SWG + Spillover = Acid for dayzzzzzzz... wat do

[wtfbliz]

Having trouble keeping pH and TA in alignment, the fight is real.

Our 10k gallon pool has floor jets that cycle to both the pool as well as 1 zone that cycles to a spa with spillover. There is no way to stop the spa zone which means there is no way to stop the spillover. The floor jets are cycled by a 6 chamber mechanical Paramount contraption. No options to change how it works without major surgery, so Id rather not try to replumb all that... I run the pump at low RPMs for about 12 hours a day.

Between the SWG and the spillover, the pH will go from 7.2 to over 8 in under a week. I added borates at about 40 PPM this year to try to buffer it some and keep it from going crazy, but its still out of control pretty fast.

To keep my sanity I will overdose acid a little and let the pH swing from 7.2 to 8 so that Im not out there every morning with acid.

Looking for a better solution than dumping 34.5% acid in the pool every other day. If I kept up on pH/TA like I should I'd be going through 3 gallons of 34.5% acid per month.
Any in-line dosing automation that could sit in the sun? Have Pentair EasyTouch & IC60

 

[Jimrahbe]

W,

I would not worry about TA unless it tries to go below 50.. Just worry about your pH. Most SWCG pools like there pH to be about 7.8... The further you take it away from 7.8 the harder it will try to bounce back.. I'd only reduce it to 7.6 and let it get to 8.0 before adding MA.

Thanks,

Jim R.

 

[Newdude]

+ 1. Think of a swing. If you give it a little push, it will come back gently. If you give it a big push it will come back just as fast and possibly slap you upside the face.

 

[wtfbliz]

+ 1. Think of a swing. If you give it a little push, it will come back gently. If you give it a big push it will come back just as fast and possibly slap you upside the face.

Maybe you can explain this, but I dont understand the chemistry as to why this would be the case.

Equating chemistry to conservation of energy in physics I'm not sure is an accurate representation of whats happening. No matter what the pH is, the SWG runs the same amount of time. Do reaction rates of offgassing or the NaOH generation go higher in a lower pH environment?

 

[Newdude]

Maybe you can explain this, but I dont understand the chemistry as to why this would be the case.

If the PH is settled in the high 7s and you force it into the low 7s, it will rebound faster than if you had only lowered to 7.6. Literally you are forcing it to swing from its desired position/location. How hard and fast the rebound is similar to an actual swing.

. Do reaction rates of offgassing or the NaOH generation go higher in a lower pH environment?

The outgassing causes PH rise so the SWG always gets blamed, but the extra aeration from the SWG (and therefore pump) runtime is what most folks notice. When they switch they go from 4 or 6 hours of pump time up to 12+. That's alot of circulation and they notice it in the raised PH.

 

[mgtfp]

Do reaction rates of offgassing or the NaOH generation go higher in a lower pH environment?

You don't have to worry about the NaOH generation. Yes, while the SWG produces chlorine, pH will increase. But as the chlorine is being used (for sanitation/oxidation of "stuff" and by UV-decay), that rise gets compensated, the chlorination cycle with a SWG is pH-neutral.

What creates the pH-drift, is CO2 outgassing. The carbonate buffer system (the main contributor to TA) means that the pool is at target pH oversaturated with CO2, i.e. more out of equilibrium with atmospheric CO2. The further away from equilibrium you are, the faster CO2 will outgas, increasing pH until the water reaches equilibrium (i.e. the same number of CO2 molecules per second is leaving the water as is re-entering the water from the atmosphere) somewhere above pH 8 - exactly how far above 8 depends on the TA (or more precisely the Carbonate Alkalinity contribution to TA).

Any aeration increases the surface through which outgassing occurs and therefore accelerates CO2 outgassing. The SWG contributes to aeration via the hydrogen bubbles coming out of the returns, and also by just running the pump longer to generate the required chlorine.

