Chlorine and acid drum feeders

[max]

I have a SWG pool which I test regularly and find it quite easy to balance.

My brother and sister-in-law are putting in a pool - they are new to pools and are after the lowest possible maintenance. They like the look of the chlorine and acid drum feeders, the type where you set a level and it adds small amounts of liquid chlorine and HCl to keep them at the set level - they see that as "set and forget". My personal opinion is that the purchasing and handling of liquid chemicals and ensuring the drums are refilled is more effort in total than spending 5 minutes a week testing the water, adding an appropriate amount of acid and pushing the Super Chlorinate button if need be. Having said that, I'm natually interested in the whole pool chemistry thing and they are not - they just want to swim.

So, in determining what's best for them, I'm interested in the following with regards the feeders which I have no experience with:

1. If algae is starting to form and use up chlorine, I assume the feeder feeds an increased amount of chlorine to keep the levels up to the set level. By doing this, is it more likely than a SWG to win the battle or does this merely use up a lot of chlorine until the pool is super chlorinated or shocked?

2. If TA is unbalanced, might the feeder use a lot of acid in order to keep the pH balanced when it would be better to fix the TA first?

If algae and TA are still a problem with feeders, I'm inclined to recommend the SWG as I think they're less effort.

Thanks,
Max.

[chem geek]

I only add about 3 cups of 12.5% chlorinating liquid to my 16,000 gallon pool twice a week and it costs me around $15 per month. I also add about 2 cups of acid about every month or two. That's pretty easy maintenance. I am able to do this because we have an opaque electric pool cover so there is minimal breakdown of chlorine from sunlight. The pool is used (open) a little more than 1 hour each day (more on the weekends).

There are automated feed systems such as The Liquidator though they can get calcium carbonate deposits if the water is saturated as with plaster pools. Peristaltic pumps can inject chlorine automatically as you indicated. You should not need to add very much acid if you keep the TA level lower -- not above 80 ppm.

You also do not need to shock the pool if you consistently maintain an appropriate FC level relative to the CYA level. For an SWG pool, that's a minimum FC that is 5% of the CYA level. For a manually dosed pool, it's an FC that is 7.5% of the CYA level. Not sure what would be best for an automated chlorine (non-SWG) dosing system.

Richard

[max]

For my own pool, the SWG generally keeps the chlorine at around 3ppm. The CYA is around 50ppm though I have a Daisy pool cover which I never remove in sunlight. It comes off for an hour each night while I swim and give the water a good stir up. I find acid is required at least fortnightly to maintain proper pH even with TA at 80mm - I've read in this forum that it is normal for pH to rise with SWG.

The only time I've shocked the pool was when I found a green tinge (I assume suspended algae) after being away for 10 days. There was no FC reading so whatever was being produced by the SWG was being used up. I was a bit surprised because I test for phosphates regularly and they were still low but I guess that's no guarantee. The tinge disappeared quickly after a good dose of chlorine - it took a couple of days with pool cover off and filter running all day before it became swimmable at 5ppm.

What I'd like to know is... by maintaining set levels of chlorine, are the liquid feeders better at handling these situations when algae is starting to form? I'd imagine if there were too many hours between the OFF and ON times (eg 16 hours) that the FC would be used up by the algae. If the FC falls below 1ppm, I assume the algae starts winning even if the chlorine levels rise back to 3ppm for say 8 hours a day. Apparently FC needs to go to a high level in this situation, eg 5ppm or more so I'm assuming both the SWG and liquid feeder would have the same difficultly. Has anyone had any experience with this?

Thanks,
Max.

[max]

PS Would a higher chlorine setting like 5ppm (assuming correct pH) be safer for both SWG & liquid feeders or is that too high for the pool cover etc?

Thanks,
Max.

