Calcium Hardness and Reverse Osmosis

[LisaDLu]

Hello - Got my pool replastered in January and calcium hardness has been creeping up since then. Todays readings are:

PH 7.6
TA 50
FC 7
CC 0
CYA 80
CH 850
Salt 3700
Temp 56
CSI -0.45

The CH of our fill water is 125. We do not add anything to the pool except for acid, which was daily for months and in the last six weeks we have only had to add it every 2 or 3 days.

I have no idea why my CH has risen so much since we replastered but I'm assuming it is the plaster curing. We live in drought stricken CA and have large penalties for going over our monthly allotment of water so drain/refill is not an option. My questions are:

1) What are the optimum levels I need to keep everything at so as not to cause problems? What number should I aim for with the CSI?
2) Has anyone ever done reverse osmosis (there is now a company in my area that offers it) and does it really work or does it cause other problems?

I'm trying to figure out the best course of action before my CH reaches 1000. Thank you!!

 

[JoyfulNoise]

My CH is near 800ppm and I'm fine. You can consider adding borates to your pool as that will help mitigate the pH rise (it adds buffering capacity to the water).

What plaster start-up process was used by the plaster company? That is most likely the cause of your high CH.

 

[chem geek]

Proper plaster curing does NOT result in any CH increase. Instead, the calcium stays in place in the plaster and bicarbonate in the water is exchanged with hydroxide. If CH is increasing due to the plaster, that is bad and means that your plaster is dissolving calcium carbonate into the water. Though this happens in an acid startup, it doesn't happen with a bicarbonate startup and also should not be happening many months after your plaster was done, certainly not after the first month.

The CSI target we say to have for an SWCG pool is slightly negative so around -0.2. If you've been running significantly lower than that, then you may have been dissolving your plaster.

You have very dry air so low humidity and high evaporation rates. You probably have around 100" pan evaporation per year. If your pool is an average 4.5' depth, then you have nearly two full pool volumes of water evaporate so would add double your fill water CH level so would increase by 250 ppm. Are you sure your CH increase isn't accounted for by this evaporation and refill?

You can distinguish between plaster dissolving vs. evaporation and refill by noting your pH rise and amount of acid you've had to add. If the plaster was dissolving to add 250 ppm CH to the water in 17,000 gallons, then your pH would have been rising frequently such that you would have needed to add cumulatively (over that time) about 4-1/2 gallons, but your TA would be high having increased 120 ppm. So you could have added more acid to combat more pH rise from carbon dioxide outgassing. Does that sound like your situation? If not, then the CH rise is more likely from evaporation and refill.

 

[LisaDLu]

Chem Geek - It probably is due to evaporation and refill (during our long hot summers I am topping off the water for at least 15-20 minutes at least twice a week but that has slowed down now the weather has cooled off). But in your reply you say something about adding 4.5 gallons of acid cumulatively over the time it takes for evaporation but since January we have actually added about 26 gallons of acid, of course we heat up significantly around April/May so I have been refilling frequently until about a month ago and not sure how much total water I've had to add since January. Never have had TA above 80 since replaster and not above 70 since April.

I've thought my high CH was due to refill but it has risen so much since January that I thought I might be doing something wrong adding so much acid with trying to keep it below 7.8.


It sounds like I need to try to bring my CSI up a little. Yesterdays calculations show me at -0.45 and your saying I should hover around -0.2.

We are down to adding acid about every 2 or 3 day now and only about 10 ounces when needed. I don't really want to get into Borates but am I adding to the problem with having to add acid so frequently?

Any opinions if I should try reverse osmosis (pros and cons) or should I just keep striving for the right CSI? When I can't keep it at the right CSI then it is time for the reverse osmosis.

 

[LisaDLu]

Just played around with Pool Math to see how to bring my CSI up to -0.2 and it says to add 72oz weight of baking soda. I've read that if you have high CH you want to keep your TA lower (mine is 50). Now I'm confused. What levels should I be looking for with TA, PH or anything else that will affect the CH and CSI adversely?

