With regard to
BABES: a better method than “BBB” for pools with a salt-water chlorine generator, I also have some comments, but overall the article is very good.
You write that "HOCl (through its equilibrium product OCl
-) breaks down in sunlight", but HOCl itself also breaks down in sunlight, just not as quickly. This is seen in
this link. The net result is that the half-life of OCl
- is around 35 minutes while that of HOCl is around 130 minutes. The direct breakdown of HOCl cannot be ignored. So you might add "
also through its equilibrium...".
In section 3.2 Sodium hypochlorite, you mention that in the assay many different chemical species have the same oxidizing power per molecule as Cl
2, but I wouldn't put it that way. The test is measuring the reserve capacity of chlorine where it is essentially counting chlorine atoms (those in the +1 state that are not in combined chlorine molecules such as chlorourea or chloramines). The way it is written, it makes it sound like chlorine bound to CYA has the same oxidizing power as HOCl, but that is not true. Chlorine bound to CYA is not only a very weak (at least 150 times less than HOCl) disinfectant, but it is also a very weak oxidizer as well.
You should emphasize that Figure 1 is not applicable when CYA is present. You show the correct graph later in Figure 7.
That is very interesting about the salt effect on indicator dye equilibrium including the phenol red pH test. Along with the roughly same order-or-magnitude effect on CSI from TDS (roughly 0.2 lower at 3000 ppm TDS than 500 ppm), this could explain the greater likelihood of plaster issues in some SWC pools. I always thought the 0.2 difference was pretty small, but combined with a second 0.2 difference then this is 0.4 lower which is more substantial.
In section 3.2.3 Salt and bromide, you write that "HOBr gives a less intense colour than HOCl (by a factor of about 2.25 according to Taylor Technologies)", but that factor of 2.25 is just the unit of measurement (molecular weight) difference between Br
2 and Cl
2. The HOBr on a molecule per molecule basis gives the exact same intensity change as HOCl. The sanitizing effect of HOBr is reasonably and correctly measured in ppm Cl
2 units as if you thought it was chlorine. Also, when CYA is present, the HOBr will be far far high in concentration than the HOCl as chlorine binds to CYA while bromine does not (Wojtowicz notwithstanding).
In spite of White's claims that bromamines don't need to be removed by shocking, spa users will disagree. Most bromine spas do need to be periodically shocked, usually with chlorine, in order to get rid of the bromamine smell.
When you were using my spreadsheet at any temperature other than 25ºC (77ºF), it wasn't clear whether you set line 230 "Use Temp. Dependent Cl-CYA" to TRUE. You should if you plan on looking at HOCl concentrations at other temperatures. Setting that line (in columns B and C) to TRUE uses the temperature dependent hydrolysis constants from Wojtowicz.
If you calculate absolute chlorine losses from the increased OCl
- at higher pH, it's not very much in comparison to overall FC losses seen on a daily basis. In spite of Wojtowicz saying that chlorine bound to CYA doesn't degrade, there is something breaking down or using up chlorine much faster than explained by photolysis of HOCl and OCl
- even at lower pH. At 4 ppm FC with 80 ppm CYA, then at pH 8.0 there is 0.087 ppm OCl
- and 0.025 ppm HOCl (at 82ºF) so even using a 30 minute half-life for OCl
- and 2 hours for HOCl, then over 8 hours of full-time equivalent noontime sun that would be a loss of 2*8*0.087 + (8/2)*0.025 = 1.5 ppm FC so at high pH this can be explained, but at pH 7.5 we still see large losses. There is some chlorine oxidation of CYA itself, but again that's not enough to explain the usual 2 ppm FC daily losses.
As for CYA helping to reduce Cl
2 amounts remaining, remember that the chlorine production amounts overwhelm CYA concentrations, but your point is well taken that downstream as the water gets diluted away from the plates the CYA may help keep the pH and HOCl concentrations in check somewhat to improve Cl
2 rates of solubility. Certainly, it binds to HOCl that forms thus buffering it away. However, the point I've made about undissolved chlorine gas outgassing is about chlorine gas that has NEVER dissolved. It is formed at the plate as bubbles and doesn't fully dissolve. So CYA isn't that relevant except that it helps to lower the HOCl concentration and therefore equilibrium Cl
2 concentration, but that has never really been much of a limiting factor in the chlorine gas bubble dissolving rate. It's a matter of physical diffusion and even having zero chlorine concentration in the water won't make it diffuse infinitely fast. It is the physical diffusion limit that appears to be at play here and that is a function of things like bubble size which determines the surface area to volume ratio, the length of pipe run that determines the amount of time for dissolving, whether returns are pointed up or down which determines the path length (so time) for the bubbles, and so on.