[chem geek]

I suspect that something in your pool got the chlorine level lower, probably below 2.5 ppm FC, at which point algae could grow faster than chlorine killed it. Perhaps sunlight was particularly strong or perhaps something fell in the pool such as pollen that used up more chlorine than usual or perhaps the SWG was off part of the time more than usual for some reason (power outage?). Perhaps the pH rose a lot as that makes the chlorine less effective (though not as much as people think due to CYA being present).

Normally if you have a 3 ppm FC level with 50 ppm CYA then this will prevent algae growth in an SWG pool. Yes, raising the FC level would lower the probability of algae and 5 ppm FC with 50 ppm CYA is not harmful to covers. Technically speaking, it is the same as 0.1 ppm FC with no CYA in the water. CYA has most of the chlorine bind to it and it is not a strong oxidizer nor sanitizer -- it just holds chlorine in reserve releasing it as FC gets used up so from a reaction rate point of view there is very little "active" chlorine (hypochlorous acid).

You could lower the rate of pH rise even more (if your pool is in direct sunlight most of the time) by raising the CYA level higher to 70-80 ppm and raising the FC level to 4 ppm. You will end up actually losing less chlorine to sunlight thereby letting you turn down your SWG on-time a little and this will lower the rate of pH rise. Other options include adding 50 ppm Borates to the pool. You can read more about all of this at the Pool School
.

Richard

[max]

Thanks Richard. Pollen is a possibililty as it was in Spring and there was more debris generally since I was away. Sometimes before I go away, I raise the chlorine levels but I didn't on that occasion. I'll make a point of doing so in future to cater for some of the things you mentioned.

Interesting about the CYA binding the chlorine - I haven't had that explained before. I can see that the CYA means a lower/shorter chlorinator setting to maintain the same FC ppm which leads to a reduced rate of pH rise - I didn't realise there was that indirect relationship between the CYA and pH with SWG. My pool gets the morning sun only and I rarely have the cover off at that time so perhaps the CYA is not having such a major effect. If I allowed the CYA to get used up, I gather that would allow a given FC level to better sanitise but at the same time it would have a detrimental effect on the cover and I assume equipment in general with increased oxidizing.

As far as the SWG vs liquid feeder goes, I'm beginning to see that the FC/CYA relationship exists regardless.

I'll have a good read of the Pool School!

Thanks,
Max.

[chem geek]

Max,

You can read more technical info about the chlorine/CYA relationship in this post and you can see the relationship of "active" chlorine (hypochlorous acid) to pH with and without CYA in this post. The science behind the chlorine/CYA relationship was definitively determined in 1973 and presented in a conference (and book) in 1974 -- this isn't new. For whatever reason, the industry doesn't teach this information even though it is critically important to understand in order to prevent algae growth and to understand disinfection and oxidation rates.

If your pool is usually covered and the cover is relatively opaque to UV, then you are right that you already have relatively low chlorine loss due to sunlight so in your case raising the CYA is probably not needed. Just boosting the chlorine level before you go away would probably be sufficient. I'm confused, however, in how pollen would have gotten into a covered pool (perhaps blown in on the sides?).

As for detrimental effect on the cover, you need to understand that the rate of oxidation (and disinfection) of chlorine has nothing to do with the FC level by itself. It is the hypochlorous acid "active" chlorine concentration that disinfects and oxidizes and this concentration is roughly approximated by being proportional to the FC/CYA ratio. So having a higher FC at a higher CYA keeping that ratio constant makes no difference in the rate of oxidation of pool covers.

My wife experiences this difference based on CYA when she uses an indoor community center pool with 2 ppm FC and no CYA in the winter. Every year, her swimsuits degrade (elasticity gets shot), her skin gets flaky and her hair frizzy. These problems do not occur in our own outdoor pool with 3-4 ppm FC and 30 ppm CYA even after 5+ swim seasons. The difference is due to the factor of 20 difference in active chlorine (hypochlorous acid) levels. In fact, indoor pools are over-chlorinated because they do not use CYA which moderates the active chlorine level and this may be part of the reason why virtually all respiratory and ocular problems associated with nitrogen trichloride are with indoor pools. It may not only be the poor air circulation and lack of sunlight, but the fact that nitrogen trichloride is produced 20 times faster and at 20 times higher endpoint concentrations when chlorine oxidizes ammonia (this is all predicted from the breakpoint chlorination models -- the only question is what exactly happens with urea).

