Where's My Chlorine?

[PitCritter]

I recently inherited the upkeep and maintenance of a small pool in the hotel I work for. Up until now, we've had a company take care of it for us so I don't know a lot about the history.

Immediately prior to taking over the pool, we had a contractor install a saltwater CL generator to make our upkeep a little easier (theoretically).

Here's the problem:

Although my pH, salt level, hardness and TA are all within normal parameters, I can't get the generator to keep up. The pool is only 14,500 gals, and the manufacturer's rep for the generator claims that it's of a sufficient size to chlorinate a pool twice that size.

There is no stabilizer in the water, it's illegal in indoor pools in my jurisdiction (Ontario, Canada).

The amperage on the generator looks like it's working at peak efficiency. If you look at the cell, it's bubbling furiously.

Unfortunately, I can not detect nearly enough CL in the water.

I have been adding a gallon of liquid CL overnight to get the levels up to something acceptable (3.2 or so), but the level drops off during the day to effectively zero by the time the pool closes 16 hours later.

I have checked for and treated sulfate in the water although even when my level was only around 200ppb I had the same problem.

Even if the generator is undersized, I should be able to detect some CL in the water. Even running it at 100% for eight hours overnight (while the pool's closed) generates no measurable CL.

The salesman from the generator wants me to empty and re-fill the pool. I told him I would if he could explain to me why I was doing it. He told me, "There's something in the water that we can't test for, it's the only explanation".

I don't buy it. I'm not saying sabotage isn't a possibility (remember we just fired a maintenance company), but I find it unlikely.

Does anyone here have any ideas what may be causing my problem? I understand that Sodium Thiosulfate is added to a pool to lower CL. Is there a possibility that it may have been added by someone? If so, can it be tested for?

Any and all ideas are welcome and appreciated.

[quantumchromodynamics]

Get a sample of water directly from a return while the chlorinator is working. You should get a strong chlorine reading.

How much use does the pool get? High usage, especially by children, will use up a lot of chlorine as the chlorine oxidizes bather wastes such as urine and other organic contaminants.

1 gallon of 12.5 % liquid chlorine should raise your chlorine by 8.6 ppm. Since you're only getting 3.2 ppm, that indicates a loss of 5.4 ppm. That would correspond to about 32 "person-hours" of usage.

You say that you can see the generating bubbling; is the cell made with clear plastic?

What is the make and model of the chlorine generator?

Is there scale on the cell plates?

Do you think that someone might be currently adding thiosulfate to the pool?

What amperage is the generator using?

What is the water temperature?

What did you do to treat for sulfate?

Are you using any hydrogen peroxide?

Can you list all chemical readings?

Can you list all chemicals that you have used?

[PitCritter]

Get a sample of water directly from a return while the chlorinator is working. You should get a strong chlorine reading.

Yes, a dip strip comes up dark purple.

How much use does the pool get? High usage, especially by children, will use up a lot of chlorine as the chlorine oxidizes bather wastes such as urine and other organic contaminants.

Very little usage. Never more than 3-4 people at a time. Maybe 20-30 person-hours/day

1 gallon of 12.5 % liquid chlorine should raise your chlorine by 8.6 ppm. Since you're only getting 3.2 ppm, that indicates a loss of 5.4 ppm. That would correspond to about 32 "person-hours" of usage.

You say that you can see the generating bubbling; is the cell made with clear plastic?

Yes

What is the make and model of the chlorine generator?

Watermaid-No model number apparent

Is there scale on the cell plates?

There has been some light scaling, but it wipes off easily with a terry towel

Do you think that someone might be currently adding thiosulfate to the pool?

Unlikely, but remotely possible

What amperage is the generator using?

28A

What is the water temperature?

84F

What did you do to treat for sulfate?

My error--phosphate, not sulphate is the issue. I added PhosFree to the skimmer as directed.

Are you using any hydrogen peroxide?

No

Can you list all chemical readings?

TA 110, pH 7.4, FC 0.2, CC 0.6 Phosphate 300PPB Salt 6000 PPM

Can you list all chemicals that you have used?

Shock n' Swim, Super Burn Out, PhosFree, Liquid Chlorine, CO2 gas injection

Thanks for the detailed reply so far...

[PitCritter]

I just finished doing a quick check of our chemical inventory. One interesting note...a 25kg pail of "ProGuard Chlorine and Bromine Neutralizer" made by Asepsis Inc. It's about half full. 12.5kg of this stuff could definitely put a dent in CL levels. The directions say that 18g/100L will lower residual by 1ppm.

My pool is just over 50,000L. 12.5 kg would lower residual by +/- 140ppm by my quick calculation. It looks like it's time to drain/re-fill and lock up the remaining half pail.

Is there a test for this stuff?

[quantumchromodynamics]

The directions say that 18g/100L will lower residual by 1ppm.

Can you double check that?

My calculations show that 0.750 grams of sodium thiosulfate will reduce chlorine levels by 1 ppm in 1,000 liters of water.

37.5 grams will neutralize 1 ppm of chlorine in 50,000 liters.

12.5 Kg will neutralize 333 ppm of chlorine in 50,000 liters.

http://poolways.com/lower_chlorine.html

You could test for sulfates, but there are other sources of sulfates, so you could only test for total sulfate levels and I don't think that you could determine how much came from the sodium thiosulfate. The missing 12.5 kilograms is a good clue though.

Based on your answers, it looks like the salt chlorine generator is working. The 6,000 ppm of salt is higher than most other salt systems use, but it looks like that is the correct level according to the manufacturer.

I'm guessing that the sodium thiosulfate is most likely the cause. You could just keep adding chlorine until all of the thiosulfate is gone, but that still leaves a lot of sulfates in the water.

[quantumchromodynamics]

You could do a test to see what the effect of adding various amounts of chlorine would be.

01) Get 1 gallon of pool water in a clean container.

02) Test the water for free and combined chlorine.

03) Make a solution containing 1 part 6.00 % bleach and 9 parts distilled water.

04) Add 6.1 milliliters of the diluted bleach to the test sample and stir well.

05) Wait 5 minutes and test the water for free and combined chlorine.

06) The free chlorine level should have gone up by 10 ppm from the original number.

07) If you get any increase, wait another 5 minutes and test again.

08) If you still have any measurable chlorine, wait another 5 minutes and test again.

09) If you do not get an increase, add another 6.1 ml of diluted bleach solution and retest.

10) Keep adding the diluted bleach solution in 6.1 ml increments until you get a steady chlorine level of 10 ppm that does not diminish over time.

11) Every addition of 6.1 ml of diluted bleach solution to the test sample will represent 10 ppm of chlorine that you would need to add to the pool to get the same result.

You can double the amount of pool water to two gallons and the amount of diluted bleach added per addition to 12.2 milliliters if that makes the test any easier.

[PitCritter]

The directions say that 18g/100L will lower residual by 1ppm.

Can you double check that?

What's a couple of zeros?

Make that 18g/10,000L will lower chlorine reading by 1ppm.

That'll teach me not to try to calculate during the last hour of a 12 hour night shift.

Oh well, no difference anyway, the pool will be drained and re-filled by morning. Then we'll see.

Quantum, thanks for the test suggestion, but I think I'll just cut my losses on this one and start from scratch.

I'll post back in a couple of days what the outcome is. I'm off until night shift on Jan 1 after tonight.