Phosphates and sudden chlorine drop

[TreeFiter]

I've mentioned this in other posts, but I thought it would be good to discuss by itself. I have seen sudden drops in FC, which have been considered a red flag to check for phosphates. I have been told repeatedly on this forum that if FC levels are adequate, algae won't grow, so phosphates are more or less irrelevant. What I would like to know is why I see this sudden drop in pools with high phosphates.

This is most often an issue with salt systems. Chemistry will be fine for weeks, and out of nowhere the FC drops, sometimes turning pools cloudy or even green. All other chemistry seems to be within normal healty ranges. This isn't something that happens only in the peak of summer, it will occur from the time we start opening in the spring right on through the when we are closing in the fall. The one common denominator I keep finding is elevated phosphates.

Before you start asking, yes the salt levels are fine and the cells are generating. As I said they run fine for weeks or even months, and suddenly the FC drops. Once we remove the phosphate they return to working fine for weeks or months.

Please don't respond asking for specific chemistry measurements. I'm talking about a broad sweeping trend here. If I told you every time I see a pool with no chlorine in it, it turns cloudy, you wouldn't ask for specific measurements, you would simply say it needs chlorine. The trend I'm talking about is based on about 8 years of experience performing about 800 maintenance visits per season. These are not situations where CYA becomes elevated or anything like that. These are pools that are well balanced, crystal clear and sparkling that suddenly experience a drop in FC for no apparent reason.

Does phosphate interact with FC in some way to decrease its ability to sanitize? Does phosphate drive off chlorine resulting in low levels of FC? Is there anything about phosphates that can result in what I'm describing?

 

[JasonLion]

FC falls because algae has started to grow faster than chlorine is killing it off. At low CYA levels this generally happens suddenly, you either don't have algae or you do have algae. But at the higher CYA levels typically used with SWGs, things can be extremely close to balanced with algae growing, but too slowly to be visible, for weeks or months before it breaks out as a full algae bloom. The higher CYA levels hold a great deal of chlorine in reserve. That chlorine is released as the active chlorine is used up, fast enough to prevent the algae from getting out of hand, but when FC levels are low, not fast enough to fill all of the algae.

You may well be running the FC level below our recommended levels. At slightly lower FC levels things are often fine for a while, even a month or more, but the level is too near to danger zone and eventually you cross over and algae gets started. Another other possibility is some event that uses up a particularly large amount of chlorine, pee in the pool, a big storm, etc pushes the FC level below the threshold and algae gets started.

Nothing about phosphate has anything directly to do with it. Phosphates have only the most microscopic of direct effects of chlorine activity. It has to do with algae. If phosphate is very low, algae can't grow regardless of FC levels, so algae isn't an issue. But at any phosphate level above very low algae can grow unless there is chlorine (or algaecide). My presumption is that you routinely lower phosphate to very low levels and have gotten used to running FC well below our recommendations, which only works as long as phosphate remains very low. When phosphate builds up, the FC level is insufficient to prevent algae, algae starts growing, and FC levels fall.

The very low phosphate approach has some possibilities, but it also tends to be more expensive and more work than simply raising FC levels just a little to the levels we recommend. There are also situations where lowering phosphate is simply out of the question, such as regions where phosphates are added to the tap water and lowering phosphate becomes a never ending battle that you can never win. Likewise, slightly elevated FC levels are more than justified by the ability to use the phosphate containing sequestrants, which are significantly more effective, assuming you need to use sequestrant anyway.

 

[JohnT]

Can you define sudden? A few hours, a day, days?

Phosphates aren't going to do that. Phosphates provide food for algae, but the don't cause it to start growing and they don't make it less resistant to chlorine.

What I would suspect is that something is happening to a pool that is near the threshold for algae to start. Either CYA or FCC are borderline low, or bather load surges or kids swim in the pool. Once the pool gets hit with anything that causes the chlorine to dip a little, algae can get started, and once it gets started it will easily consume all of the chlorine produced by the SWCG. You can pour chlorine in like crazy a few times a day and never measure any FC if you aren't getting near shock level and you aren't hitting it every hour or so. It is stunning to see how fast algae can consume 20ppm of chlorine in a pool.

