Saturday, March 05, 2011
Salt Peddlers Respond to IPSSA report. Send out Hired Guns and Crackpots to take Pot Shots.
I'm curious as to why the much lower level of salt in these pool systems would be so damaging to pool components yet my glass heater, pumps, etc all work fine in very high levels of salinity. They are all also designed to work in both fresh and saltwater.
Are pool heaters, pumps, filters metal or have metal components? Modern aquarium equipment is made of glass, plastic, or acrylic, no metal at all.
Good questions. The problems we're having with stone and concrete have to do with splash zones, where the salty water evaporates, leaves the salt behind, which builds up until it recrystalizes. When it does that, it exerts crystallization expansion pressure on the interior of the stone or concrete, in the pores. That causes the stone/concrete to crumble or delaminate in layers. The metal corrosion problems are manifold, but a couple of the mechanisms that are giving us issues that you wouldn't see in your application are:
The same splash, drying cycle, causing a buildup on the surface of the metal that causes oxidation and eventually rusting. Our metal parts that remain submerged don't have the problem.
The only time we see submerged metals corrode is when two dissimilar metals enjoined ubderwater, setting up a galvanic cell that results in deterioration of the softer metal. Or when the metal part is bonded to the pool's electrical bonding grid, which all metal components ar supposed to be. This is different than simple grounding, like three prong plug grounding. This is bonding, which we suspect makes the metals susceptible to stray current corrosion. The salty water creates a better conductor, amplifying any stray currents that may be on that grid.
That's it in a nutshell. Look over to the left and you'll see tags that will take you to past blog pieces where I talk more in depth about all this.
Good luck win your pool and stay away from salt. Look up my blog piece How Low Can You Go. It talks about salt systems for above ground pools. They are scary.
And sorry about the disjointed comment. After losing what I'd written I tried to just sum up the main points. I'm totally new to owning and maintaining a pool. Having some experience with synthetic saltwater systems the idea of a chlorine generator appealed to me. The cost of the systems was a big red flag, plus the hype on the the various pdf brochures.
My wife and I were all but set on the Trevi all-resin above-ground pool, which is supposedly perfect for saltwater systems and carries a warranty for it. The kit was expensive and we also could not get it in our desired size, 15' round.
Perusing your blog as well as poolnerd's (how I found you) has been very enlightening.
We have something called "salt creep" in marine aquarium parlance, which is the residual product of evaporation that crystallizes around the top of the aquarium, upper part of filter intakes, powerheads, and even around the top parts of power cords. It looks like salt but of course the salinity does not evaporate out with the water.
In aquariums we're only dealing with glass, epoxy, and plastics, rarely stone material and never metal (though it used to be in the early days of fish keeping). In fact, regarding metal: I have one of my saltwater fish-only tanks on an iron stand. Undetectable mist from the water surface being broken is enough to rust the iron. You can even see the results in days, even from that minute level of saltwater.
We're going to stay away from salt. The more I read about it I think the only applicable way it could be used is for some cheap above-ground wal-mart pool, something that's not going to last more than 2-3 seasons. Definitely not for something nice and longterm like we want to install.
Very glad I found your blog before we built our deck around this thing, lol
The primary source of pH rise in most pools using hypochlorite sources of chlorine or SWCG pools is carbon dioxide outgassing because pools are intentionally over-carbonated to provide a pH buffer and to saturate the water with calcium carbonate to protect plaster surfaces. Total Alkalinity (TA), specifically the carbonate alkalinity portion, is a SOURCE of rising pH. You can see in the following chart the degree of over-carbonation of water at various TA and pH levels:
There are many pool owners using chlorinating liquid or bleach that use little or no acid in their pools because they keep the TA low enough to minimize carbon dioxide outgassing. In my own 16,000 gallon pool with a mostly opaque electric safety cover with the pool used every day for 45-90 minutes the daily chlorine demand is around 1 ppm. If chlorine usage/consumption were not acidic, then the pH in my pool with 80 ppm TA (and assuming no carbon dioxide outgassing) would rise from the high pH chlorine addition from 7.5 to 7.9 in the first week then to 8.4 in the second week and to 8.6 in the third week, etc. Of course, that's not what happens because chlorine consumption/usage is acidic, mostly from breakdown from sunlight but also when oxidizing ammonia and urea from bather waste. In my own pool, the pH rises much more slowly where I only add a few cups of acid once a month.
