Combined Chlorine Confusion

[HOApoolmanager]

I work at an outdoor swimming pool and have been having issues with controlling the combined chlorine in the spa. The combined chlorine usually rises at least 1 ppm per day throughout the day. The spa sits in sunlight most of the day. Here are the stats (as we speak):

FC: 4.6

CC: 1.6

CYA: 110

Alkalinity: 90

PH: 7.2

Calcium: 270

300 gallons

Combined chlorine is the only chemical I've had trouble balancing all summer. I've tried shocking at 10x the FC level (something I read) while exposing the tub to sunlight all day and this gradually works, but as soon as it gets low enough I set FC down between 3-5 and combined rises once again. I've tried thio-trine Cl neutralizer and this works to get combined chlorine to zero, but again once I get FC back to the desired range whaddaya know the CC rises sharply once more. It's a busy spa which gets many users over the course of a day so super-chlorination is a frustrating procedure as it requires us to close the spa. Our health regulations mandate a CC of no greater than 0.5

[chem geek]

Part of the issue is that your CYA level is too high so when the FC gets lower the active chlorine level that oxidizes bather waste gets too low and can't keep up with getting rid of CC. So you see more of the CC intermediates. They will eventually drop if you don't have more bather load, but to be able to keep up you need a higher FC/CYA ratio. You can either do this by maintaining a higher FC so long as you have this high CYA or you can dilute the water to lower the CYA level.

You must be using or have used Dichlor or Trichlor for the CYA to climb. You should instead be using primarily chlorinating liquid or bleach. You don't want to use only unstabilized chlorine without having some CYA in the water or else the chlorine will be too strong, but 30-40 ppm CYA is reasonable. If this is a commercial/public spa, then 20-30 ppm would be better though you need to check your regs since they might not allow any CYA at all in the spa.

Also, your CH is too high and risks scaling especially in the gas heater heat exchanger. The main thing preventing that is your pH is low and I presume you have automatic acid feed for that because normally your pH would be higher in a heavily used spa, especially if there are jets making aeration. I recommend a CH of 120-150 ppm to prevent foaming. Note that if you switch to chlorinating liquid or bleach that you'll probably need to lower your TA to have the pH not rise as quickly -- even a TA as low as 50 ppm if the regs allow (they might only allow down to 60 ppm). In residential spas, I recommend adding 50 ppm Borates (usually from boric acid) for additional pH buffering, but I don't know if that is allowed in what sounds like a commercial/public spa.

[HOApoolmanager]

Thanks for your help! It is a neighborhood public spa. Does the dichlor come from the chlorinating tablets in the chlorinator, the shock, or both? Also, our health regulations mandate a minimum CH of 200 so I'm not sure if I can go that route.

[chem geek]

Chlorinating tablets are Trichlor. Some "shock" called chlorinating granules is often Dichlor but can also be Cal-Hypo. Note the following chemical facts independent of pool/spa size and of concentration of product:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.

If you got your Certified Pool Operator (CPO) certification, I suggest you read Certified Pool Operator (CPO) training -- What is not taught.