ETA: Jason beat me, but I'm not wasting all that iPad typing!

 

[chem geek]

Please don't respond asking for specific chemistry measurements.

Let's put this another way. We've gotten people, including service folks, coming on to this and some other forums saying that they can't get any FC reading in the bulk pool water but that the cells are working and they get measurable FC coming out of the returns and that the water is clear (sometimes they say it's dull or cloudy). They attribute the problem to "phosphates eating/consuming chlorine", but that's not at all what is happening. It's simply that the phosphate level is enough to have algae grow at a rate where chlorine is just barely able to kill it because the FC/CYA ratio is too low and the chlorine gets consumed doing so. It's right on the edge, like a knife balance. In every single case when this has occurred, and I mean EVERY single case, we have just had the people shock the pool to relatively quickly get ahead of the algae growth and kill it all off and then we have them lower the FC to NO LOWER than the minimum FC recommended for the CYA level (and if the CYA is too high, then they do a partial drain/refill to lower it). After that, they don't get any repeat occurrence of mysterious chlorine demand or algae growth (for green algae -- yellow/mustard algae is a different type of problem requiring a different "eradication" treatment since it is more chlorine resistant and hides out in shaded areas including behind light niches).

The thing with algae nutrients, such as phosphate and nitrates, is that there is a limit to how much they can contribute to algae growth. At some point, the amount of sunlight and the temperature of the water (too hot as well as too cold) become limiting factors. The maximum rate of growth for algae is around 3-8 hours to double in population depending on species, but when the algae has grown enough for the water to still be somewhat clear yet have a lot of algae available, it can take only a couple of generations, so less than half a day, to become dull and another half a day to become cloudy or green depending on species (eventually all green algae species will look green, but some clump more than others and the clumpy ones look green while the diffuse ones look more cloudy. That's what makes it seem like it goes bad overnight. It actually hasn't and was building up to the edge in the days before.

I've seen this slow buildup starting out with a mysterious chlorine demand in a seemingly clear pool myself in my own pool 8 years ago when I was using Trichlor pucks in a floating feeder. I had 3 ppm FC but my CYA had risen to 150 ppm after 1-1/2 years and I had a harder and harder time maintaining the chlorine level, resorting to putting pucks in the skimmer (yes, I now know that's bad, though I did have the pump running all the time) in a vain attempt to keep up with the demand in spite of using Polyquat 60 algaecide though only every other week. Things slowly got worse and the water got dull and then cloudy by the time I found The PoolForum and started to learn pool water chemistry and take charge of my pool. I lowered my CYA level, have used 12.5% chlorinating liquid, and have never had an algae problem in spite of 3000+ ppb phosphates at times except for one spring opening when I was sloppy and let the chlorine get to zero resulting in bacterial conversion of CYA into ammonia (see this thread).

Quite frankly, until you start making measurements using a high quality test kit, such as the Taylor K-2006 or TF-100 where with the FAS-DPD kit you can measure chlorine to within 0.5 ppm (even 0.2 ppm, with a 25 ml sample size) and where the CYA is measured more accurately (test strips are practically useless in that regard -- see this post), then there's no point in discussing what you see since we are just guessing as to what is going on, though the pattern is the same as many others where we do have clear explanations from accurate testing (after they've gotten the good test kits we recommend).

No one here is saying you have to stop using phosphate removers and only follow the Chlorine / CYA Chart or the Water Balance for SWGs, but since you haven't been following those and are simply reporting how pools seem to mysteriously go bad after being fine for quite a while after using phosphate removers, you can't really make any valid criticisms of the method. Phosphate removers should be seen in the same vein as algaecides. They slow down algae growth enough so that chlorine can kill it. If things change, such as phosphate levels increasing, then that balance can be thrown off and algae can grow faster than chlorine can kill the algae. So if you want to maintain FC/CYA ratios lower than recommended on this forum, that's fine but to prevent algae in all your pools you'll need supplemental products (algaecides or phosphate removers) to prevent algae growth. I've written before about the pool service in my area that makes way more than 800 maintenance visits per season since they have over 2000 customers (yes, it's a big operation with many service personnel) and they target 4.5 ppm FC and do a partial drain/refill when the CYA hits 100 ppm and if algae develops in pools they first shock and if that doesn't work (i.e. the algae comes back again) they use a phosphate remover. They don't follow the FC/CYA minimum, but they do know (and have told me) that they understand that higher CYA is a problem as they've seen for themselves (they don't see any issues in pools until the CYA gets closer to 100, which of course is not surprising to us) and that they've struck a balance between setting a CYA limit, shocking and using phosphate removers that works economically for them. We have 400 ppb phosphates in our fill water and we have water restrictions where most pools have cartridge filters.