At some pool forums we've had people lower the TA level in SWCG pools and have reduced the rate of pH rise significantly. In addition, some use 50 ppm Borates that not only acts as a pH buffer, but also as a mild algaecide (more on why that is important later). The CH is raised higher to compensate for the lower TA to keep the saturation index reasonable. Now even after doing all of this, most SWCG pools still have a greater pH rise than non-SWCG pools and though part of this is due to the increased aeration from the hydrogen gas bubbles (which drives carbon dioxide out more quickly), my best guess is that this is also due to undissolved chlorine gas outgassing. The problem does seem to correlate with shorter pipe runs from the chlorinator to the pool which would be consistent with this. So cutting down the SWG on-time can help reduce this problem, but that means reducing chlorine demand. So using a higher Cyanuric Acid (CYA) level such as 80 ppm protects chlorine from breakdown from sunlight better, though one needs to have a Free Chlorine (FC) target of 4 ppm in this case to prevent algae growth. The use of 50 ppm Borates also helps by being a mild algaecide so reducing nascent algae growth and corresponding chlorine demand. These techniques are described in the following Pool School article at Trouble Free Pool:
So the IPSSA study really was flawed since they weren't looking at the known ways of reducing the pH rise in SWCG pools, especially having the TA be lower (around 70 ppm). Though there are real issues of corrosion from the higher conductivity at higher salt levels as well as higher chloride levels with regard to (especially lower-quality) stainless steel and to splash-out salt recrystallization, the issue of rising pH can be mostly though not completely controlled.
I just wanted to correct what you said technically because what you said was not exactly true and I don't want the main point of running with lower TA helping to reduce the pH rise get lost because of technical inaccuracies. The salt cell has the following two primary reactions:
2Cl- --> Cl2(g) + 2e-
2H+ + 2e- --> H2(g)
2H+ + 2Cl- --> Cl2(g) + H2(g)
The chlorine gas almost immediately dissolves in the water with the following reaction to have a net reaction as shown:
Cl2(g) + H2O --> HOCl + H+ + Cl-
H+ + Cl- + H2O --> HOCl + H2(g)
Because water dissociates, the above reaction is normally written with a net reaction as follows:
H2O --> H+ + OH-
2H2O + Cl- --> HOCl + OH- + H2(g)
So the bottom line net reaction with the generation of chlorine in a salt cell (ignoring side reactions) is that water and chloride ion (from salt) combine to form hypochlorous acid (HOCl) plus hydroxyl ion (which is basic or alkaline) and hydrogen gas. Since hypochlorous acid is a weak acid, this net reaction is weakly basic (alkaline). This is where most SWG manufacturers (at least their salespeople) believe that the rise in pH comes from, but they are wrong (keep reading to find out why).
This is pretty much exactly the same thing that happens when you add chlorinating liquid or bleach to a pool as follows:
NaOCl --> Na+ + OCl-
OCl- + H+ --> HOCl
H2O --> H+ + OH-
NaOCl --> Na+ + HOCl + OH-
except that you get some sodium ion as well (plus some extra salt, NaCl, that is in sodium hypochlorite solutions due to how they are made) and you don't get the hydrogen gas.
