Phosphates and Sequestrants

[TreeFiter]

Hi,

For years, my company has been using a product called Conquest to treat pools with metals in them. Within the last few years a bunch of our guys have convinced themselves that using this product causes high phosphates in pools. I did a little bit of research on the product and there is phosphorus in its chemical makeup, but I don't know if it would result in elevated phosphate levels in a pool. Has anyone else had any experience that would suggest that sequestrants containing phosphorus will result in elevated phosphate levels?

 

[jblizzle]

No experience with it but ...

Even if it did raise the phosphate level ... if you are keeping the FC in the correct range for your CYA, it does not matter what the phosphate level is since algae can not survive.

 

[chem geek]

Phosphonate-based metal sequestrants, including the one we usually recommend, HEDP, will break down slowly from chlorine and produce orthophosphate in the water. However, as was pointed out, the phosphate level doesn't matter if the chlorine level is maintained appropriate to the CYA level since chlorine will kill algae faster than it can grow regardless of phosphate level. Algae growth is ultimately limited by sunlight and temperature.

Now since you appear to be in the industry and may not be able to service pools more than once a week, you have different challenges than the residential pool owners on this forum who maintain their own pools and can add chlorinating liquid or bleach every day or two if needed. You might take a look at the Pool Professionals thread.

 

[TreeFiter]

I appreciate what you guys are saying, but I'm only there once a week, and when I am the levels are typically fine. But my original question was asking if Phosphonate based sequestrants will cause high phosphate levels. Chem geek mentioned that they break down to form Orthophosphates. What I'd like to know is to what extent. If I add a 32 oz. bottle to a 25,000 gallon pool will I see phosphate levels at 200 ppb, or will they be at 1000 ppb? Since phosphates ARE a problem for pools that receive weekly maintenance only, I want to make sure that by treating metals, I'm not i ntroducing phosphates.

 

[JasonLion]

One bottle of sequestrant won't add very much phosphate. But people who use sequestrant constantly can build up a fairly significant phosphate level over time, since sequestrant must constantly be replenished.

It isn't a question of daily vs weekly treatment. Phosphates are never an issue if proper FC levels are maintained. That can be done with weekly treatments, though it is easier with daily treatments.

The other thing to keep in mind is that the sequestrates that do not contain phosphates are dramatically inferior, you will need to use more and as a result they will cost more and still be less effective.

 

[TreeFiter]

Well what I have seen fairly regularly is that pools will turn cloudy or green despite FC levels being held at 3-5ppm. Things will be fine for weeks, and suddenly the pool will turn. I actually saw this today. Everything was within their normal ranges, FC at 3ppm, CYA at 30-50ppm, pH at 7.6. I don't maintain this pool on a weekly basis, but I help out whenever they need it. (Its a friend's pool, not a client). The pool has a Pool Pilot Salt System, salt levels are fine (about 3000ppm.) The pool was fine last weekend. Today I got a call saying it was cloudy. It tested very high for phosphates (greater than 1000ppb.)

With salt systems, the FC levels typically stay high enough with little or no attention for weeks on end. At least this is what I have seen in my experience. So what causes these kinds of issues. Is it that I'm just there at times when the FC is high enough, but when it gets a few more hours of sunlight it drops off allowing algae to grow?

The symptoms I see that usually tip me off to phosphates in a pool are low chlorine levels in a pool that usually has no problem maintaining FC levels. Often this is quite sudden. I run a lot of my pools on the high side (5ppm) so there is a little wiggle room if it gets hot and sunny for a week straight. So when I get to one of these pools and see the FC has dropped down to 1ppm or sometimes less, I know to check for phosphates.

Everybody keeps saying that if I maintain the FC, phosphates don't matter, but I'm seeing the opposite. It appears in my experience that high phosphates will drive down the FC resulting in algae growth. Is there something I'm missing?

 

[duraleigh]

Is there something I'm missing?

You are missing that algae cannot grow in the presence of adequate chlorine. Thousands of people on this forum have pools that do not turn green because they keep the chlorine adequate and they pay no attention to phosphates.

 

[TreeFiter]

I'm understanding that completely, but I'm not understanding how I am getting "adequate" chlorine readings for months and suddenly seeing rapid algae growth. And as in the example I mentioned above, the water was crystal clear on Saturday with excellent chemistry and today the water is cloudy with good chemistry and FC of 3ppm. So if what you are saying is true, how is it possible that this happened.