 

[TreeFiter]

Thank you guys so much. You have really made it pretty clear what is actually happening. My reason for joining this forum in general is because I want to get good scientific information that can be tested and verified. With the company I work for, I'm pretty sure I'm the only guy there that can even spell "science". My boss has been running this company for over 30 years, and I'm convinced he doesn't really know what hes doing. Aside from that training is minimal, and is usually wisdom passed down from another tech that doesn't know what they are doing. I've never been told about the relationship between CYA levels and FC. I've heard all kinds of hocus pocus around phosphates and how they consume FC. It is very hard to do a good job when there is noone available to help you become better at what you do.

In the past few days since I joined the forum, I have learned more useful information than I have in the 8 years working on pools. As I learn it, I start looking to apply it, and I have already come to the conclusion that test strips are pretty much worthless. Unfortunately this is how I am expected to test pools, and I don't see my boss trying anything better. I have already found that the pH is way off when compared to a traditional phenol red test. The rest of the parameters on the strips are inadequate because they only provide a ballpark range. For example the color chart for CYA jumps in 50 ppm increments.

From what I have seen so far, it seems like the point of this forum is that swimming pool maintenance is fairly simple as long as you can maintain adequate FC levels. I've always been a believer in keeping FC levels up, but I was always worried that it would be too high. I'm thrilled to be learning that I was on the right track.

 

[rubberboots]

yes Treefiter...can i ask if you are using any pool perfect+phosfree?

 

[ps0303]

chem geek, your reply is great and has lots of info in it. The only problem is that the OP is working for a service company and when he goes to a customer that has this algae problem and the CYA levels are thru the roof and he tells them they really need to drain and fill some water, they are going to tell him he is nuts. So many people out there don't understand things and refuse to even listen to someone that services their pool. I know this myself as I have a customer that barely understands English, old Italian lady, and when I talk to her about her problem she thinks I am insane. However she has no problem calling me every little time there is a green spec in the pool. I've even tried to convince her that she doesn't need the cartridge cleaned weekly but she has done this for years and won't listen to me. However I listen to all the time as she complains about how it hurts her back to clean the filter. Plus she watchers her money like a hawk and won't pay me a little extra to clean it for her.

Thanks again for the great post. I really wish more people understood pools before they had them installed.

 

[TreeFiter]

So I have been keeping this thread in mind while I work on pools in the heat. The weather has been pretty hot and sunny with virtually no precipitation for the past few weeks. This is the weather where pools demand more FC. I've noticed this sudden FC drop in several pools over the past few weeks, and I have been shocking them and turning up the salt systems. This should take care of the problem regardless of phosphate levels, which in almost every case has been high. The ones that concern me most, and that I'm not sure how to handle other than to use phosphate removers, are pools that already have the SWG turned up to 60 or 70% and still experience the sudden drop. Often I run my pools at about 20% for the first part of the season, and when it gets hotter I turn them up to 40 or 50%. When they are already at 70% and can't keep up with the chlorine demand, there is very little that can be done to prevent the algae bloom. If I understand correctly, the higher the output, the more it wears out the cell. So how can I keep a higher FC level in these pools without burning out the cell? Could I avoid this problem by keeping FC levels higher earlier? I think what I'm taking away from all of this is that as the algae progresses, the consumption of chlorine goes up. So if I am able to prevent the START of algae growth I won't have to run at higher levels to prevent it from reaching a full bloom. Is my understanding correct? I realize that CYA levels are also a factor, but I haven't been seeing an issue with high CYA levels or anything like that.

 

[JasonLion]

Running at a higher percentage doesn't "burn out the cell". Cells have a lifetime measured by the total amount of chlorine they can produce. If you turn up the percentage you are producing more chlorine, which means the cell won't last as many calendar months. But it will still produce just as much total chlorine over it's life time as it would otherwise. The whole point of a SWG is to produce chlorine, you might as well use them for what they were designed to do.

