I have a question: If we put CYA in pools to protect chlorine from degradation, and suggest levels of 30-50 ppm in a "regular" chlorinated pool that gets dosed weekly (assuming a pool man in servicing the pool), why does the CYA level increase to 60-80 ppm in a salt pool that generates chlorine every day?
It would make sense to me that the pool that gets chlorine weekly would need a higher concentration than the pool that gets dosed daily. What is the purpose of the higher residual in a salt pool (or liquid injection pool)?
Replies
To be clear, I agree with Steve Dunn as to why the SWG manufacturers recommend the higher CYA levels. If they had sized their systems larger then a lower CYA level would work fine. More chlorine would be lost from sunlight at a lower CYA level and the generator would have to be larger to make up for that. Unfortunately, the same manufacturers that recommend 60-80 ppm CYA also recommend 1-3 ppm FC and that is not sufficient to prevent green algae growth in all conditions (i.e. independent of algae nutrient levels). They should be recommending a minimum FC that is 5% of the CYA level.
I also wanted to comment further on the statement that at 30 ppm FC that 98% of the chlorine is protected from sunlight. The amount of chlorine bound to CYA is not solely a function of the CYA level, but also of the FC level, the pH and the temperature. At a pH of 7.5, 77ºF temperature, 3 ppm FC and 30 ppm CYA, 97.2% of the chlorine is bound to CYA, 1.4% is hypochlorous acid, and 1.4% is hypochlorite ion. However, the chlorine bound to CYA is not immune from breakdown from sunlight -- it just doesn't break down as quickly as hypochlorous acid. A low-bather load residential pool in full sun with only 30 ppm CYA will lose more than half the FC in a day. If the loss only came from hypochlorous acid and hypochlorite ion, then the loss would only be a little more than 10% for the day (about 1.4% per hour instead of 50% per hour). This is why most non-SWG pool owners with full sun on their pools have at least 50 ppm CYA. The actual daily loss rate as a function of CYA depends on the amount of sunlight, depth of pool, and other conditions, but is roughly the following and is mostly determined by the competing factors of the linear loss rate of chlorine bound to CYA assuming the FC/CYA ratio is kept constant vs. the shielding of UV by CYA protecting lower depths in a non-linear fashion. An example table that gives the order of magnitude of what goes on is the following where the FC is 5% of the CYA level for roughly the same active chlorine (hypochlorous acid) level as 0.1 ppm FC with no CYA.
CYA ... FC ... % Loss/day ... % Loss/hr ... FC loss/hr .... % ontime (0.5 ppm FC/hr = 100%)
... 0 ... 0.1 ....... 99.6% ......... 50.0% ........... 0.07 .............. 7% (but can't realistically maintain 0.1 ppm FC everywhere)
... 0 ... 1.0 ....... 99.6% ......... 50.0% ........... 0.69 .......... 138% (can't keep up)
. 30 ... 1.5 ........ 75% ........... 15.9% ........... 0.26 ............ 52% (say, 50%)
. 50 ... 2.5 ........ 40% ............. 6.2% ........... 0.16 ............ 32% (say, 30%)
. 80 ... 4.0 ........ 22% ............. 3.1% ........... 0.12 ............ 24% (say, 25%)
100 ... 5.0 ........ 15% ............. 2.0% ........... 0.10 ............ 20%
160 ... 8.0 .......... 7.7% .......... 1.0% ........... 0.08 ............ 16%
Note that the losses per hour assume the equivalent of 8 hours of direct noontime sun though in practice there is sun longer on the pool though at lesser intensities. The "FC loss/hr" is the rate of loss at the initial FC level (the absolute loss rate declines as the FC level drops). The 0.5 ppm FC per hour is equivalent to 1 pound chlorine (Cl2) in 24 hours in 10,000 gallons. The residential SWG industry typically quotes "sizing" of around 0.3 to 0.4 pounds chlorine in 24 hours per 10,000 gallons so around 1/3rd what I show above. That sizing would make the SWG in 50 ppm CYA just barely able to keep up and would likely get behind around noon or with extra bather load, chlorine demand from pollen, leaves, etc. Since the price of SWG cells is not linear, one should generally get a larger cell size not only for a safety margin, but to extend the cell life (so more economical) since the life is mostly a function of on-time regardless of cell size.
The main reason that salt pools carry a higher level of CYA is that at lower levels the generator can not keep up with the Cl loss due to sunlight. Most salt systems are undersized and produce a specific amount of Cl over a 24 hour period and that is if the system is on all the time.
Take the example of a standard residential 1.5lbs cell. This unit will generate a total of 9ppm over a 24 hour period for a pool of 20,000 gallons. That means it is only capable of producing 1ppm every 2.6 hours when it is on, and most operators will set the unit to run at 50% to 75% of the day. Hence the perceived need to increase the CYA level to hold the CL in the pool. However, I would suggest increasing the cell size and keeping the the CYA at 20 to 30 ppm.
