Truth About Phosphates

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C

chem geek

TFP Expert
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Mar 28, 2007
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San Rafael, CA USA
I'd like to figure out, once and for all, the truth about phosphates. I don't mean the fact that it's a nutrient (along with nitrates and carbonates) for algae. I mean whether 1) it somehow consumes chlorine on its own, not just as part of faster algae growth and 2) at what level of phosphates (if any) does the standard FC/CYA chart not work anymore.

To address these issues, I think one of the easiest things to do is to have TFP users actually measure their pool's phosphate levels. We may very well find many pools with what the industry believes are "high phosphate levels" actually be clear and algae-free at the recommended FC/CYA levels and some such pools start to develop algae at expected lower FC levels and be able to be shocked back to being clear again. We already know some users (waterbear, for example) whose pools have high phosphates (3000 ppb, I believe) yet are algae-free using from the standard FC/CYA ratio recommendations (in waterbear's case, the standard FC/CYA is before he got an SWG and used borates). To this end, there are phosphate test strips from AquaChek (called AquaTrend Phosphate Test, I think) with 50 tests for $10-$18 depending on where you buy it (some descriptions say it's only 10 tests, not 50, but the package says 50). Taylor sells a test kit, the K-1106, that sells for $27.25 and tests for phosphate levels from 0 to 1000 ppb though I suppose that the dilution method could be used to test for higher levels (same with the AquaChek/AquaTrend test strips). Does anyone know if either or both of these are accurate? Pool store tests are free, but I'm not sure I'd trust them.

I suspect that there may be some very, very high phosphate level (10,000 ppb?) where higher FC/CYA ratios than what we recommend may be required, but I could be wrong about that.

I think that one way to convince those that high phosphate pools can still be managed with chlorine alone is to show that many such pools are indeed OK. We already know that pools that get algae are often tested for phosphates, but the converse typically isn't done -- that is, well-managed pools aren't usually tested for phosphates. If they were, then it might show that well-managed pools with sufficient chlorine won't get algae even with "industry" high (what is that anyway -- >1000 ppb?) phosphate levels. At that point, phosphate control would become something more like using an algaecide or copper -- another method of controlling algae, but not the only nor required nor inexpensive way to do it.

Any thoughts about this?

Richard
 
We reguarly get people coming here who have had pool store tests show phosphate levels between 1000 and 2000, who start following our directions and stop having algae problems.

AuqaChek says phosphates at 250 ppb or higher promote algea growth and that phosphates should be held at 100 ppb or below to prevent algae. Natural Chemistry says to keep phosphates below 125 ppb. Natural Chemistry also appears to have a test kit, but I can't find anyone who sells it. The Natural Chemistry test says it shows levels up to 2500 ppb.
 
I have some strips from Natural Chemistry that I bought locally - sort of a pain to test, even with the strips - I'm not sure they are worth a lick, but they are supposed to test from 0 - 2500 ppm.

The strips are expensive, too. There are only 10 strips in the container and while I cannot recall exactly what I paid for them, I remember thinking it was a lot of money for only 10 strips. I'm thinking it was over a dollar per strip. Then, I messed up during the first test, so wasted one strip.

The way you test with these strips is to bend one in half - there are 2 pads, one on each end of the strip.
Place sample water in a tube, place bent strip into tube, cap and invert 5 times, remove strip, then site the water color looking into the tube placed next to the color samples on the container.

So, with what I have, if you'd like me to help you test, I'd be happy to do so.
 
Sorry Buggs, I must have missed your post the first time. Yes, if you can measure your pool that would be great. I'd like to hear from waterbear (Evan) on what he thinks the best phosphate test is (AquaChek/AquaTrend or Taylor) so I can purchase one for myself and test my own pool. I did see a report on another form of a use who's had 4000 ppb in his pool for years and never had any problems, but I don't know the FC and CYA levels for that pool and also wanted to confirm no use of algaecide. I think I've seen the test strips for $10 for either a pack of 10 or 50 -- it's confusing since I've seen both. I may just go see what my pool store has this weekend and/or have them test the level as well.

My hunch at this point is that Ben's chart for minimums is probably good for phosphate levels up to somewhere in the neighborhood of 3000-5000 ppb (which is very high and is why his chart is good for nearly all pools, at least for green algae), but a pool with 10000 ppb which would be extraordinarily high *might* need higher chlorine levels to prevent algae. Also, it seems that the algae bloom comes on more quickly, due to faster algae growth, if the chlorine gets low in a higher phosphate pool so that will make it seem harder to combat unless caught quickly or prevented in the first place. But we'll see...

Richard
 
LaMotte has Insta-Test® Phosphate Test Strips, which test to 2500, around $10 for 25 strips. They are listed as a new product at their web site and the description matches the description of the Natural Chemistry test strips (which appear to no longer be available).

AuqaChek recently changed their phosphate test packaging. The old test came in a box of ten and the new one comes with 50 "powder pillows" in a plastic jar. The price doesn't seem to have changed. Their web site says "Previously packed in a cardboard box".

