Pool/Spa combo with spillover-pH higher in spa. Why?

[salinda]

I have an issue with my pH constantly rising and I alway have had this issue. My TA is a little high, at 100, but no big deal. I have maintained the pool well over the past several years and I regularly pour in 1/2 gallon 1-2 times per week. The SWG is constantly bubbling and I believe it is the greatest source of aeration because recently, when I SLAMmed my pool (due to some green algae issues) and turned off the SWG, pH before and after the shock were fairly similar over several days. After I turned the SWG back on, the pH started increasing as usual.

I also have very high CH--about 750. We are perpetually in a drought and our fill water is pretty hard too so I only drain the pool in the winter when we get rain. We didn't get much rain this winter. I have run my pool with high CH for years and simply try to keep the pH <7.8. What I do have (and always have had) are what look to be Calcium flakes that settle to the bottom of my spa. I always keep a little pool water returning to my spa (I have a manual bypass that I use for that). The returns in the spa are near the floor and the spillover is on the opposite side and (of course) at the top of the spa. Thus, my spa is basically a settling tank where only the "scum" gets removed regularly but stuff that is in suspension will settle out to the bottom over time, forming "sludge." (I did some work years ago designing wastewater treatment plants and those really are the technical terms.)

The Calcium flakes are no big deal but I dislike that when I get in the spa, they get suspended and make the water turn cloudy. To mitigate this, every now and then, I manual drain the spa into the body of the pool using the controls at the Jandy control center and let it refill with nice clear pool water. I also vacuum them carefully with a manual spa wand. I wish I could drain the spa to waste but I have a cartridge filter so I don't have a valve for that.

To make things worse, my variable speed driver on my Ikeric filter pump is broken and doesn't turn the pump on consistently, so I have the pump (and SWG) running 24 hours, most of that time on a VERY low speed to save energy. My FC is 7 and my pool is very clear because I have a main drain and a suction-side PoolCleaner that sucks up any flakes that settle in my pool. Keeping the FC on the high side because I get a lot of sun and am expecting a large bather load with 2 parties coming up in the next 2 weeks.

Today it occurred to me that I might want to test the pH of the spa water to see how different it is from the pool water. I had just added acid and the pool water was about 7.1. The spa was 7.4. This was after letting the acid mix in for an hour.

Questions:
Why is the pH in the two vessels so different? Aren't they one body since I have the spillover trickling all the time? I think I know the answers but I'd love to see if my theories pass the smell test with some of you:

Maybe it takes longer for the pH to change in the spa after an acid addition? Should I add some directly to the spa when I add to the pool? Is the pH always higher in the spa because the water is a little bit warmer (pool around 85 degrees and spa around 93 degrees)?

 

[JasonLion]

The PH goes up because of aeration. The spa presumably has way more aeration than the pool, so it's PH goes quickly. It can't get too far out of sync with the pool, because they are constantly mixing, but it is easy for the spa to be higher all the time.

You can dramatically reduce or even eliminate the PH increase by lowering your TA to around 60, or letting it drift down over time.

 

[chem geek]

Hotter water outgases carbon dioxide faster so will make the pH rise faster. It also increases the saturation index so between the hotter temperature and the higher pH there will be more likelihood of having calcium carbonate form or for existing flakes to take longer to dissolve (if they dissolve at all). If you run spa jets or even circulation in the spa then that is proportionately more aeration per volume than found in the pool.

I would not say that your TA of 100 is only a little high. In an SWG pool, most people find they need to have the TA down much lower to around 70 ppm (and that's with 80 ppm CYA so the carbonate alkalinity is quite low) in order to have more stable pH. Also, having a slightly negative saturation index will help reduce the calcium carbonate flakes forming in the SWG. Finally, using 50 ppm Borates will also help reduce the rate of pH rise and also reduce the formation of calcium carbonate flakes in the cell a lot since it cuts down the amount of pH rise in the cell roughly in half.

The saturation index in your pool, assuming 80 ppm CYA, and 3000 ppm salt (and your 750 ppm CH and 100 ppm TA and 85ºF temperature) is +0.14 when the pH is 7.5 (you won't keep your pH at 7.1 for very long). In the spa at 93ºF, it would be +0.20 (again at pH 7.5).

Is there a reason why you don't try lowering your TA level (perhaps your fill water is high in TA so it's pointless with evaporation and refill)? Are you using 50 ppm Borates?

 

[salinda]

I did lower my TA over a couple of summers. This spring we had serious health issues in our house and I had no bandwidth for the pool. I called in a service company and I think they INCREASED THE ALK!!!! I didn't realize that until I got back in the swing of things this past month.

I'm trying to get TA low again. It will happen with patience and A LOT of acid.

FYI the spa is not actively heated most of the time but holds a higher temp with the little bit of solar heated fill water going into the smaller body.

I really appreciate your help. Should I add some acid directly to the spa when I add it to the pool?

 

[Richard320]

I'd just bite the bullet and run more water through the spa and aggressively lower the TA that way. When it settles down, then reduce the spa flow back the way you've been doing.

About once a week I move the valves around and empty the spa into the pool and brush any thing in the spa at the drain. Then I don't need to vacuum it. And then the real beauty: I vacuum the pool while returning all the water to the spa. There are no cross currents or ripples to interfere with my vacuuming. If I move fast, I can get it all before it starts to spill.

 

[salinda]

Richard, that sounds like what I usually do. I WILL open the valves to the spa more for a little while. Unfortunately, I can't turn my pump off to change it to service mode to drain the spa right now. It's complicated — my Ikeric controller is dying so as long as it's running, we are leaving it on until the new pump gets installed this week. Draining/refilling the spa is ideal but I can't do it right now.

 

[Richard320]

Richard, that sounds like what I usually do. I WILL open the valves to the spa more for a little while. Unfortunately, I can't turn my pump off to change it to service mode to drain the spa right now. It's complicated — my Ikeric controller is dying so as long as it's running, we are leaving it on until the new pump gets installed this week. Draining/refilling the spa is ideal but I can't do it right now.

I'm so happy I have primitive manual valves on my pool.

 

[salinda]

Chem Geek: Forgot to respond--I can't use Borates because one of my dogs swims a lot and seems to inhale water as she swims--I know because it comes out later when she is either vomiting water, peeing like mad, or incontinent. My TA is slowly drifting down. It is now between 80-90. The pH is already more stable.

- - - Updated - - -

Richard: Yes but my pool was exceptionally clean and my power use was not very high because I just turned the non-solar speed on the pump to VERY low. As of yesterday, I have a new pump.