Commercial Pools - Need to Cut Down on Acid Use

[bgriffith1990]

I am the Aquatic Coordinator at a collegiate fitness center with two indoor pools and I have worked here for around 6 months. Maintaining proper chemical levels has been easy, but in many ways I am still learning the ins and outs of how these pools truly operate. My signature contains the specs for my pools.

I manually test the chemicals twice a day (Taylor K-2005), but our chlorine and acid are regulated through an automatic feeder Chemtrol machine. The acid is pumped from a 65 gallon tank that is diluted with a 5:1 water-acid ratio. I have recently noticed that we seem to be going through a lot more acid than we have in the past. In the beginning, dropping the tank 7 gallons from one day to the next seemed to be pretty standard. Lately, there have been numerous days where the tank is dropping 15 gallons overnight. The PH has steadily remained between 7.5-7.6 without hardly any change. Alkalinity in both pools is about 90-100. I am having to re-fill this tank twice a week and just feel like I should be using significantly less acid. Any suggestions?

 

[TomBorg]

Did you recently boost your Total Alkalinity? The higher your TA, the faster the pH will rise, and the more acid you use.

Sadly, with a commercial pool, you have to follow your state/county guidelines for minimum TA, usually around 80ppm. This level is fine if you are using an acidic form of chlorine like trichlor tabs, but it is a pain in the rear if you are using a relatively-pH-neutral form like liquid chlorine. Just don't raise your TA much above what your local health code requires.

 

[TomBorg]

Another thought... Did you recently adjust your control system to maintain a slightly lower pH? The lower your pH, the faster the pH will rise, until it reaches an equilibrium defined mostly by your Total Alkalinity. Perhaps setting the controller to maintain the pH between 7.6 and 7.7, instead of 7.5 to 7.6, would reduce the acid requirements sufficiently?

And increased bather load will create increase aeration, which will increase the rate of pH rise due to off-gassing CO2. But since you have indoor pools, I imagine your bather load has remained fairly consistent?

 

[ping]

Another slim possibility is that they delivered 1/2 strength acid with what sounds like double the amount of acid use.

 

[bgriffith1990]

Thanks for the replies!

The alkalinity may be the answer. I did recently add a significant amount of baking soda in an attempt to raise it. I knew that raising TA also slightly increased PH, but I did not think I would notice such a change in acid use. My whole hope was to raise TA around 120 and that it would keep the PH level steady longer, therefore causing me to use less acid. It seems the opposite has occurred and my thinking was backwards. Our pools are in Illinois and code states that TA must remain between 50-200. Would you suggest I allow my TA to drop closer to 50-60? We do indeed use chlorine tabs.

To address the other points...Yes, our bather load has remained consistent. In fact, the biggest change was over this past weekend and we are completely closed on Sundays. As for the Chemtrol machine, my PH sensors have been fairly sporadic ever since I arrived. I am constantly needing to re-calibrate them based on my testing results. I have cleaned the sensors, I just think I need completely new ones...something I plan on obtaining in the new fiscal year. However, I keep a pretty close eye on them and don't think that has caused increase in acid use. I definitely get what you are saying....if one of my pools was re-calibrated to 7.8, it would then start pumping more acid since my setpoint is 7.5.

Which I suppose brings another question....I have seen many posters here suggest 7.7 - 7.8 as the recommended level. Would you recommend I keep my setpoint at 7.5?

As for the last comment about 1/2 strength acid....that is definitely a possibility because we just got a new shipment in last week. The boxes looked the same, but I will look at them more closely when I return tomorrow. But I am thinking my attempt at raising alkalinity may be the answer to my issue.

 

[JasonLion]

Higher TA has two effects, it both stabilizes PH, and puts upward pressure on the PH. If you have a fair amount of aeration, higher TA will cause the PH to rise more quickly. Raising your PH set point will reduce the amount of upward pressure on the PH at least a little.

With an acidic chlorine source, you don't want to lower TA as low as 60, but you can certainly come down to the 80 to 100 range.

 

[chem geek]

As Jason notes, TA is a SOURCE of rising pH. You may find the presentation I gave at the last World Aquatic Health Conference (WAHC) helpful: Reducing Facilities Chemical Costs By Proper Management of Total Alkalinity (TA) and pH. The bottom line is that you should target a lower TA level and a higher pH level. If your calcite saturation index is still too low with this new combination, you can raise the Calcium Hardness (CH) level to compensate. If you want to save even more money (after lowering TA and raising pH), then you can use carbon dioxide to lower pH in which case you won't need any bicarb at all and only a very small amount of acid.

 

[bgriffith1990]

Thank you for the responses.