As for your question regarding liquid feeders vs. SWG, they would both have roughly the same issues if the chlorine demand outstripped their dosing levels. Only an automated feedback system that measured chlorine levels (or a rough proxy such as ORP) and adjusted liquid feeder or SWG output would prevent the sort of problem you saw (or you can just turn up the dosing level before you leave). The higher salt level in an SWG pool can lead to problems associated with the higher conductivity (metal corrosion) and splash-out of salt (degradation of soft stones). These problems can usually be mitigated by use of appropriate materials or use of a sacrificial anode or by sealing stone, etc.

Richard

[max]

Hi Richard,

Thanks for all the great information - this has got to be the best pool site in the world!

Looking at the charts, I'm amazed at how little "active" chlorine I have had all this time. I can see how algae could easily form with the levels I was maintaining. I plan to adjust the SWG such that it averages 5ppm rather than 3ppm by having it run longer (currently it is only running 4 hrs/day at a setting of 2 of a possible 5). I may allow the CYA to fall a little over time - the SWG manufacturer recommends 30-50ppm so I think I'll aim for 40ppm given my circumstances rather than the higher levels.

With regards the cover, your assumption is correct - leaves and debris are good at getting in at the sides even though there is only an inch or so gap. Initially I trimmed the cover as per instructions so that it was bigger than the pool but it was very awkward handling it and difficult to get the debris off prior to taking the cover off to the point that I didn't always put the cover back on. Having it trimmed to float in the pool I find is the better option even if it isn't perfect.

The liquid feeder being considered is the Chemigem DM-52 and it does have an automatic feedback system for both FC & pH, perhaps using some rough measurement as you suggest. None of the SWGs I researched in the past have feedback systems which is why my brother and sister-in-law were interested in the Chemigem. I still think it doesn't take much to get it right with a SWG - I just very much underestimated the effect the CYA was having on the FC.

Thank you very much for your help. I plan to do much more reading of this site!

Max.

[chem geek]

Max,

It fortunately takes VERY low levels of "active" chlorine (hypochlorous acid) to kill bacteria and it takes more to prevent algae growth, but still not very much. The green algae inhibition amount of hypochlorous acid that kills it faster than it reproduces so prevents its growth is somewhat less than 0.025 ppm which is roughly 0.05 ppm FC at a pH of 7.5 with no CYA and corresponds roughly to an FC that is 5% of the CYA level. Though you can certainly aim for a higher target, it's probably not necessary except when you believe the FC may drop for some reason due to higher bather load or organics getting blown in or something like that. During the day, the SWG (and pump) should be run when the sun hits the pool to ensure that it keeps up with the chlorine breakdown from sunlight.

Richard

[max]

0.025ppm HOCl is more encouraging - many of the green "possible algae growth" figures in the HOCl chart are higher than that but I realize that was all a guess. I was below the green line in that top chart but I do just make 0.025ppm HOCl. I'm still taking in that page - it is excellent. The % FC vs pH is great too because I've always wondered just how much the pH really influences the chlorine - your comment "not as much as people think due to CYA" holds true - far less variance in the 7 to 8pH range with CYA present. That alone is enough to convince me that CYA is a good idea no matter how much sunlight hits the pool.

I'm in pool heaven here!

[chem geek]

The green line in those charts was just the HOCl level for the example I gave of 3 ppm FC with 30 ppm CYA. That's higher than needed to prevent green algae growth. Ben Powell came up with the initial estimates in this chart based on his experience, but this also has some leeway since it's for manually dosed pools. Roughly speaking, it's around a minimum FC that is 7.5% of the CYA level. We've found that SWG pools can have a minimum FC that is 5% of the CYA level. Much of this is described at the Pool School.

