Need to lower CH from 750 to around 400

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T

TheOne

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Mar 28, 2007
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Houston, TX
This past Friday (2 days ago) I had my aggregate stone finish IG pool acid washed to smooth out the color consistency of the interor pool finish. I was out of town when the work was completed but I know they lowered the PH down considerably during the process.

Since yesterday afternoon I have had my filter pump on high speed (80+gpm). My CH (750) and TA (225) are both way high due to the work that was done. I am doing the aeration cycle along with acid to lower my TA. I am thinking I should just drain a portion of the pool and attack the CH first. The only way I know to lower CH is by draining water. My fill water has the following specs PH=8+, CH=100, TA=140.

Does anyone know an approx amount of how much water I should replace if I have a 15250gal pool with CH of 750 and I want to bring it down to 400? My fill water has a CH of 100.

Any help would be appreciated thanks.

Currently the water is at ph 7.4, TA 225, Ch 750. I am not concerned with FC at the moment but it is at 5ppm+ in a shock cycle since the filter pump was off for a few days.
 
Does anyone know an approx amount of how much water I should replace if I have a 15250gal pool with CH of 750 and I want to bring it down to 400? My fill water has a CH of 100.
Click to expand...
400 = Fraction * 750 + (1 - Fraction) * 100
400 = Fraction * (750 - 100) + 100
400 - 100 = Fraction * (750 - 100)
300 = Fraction * 650
Fraction = 0.46 or 46%

So you would need to dilute more than half of your pool water to lower the CH from 750 to 400. If you used continuous dilution instead of a partial drain followed by a refill, it would take 1 - LN(0.46) = 0.78 or replacing 78% of the pool water volume by simultaneously draining from one end of the pool while filling from the other (assuming good mixing). Clearly, the partial drain and refill, using the plastic sheet or silage bag methods if desired, uses less water.

Richard
 
completed the (re)fill late last night and aerated all night.

Tested the water this morning..

CH was only down to 500 which was a little dissapointing. My pb has 500 listed as the max so I may leave the CH alone for now.

PH was at 7.6 so I added enough acid to bring it down to 7.0. I will be aerating all day with my filter/circulation pump at 80gpm and test again later today.

TA was down to 160 after the water change. Currently working to get this down with the acid/aeration cycle.

I added Borax previously to stabilize ph but never noticed any benefits. Is borax still recommended to stabilize ph?
 
I don't think there is anything wrong with a higher CH so long as you lower the TA to compensate. In fact, having the higher CH will let you operate at a lower TA. I don't know your current Cyanuric Acid (CYA) number -- the target TA is dependent on the CYA since it is the adjusted TA (that is, the carbonate alkalinity) that is important for water balance.

For example, if the CYA was 80 ppm, then you could have a pH of 7.5 with a TA of 80 ppm and be slightly negative in the saturation index (-0.15) which is where you want to be to avoid scaling in the salt cell. Alternatively, if you target a pH of 7.7 with a TA of 60, then this would have an even lower tendency of the pH to rise. On the other hand, if you had a lower CYA level, then you would have lower TA levels to be in balance -- at a CYA of 30 and pH of 7.5, the TA would be 65 ppm insetad of 80 ppm.

The Borates will reduce the frequency of pH rise, but not the amount of acid needed per time (say, per week) unless the Borates are inhibiting algae growth that was using up some of the chlorine that is generated. In other words, the main way that the Borates help reduce the rate of pH rise and amount of acid you have to add is by letting you lower your SWG output since the Borates are an alagecide so help lower chlorine consumption.

The even bigger factor is the CYA level. If it's in the 70-80 ppm range, then it protects chlorine from breakdown from sunlight much more than one might expect and that let's one lower the SWG output. Lowering the SWG output lowers the hydrogen gas bubble production so there is less aeration and therefore less carbon dioxide driven out of the water since that is what causes the pH to rise. Though such a high CYA level is riskier in manually dosed pools since it's harder to fight an alage bloom at such levels, in an SWG pool the risk is low if one maintains an FC level of at least 4.5% of the CYA level (so 3.2 - 3.6 ppm FC for 70-80 ppm CYA).

