How to lower alkalinity in my work (commerical) pool?

[dmccarthy]

Please see below for the specifications of my work pool. It is all automated and have the pH set for 7.5 and ORP is currently set at 675mv (jumps up and down a little depending on bather load at the time) and that seems to keep the free chlorine levels right between 2.0 and 2.5 ppm give or take depending on bather load and the dilution of sodium hypochlorite to water that I have in the tank at any given time. We test with the taylor testing reagent kit (recommended on this site) 4x/day so the numbers should be accurate and the water is very clear and blue. The system has been working great in my opinion since I installed 3 months ago.

One thing I have been noticing is that TA has started to increase (we were trying to keep it right around 100ppm) and has crept up to 140-150 over past several weeks and would like to know #1 the best and safest way to bring it down without throwing off or affecting the controller readings or with the automation of the liquid chlorine and CO2 gas and #2 if having it up that high poses any problems with sanitizer levels, swimmers and/or what might be causing it? alll the other numbers are right in line and we use a very small amount of CYA (about 20ppm) just becuase of the UV lamp and to aid in bather comfort, but like to keep the sanitizer levels as close to 3.0 as possible as we have a large number of bathers/day/week.

Any advise on how to best addresss the TA rise issue would be most appreciated.

Darin

24,000 gallon indoor, commercial, gunite pool
hayward pro series 500lb HR sand filter
Hayward TS VS 1.85hp programmable pump
Hayward CAT-4000 automatic pH/ORP Controller
15 gallon liquid chlorine tank with stennar pump injection
50lb CO2 gas tank injection
Paramount 99gpm UV inline sanitizer light
Raypack 333k btu gas heater

 

[dmccarthy]

Also, failed to mention I have not checked the TA of the fill water (town water supply) which we add 1-2 inches of water/week especially after backwashing which could be contributing.

 

[cfherrman]

Muratic acid, them I'm guessing the co2 injection is to raise pH? If so use that or areation to raise pH, then more acid, and so on untill you get required ta.

 

[mknauss]

The CO2 is in place of acid. It is used to control pH.

Fill water TA needs to be looked at. You may need to lower the pH at night and aerate somehow to keep the TA in check.

 

[JamesW]

You need to use muriatic acid for ph control for a while instead of the CO2.

You would probably do better trying to maintain a pH of 7.8 and a TA of closer to 60.

What are all of your readings?

 

[dmccarthy]

Thank you,
Yes the CO2 injection is used to lower the pH in place of an acid injection system. I will check the TA of the fill water this week and it sounds like the best solution would be to temporarily suspend the CO2 injection and use muriatic acid to lower the TA and resume once it is in check. The other readings are as follows:

Pool Temp 92 degrees
FC 2-3
CC .2ppm
pH 7.5
TA 140
CH 220
ORP set point 675mv
CYA 20

Just curious why raising the pH to 7.8 and and lowering the TA to 60 would be better? I thought as the pH climbed it lowered the killing effectiveness of chlorine so I wanted to keep the pH around 7.4-7.5 and the FC as close to 3ppm as possible with about 20ppm of CYA as we have a high bather load at any given time. If you think lowering the TA closer to 60 and raising the pH is a better idea I would certainly am willing, I would just like to know why so I understand what I am doing. I just need to keep in mind that because we are considered a "commercial" pool the regs in MA are that the TA needs to be between 80-120 (technically) if it was to be inspected. The TA was below 120 when the inspector came out a few months ago, so I am not sure if they would give me a hard time if they came out again and it was 140, but I am trying to address the problem before it becomes one.

Thanks again for any info

 

[duraleigh]

I thought as the pH climbed it lowered the killing effectiveness of chlorine

An often repeated thought. There is SOME small degree of truth to that but it should not enter into you pool management techniques.....it's too small. chem geek published some data on this subject within this website but I couldn't locate it just now.

Like JamesW suggests, 7.8 and 60 just might give you quite a bit more stability and it's inexpensive, easy and also easy to change back if you don't like it........there is no downside.

 

[dmccarthy]

Great thank you I will try. If for some reason I need to keep the TA at a minimum of 80 (state regs) is there a different ph I should be shooting for? 7.6, 7.7?
Thanks,

 

[JamesW]

Lower pH levels cause the carbon dioxide to come out if solution and leave the water. This greatly increases the amount of carbon dioxide that you have to put back into the water.

Higher pH level reduce carbon dioxide loss, so, I would target 7.8.

Higher TA levels also cause carbon dioxide loss. So, you should keep it as low as allowable.

 

[cfherrman]

Go over to pool math here and play with you pH levels a bit and check your csi factor to make sure you keep your plaster in check