Having a CYA above 30ppm is a waste of money as this amount protects 98% of the chlorine from the sunlight. However, this same 20ppm of CYA reduces the ORP or chlorine effectiveness by almost 80%. One study has shown that 1.5 ppm of FC without CYA will produce an ORP of of ~820mV, and this same 1.5ppm of FC in water with 20ppm of CYA produce an ORP of ~700ppm which is a same killing power of .3ppm w/o CYA.
I have seen pools with CYA at 80 ppm with a FC level above 5ppm grow algae like crazy. It does no good to use chlorine to sanitize and oxidize if it is "stabilized" to the point it can not do what it was designed to do regardless of the chlorine source.
I am confused by the discussion of CYA " buffering" against pH rise. The rise in pH on SWCG or bleach systems is due to the sodium hydroxide produce by the process. Although salt systems effect the pH less than a straight bleach system because of the hydrochloric acid generated by the SWCG counteracts the effect of the sodium hydroxide also produced by the same process. As a result the pH of the product of most salt systems is between 9 to 10 where the pH of a bleach system is about 12.
I am also confused by the need to use CYA for a higher chlorine residual. If the need is due to bather load then increase the size of the chlorine delivery system e.g. bigger salt cells or faster feeders. Additionally, you may want to consider automatic controllers to monitor and meet demand as needed.
This process of loading up a pool with FC and keeping it in the pool with higher levels of CYA is at minimum questionable, and at worst dangerous.
For a residential pool, or any pool with fairly low bather load, it is absolutely positively not true that "having a CYA above 30 ppm is a waste of money as this amount protects 98% of the chlorine from the sunlight." This is one of those myths in the pool/spa industry that somebody states, usually out-of-context, and that then gets repeated over and over again as if it's absolutely true. The chlorine/CYA relationship has been known definitively since at least 1974. At Trouble Free Pool (TFP) with nearly 40,000 members, experiments were made showing very clearly that higher CYA levels lower the absolute chlorine loss even with proportionately higher FC levels so that the FC/CYA ratio is kept constant to keep the active chlorine (hypochlorous acid) level constant in order to prevent algae growth. It has also been shown that an FC that is 5% of the CYA level for SWG pools is enough to prevent algae growth, so 4 ppm FC with 80 ppm CYA, for example, and that this is true even for pools high in phosphates (even tens of thousands of ppb!). However, one must kill off any nascent algae by shocking first (though not needed to be done subsequently) and this FC/CYA ratio is a minimum that must never go lower, so in a pool with varying demand or chlorine losses higher levels may be needed.
It is true that the ORP is lowered when CYA is present because the active chlorine (hypochlorous acid) level is significantly lowered, but it is not that the chlorine level gets too low, so long as the FC/CYA ratio isn't too low, but rather that FC without CYA has too high a chlorine level with the result of a faster creation of disinfection by-products, faster oxidation of swimsuits, skin and hair, faster outgassing and corrosion of equipment, etc.
The CYA is a pH buffer, but it's main effect at higher levels is the additional protection from sunlight through it's UV shielding of lower depths (not just its lowering of the active chlorine level). This lets one turn down the SWG on-time and THAT is what lowers the rate of pH rise due to less hydrogen gas bubbles so less aeration of the water and also less undissolved chlorine gas outgassing. As you wrote, hydroxyl ions are produced by the salt cell and that the chlorine gas produced at the other cell is acidic when dissolved in water so the net result is the same as adding hypochlorite ion. Though the pH rises upon addition, the usage/consumption of chlorine is an acidic process so the net result is pH neutral (see this post for technical details). The rise in pH comes from carbon dioxide outgassing, which can be minimized by lowering the TA level (usually to 70 ppm is sufficient), and by undissolved chlorine gas outgassing, which can be minimized by better design of piping/returns for longer contact time. It is also true that chlorinating liquid and bleach have "excess lye" which is the main contributor to their net pH rise, but in practice many SWG pools tend to rise in pH more than those using chlorinating liquid or bleach, at least for residential low-bather load pools.
It isn't that one needs to use CYA for a higher chlorine residual. It's the other way around. The higher CYA is so one loses less chlorine from sunlight, but the higher FC is to keep the FC/CYA ratio constant as that roughly determines the active chlorine (hypochlorous acid) level which determines the rate of disinfection, oxidation, and alage prevention. For non-SWG residential pools, TFP usually recommends 30-50 ppm, but some in hot desert areas use a higher CYA level, but the FC is raised proportionately to prevent algae growth.
