New here and this is my first post. In fact I'm new to the whole pool/spa maintenance thing, but I just completed the CPO class. I have a 1,000 gallon hot tub, which is using hydrogen peroxide and U/V light to sanitize.

I cannot get the pH to come down and stay down at a reasonable level. In fact I cannot get it down even to 8.2. If I drain the tub and refill, which I have done twice in a month, the pH of the new water starts at 7.4/7.5.
Here are the values I measured just after filling the spa:
TA: 100
Hardness: 220
H2O2: 0
pH: 7.5

I turned on the heater and the circulator pumps, left it for 24 hours, and the pH has risen off the chart. Other than temperature (95 degrees,) the other numbers have not changed.

What is causing this, and what do I need to do to keep the pH down?
I'm beyond measuring for the "correct" amount of chemical to add. I've added up to 8oz. of muriatic acid at a time, no help, and one pound of sodium bisulfate. No change.

Hydrogen peroxide is alkaline upon usage and will account for some of the pH rise.

[edit]It depends on what the hydrogen peroxide is oxidizing. If it's something like ammonia, then it will end up being more pH neutral.[end edit]

The other source of pH rise is the TA. Keep adding acid until the pH and TA go down and stay down. The TA will probably need to drop to about 40 to 60. As long as you're using 31.45 % acid, the pH and TA will go down. If the pH and TA won't go down, then there is something wrong with the acid. Do not lower the pH to less than 7.2.

It's also likely that you have some scale built up. Some of the acid will be used up dissolving the scale until the scale is mostly dissolved.

Is the spa plaster, tile, fiberglass or what?

Is the surface smooth or rough with scale?

I'm guessing that you're probably in South Australia where hydrogen peroxide + UV is approved for commercial use?

It's a fiberglass tub. The surface is smooth and not rough with scale. There is no hydrogen peroxide in the tub as I just drained it. Nothing but tap water, which was 7.5 when it went in yesterday and jumped to 8.2+ overnight with nothing but the heater and the jets. The spa is outdoors though, in the sun.
I'll start dumping acid in there today, keep the pool closed for the week, see if I can get the numbers down. I thought TA was supposed to be 80-120 though?
And South Australia? No, I'm in Colorado.

I didn't know hydrogen peroxide + UV was approved in the US for use as a primary sanitizer. The TA does not need to be 80 to 120 ppm; it can be as low as necessary to maintain the pH.

There is no need to keep the pool closed for a week. With hot water and good circulation, it won't take very long to get the TA down. Add the muriatic acid and then recheck the pH in 10 minutes.

Keep lowering the pH to 7.2 until the TA gets down to about 60 and then only lower it to about 7.6 until the it stops rising.

When you're reporting Hardness as 220, is that Total Hardness or Calcium Hardness?

I'd expect that. Tap water has a lot of CO2 dissolved in it as Carbonic Acid; same stuff that gives CocaCola its fizz. Spas get a lot of aeration, which drives out the CO2, lowering the acidity, and making the pH go up.

Keep adjusting pH as it rises; eventually your spa will hit the sweet spot where it all holds fairly stable. Draining and refilling it justs wastes water.

Just to reiterate what JW mentioned, it is not, nor do you have any way to confirm your water is sanitary.

Agressive aeration from the spa jets and TA over 60 is causing the PH to rise.

Colorado is one of 28 states whose code requires that all Pool and Spa Circulation System Components comply with NSF/ANSI Standard 50, which requires that UV be used with a residual level of an EPA registered chemical disinfectant.

Hydrogen peroxide is an oxidizer, not an approved EPA registered disinfectant. NSF 50 requires that UV be used with not less than one ppm free chlorine or two ppm bromine.

This Colorado pool company says that UV + H₂O₂ meets code, but I don't think that they're correct.

I'm assuming that this is a commercial/public pool, is that correct? Has the health department approved your facility to use UV + hydrogen peroxide without chlorine or bromine?

[edit]Recent NSF activity seems to indicate that they are now changing the language to indicate that UV systems are intended for use with appropriate residual levels of EPA registered disinfecting chemicals, which seems to be vague, but getting away from any sort of requirement. Colorado code is also somewhat vague as to exactly what they require.
Also note: UV + H2O2 is approved in South Australia only for indoor pools.
[end edit]

So technically we're a public pool. It belongs to our condo association, so condo owners and their guests/renters are authorized to use it. We'll go for days or a week at a time with no one in it at all. It's busiest may see 6-8 people per day.
I don't know whether it's "legal" or not - it's been using H2O2 and U/V for probably 15 years. I just inherited the maintenance job this summer.
The answer to someone's hardness question is, it's calcium hardness I'm reporting.
So here we go: Per the advice in this thread, I have been dumping acid in the pool and re-checking pH for the last two days. pH started at above 8.2 (off the chart) and after adding 46 fluid oz. of acid, we're still above 8.2. Do I keep going?

Yes, add enough acid to lower PH to 7.2 and when it get to 7.8, lower it again. Eventually when TA drops to around 50-60 the PH rise dhould slow considerably.

Something does not add up. Your first post says that the tub is 1,000 gallons and that your TA was 100 ppm. 46 ounces of 31.45 % muriatic acid should lower the TA by 180 ppm, which would drop your pH well below 4.5.

Your Volume, pH reading, TA reading and/or acid strength are wrong. What is your current TA?

As a CPO, it's your job and responsibility to make sure that it is "legal" and safe. Have you ever applied for a health department permit to operate this tub, or been inspected by the health department?

Note this paper comparing sodium hypochorite vs. hydrogen peroxide. Notwithstanding the limited Australian approval, it is not approved by the EPA as a disinfectant for use in commercial/public pools.

According to the Colorado code, this hot tub might not be considered as public, semi-public or commercial.
Therefore, if only the residents use the pool, then it might not be considered public. Whether or not the residents are allowed to have non-resident guests accompany them to the pool might affect the classification.

Either way, as a CPO being paid to maintain the pool, you are still responsible for making sure that the tub is safe to use.

Checked this morning... TA has finally dropped from 100 to 70, and calcium hardness from 220 to 180, no change yet in pH. Added another 22oz. of acid and will check again this aft.

You should double check the volume of the tub and the strength of the acid. Something has to be off for your TA to only drop by 30 ppm after adding the amount of acid that you have.

What else is being added to the tub?

Holy ****! Everything dropped today. I think it may be because I used a different bottle of acid. I'll bet that other bottle, which I finished yesterday, had gone bad or lost strength. My pH is now down below 6.8, and TA is down to 10. Now we're getting somewhere!

If TA is down to 10, your PH is dangerously low. Raise PH to at least 7.2 promptly.

What test kit are you using?