Richard
 
I ran out of my CYA test kit and need to get another one. Since it has turned colder here this week I considered just using pucks since I know my CYA and salt is way low due to the water change. I didnt realize the CYA reading had a big impact on TA. My swcg manual recommends a 75ppm CYA level.

This past summer my IC40 salt cell was set at only 20% and I usually had 3+ppm FC. Next season I may try running it even lower to minimize the ph rise. I didnt have any algae problems at all this season and I never shocked all season except after my recent pool finish work.

The aeration/acid process is working. My PH was back up up to 7.6 this afternoon and my TA has dropped further to 145 from 160 yesterday. I added more acid and will check it in the morning. I would like to get the TA back down to around 100.
 
PH at 7.4
TA at 90
CH at 450
CYA at 40
FC at 1

this morning....process complete. Thanks for the info on how to properly lower TA. The pool water looks great.

Also Im glad my PB installed the 2 sprinklers in the pool so I can aerate easily.
 
Congratulations on getting the TA lowered. As for your chlorine level, I would keep it at least at 4.5% of the CYA level so around 2 ppm in your case (with an SWG). If you find that the SWG is on a lot to maintain that level and your pool is in sunlight, you should consider raising the CYA to the 60-80 ppm range with an FC of around 3.5 ppm. Many users find they can actually lower their SWG output even with the higher FC as the high CYA has some non-linear extra protection effect (that we're still trying to figure out). Between the lower TA level and the lowest possible SWG output, your pool's pH should be more stable and the amount of acid you need to add will be significantly less.

It is also possible for the ozone to be breaking down chlorine (oxidizing it to chlorate) and with an SWG there should be absolutely no need for an ozone system. If possible, you might try turning off your ozone system and seeing if the FC level rises in which case you can lower your SWG output level.

Richard
 

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Using the ozone system as a supplemental oxidizer is supposed to allow me to lower my FC levels as my FC is mainly used as just a sanitizer. I know there is much debate about that and am certainly not an expert on the chemistry behind it. On my system the ozone is injected into the flow just prior to the pump/filter then the water passes through the heater then swcg on the way back to the pool.

I adjusted my SWCG setting down multiple times this past summer to try to minimize ph rise. My fill water is also ph 8+ so I have that issue as well. I turned off my autofill for now so I can see how that affects ph rise. I also normally have some return flow directed to the spa for spillway effect. That may also be causing the ph to rise as well. I may limit that more and see how that affects ph rise. My pool is apparently pretty low maintenance as compared to others but if I can slow my ph creep that would be great.

Since my salt and cya is low (from the water change) I am considering using my erosion chlorinator or bleach over the winter. This is my first winter with a pool. I would think with the water temps lower there would be less chlorine demand due to less potential algae growth and minimal if any bather load. It looks like I am only dropping .5ppm a day with the SWCG and Erosion chlorinators completely off. The past couple days have been full sun with water temps in the low 70's.
 
The superchlorination in the SWG cell also oxidizes organics, but you are right that ozone is a more powerful oxidizer and in theory that should reduce chlorine consumption. But many users with ozonators report chlorine loss and ozone can theoretically oxidize chlorine to chlorate. Also, the ozone will turn to oxygen and those bubbles also aerate the pool water so it doesn't really help in that regard. That's why I suggested at least trying it off to see what happens. It sounds like that's something to try for next year as you are about to close the pool. Yes, the cooler temperatures will lower the chlorine requirement and at some point the temps will be too cold for the SWG to run at which point manual chlorination will work. If you have a pool cover, you may find that chlorine lasts over a month at a time in colder water. That's the situation with my pool -- if sunlight is kept away (via an opaque cover -- at least opaque to UV) -- then chlorine consumption is very, very low in cool water.

Richard
 
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