At pH 7.2 you are further away from equilibrium than at pH 7.6, the rate of pH-rise is therefore faster at 7.2 than at 7.6. Lowering pH to 7.2 buys you only very little extra time until the next acid addition.

Try to maintain your pH more towards 7.8. Add acid only once your pH is above 7.9, and only lower it to 7.6. That should slow things down.

What is your TA? If it is very high and the pH-rise is too fast, it can make sense to get TA down. This is something that often gets misunderstood - even though you get more pH-buffering at higher TA, i.e. a better protection against pH crashing too low when using acidic forms of chlorine like trichlor tabs, it also increases the rate of pH drifting up due to more CO2 being dissolved in the water.

Don't worry about pH being in the higher 7s, that is only an issue for pools without CYA (in this case you have not enough active chlorine left at pH 8). But with CYA in the water, the pH-dependency of the active chlorine concentration (HOCl) is much flatter. The FC levels in the FC/CYA chart are designed so that there is still enough active chlorine at pH 8. Just don't go above 8 as that increases the risk for metal staining.

 

[wtfbliz]

You don't have to worry about the NaOH generation. Yes, while the SWG produces chlorine, pH will increase. But as the chlorine is being used (for sanitation/oxidation of "stuff" and by UV-decay), that rise gets compensated, the chlorination cycle with a SWG is pH-neutral.

What creates the pH-drift, is CO2 outgassing. The carbonate buffer system (the main contributor to TA) means that the pool is at target pH oversaturated with CO2, i.e. more out of equilibrium with atmospheric CO2. The further away from equilibrium you are, the faster CO2 will outgas, increasing pH until the water reaches equilibrium (i.e. the same number of CO2 molecules per second is leaving the water as is re-entering the water from the atmosphere) somewhere above pH 8 - exactly how far above 8 depends on the TA (or more precisely the Carbonate Alkalinity contribution to TA).

Any aeration increases the surface through which outgassing occurs and therefore accelerates CO2 outgassing. The SWG contributes to aeration via the hydrogen bubbles coming out of the returns, and also by just running the pump longer to generate the required chlorine.

At pH 7.2 you are further away from equilibrium than at pH 7.6, the rate of pH-rise is therefore faster at 7.2 than at 7.6. Lowering pH to 7.2 buys you only very little extra time until the next acid addition.

Try to maintain your pH more towards 7.8. Add acid only once your pH is above 7.9, and only lower it to 7.6. That should slow things down.

What is your TA? If it is very high and the pH-rise is too fast, it can make sense to get TA down. This is something that often gets misunderstood - even though you get more pH-buffering at higher TA, i.e. a better protection against pH crashing too low when using acidic forms of chlorine like trichlor tabs, it also increases the rate of pH drifting up due to more CO2 being dissolved in the water.

Don't worry about pH being in the higher 7s, that is only an issue for pools without CYA (in this case you have not enough active chlorine left at pH 8). But with CYA in the water, the pH-dependency of the active chlorine concentration (HOCl) is much flatter. The FC levels in the FC/CYA chart are designed so that there is still enough active chlorine at pH 8. Just don't go above 8 as that increases the risk for metal staining.

Exactly the level of detail I was looking for. Makes sense, thanks so much!

TA has crept up with pH, working on getting it sub 100, this morning when pH was 8+ TA was up to 180. Lots of evaporation out here in the desert so trying to keep LSI neutral because the calcium can get pretty bad pretty fast.

 

[mgtfp]

Maybe just one more comment: The only situation where keeping pH in the lower 7s is important, is when you have very high CH, especially when in combination with high TA. In this case, a lower pH can be the only way to keep the CSI in range to avoid scaling. This can be a constant battle for those living in areas with very hard fill water, especially when it's also very hot with little rain, requiring at lot of fill water additions to replace evaporation losses.

But as long as you are able to maintain all water parameters in the TFP-recommended ranges, pH in the higher 7s is preferred.