All your points about reasons why the pH would net rise due to chlorine not getting all used/consumed are valid, but they are mostly all true for non-SWC pools yet we fairly consistently see SWC pools have a greater tendency for the pH to rise than non-SWC pools. So that is where the speculative hypothesis of undissolved chlorine gas outgassing and increased aeration causing greater carbon dioxide outgassing come from.
I didn't see you mention anywhere borates positive effect on reducing scaling in the SWC cell. I think that is very important. Because the borates are strong buffers against a rise in pH, the effectively cut down the amount of pH rise at the hydrogen gas generation plate about in half and that helps to prevent calcium carbonate scaling as well as calcium phosphate scaling.
I would not say that > 50 ppm CYA is too much in a residential SWC pool. 80 ppm seems to work very well in many such pools and the higher CYA seems to help reduce the SWC on-time with its associated benefits of slower pH rise.
As for pH rise in borate-buffered SWC pools, there is still CO
2 outgassing unless the TA is very low and/or the pH quite high (above 8.0 unless the TA is low). The other factors you mention regarding not all chlorine getting used/consumed are of course still relevant.
You wrote "But Langelier’s brief was to reduce pitting in cast-iron water pipes, a far cry from the challenge of ideal solute composition for a SWC swimming pool!" However, if you looked at the bottom of my Pool Equations spreadsheet, you would find that the derivation of the Langelier Saturation Index has absolutely nothing to do with boilers, closed systems, cast iron corrosion, or any other such nonsense. It is a chemical equilibrium equation solution for the saturation point of calcium carbonate, nothing more. So please stop perpetuating this falsehood that gets repeated over and over again by many in the industry that somehow because Langelier wasn't looking at pools that his equation somehow doesn't apply. That is simply not true at all.
I also would not say that "There is no strong indication from experience that any calcium saturation index is important in modern pool concrete etching." onBalance has done tank experiments with plaster coupons that show that the index plays a strong role in etching of poorly made plaster but that even plaster that is will made will etch at more extreme saturation indices such as those in the -0.7 to -1.0 range. I would say that low pH is more detrimental than a low calcium level. Even at zero calcium levels in the water, there is a limit to dissolving/pitting rates and those rates are more of a function of pH and temperature.
You write "concrete is damaged by the formation of CaCO3, through reaction of carbonic acid with bonding agents including calcium silicate hydrates." That doesn't make sense since pool plaster (concrete) curing requires the conversion of calcium hydroxide produced from that curing to calcium carbonate. The bicarb startup works best to form the strongest pool plaster surface.
You write "If the protection from ‘balanced water’ is primarily from a protective surface coating of CaCO3, then MgCO3 or other low-solubility salts may be similarly effective." This is again simply not true. The saturation of calcium carbonate is much lower than that of magnesium carbonate so trying to trade off by having more magnesium instead of calcium simply does not work. Calcium carbonate in pool plaster dissolves into the water and you can't practically have enough magnesium concentration to form magnesium carbonate in its place for an equally strong surface. Total Hardness is therefore pretty much irrelevant. It is Calcium Hardness (CH) that matters since it is calcium carbonate saturation that matters.
Your Box 2 approach of low calcium and high pH isn't bad but remember the risk of metal staining at higher pH. If you've got metal ions in the water such as iron, copper, manganese, etc. then you risk staining at higher pH levels. You referenced Ben Powell's high pH poolsolutions.com page so that talks about the pros and cons of the approach.
The catalytic breakdown of chlorine from some metal ions is something I don't think applies practically in pools due to the much lower concentration of chlorine in the pools. Such problems with metal ion impurities are a problem for chlorinating liquid but the auto-breakdown of chlorine (not photolytic) at pool concentrations should be negligible.
You wrote "Polyquat algaecides are sometimes claimed not to deplete pool chlorine, without reference to independent scientific tests." I don't know where you heard that claim. In fact, we have most definitely seen where the two react with each other where high concentrations of Polyquat most certainly deplete chlorine. What has been said by observation of many pools is that the chlorine reaction with Polyquat is slower than that of linear quats. At recommended Polyquat levels, the increased chlorine demand is relatively low but when using linear quats it is much more noticeable. Specific quantification of these observations has not been done.
As for copper stains, they may be from copper carbonate formed in the pool plaster itself by copper displacing calcium so forming copper carbonate where calcium carbonate existed.
If there is the risk of salt splash-out problems such as damage to concrete or other surfaces, I would not recommend running at the highest salt levels the manufacturers recommend but rather the lowest levels that still give reasonable SWC efficiency.
Your 30-50 ppm CYA recommendation may not be enough for SWC pools in a lot of sunlight or in desert areas, at least not unless the SWC cell is sized higher, but then you have the other problems of higher SWC output including faster pH rise.
Again, overall, a very good article -- the only reason I had so many comments is that there was a lot of content in this article on which to comment.
