The alkalinity keeps rising-idk why.....?

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Pmchick

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Mar 13, 2010
39
Hello,
My alkalinity has been on the rise, it went from around 100ppm to 140ppm in the past few weeks. I've been adding MA on a regular basis to try to lower it, I get it down to around 100, and by the next morning, it's hovering between 120-130ppm.
I wait 2 hrs after adding MA before I test to ensure that it mixes.
My pump is on 24/7 (commercial pool).
I use Co2 as the pH control, I know that will raise the alkalinity, but, why does it keep bouncing back after the addition of MA?

Current Readings using the ColorQ:
FC: 3.44
TC: 3.53
pH: 7.4
Alk: 126(I've tested this using the Taylor drop-count, and got 130ppm)
CH: 278
CYA: 0 (commercial "indoor" pool-cannot use CYA)
Temp: 86 degrees

This is a 126,500 gallon commercial pool with a retractable roof-which is open all day since it has been so gorgeous. The pool sees about 150 people/day(mostly kids)
Thank you for your help!
The pool uses liquid chlorine.
 
Injecting CO2 does not raise the Total Alkalinity (TA); it only lowers the pH. The TA is rising from the chlorine source you are using since chlorinating liquid has excess lye in it. The Muriatic Acid you are adding compensates for that and lowers the pH and TA.

Fundamentally, the problem is that your TA level is too high so there is a lot of outgassing of carbon dioxide that causes the pH to rise. You may also have a lot of splashing and aeration in the pool that accelerates such outgassing. If you were to let the TA drop to 80 ppm or even somewhat lower (assuming you are using a hypochlorite source of chlorine), you should find that the amount of CO2 you need add drops and the rate of pH rise may slow as well, especially if you target a pH of 7.6 instead of 7.4. If you find that a lower TA level helps, then you may need to raise the Calcium Hardness (CH) level to compensate for the saturation index to protect plaster surfaces.

The amount of Muriatic Acid you need to add, however, will not change since that is counteracting the excess lye in the chlorinating liquid and needs to be added to keep the TA from rising. The only way to prevent that is to either lower your chlorine demand through supplemental oxidation or to use a better brand of chlorinating liquid that has less excess lye in it. If you are using 12.5% chlorinating liquid, it should have a pH of 12.5 or lower. If it has a pH that is significantly higher, say 13 or more, then that will require more Muriatic Acid to compensate.

A table may be helpful to understand what is going on:

.............................. pH ... TA
CO2 Outgassing ..... + ..... 0
CO2 Injection ......... - ..... 0
Lye in Chlorine ...... + ..... +
Muriatic Acid .......... - ..... -
-------------------------
Net Result .............. 0 ..... 0

The first pair is reduced by lowering the TA level since TA is itself a source of pH rise due to carbon dioxide outgassing (it is also reduced by targeting a higher pH level as shown in this chart). The second pair is reduced by using a source of chlorinating liquid that has less excess lye in it so is lower in pH. You can see that the combination of carbon dioxide outgassing with the excess lye in chlorinating liquid would have the pH and TA rise over time and using CO2 injection alone would still result in a TA rise over time but that this does not come from the CO2 itself; it comes from the excess lye.

If you have actual chemical amounts added each day and their strength (for Muriatic Acid and chlorinating liquid), I can model what's going on, especially if you can find out the pH of the chlorinating liquid you are using.

Richard
 
Richard,
Thank you so much for your reply. :)
I use sodium hypochlorite as my sanitizer. I have a chemical controller that controls the chlorine & CO2 injections. It's meausures ORP, not ppm. How would I go about measuring the amount of chemicals that are injected into the pool each day?
I checked out your chart with the pH/alkalinity. If I raise my pH setpoint to 7.6, will that cause my ORP measurment to drop? I have a tough time with the ORP dropping when the roof opens, and than the FC rises to around 4ppm-the weekly lab gives me a hard time about this because my pool is considered "indoor" even though the entire thing is surrounded by glass & has a retractable roof...indoor pools in NJ must keep the FC at 3ppm max.
Anyway, will upping my pH to 7.6 decrease the amount of CO2 being injected which will in turn help to reduce my alkalinity level?
Thank you so much :).
 
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Well, part of the reason that indoor pools in NJ must keep the FC at 3 ppm max is that there is no CYA in the water and apparently they don't allow any so you have a real catch-22 there. You want a high enough FC to be able to manage and regulate the chlorine level more easily and keep up with the chlorine demand, but you don't want the active chlorine level too high since that results in faster oxidation of skin, swimsuits, hair, equipment and faster production of disinfection by-products. Oh well, maybe someday NJ and other states will realize the folly of their regulations. 4 ppm FC with 20 ppm CYA would probably work well and be equivalent to 0.2 ppm FC with no CYA but have plenty of chlorine in reserve, but you've got to follow your state and local regs.

