Huge Chlorine Demand Why!?

[Dietitian]

My plan is to shock to 100ppm chlorine over the weekend, drain and refill, but I'm wondering why the tub has such a huge chlorine demand. Is it the high TDS?

I have a 280 gallon tub. I'm using chlorine, MPS, Nature 2, Ozonator (50mg/hr), dry acid, baking soda, Leisure Time enzymes, and pool calculator. Last water change was 2 months ago and new filter installed. Total bather load was about 40 person hours during that time.

I shock after every soak. Lately I have been shocking to 21ppm daily and all the chlorine is gone the next day. MPS shocks daily as well at 16 grams MPS per day. Last night the tub was shocked to 24 ppm total chlorine with PH 7.0. 15 hours later we are at the following:

PH:7.7

Borate: 30 ppm

TA: 45

Calcium hardness: 550

TDS: 3,060

Free chlorine: 4 ppm

Combined chlorine: 1 ppm

MPS: Test strip indicated "ok" MPS levels (All other tests done on Taylor K-2006)

I recently raised the TA to 100, so the rising PH is probably normal while it stabilizes. Chlorine demand is 29 ppm per 24 hrs, or about 300% as per the 10ppm test.

[chem geek]

Sounds like you've either got biofilm or some other organics that built up in the tub. When you are ready to do the drain and refill, use Spa System Flush or similar product to ensure you get rid of any organics or biofilm in your tub.

It's also possible you aren't using enough oxidizer after each soak and got behind, but with an ozonator (if it's working) that would be unusual.