If you run at appropriate FC levels, algae will never get started, and there will never be any excess chlorine consumption because of algae. With a SWG and high CYA levels algae can get started and not be obviously visible except as increased chlorine consumption. But that can only happen if the algae can get started in the first place.

 

[TreeFiter]

Thank you for your explanation. The problem is that it can be very difficult to explain to a customer why their salt cell is burned out after a year, and costing them several hundred dollars to replace.

I'm struggling with this whole concept a little bit. What I'm being told makes total sense. I'm convinced that it should work. I just keep running into problems that don't seem like they should be problems based on what I've been doing.

I just started to type out what I was doing and figured out that if I use the rule of thumb that FC should be 10% of the CYA, the FC needs to be above 5 if I'm seeing CYA levels of 30-50 on a test strip. If I go by 7.5% I would need to have a minimum FC of about 4. These are bordering on what would be considered "high" chlorine levels according to the guidelines provided by test strip manufacturers, and most traditional water testing kits. Even the basic pH and Cl tests where you add the drops and match the color typically only go up to 3.2ppm Cl. So it seems that I need to keep pools at a level that would be normally considered high.

So why are levels greater than 3ppm considered high? How high is too high? I'm trying to beat some of the conventional wisdom out of my head and understand how it should work rather than just rely on what I've been told by the pool industry. What are the risks of keeping chlorine levels too high?

 

[Bama Rambler]

All those recommendations are based on a pool with zero CYA. The pool industry is very slow to recognize the chlorine/CYA relationship.

 

[chem geek]

Are the SWG pools you describe running with 4 ppm FC and 80 ppm CYA? You don't need the FC to be 10% of the CYA level. For SWG pools, 5% should be OK. That should let you have a lower on-time. Now there my be some pools where the SWG that ws purchased is undersized. It is much better to buy somewhat over-sized SWG systems since larger capacity systems don't cost proportionately more, but they last proportionately longer. The use of 50 ppm Borates is also helpful both as an additional pH buffer and as a mild algaecide. You'll also want the TA lower at around 70 ppm to slow down the rate of pH rise. See Water Balance for SWGs for more details.

 

[TreeFiter]

If I were to adhere to 5% of the CYA, it would put me right about where I keep my pools. The majority of them stay right around 5ppm. A few may get down to about 3ppm. The CYA levels are typically 30-50 according to test strips. I usually assume that means 50ppm. I don't think the SWGs are undersized. The vast majority of situations they are able to maintain for most of the year at less than 30%. I think I'm just seeing the chlorine being consumed by algae to the extent that the SWG can barely keep up.

If that is the case, wouldn't a good shock, followed by turning the SWG to a higher setting than I started with, most likely kill off the algae and in a sense reset the chlorine demand? For example if I have been running a SWG at 30% and started noticing low FC levels, and I've had to keep bumping up the percentage incrimentally each week and have reached about 80% now. If I were to shock the pool bringing FC to between 10 and 20 ppm, then set the SWG to 40%, is it reasonable to expect the pool to maintain at that level?

I realize that it might not work with these exact numbers, but is the general idea on track?

 

[chem geek]

You assume that it is algae that is causing this issue of not being able to keep up, but the problem occurs during warmer temperatures and the longer days of peak summer sun. What I am saying is that your lower CYA levels (if that is truly the case) are creating more chlorine loss during the day from sunlight, the SWG isn't keeping up, and when it dips low then algae can start to grow faster than chlorine can kill it. If you were at 80 ppm CYA, then with a target 4 ppm FC MINIMUM AT ALL TIMES you should be OK (this means a minimum of still having 4 ppm FC in the morning when the SWG turns back on). Yes, you would still need to increase your SWG on-time percentage as peak summer approached, but the percentage would be lower than it is today because the higher CYA level significantly lowers the amount of chlorine loss from the sun which is the largest part of chlorine loss.

With your current situation, shocking would kill off the algae, but would not solve the fundamental problem which I believe is that you are 1) losing too much to sunlight and 2) are using test strips that are useless so you don't really know your true FC and CYA levels.