As for figuring out your chemical usage, I assume you keep track of how often you refill the drums or containers with chlorinating liquid that feed the system and how much that is; same for the Muriatic Acid; number of cylinders of CO2 and quantity in each (pounds CO2). I wasn't asking for specific hourly usage -- just overall usage over an extended period of time since that's enough for me to figure rough averages.

You need to stop trying to hit an ORP target and instead set an FC target instead, such as 2 ppm, and then figure out what ORP setting is associated with that FC level under different conditions -- roof closed vs. open, no bather load vs. bather load. You would then change your setpoint as conditions change (or program different setpoints by time if your controller allows for that). I'm sure the regulations require you to test FC regularly (every 2 hours when the pool is in use?) and not just rely on ORP. Think of the ORP number as a control knob for regulating the FC level that you measure directly with your FAS-DPD chlorine test. The absolute ORP number itself is mostly irrelevant.

I would first let your TA drop to 80 ppm and then set the pH to 7.6 and yes, the ORP level will be lower but that just means you need to determine a new ORP setpoint that is lower. The important thing is to maintain a certain FC level -- not an ORP level. The amount of active chlorine in the water is WAY more than needed for adequate sanitation even at the higher pH. 2 ppm FC with no CYA at a pH of 7.6 has over 9 times the active chlorine (hypochlorous acid) concentration as a pool with 4 ppm FC and 20 ppm CYA. Even at a pH of 8.0, it has over 5 times the active chlorine concentration. This is part of the fallacy of the whole FC vs. pH and active chlorine level -- with no CYA the absolute level is so high that the large swings with pH don't get it very low while with CYA the pH swings are less but the absolute level is far lower (but still fast enough to kill pathogens).

In the worst case if the lower TA and higher pH target doesn't reduce your CO2 usage, you can easily go back to the way things were.

Again, I'd see if you can get an MSDS for your chlorinating liquid or ask the manufacturer for the pH of that liquid and if it's higher than 12.5, see if you can find a source with a lower pH that is comparably priced -- that will let you reduce the amount of Muritatic Acid you've been using. If this isn't a big deal for you, then don't worry about it.

Richard
 
Thanks, Butterfly :)


Richard,
I have a 150 gallon chlorine tank-it gets filled every 3 weeks.as for the CO2, I'm not sure how many pounds get filled, but, the company comes out every three weeks & fills it, I've never seen them use barrells. I will see if I have an invoice or if there's a label on the CO2 canister indicating the amount.
I do have the MSDS sheets for all chemicals, I will check yo see what the pH of the sodium hypochlorite is. Does the fact that the chlorine tank is stored outside in direct sunlight have any significance?

If I can get my alkalinity to 80ppm, I will definitely try raising the pH to 7.6, and hopefully that can help with
the chemistry swings. In the meantime, it seems that I would have to add alot of MA in order to get it that low, it's 130ppm today.
I completely understand what you're saying about the ORP, unfortenately, the AK-100 does not have a time setting where the setpoint could be changed based upon the time of day in order to maintain a certain FC level. Is it true that ORP systems are not really meant for outdoor pools as it is not an accurate reading of the actual FC level.
The readings are taken & recorded every 2hrs(NJ law). I do wish that they would permit CYA in indoor commercial pools, like you've said, maybe one day they will have someone with your knowledge writing the laws :)
So, should I drop my alkalinity to 80ppm? That would take probably about 8 gallons of MA....I'm just concerned about bringing the pH down too low. (pool must be opened 5:30am-10pm) I add my chemicals at night after closing & stick around for an hour to see the results, and than arrive before opening (5am) to make sure everything is ok prior to opening.
 
To lower TA you need to add enough acid to bring the PH down to 7.2, then wait for the PH to drift up again (possibly aerating to speed up the process), and repeat. This will cause large PH swings, which will require paying a lot of extra attention to the ORP system to maintain the correct FC level.

ORP systems are far from ideal. ORP levels don't correspond to FC levels unless you hold a large number of other things constant. This is simpler to do in an indoor pool than it is in an outdoor pool, as sunlight can affect the ORP reading to some extent (even assuming you were holding the FC level constant, the ORP would still vary based on the amount of direct sunlight on the water).
 
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