Now we need to look at what happens to chlorine (regardless of source) when it gets used up. Most chlorine in pools gets broken down by sunlight and even though Cyanuric Acid (CYA) combines with chlorine to form a chemical compound that slows down this process (and is not an effective disinfectant or oxidizer), it still happens as follows:
2HOCl --> O2(g) + 2H+ + 2Cl-
Continued in next comment
The next most common thing that happens to chlorine is that it combines with ammonia or related compounds such as urea from sweat as follows where I show the reaction going all the way to "breakpoint" assuming that shocking occurs (which it usually does if you have sufficient chlorine in your pool and especially when exposed to sunlight which helps the breakpoint process). I'm not going to show what happens when chlorine combines with organics, but the process is somewhat similar (carbon dioxide is produced if the organic is fully oxidized, but more typically intermediate compounds are produced that don't breakdown quickly).
2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
So, even though the generation of chlorine resulted in hydroxyl ions, the usage of chlorine results in hydrogen ions and these cancel out forming water:
OH- + H+ --> H2O
So the bottom line in an SWG pool is the following reactions:
4H2O + 2Cl- --> 2HOCl + 2OH- + 2H2(g)
2HOCl --> O2(g) + 2H+ + 2Cl-
2H2O --> 2H2(g) + O2(g)
6H2O + 3Cl- --> 3HOCl + 3OH- + 3H2(g)
2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
2NH3 --> 3H2(g) + N2(g)
So the bottom, bottom line is that the net result in an SWG pool from the creation and usage of chlorine is that water is split to produce hydrogen gas and oxygen gas or that ammonia (urea) in the water is broken down (oxidized or "burned" in some sense) to produce hydrogen gas and nitrogen gas. These net, net reactions, as you can see, are neutral.
At this point, you can now talk about the source of rising pH being the outgassing of carbon dioxide from the pool. You should explain that pools are in essence intentionally over-carbonated, similar to a lovely tasty carbonated beverage! This is done when you initially added baking soda or sodium bicarbonate to your pool (it also happens when you add some pH Up products that have sodium carbonate). The purpose of having extra carbonate in your pool is to act as a pH buffer and to provide carbonate ion that, along with the calcium you added to your pool with calcium chloride, saturate the water with calcium carbonate so that this compound does not get dissolved out of plaster/gunite/concrete/grout. If too saturated, scaling would occur essentially precipitating calcium carbonate on to pool surfaces. Also, the calcium carbonate tends to form a thin film layer on metal surfaces that help reduce corrosion, though pH is a much more important factor for metal corrosion. Pool water chemical balance attempts to keep a balance between corrosion and scaling.
The downside of having a pool over-carbonated is that there is more carbon dioxide in the pool than in the air so there is a tendency for it to outgas. When this occurs, the pH rises while for technical reasons I won't get into here, the Total Alkalinity (TA) remains the same. If you then add acid to restore the pH, you lower both the pH and the TA with the net result of having TA get lowered -- which makes sense since TA is partly a measure of the amount of bicarbonate in your pool. The carbon dioxide outgassing, and therefore the rise in pH, is increased when the TA is higher, when the starting pH is lower, and when there is more aeration. So the easiest ways of reducing this rise in pH are to lower the TA, keep the pH higher, and reduce aeration (waterfalls, spillovers, etc.) including using a pool cover.
For an SWG pool there is another way, in addition to lowering TA, that can help reduce the pH rise. That is to add an additional buffering system to the pool that is also an algicide that will cut down chlorine consumption. Adding 50 ppm Borates (from Borax, the 50 ppm technically being Boron) to your pool will add additional pH buffering capability so that you can keep the carbonate part of the buffer lower. To compensate for water balance, you need to keep either your calcium level or your pH higher (or both). The algicidal properties of the borates lower the consumption of chlorine which will let you lower the output of your SWG which lowers hydrogen gas production so less aeration so less carbon dioxide outgassing and less pH rise.
