Can I calculate the added copper?

[loop_pea]

We use trichlor tabs to chlorinate the pool on a daily basis (I wish we didn't but hand dosing is not allowed in a semi-public pool and money is not available for the automatic options).

I am required to use tablets with added flocculant to improve the filtering (cryptosporidium control). Last year I could buy a trichlor tablet with aluminium sulphate in it to meet that requirement. This year the only tablets that I can buy with added aluminium sulphate also contain copper sulphate - about 5% of each by weight. I am concerned about a build up of copper in the pool. I know how easy it is for CYA to get high, and it seems that copper could easily go the same way.

Each tablet adds ~2PPM of chlorine to my pool and ~1.6PPM CYA, which has now reached 50PPM. I'm sure it's possible to calculate the added copper as well, but I can't seem to work out how to get started. Can you help?

Numbers right now are typically:

FC=6
CC=1.5
pH=7.6
TA=110
CH=250
CYA=50

We're having a problem with a small amount of free floating brown algae, probably because of the wettest British summer in living memory, which is why the CC is up. I was advised to shock the pool and then add algaecide. I still trying to work out how best to do the shock step - do I drain half the pool first because it will go a lot easier and quicker at lower CYA levels?

And I am planning to skip the algaecide step. I think there's a reasonable chance that there's already too much copper in there, and so adding more would be really bad. I don't know how to test copper levels though, and thought it might be easier to work it out. I could just skip that step regardless, but I would still like to know how much copper is in the water.

Any help or advice would really be appreciated. I think this forum is amazing and Richard Falk's Pool Equations spreadsheet even more so. I have yet to meet any pool people here who understand just what an impact the CYA has on chlorine's ability to do its job.

 

[chem geek]

Let's say that your Trichlor pucks/tabs have 90% Trichlor, 5% copper sulfate, and 5% other such as aluminum sulfate. 10 ppm FC would be 109.3 grams of Trichlor in 10,000 liters so you'd also get 109.3 * .05 / 0.90 = 6.07 grams of copper sulfate (which I assume to be pentahydrate, not anhydrous). This is equivalent to 6.07 * 63.546 / 249.70 = 1.54 grams copper. So this is 1000 * 1.54 / 10000 = 0.154 mg/L so around 0.15 ppm.

So if you have a chlorine demand of 2 ppm FC per day, then the copper buildup will be 0.9 ppm per month which is pretty high and would stain if the pH got towards 8.0. It may be that there isn't quite as much copper in the tablets as I calculated, but then again these manufacturers tend to think that the higher copper levels just mean greater sales of metal sequestrant and stain removal products.

 

[Richard320]

We use trichlor tabs to chlorinate the pool on a daily basis (I wish we didn't but hand dosing is not allowed in a semi-public pool and money is not available for the automatic options).

You might want to carefully re-read the regulations. They may actually say no manual dosing while the pool is in operation. Which will allow you to run FC up to max with bleach before opening, and then blast it after closing. And let the pucks keep you above minimum during the operating day.

 

[loop_pea]

Richard320. That's moreorless the approach that I take. I use tablets to keep the chlorine level up during the day, but if it needs topping up when the pool is not in use at the end of the day then I usually use liquid chlorine. If I can check it at the end of the day then we start the following day with plenty of chlorine so there's no need to add more in the morning. We've been open nearly 8 weeks and only got to CYA of 55, so it has slowed down the rise in CYA quite a bit.

chem_geek. Thanks for help with the calculation. It's the weights that got me. I already hedged my bets a bit by using both plain tablets and tritabs with copper in, so the copper will go up roughly half as fast as you calculated. As CYA has reached 56 and about half of that came from tablets with copper in, I think it's possible to work out how much copper has been added. I make it 27ppm of CYA corresponds to ~40ppm of FC, which has also added 0.6ppm copper so far.

I think that's a definite "no" vote for the algaecide!