Chem geek.... very interesting presentation. I would definitely like to discuss this further. Thought about privately messaging you, but I supposed I can keep these questions on here in case others look in the future.

My facility recently came very near to a shutdown a couple months ago due to high overall costs. Maintenance supplies run us around $17,000 a year and I would like to cut that down.

1. What would be the suggested method for delivering CO2 into my pools? Would new equipment be needed? I see you mentioned a "booster pump with a properly sized efficient gas transfer Venturi injector", but I am not sure exactly what that is.

2. Can you a little bit further explain the statement "TA is a source of rising pH"?

3. I am also having a little difficulty understanding slide number 5...."Net pH of Chlorine Sources".

4. In slide 7, you state that a pool with TA 80ppm and pH 7.5 has over 7 times the CO2 in water compared to balance with the air. Those are typically the same levels I keep my pools at. You then continue on to say that most pools are over-carbonated in order to reduce swings in pH from external sources and to protect plaster surfaces. In short, would I be exposing myself to potential pH swings or plaster damage if I were to start using CO2 instead of baking soda and acid?

5. Would there be any other negatives to potentially making this switch?

6. What would be your (or anybody's) recommendation on keeping our chlorine tabs versus using liquid chlorine (bleach)?

7. I do see that you acknowledge plaster damage in slide 13....but I am very unfamiliar with the saturation index. Could you give me a briefing on that?

Thanks again for the responses. I am being promoted from Aquatic Coordinator to Facility Director next month. Researching how to cut down chemical costs is one of the first things on my list.

 

[chem geek]

Yes, it is best to have these discussions on the forum so that others may benefit from them. Please add the state to your location (I presume Carterville is in Illinois).

1. What would be the suggested method for delivering CO2 into my pools? Would new equipment be needed? I see you mentioned a "booster pump with a properly sized efficient gas transfer Venturi injector", but I am not sure exactly what that is.

Yes, new equipment would be needed for storing and injecting the carbon dioxide so you'd need to do a cost/benefit ROI analysis. It will definitely pay off longer term, but you'll have to see what that would be in your situation. The best way to figure this out is by talking to others who have gone through this before. PM me with your E-mail and I'll hook you up with some people who have been using CO₂ successfully and who can explain the equipment side better than I.

2. Can you a little bit further explain the statement "TA is a source of rising pH"?

TA is mostly a measure of bicarbonate in the water and bicarbonate and carbon dioxide are in equilibrium (balance). So a higher TA (at a given pH) means a higher level of carbon dioxide in the water. Pools are over-carbonated so there is more carbon dioxide in the water than the equilibrium (balance) amount with the carbon dioxide in air. That means that carbon dioxide outgases from the pool. When this happens, the pH rises.

The carbon dioxide outgases more quickly from the pool when the TA is higher, the pH is lower, and/or there is more aeration of the water (waterfalls, spillovers, fountains, splashing, etc.). So to minimize the rate of pH rise, you want to target a lower TA level, higher pH level, and minimize aeration (if possible).

3. I am also having a little difficulty understanding slide number 5...."Net pH of Chlorine Sources".

There is a myth that using hypochlorite sources of chlorine such as chlorinating liquid (sodium hypochlorite) is the cause of pH rise in pools. This is not really true. There is some amount of pH rise from the "excess lye" that is in bleach and chlorinating liquid, but the vast majority of pH rise in most pools is from carbon dioxide outgassing. Though it is true that when adding chlorinating liquid the pH rises, when the chlorine is used/consumed the pH falls back down. The reason for the slide is to get people associating pH rise in pools with carbon dioxide outgassing, not with using chlorinating liquid.

4. In slide 7, you state that a pool with TA 80ppm and pH 7.5 has over 7 times the CO2 in water compared to balance with the air. Those are typically the same levels I keep my pools at. You then continue on to say that most pools are over-carbonated in order to reduce swings in pH from external sources and to protect plaster surfaces. In short, would I be exposing myself to potential pH swings or plaster damage if I were to start using CO2 instead of baking soda and acid?

The risks you describe have nothing to do with using CO₂ instead of baking soda and acid. Injecting CO₂ is chemically identical to adding baking soda and acid except the latter also adds/produces salt and water (see slide 3).

If you lower the TA level then you will have less pH buffering and will lower your saturation index, but if you raise your pH target then that will raise your saturation index. So depending on where you start and end up, your saturation index may not change much. Even if it does, you can compensate by changing your Calcium Hardness (CH) level. So you can lower your costs by lowering TA and raising pH and still protect your plaster surfaces.

