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 CO
2 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 CO
2 instead of baking soda and acid. Injecting CO
2 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 CO
2 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 CO
2. 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 CO
2. 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.