I don't see the contradiction. In my post in this blog and in the earlier more detailed post in 2006 I wrote that the chemistry of SWCG's is pH neutral and that the rise in pH is primarily due to carbon dioxide outgassing which can be lessened by having a lower TA. With SWCG there is also the possibility of having some chlorine gas remain undissolved and outgassing directly, especially if the pipe run from the SWCG to the pool is short which is something I had not mentioned before in either post. Where is the contradiction in the two posts?
That's why, when I first met you on the internet, I was sure you were just an industry shill masquerading as a really bright consumer. Because you would lead with these statements that seemed to back up what the slimeballs - I mean, salesmen - were saying.
"The use of any hypochlorite source of chlorine including that from saltwater chlorine generators (SWCG) when accounting for chlorine usage/consumption is pH neutral except for the small amount of excess lye in the hypochlorite products. Yes, the addition of these sources of chlorine has the pH rise, but the usage/consumption of chlorine is an acidic process and brings the pH back down. This is described technically in the following post:
I promise you that no one clicks on that link. No one goes and reads the follow up explanation. So, they're left thinking that guys like me are lying when we tell them things like; using a steady diet of 3" tabs will result in low pH unless they run their TA up to the industry prescribed 100 - 120 ppm, and that they better allow for the CYA correction factor. Or that using a salt based chlorine generator will result in high pH EVERY TIME IT'S USED AND SO WATCH YOUR pH SO YOU DON'T END UP SCALING THE CRAP OUT OF YOUR POOL.
All they take away is the memory that this really smart guy said, "chlorine usage/consumption is pH neutral".
That's exactly why I cut and pasted your first explanation. It was meant to be tongue in cheek, a bit, in that I'm pretty sure that you and I are the only ones who have read all of your explanation in that first post, and of the two of us, you're the only one who understands it. I "get it", in that I read through it, skip over the chemical reactions and read your continued explanation, assuming that you wouldn't intentionally lie about it so your assumptions must be right, and that's about a country mile farther than anyone else goes in comprehending most of the things you post.
You're a great guy and a brilliant, beautiful mind, but you ought to consider your audience more - pool owners, who don't care about the science behind it, and poolmen, who can't understand it - and stop posting things that really don't matter in the grand scheme of things.
The truth is, if you use a salt based chlorine generator, you will note, week after week, the rise in your pool's pH. Period.
Finally, I think it's wholly irresponsible of you to recommend that pool owner's run their TA around 70 ppm when everything that their warranties are based on tells them that a corrected 80 ppm is the absolute lowest they're allowed to run it. It really doesn't matter if you're right or wrong on that, because a homeowner pointing to your forum posts or this blog and saying "because the Chem Geek told it would be alright" will not result in them getting a warranty repair performed. In fact, the opposite is true, as that manufacturer looks for any way possible to void the warranty and blame "bad water chemistry" for the shortcomings of their lousy Trojan Horse of a salt box.
Having the TA be lower than 80 ppm and using 50 ppm borates as a supplemental pH buffer is not at all irresponsible. There has not been a single report of anyone having a problem with scientifically sound recommendations because these make scaling in the salt cell much less likely (especially the use of borates which buffer strongly against a rise in pH in the salt cell). So no warranty issues for the SWCG ever come up.
I don't have time to pull up every single report, but a couple in the same thread at TFP are here:
The following long thread in 2006 at The Pool Forum is where we first figured out that TA was a primary source of pH rise and that even SWCG pools could be managed.
and the following long thread
is where waterbear figured out the multiple benefits of borates including pH stability for SWCGs. This also included raising the CYA level (with proportionately raised FC level) to reduce chlorine losses letting one turn down the SWG on-time.
And this isn't just me making these recommendations.
There are absolutely very legitimate concerns with SWCGs mostly related to the corrosion issues from the higher salt levels such as stone degradation described in this recent thread:
So while it is reasonable to tell people that SWCG pools will have a greater tendency for the pH to rise, it is incorrect to state that such rise will occur week after week and be inevitable and that it cannot be at least partially mitigated and in some cases almost completely mitigated.