As for pH swings, the lower TA theoretically would have pH swing more, but if your pool is mostly rising in pH and if your feedback systems for monitoring and dosing are responsive and do not overshoot, then the swing should be small. Let's compare a pool with TA 80 ppm (I'll assume 0 ppm CYA) and pH 7.5. As shown in this link for the Illinois state code from swimming pools, the minimum TA allowed is fortunately a low 50 ppm, but the maximum pH allowed is unfortunately only 7.6 (the minimum pH allowed is 7.2). So let's use these numbers for comparison and I'll use 60,000 gallons for your therapy pool. The following shows the effect of adding half a gallon of full-strength Muriatic Acid (31.45% Hydrochloric Acid).

pH 7.5, TA 80 ---> pH 7.22, TA 75.8
pH 7.6, TA 50 ---> pH 7.14, TA 45.8

So you can see that indeed the lower TA does result in a larger drop in pH when acid is added, but if you are adding the acid more slowly with a tighter feedback loop and not overshooting, then the pH swing should not be too large. There is still significant pH buffering and the pH shouldn't "crash" unless your acid feed system went nuts. You would add 3.5 pounds of bicarbonate to restore the TA assuming the reason for the acid addition was that the pH rose due to 1.8 pounds of CO₂ outgassing.

However, at a pH of 7.5 and TA of 80 ppm, there is 6.2 times as much carbon dioxide in the water compared to equilibrium with air while at a pH of 7.6 and TA of 50 ppm, there is only 2.6 times as much carbon dioxide in the water compared to air. The rate of carbon dioxide outgassing would be at least a factor of 2.4 lower (it's actually more because the outgassing varies as the square of TA).

The saturation index is -0.1 lower at pH 7.6, TA 50 compared to pH 7.5, TA 80 but this is easily compensated by having the Calcium Hardness (CH) be 30% higher so closer to 400 ppm than 300 ppm.

You should certainly do this first before switching to CO₂. This would be the baseline from which any cost savings would occur and you might get most of the benefit just from these new pH and TA targets.

5. Would there be any other negatives to potentially making this switch?

For lowering the TA and raising the pH targets the only downside would be if your acid controller wasn't responsive and you were already having large pH swings since those would get somewhat larger. If your pH is already remaining fairly well controlled, there should be no downside. If you adjust your CH then your plaster will be just as well protected as before.

As for switching to use carbon dioxide, there should be no downside since it is chemically identical, but you'd have to have the equipment to handle this CO₂. You'll need to make sure the cost savings justifies the equipment expense.

6. What would be your (or anybody's) recommendation on keeping our chlorine tabs versus using liquid chlorine (bleach)?

Your state code restricts Cyanuric Acid (CYA) levels to no more than 100 ppm and it apparently allows CYA in indoor pools (not all states do). It's actually reasonable to have a small amount of CYA in an indoor pool, say 20 ppm, and then target 4 ppm FC with it. This gives the equivalent active chlorine level of roughly 0.2 ppm FC with no CYA. If you use chlorine tabs, then the CYA level will likely get much higher (unless you have extraordinary water dilution) and that will make the chlorine less effective since the active chlorine level is proportional to the FC/CYA ratio. So generally you should be using chlorinating liquid or bleach though if you have water dilution reducing your CYA level then you could supplement with some chlorine tabs to maintain that CYA level (and provide some of the chlorine).

If you were to have no CYA in the water, then you'd want to target much closer to the minimum of 1.0 ppm FC (but your state code requires 2.0 ppm FC when the pool water temp exceeds 85ºF).

7. I do see that you acknowledge plaster damage in slide 13....but I am very unfamiliar with the saturation index. Could you give me a briefing on that?

The Calcite Saturation Index (that some call the Langelier Saturation Index) is an index that computes the saturation level of calcium carbonate that is used to protect plaster from dissolving (if undersaturated) or from precipitating as scale (if oversaturated). You want to target your pH, TA, CYA, CH, and temperature combination so that the index is close to 0.0 though being off a little from that is no problem. We generally don't see scaling in pools until the index reaches +0.7 (though in spas it's been seen at +0.3) while for degrading plaster it's been seen to have problems in tank tests at -0.7 or below but such tests were not over years so problems may still occur with other negative saturation index levels. Generally speaking, target around 0 or slightly positive unless you have a saltwater chlorine generator (SWCG) since those tend to scale in which case a slightly negative target (around -0.2) is more commonly recommended.

We have a set of Recommended Levels but the range of saturation index for those levels is somewhat broad and is offset too negative for SWCG pools. You can easily calculate this index by putting your water chemistry parameters into PoolMath.

 

[bgriffith1990]

chem geek,

Thanks so much for the thorough response! I really appreciate it!