For commercial/public pools or other high bather-load pools, it doesn't make sense to have the CYA be above around 30 ppm (which is probably where the "rule" came from) because the chlorine consumption from bather load is higher than the losses from sunlight so any higher CYA results in only a marginal lowering of overall loss. Also, in commercial/public pools, one wants a higher disinfection rate and oxidation rate, though still not too high in order to avoid disinfection by-products and the other side effects I mentioned. An FC that is up to 20% of the CYA level is probably reasonable.
Unfortunately, many indoor pools are regulated to have no CYA in them (and New York bans CYA entirely for commercial/public pools, even outdoors), but they often have 1-2 ppm FC or higher which makes them have 5-10 times higher chlorine than an outdoor pool with an FC that is 20% of the CYA level or 10-20 times higher than a residential pool with an FC that is 10% of the CYA level. My wife personally experienced this difference every year when she swam at an indoor commercial community center pool that had 1-2 ppm FC with no CYA where her swimsuits degraded every year and her skin was flakier and hair frizzier during the 5-month winter season. During the 7-month summer season in our own residential pool with an FC that was around 10% of the CYA level, the swimsuits only started showing signs of degradation after 7-8 years and her skin and hair were not affected nearly as much. The difference is the 10-20 times higher active chlorine (hypochlorous acid) level in the indoor pool compared to our outdoor pool.
Read Certified Pool Operator training -- What is not taught for more info including links to peer-reviewed scientific papers that detail the known science that unfortunately does not get translated into practice in the pool/spa industry as a whole. There are thousands of residential pool owners at TFP with SWG systems that are able to readily keep their pools algae free by following Water Balance for SWGs.
Hello all,
My take on the CYA higher requirement in a salt water chlorine generation (SWCG) pool is simply this.
Whether a pool is chlorinated by SWCG, bleach, calcium hypochglorite or even chlorine gas they all produce sodium hypochlorite when added to water. There is no difference in the chlorine product once added to the water. The only difference is the by-product that is produced. The only reason to have higher levels f cyanuric acid in the water for SWCG systems is so that the systemsw can produce enough chlorine to handle the pool or spa with a usual bather load. In the hotter climate areas the SWCG systems would have to be doubled in size if not greater so that they could produce enough chlorine to maintain a pool/spa. It's all sales - how to get around the dilema of selling SWCG systems, raise the CYA, smaller systems required, lower cost sell more units. We've seen this game played for many many years in the commercial business.
Steve Dunn
Commercdial Pool Systems, Inc.
Steve,
Obviously you meant hypochlorous acid not sodium hypochlorite. Other that that I agree with every word.
Hi, Steve.
I've been scrolling through all the remarks about dealing with SWCG pools and one phrase in your reply caught my attention: "...the dilemma of selling SWCG systems...". Is it just me, or does any other POOL CLEANING SERVICE out there feel that salt systems are more trouble than they're worth? Having a pool service here in Florida since 1994, chlorine tabs dropped in inline chlorinators and keeping everything else balanced has always been a helluva lot easier than the consant acid/water soaks; constant Ph and TA shifts; constant adding CYA not produced by salt; high calcium levels; mineral deposits and corrosion; salt deposit rings on the tile; hauling 40lb bags to the pool--who needs it? Am I missing something?
Tal,
LOL. I am right there with you buddy. Who need em? The only thing you are missing is the profit on the sale. Most people who sell them do not have to deal with them on a weekly basis. The only other thing that is more aggravating is the silver/copper ionizer.
Lowering the TA to 65-70 ppm, raising the CYA to 60 ppm, and maintaining the pH at 7.7 will help reduce the treatments of acid (MA and/or dry) needed. Also, allow the CH to raise to balance the LSI, and raise the chlorine level to 4 ppm. This program will reduce pH rise for most daily and/or weekly chlorine applications, as Richard suggests, and will make maintenance easier and may reduce chlorine costs.
Maintaining and allowing the pH slightly higher also reduces pH rise and acid treatments.
Since I run my pool through my RO rig every couple of years, sulfate build up shouldn't be a problem. I do have a lot of aeration from spillways and waterfalls, so I do have quite a high acid demand.
I've been using the guidelines from the Pool Calculator for TA, and 80 seems to be where I try and stay. What would you suggest my level be?
So if I am using hypochlorite now with a LQ, and I am using sodium bisulfate with the pH Adjuster, should I see a greater demand or lesser demand than if I was controlling pH with MA? I was dosing with MA every 2-3 days prior to installing the pH Adjuster and switching to dry acid. Since I tend to get busy and "forget" to add MA occasionally, I needed to simplify this part of my maintenance.
I have to leave for Oregon for a week on Friday and I hope to have all these nagging questions handled so that I can relax while I'm gone (one reason I installed the pH Adjuster; probably should have done that a couple of weeks ago!).
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