Accuracy of FC test with a rising pH

[ringfinger]

Apparently my boss quit paying the Co2 bills and our tanks ran empty right before a water polo tournament.

How accurate are my FC tests going to be as the pH rises? I know the chlorine is less effective, but I want to know how I should interpret the test results.

Any other short term concerns until we can get some acid or a Co2 refill?

Currently:
FC: 4.2
pH: 8.0
TA: 170
CH: 275
CYA: 0 (indoor)

Thanks!

 

[JasonLion]

It looks like you are using a FAS-DPD chlorine test, which works just fine at any swimmable PH. Given that CYA is truly zero, your main issue is going to be that less and less of the FC is actually available to disinfect as the PH goes up. Still, given FC is currently 4.2 you should be just fine and good for some additional PH increase.

You can use the chart in this post to estimate the percentage of active chlorine.

Your next problem is that you are moving into range of significant chances of calcium scaling. If the PH goes up to 8.4 you will almost certainly see calcium scaling, if it hasn't started already. Calcium scaling takes some time to happen, so you have a little while, but if this is not dealt with fairly soon there is going to be an expensive problem to deal with.

 

[ringfinger]

Thank you!

If I'm understanding the charts correctly, I should continue to keep FC in the 4 ppm range to ensure a high enough percentage of active chlorine, right?

We should have acid and/or Co2 by Monday. Hopefully that is soon enough. Maybe I could run to Home Depot for some 1 gallon jugs....but this is a 600k gallon pool.

 

[JasonLion]

I don't know what your normal PH and FC levels are, but FC around 4 seems reasonable. Chlorine is just a little less than half as active as it would be if the PH was 7.5.

You might want to mention that you are managing a commercial pool in your signature. In some ways just saying 660k gallons is nominally enough, but we deal with residential pool nearly all the time, so making it clearer that this is a commercial pool will help people coming along to read these posts.

 

[chem geek]

You needn't worry about your active chlorine level since it is far, far higher than needed for adequate disinfection, even when the pH is high.

For a commercial/public pool, let's use an FC that is 20% of the CYA level as a standard -- that's equivalent to an FC that is 0.2 ppm with no CYA at a pH of 7.5. This is half the EPA DIS/TSS-12 standard that is roughly 0.4 ppm FC and is overkill with 6-log (99.9999%) reductions in two common bacteria in 30 seconds. The pH would have to get to around 9.1 to have equivalent disinfection with FC at 4.2 ppm. So, bottom line, your concern shouldn't be about disinfection.

As Jason noted, your concern should be more about calcium carbonate scaling since with your numbers and assuming 80ºF water temperature your calcite saturation index is +0.74 which is pretty high.

Is there any particular reason you keep your FC so high especially with no CYA in the water? It's going to oxidize swimsuits, skin and hair and have metal corrosion rates around 2-4 times higher than a pool operating at 1-2 ppm FC with no CYA and around 10-20 times higher than a pool with an FC that is around 20% of the CYA level. Are you in a state that bans the use of CYA in indoor pools? Do you have any forms of supplemental oxidation in your pool such as ozone or UV (really more for chloramine reduction and Crypto control) or non-chlorine shock or enzymes? Do you use any coagulants such as alum floc? Do you have air systems for blowing air across the pool surface to remove volatile chlorinated by-products?

 

[ringfinger]

FC is high because everything is automated. pH started to rise when the Co2 tanks ran empty, and as a result extra chlorine was added to get the change in ORP the computer shoots for. We usually run FC at 1.5 ppm.

We can use CYA as far as I'm aware, I just never considered it for an indoor pool.

The only supplemental oxidation we use is a UV system. It's big, runs 24/7.

We don't use any coagulants.

Air circulation is very good, never any hint of chloramine odor.

 

[chem geek]

So ORP is affected by pH beyond just the change in HOCl level so until your CO₂ is working again or you otherwise lower the pH, you can adjust your ORP setpoint so that your FC doesn't get so high -- you can still have your FC in the 1-2 ppm range and you'll still have more than enough disinfection. The 0.2 ppm equivalent at 1 ppm FC would occur at a pH of 8.4; at 2 ppm FC it would occur at a pH of 8.75. Of course, as we noted, you'll want to get your pH down to prevent scaling anyway.

Some ORP sensors don't do well at lower active chlorine levels or with CYA in the water so probably best not to chance that unless you know it will work.

For the UV to reduce chloramines in a high bather-load pool, you probably have a lot of turnovers -- 24/7 helps, but you may have fast turnovers as well. Each turnover may only reduce chloramines by around 10-15% unless it's much larger than normal systems (i.e. in the many hundreds of mJ/cm²; most systems are in the 60 mJ/cm² range though 120-140 when new).

 

[JamesW]

One thing that I think would help is to lower your TA by quite a bit. Running it at 170 will make your pH rise like crazy and cost you a fortune in CO2. I would recommend that you use muriatic acid to lower the pH until the TA gets down to about 60 ppm.

See the Lowering Total Alkalinity post in Pool School.

 

[chem geek]

Note that CO₂ will lower the pH without lowering the TA. So by using hypochlorite sources of chlorine that have a small amount of excess lye in them, the TA will rise over time. So the proper way to manage a pool in this situation is to use an acid such as Muriatic Acid to counter the lye in the chlorinating liquid and then use CO₂ to counter the outgassing of carbon dioxide. As James noted, if you maintain a lower TA, then there will be less CO₂ outgassing and therefore less CO₂ needed to be injected. So a facility should be using both forms of pH control -- acid to keep the TA down and CO₂ to keep pH low beyond that which the acid takes care of. Perhaps the following table can help in understanding this:

Action ............................... pH . TA
Lye in Chlorinating Liquid ....... + ... +
Acid Addition ........................ - .... -
High TA, Low pH, Aeration ..... + ... 0
CO₂ Injection ....................... - .... 0

So to reduce the amount of pH rise from CO₂ outgassing, you want your TA to be lower (80 ppm or below) and your pH target to be higher (around 7.7 or 7.8 rather than 7.5) and to reduce (if possible) sources of aeration (spillovers, fountains, waterfalls, etc.).

 

[JamesW]

I would even suggest that you could probably eliminate the CO2 altogether. Any hydroxide from excess lye could be neutralized with muriatic acid with no net change in pH or TA.

 

[chem geek]

Usually in an active pool there is enough aeration of the water to have carbon dioxide outgassing and therefore require carbon dioxide injection to balance the pH. One may need to have the TA be quite low to minimize this and for a plaster pool that means a higher CH and pH target. One could try that, but in commercial/public pools there are cost considerations and one must also follow the state (or county) regs that may prevent very low TA levels. At any rate, the important thing is that ringfinger now understands the relationship and can use that knowledge to tune his use of carbon dioxide vs. acid to what works well for this pool.

 

[ringfinger]

Excellent. Thank you all for the help.

At the moment we only use muriatic acid once every couple months to bring TA back down around 120. Based on the advice here, I think we'll start moving in a direction where we use Co2 and acid together to keep TA and PH a little more stable. I know our Co2 tanks work really hard as it is to keep PH down.