mark since above that mark would be between 90 and 100 ppm. The marks go from high (100) at the bottom to low (30 or 20 depending on test kit) at the top.

[jereece]

You are right ChemGeek. On my kit 100 is an inch or so up from the bottom then the measure marks start. And you are right WaterBear the marks are not linear. So I should have diluted. Now, you mentioned using distilled water. Why can't I use tap water? Tap water should not have any CYA...should it?

Thanks guys.

Jim