High Acid Demand

[lnorris]

I am having to add acid almost daily with a pH rise of .2-.3 points a day. Is this normal for a newer pool? I tried to lower the TA but the pH and TA rise so quickly the additional aeration doesn't allow me to gain much on the TA. SWG is set at 30% and I have a slight trickle out of the spa most of the day with the pump on low. Is lowering the TA the best option or should I adjust some of the other numbers? Thanks.

FC 4.5
CYA ~75
TA 80
CH 425
pH 7.6
Salt 3300
Borate ~45
Temp 84f

 

[JasonLion]

For the first three or four weeks after a fresh plaster/pebble surface is applied the PH, TA, and CH levels will all rise rapidly. This rise continues, though at a much slower pace, for up to a year.

 

[lnorris]

The pH rise has actually increased since the first month with the temperature. The pool is about 3 months old now. Will attempting to drop the TA to 60 help?

 

[JasonLion]

Yes, dropping the TA down a bit, to say 60, should reduce the amount of PH increase you are seeing. But there will still be some PH increase from the plaster.

 

[flintstone]

It is normal to chase rising pH with new plaster. Add an SWG and your rate of rise increases a bit more. We have similar pool equipment, and I have noticed a few others on this site who also have similar finish and equipment are always chasing rising pH. Always, as in forever. There has been no definitive reason for the continual rise other than the usual suspects. I have found as the water temp cooled and I adjusted the SWG downward, my demand for acid also decreased. I went from a cup a day in the spring and summer to a cup a week or less over the winter.

So, I keep my CYA level up to reduce chlorine demand thereby reducing rising pH and the need to add more acid. I have come to accept my pool as an acid user. Other than one series of unfortunate events, I have never had to shock or add bleach. I've registered .5 CC only twice (when I neglected to clean the pool cleaner).

Goldline recommends an FC level of 1-3, but this site, which is full of conventional wisdom, recommends a bit higher level. I have decided to live dangerously and try an FC level of 2 and see if I can keep acid demand down a bit more. I do have a light swimmer load. We'll see.

 

[Guest]

rising pH and TA is to be expected for the first year with new plaster. Nature of the beast! It will settle down and then you can tweak the water chemistry to lower your acid demand as much as possible. Your CYA and TA are the key here once the plaster cures. For now all you can really do is keep on top of it and keep a lot of acid on hand!

 

[dschlic1]

It is my belief that on some pools there is very little extinction of chlorine, rather all of the chlorine demand is used in combining with complex organics. This results in a net loss over time of chlorine ions from the pool. This loss of chlorine causes the pH to rise. I have found that my acid demand is linear with my chlorine production. If I double my chlorine production, I need to double my acid addition. I geese this is something I will just have to live with. As it is I spend less than $3 per month on acid. My daily chlorine demand is between 1.5 and 2.0 ppm.

 

[Guest]

It is my belief that on some pools there is very little extinction of chlorine, rather all of the chlorine demand is used in combining with complex organics. This results in a net loss over time of chlorine ions from the pool. This loss of chlorine causes the pH to rise. I have found that my acid demand is linear with my chlorine production. If I double my chlorine production, I need to double my acid addition. I geese this is something I will just have to live with. As it is I spend less than $3 per month on acid. My daily chlorine demand is between 1.5 and 2.0 ppm.

This is not quite right.
The reaction of hypochlorous acid to chloride ions (when chlorine gets consumed oxidizing organics) is actually slightly acidic and will cause pH to drop. It is nearly equal to the pH rise caused by using chlorine in the form of sodium hypochlorite thus the net effect of using liquid chlorine is nearly pH neutral! Also, the degradation of hypochlorous acid from UV is also slightly acidic if I am not mistaken (Richard (chem geek), correct me here if I am wrong!) so it really doesn't matter if the chlorine is consumed by UV or oxidation of organics.

 

[MikeInTN]

It is my belief that on some pools there is very little extinction of chlorine, rather all of the chlorine demand is used in combining with complex organics. This results in a net loss over time of chlorine ions from the pool. This loss of chlorine causes the pH to rise. I have found that my acid demand is linear with my chlorine production. If I double my chlorine production, I need to double my acid addition. I geese this is something I will just have to live with. As it is I spend less than $3 per month on acid. My daily chlorine demand is between 1.5 and 2.0 ppm.

I would think that your increased acid demand is more a function of your increased use of your SWG. The additional aeration from the increase in use would be the culprit.

 

[chem geek]

Yes, the consumption/usage of chlorine is acidic and exactly compensates for the rise in pH from initial chlorine addition from hypochlorite sources of chlorine or from an SWG. So normally such chlorine addition and usage would be pH neutral over time (technical details are in this post). This is generally the case in pools using hypochlorite sources of chlorine and in pools with minimal aeration, such as those with pool covers (including my own pool). Some chlorinating liquid and bleach has more "excess lye" in it so this can lead to a small rise in pH over time of around 0.1 to 0.3 units per month, depending on specific product and quantities that are used. Any higher pH rise is due to carbon dioxide outgassing which is faster at higher TA, lower pH and with more aeration.

As for SWG pools, we don't for certain know the source of rising pH, but two factors that are the most likely include the increased aeration from hydrogen gas bubbles and the escape of undissolved chlorine gas. So turning down the SWG on-time would reduce both of these effects thereby lowering the rate of pH rise. This is why we suggest using a higher CYA in pools exposed to sunlight so that there is less chlorine loss and therefore less need for higher chlorine production (i.e. SWG on-time). The same is true for the use of 50 ppm Borates which may help inhibit algae growth thus allowing for lower chlorine loss and therefore lower chlorine production that is needed (again, lower SWG on-time).

Richard