And how do you explain the sudden dramatic drop in FC levels that I have found to be indicative of high phosphate levels. This does not happen in pools with no phosphates.

 

[duraleigh]

Simply because the chlorine went from barely adequate to inadequate in that time period. There are many, many factors that allow an environment where algae can grow...it is not as perfectly simple as maintaining 1 fixed set of values each day because the conditions change.....especially temps in the summer.

If 3ppm FC in that pool allows algae growth, then hold the FC level @ 5ppm and it won't.

Has the CYA increased in that pool? How accurate is your testing?

 

[chem geek]

Jack Magic's The Pink Stuff has HEDP. The MSDS says it is a proprietary formulation and they don't say how much of that is HEDP, but if we assume it's 60% then with it's molecular weight of 206.02 g/mole and having two phosphate groups per molecule and with phosphate having a molecular weight of 94.9714 g/mole the initial dose of 1 quart per 10,000 gallons would be 1,000,000*((1/4)/10,000)*2*94.9714/206.02 = 23 ppm (23,000 ppb), but more likely it's in the 1-10 ppm range with maintenance. So if all the HEDP were to break down, then it would add a lot to phosphate levels. I know from personal experience that it can't be breaking down very fast or so completely, but my pool got to 3000 ppb phosphates after a combination of 1-1/2 years of using a metal sequestrant regularly (in a preventative maintenance dose) along with 400 ppb phosphates in my fill water (but I have a pool cover on most of the time so minimal evaporation/refill).

However, the phosphate level is irrelevant if the chlorine level is properly maintained relative to the CYA level. There are those on this forum with even higher phosphate levels of 5000 ppb. Now it is true that a pool is very reactive at high phosphate levels -- IF the chlorine gets low then algae grows more quickly but it still takes algae 3-8 hours to double in population under ideal conditions (i.e. with as many nitrates and phosphates as they need for optimum growth) because algae is ultimately limited in growth rates based on temperature and sunlight.

If you are using test strips for your 30-50 ppm CYA estimate, then that can be completely bogus. You have to use the turbidimetric test for any sort of accuracy. Also, 3 ppm FC with 50 ppm CYA is below the minimum FC of 7.5% of the CYA level and it likely fluctuates lower at which point algae growth increases chlorine demand and things fall apart fairly quickly. For saltwater chlorine generator (SWCG) pools, the FC needs to be a minimum of 5% of the CYA level and usually the CYA needs to be 60-80 ppm or else the SWCG is on too long which causes other issues of faster pH rise and wearing out the cell faster. So figure 4 ppm FC with 80 ppm CYA for SWCG pools and 3.8 ppm FC (so say 4 ppm) with 50 ppm CYA for non-SWCG pools.

We've seen this pattern over and over again in thousands of pools from new members to this and other forum (35,000 members on this forum, tens of thousands on others as well). With non-SWCG pools that you aren't visiting more than once a week, having the FC be at a good level one day doesn't mean anything because the next day or two it is guaranteed to be too low unless you are dosing with a peristaltic pump or The Liquidator. If you are dosing using Trichlor tabs/pucks, then they build up CYA. For every 10 ppm FC, they also increase CYA by 6 ppm so even with a 2 ppm FC per day chlorine usage that's an increase in CYA of 36 ppm per month if there is no water dilution.

As a pool service, if you want to use a phosphate remover or a weekly algaecide product then those are options for you (at extra cost, of course), but just realize that you can have the pools be algae free using chlorine alone but will take either automated dosing or a large range in FC swings (see the Pool Professionals thread I linked to earlier). The pool service in my area servicing 2000 pools uses Trichlor pucks/tabs, but with a 4.5 ppm FC target and they do a partial drain/refill of the pools to keep the CYA below 100 ppm. Even so, they still have some pools get algae, which of course is not surprising since their FC target only works up to around 60 ppm CYA, so they shock with chlorinating liquid and if that doesn't prevent a recurrence, then they use a phosphate remover. They know, however, that they can't just let the CYA keep climbing or else even algaecides and phosphate removers won't be enough. I also know this from my own experience in my first years with my pool where 3 ppm FC and an eventual 150 ppm CYA couldn't stop algae from growing even with Polyquat 60 being used, but only every other week.