CYA should probably be used in small amounts (around 20 ppm) in indoor pools, but the industry mantra is not to do this because "CYA doesn't matter; only FC matters" and "CYA only protects chlorine from sunlight; it shouldn't be used in indoor pools". Both of those statements completely ignore the actual chemistry that is involved. It may even turn out that part of the reason for respiratory and ocular illnesses that are all reported in indoor pools may be not only from poor air circulation and lack of sunlight, but from the 20+ times greater active (hypochlorous acid) chlorine concentration that produces 20+ times the amount of nitrogen trichloride at a 20+ times faster rate. This is shown in all the breakpoint chlorination models of ammonia by chlorine -- the only open question is what really happens with urea.

Richard

[max]

I'll aim for the 5% minimum for SWG which for 50ppm CYA is 2.5ppm FC. With my current SWG settings, FC sometimes gets to 1ppm so I will have to lengthen the chlorination time a little to ensure a minimum of 2.5ppm is maintained. In doing so, there will be occasions when it exceeds 5ppm or 10% but the HOCl level won't be so bad. I'm still amazed at how much the chemistry changes with CYA.

The green line I was looking at was from the chart "HOCl vs FC at different levels of CYA". I'll keep reading!

Thanks,
Max.

[chem geek]

You might also consider raising the CYA level to 70-80 ppm and having a minimum target of 4 ppm FC. That way the change in FC will be a smaller percentage change in HOCl "active" chlorine. In fact, the recommendation for most pools with an SWG (those with a lot of exposure to sun during the day) is to have the higher CYA level since it protects the chlorine from sunlight more. This is another area where the traditional industry advice is wrong. They were probably looking at commercial pools where the bather loads are high and overwhelm the loss of chlorine from sunlight so it seems like anything above 30 ppm doesn't do any good. However, careful experiments by a forum user showed that the higher CYA levels protect chlorine in a non-linear way probably related to the CYA shielding effect of UV from lower depths.

So you might end up going from 5.5 to 4 in a day or even less such as 5 ppm FC to 4 ppm FC, but at 70-80 ppm CYA that's not very much of a change in HOCl. Remember that the FC number by itself is meaningless. The amount of active chlorine is proportional to the FC/CYA ratio.

Richard

[max]

I see what you mean. First I'll experiment with the SWG and see if I get less variance at a higher level, in percentage terms especially. Eg it is usually around 3ppm but occasionally 1ppm and 5ppm which in percentage terms are major swings. The same 2ppm variance with 5ppm average would be better because as a percentage against itself and the CYA the variance is less. It will take a number of months to assess properly - I record test results to a spreadsheet so I'll be able to make a reasonably accurate comparison.

Because it is FC I am measuring and not HOCl, I gather that the level of CYA has no bearing on the FC readings at all, so the FC range I observe at 50ppm CYA will be the same range I can expect at 70-80ppm - is that correct? (Sorry if this is covered in the long Pool Water Chemistry sticky - I'll have more time to read it properly on the weekend!)

Thanks,
Max.

[chem geek]

I'm not sure what you mean by the FC range you would observe. If you are talking about the expected amount of FC drop per day, that is a function of several different sources of losses, some of which are affected by the CYA level while others are not.

For example, the amount of FC drop from sunlight is a proportion or percentage of the FC, BUT this percentage drop is lower at higher CYA levels. Somewhere between 40 and 70 ppm CYA, this percentage drop gets low quickly such that even keeping a higher FC at a higher CYA to keep the FC/CYA ratio constant, there is less absolute FC loss (not just percentage) at the higher CYA level.

Another source of FC drop is from oxidizing ammonia/urea and organics in the pool. This is mostly a fixed FC ppm drop per day relatively independent of the FC and CYA levels.

Richard