If you are reading 30-50 ppm on CYA test strips that is completely and totally meaningless. We have seen over and over again that test strips are inaccurate, most especially for the CYA test (though pool test strips don't even test for CH at all -- they test total hardness). If you knew how the chemistry of the CYA test works on the strips, you'd realize that they can't be accurate since their result varies depending on TA and other levels (it's essentially a pH measurement after precipitating CYA with melamine in the strip).

Please, please, please get yourself a proper test kit, either the TFTestkits TF-100 or the Taylor K-2006. Until you accurately measure the CYA levels AND the FC levels in your pools (the FAS-DPD test is far more accurate than a visually compared DPD test), you can't really know what is going on. I suspect that your true FC level is too low relative to your true CYA level.

 

[TreeFiter]

I think you make a great point about using a good test kit, but its not up to me in this case. My boss wants us to use test strips because its the cheap and easy way. I'm the only person at the company that is attempting to understand what is going on inside a pool on an academic level.

What is the ideal CYA level? If I understand correctly at some point the CYA does more to inhibit FC than it does to preserve it. At what concentration does this happen?

 

[JasonLion]

at some point the CYA does more to inhibit FC than it does to preserve it.

No, that is a myth. Total chlorine usage goes down as CYA goes up, even when you increase the FC level to maintain the same active chlorine level. The problems with high CYA levels come from elsewhere.

We recommend CYA be between 30 and 50 without a SWG, and between 70 and 80 with a SWG.

There are a couple of different tradeoffs, but the main one is the amount of chlorine used day to day, vs the challenges of shocking the pool at higher CYA levels. There are also more complex issues when CYA goes over 90, for example none of the tests are good at measuring high CYA levels, so if it difficult to know what your CYA level really is when it is 100+. The PH test also starts to get problematic when FC is above 10, which is required when CYA is very high. And so on, there are a number of issues like that.

 

[chem geek]

I'm the only person at the company that is attempting to understand what is going on inside a pool on an academic level.

Then I suggest you buy a proper test kit just for yourself to use on problem pools (so presumably limited in number so you don't burn through reagents). Otherwise, we can't really help you with your questions except on a theoretical level. The numbers you provide, especially for CYA and to a lesser extent for FC, won't be much better than not providing any at all.

 

[TreeFiter]

I'm not sure I understand how this works. Isn't the whole point of the FC and CYA relationship that if you have more CYA, you need more FC? You use the term "active chlorine". I'm assuming this refers the the chlorine available to actively sanitize. The amount of active chlorine is based on the CYA/FC ratio. So by raising the CYA you reduce the active chlorine, unless you increase the FC. So in order to maintain an adequate active chlorine level with a higher CYA level it is necessary to raise the FC. With a SWG this would mean that the more CYA in the pool, the more you need to run the SWG (higher percent duty). So if I'm struggling to keep up with FC levels, wouldn't it allow me to turn down the SWG if the CYA is kept on the lower side.

 

[TreeFiter]

I am considering investing in a test kit, but since it would have to come out of my pocket, its tough to justify. Since there is money to be made in constantly shocking pools and doing phosphate treatments, I don't see my boss having much interest in avoiding these problems. I know its a bad way to do business, but its the game I'm stuck playing to keep the paycheck coming in.

As far as there being a limited number of problem pools, this is true, but since I'm the only guy who looks beyond the color on the test strip and thinks about why things go wrong, I've found myself a niche of fixing the problem pools. So I do see quite a few.

 

[JasonLion]

Isn't the whole point of the FC and CYA relationship that if you have more CYA, you need more FC?

You need to distinguish between the FC level and the total amount of chlorine you need to add to the pool. As CYA goes up you need a higher FC level, but you add less total chlorine to the water to maintain that level.

CYA protects chlorine from sunlight. Higher CYA levels mean less chlorine lost to sunlight each day. In an outdoor residential pool, sunlight is the primary consumer of chlorine, so CYA levels have a large impact on total chlorine usage. It is somewhat counter intuitive, but it works.

The net effect is that you need to turn down the percentage setting on the SWG as the CYA level goes up, and you still maintain a higher FC level, even though the SWG produces less chlorine.