 

[TreeFiter]

Thank you Chem geek, your calculation is just what I was looking for. Its a bit alarming though. Phosphates are considered a problem starting at 200ppb and your calculation shows results in the tens of ppm.

We do typically use phosphate remover once we run into a problem, but it initiates a bit of a cyclical repetition of adding sequestrant, which raises phosphates. Phosphate treatment requires vacuuming on waste, which lowers the water level. This means we need to add water, typically from the customer's well, which often has Iron which was the reason for the sequestrant in the first place. This is why I wanted to know if sequestrant did in fact increase phosphates. Most of the guys I work with are not educated men and don't think scientifically at all. So their logic goes as far as we put in sequestrant, then that pool had phosphates, so sequestrants must cause phosphates. You could easily make the same leap by saying we vacuumed the pool, then the pool had phosphates, so vacuuming must cause phosphates. Same logical path, but one happens to be partially right. I personally use sequestrants fairly often, and I haven't seen many pools with phosphate problems. It is also possible that I'm doing a better job of maintaining adequate FC levels in my pools so I don't see the tell-tale drop in FC that tells me to test for phosphates.

 

[TreeFiter]

Simply because the chlorine went from barely adequate to inadequate in that time period. There are many, many factors that allow an environment where algae can grow...it is not as perfectly simple as maintaining 1 fixed set of values each day because the conditions change.....especially temps in the summer.

If 3ppm FC in that pool allows algae growth, then hold the FC level @ 5ppm and it won't.

Has the CYA increased in that pool? How accurate is your testing?

Well this is why it is so tricky for me. I cannot be at every pool all the time to constanly monitor the chemistry. I get a quick glimpse once a week. There are many factors, but most of them can be reasonably controlled on a weekly basis. This is why the majority of my pools never have an issue. If I can balance the chemistry and make minor adjustments as needed once a week, they stay clean. What seems to be a variable present with all of the pools that suddenly turn is elevated phosphates.

As far as raising the FC level, isn't there a limit to how high you can push it? What if I do raise it to 5ppm and still see algae? Just how far can I go before it begins to be a health risk for swimmers?

As for the CYA in the pool I mentioned, I went back there and tested again. The CYA was in fact low, which would have allowed the FC to drop when it got hot and sunny. Couple that with a heavy bathing load (they had a party over the weekend) and the algae is given an opportunity to take over.

The idea of not treating the phosphates is new to me, as the company I work for has always just treated them. I may use this pool as my guinea pig. I shocked the pool and within a few hours I could see a noticeable difference. What concerns me is that the salt system was already set to 100% output, and the FC was only 3ppm. But with low CYA, there is a good chance that has something to do with it.

 

[JamesW]

I think that the most important thing you might not understand is that the "right" FC level is completely dependant on the cyanuric acid level.

I have found that maintaining the FC at 7.5 to 15 % of the cyanuric acid level works very well. As long as you do this, you should never get algae, regardless of the phosphate levels. If the pool gets algae, then the FC was not kept high enough. Phosphates are only relevant when inadequate chlorine levels allow algae to begin growing.

It is a difficult thing to comprehend for someone who has been told for years that the chlorine should not go above 5 ppm, that they can safely raise the FC to 15 ppm without problems.

For example, if the CYA is 60, then the FC should be kept between 4.5 and 9 ppm, and the FC should never be allowed to go below the minimum.

Of course, there might be regulations that you have to follow regarding FC limits, especially for commercial pools. In those cases, careful control of the CYA is critical. Even if there are no regulations, the lack of general knowledge in the industry regarding the FC/CYA equilibrium makes keeping the FC higher than 5 ppm problematic.

Are you checking the salt cells for scaling? If yes, how often, and what are you finding?

 

[TreeFiter]

Depending on which type of salt system we are dealing with, we typically check them when we suspect a problem. Ecomatic systems (which I love) will let you know when something is wrong. It won't tell you what the problem is, but when the light comes on you check the salt level and inspect the cell. Compu-pool systems aren't as informative, and we usually end up checking them when we find things aren't right. We deal with a few others, but those two cover the majority of our pools. For the most part we don't see a lot of salt cell issues (maybe 5% of our pools will have a problem in the course of the season). Sometimes there will be visible scale on the salt cells, but unless it affects output we let it go. Most of our pools are easily maintained at 20% output.