 

[duraleigh]

Now I'm confused.

Go back to the basic ranges we teach in Pool School.

Since your CH is outside what we suggest, lower your pH down to around 7.2 - 7.3 to compensate somewhat.

You will then have to bring TA back up with baking soda to get it back around 50-60.

I have no idea what your csi will then be but I think it will be negative but not enough to be in any danger.

Ultimately, reducing the CH with a water exchange will get your pool in even better shape.

 

[chem geek]

So 26 gallons of acid for a 17,000 gallon pool since January is 2.4 gallons per month or a little more than 1/2 gallon per week. It's understandable to add a lot of acid soon after a replaster with most being added in the first month but then should noticeably trail off over the next months. If your plaster was curing properly, then the pH would rise with no change in TA, just as if carbon dioxide outgassing were occurring -- the two processes are indistinguishable because in effect the proper curing of plaster takes carbon dioxide out of the water.

Another possibility, which would not be good, would be that you had calcium hydroxide from the plaster curing go directly into your pool water so not cured in place. That has the pH rise the most with no change in TA. With CH increasing by 250 ppm in your case, this would take 8.5 gallons of acid. So you seem to have been adding a lot more acid than would be explained by even the worst-case scenario with the plaster. So while the rise in CH wasn't good if it were from plaster, at least some of that could have been from evaporation and refill. If the rest of the acid were due to plaster curing in place, then that would be a good thing. To sum up the possibilities in your pool, there's the following (each scenario assumes it is the sole source of CH rise and shows acid in gallons):

.................................................................................. CH .. Acid .... TA
GOOD: Plaster Curing in Place ...........................................0 .... ? ........ 0
BAD: Plaster Calcium Carbonate Dissolving.................... +250 .. 4.4 . +120
BAD: Plaster Curing and Calcium Hydroxide Dissolving ... +250 .. 8.5 ....... 0
OK: Evaporation and Refill .......................................... +250 ... ~0 ...... ?
OK: Carbon Dioxide Outgassing ........................................ 0 .... ? ........ 0

The "?" means there will be acid needed but the amount is variable and unknown. You can see that you can't distinguish between the desired plaster curing in place and carbon dioxide outgassing. The only way to tell is to eliminate the outgassing possibility such as using a pool cover in which case if the pH rise stays roughly the same then it's due to plaster curing in place.

I think you were lowering your TA because you thought it was the cause of the rising pH when instead most of the rise may have been due to the plaster curing (most of it in place as it should be) and lowering the TA wouldn't help with that and would actually hurt in that the pH would rise faster (though the amount of acid added over time would not change in that case). Just to be sure, do you have large sources of aeration in your pool such as waterfalls, spillovers, or fountains? Do you use a pool cover? If you don't have lots of aeration or if you have a pool cover, then raising your TA to the normally recommended level would be helpful. You should not need to lower your pH as Dave suggests even if you were to get the TA to 70 ppm. Your water is cold and should have a higher pH naturally. No need to go lower than recommended in Pool School since those recommendations do not account for winter cold water temperatures. When the water warms up next season, then you can consider adjusting your pH down if necessary, but right now that's not needed and even if you do that next season you wouldn't lower it below around 7.4 anyway -- 7.2 to 7.3 would be too low to compensate for a CH that is 850 ppm compared to the Recommended Levels (mid point of 400 ppm CH). A doubling of CH increases CSI by 0.3. To lower it via pH means lowering pH by 0.3 (so from 7.7 to 7.4).

 

[LisaDLu]

Thank you everyone for the detailed responses. I am going to assume my rising PH is from the "good" and "ok" scenarios Chem Geek describes. I do not have any waterfalls, spillovers, or a pool cover. I'm going to add the recommended amount of baking soda and take it from there. Hopefully getting the TA up some will help with the CH rising especially since I'm not refilling much at all now during the colder weather. I am going to call the reverse osmosis company and see how much they charge for that because I'm sure that will be in my future. Thank you everyone!