I am happy to order which ever you pick and test my water. It might take quite a lot of people doing testing before we find anyone with levels above 4000, though that is of course a guess.
 
I've said it before and I'll say it again...
My pool and many of my customer's pools have orthophosphate levels in excess of 1000-2000 ppm and, with properly maintained water chemistry, do NOT develop algae (nor is an algaecide needed either!)
 
waterbear,

I know that -- I'm more interested in what really high levels do -- 3000+ (though I believe your pool was at 3000 so that's clearly OK). Maybe Ben's chart is fine for all levels as it may be OK at even optimum algae growth rates, but I'd like to know more definitively. Otherwise, if there's a 5000-10,000 ppb pool that doesn't respond to high chlorine levels and continues to get green algae even with Ben's chart being followed, then our "phosphates don't matter" argument won't hold and we'll have egg on our face. I'd like to know that before going down that road so I can say that "these chlorine levels will work for almost any pool unless the phosphates are at extraordinary levels above XXXX ppb at which point higher chlorine levels are required".

This post is from a guy who has had 4000+ ppb phosphates and never had a problem, but he never responded with his FC and CYA level nor whether he used an algaecide. mr_clean's comment about 1000 being a problem isn't relevant since he didn't manage the FC/CYA relationship in those pools. I just want a more reasonable cleaner cutoff value, if any. I am not detracting from the main point which is that phosphates never matter if there is sufficient chlorine and pool stores are just using this as another way to sell more expensive product. The question is whether Ben's chlorine levels are sufficient for all levels of phosphates or just below some high level.

And then there's still the nagging possibility of organics that act like CYA, but aren't measured, as seemed to be implied in the Pinellas study. But I'd like to tackle the phosphates first before moving on to these even more rare cases.

And Jason's point is a good one -- we may not find anyone with high enough phosphate levels to determine the truly highest limit (if any) to Ben's chart. Perhaps this would be better to do with experiment, but I thought I'd at least ask, just in case we get some good info right away.

Richard
 
This thread got me curious, so I got some LaMotte phosphate test strips. They say low range on the package but there is no sign they sell another kind. The LaMotte test strips test up to 2500, but they have the most resolution between 0 and 500. The seven shades on the comparison chart are 0, 100, 200, 300, 500, 1000, and 2500. You mix the strip with a vial of water, remove the stip, and the judge the color of the water (sighting through 4" of water in a long vial).

The pool water maxed out at 2500, so I did it again with a 3:1 dilution with tap water and it maxed out at 2500 again. That seemed strange so I tested my tap water. My tap water tests at 800-1200! That surprised me so I checked with my water company. My water company, DC-WASA, says they have been delivering water between 2200 and 2600 recently, see this chart, so I am actually low for this area! Keep in mind that phosphates are measured as ppb, while mg/L is equivalent to ppm. Natural Chemistry must love this area. I can't imagine how much PhosFree I would need to keep the phosphates down given that level in the fill water.

Did the pool again at 3:1 with distilled water and got 4000+, the shade was darker than 1000 but no where near 2500 so hard to say 4000-6000 perhaps? I have no idea how accurate the test strips are, particuarly after a 3:1 dilution. I did each test twice and got very similar results.

I normally run FC between 3 and 5, CYA around 40, with a SWG and don't have any problems with algae. When FC falls to 2 the water has a dull look to it and something that looks like dead algae accumulates slowly in drifts in the shallow end. The one time FC fell to 1 (the percentage seems to drift around on my AutoPilot sometimes) I had the start of an algae bloom within hours (light green very mildly cloudy water) but killed it off easily without even having to go all the way up to shock level. I dumped in the bleach I had (FC of perhaps 8 plus the SWG) and by the time I got back from the store to buy more the water was already clear.
 

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Jason,

That is fantastic information. Very detailed and a pool at the higher end of the range. Most SWG pools operate with an FC of 4.5% of the CYA level as with 2.7 to 3.6 ppm for 60 to 80 ppm CYA so their "dull" level is mostly below that. But a small number of users found that they needed to keep higher levels closer to Ben's Min column and it sounds like you are in that situation as your minimum is around the non-SWG minimum of 7.5% of the CYA level. This is all very consistent (and I love consistency!).

The only wrinkle to this is that an SWG pool not only has more consistent dosing, but it also has some superchlorination of part of the pool water passing through the SWG cell, and this is acidic and high in disinfecting chlorine concentration near the chlorine generating plate (unlike manual hypochlorite addition to the pool which is basic/alkaline and lower in peak disinfecting chlorine concentration).

So, I would say that your data implies that Ben's minimum may not quite hold in that 4000-6000 ppb phosphate range for a manually dosed pool, though for an SWG pool it's OK. An educated guess, based on waterbear's 3000 ppb level and other reported evidence has me think that Ben's chart and our recommendations are reasonable up to somewhere around the 3000-4000 ppb range, but above that somewhat higher chlorine levels will be needed. At some point, algae growth should max out regardless of food source (they can only divide so quickly), but I don't know if that limit is far higher than these ranges you are seeing.

And, as waterbear has pointed out before, nitrates are also a limiting food source so the single phosphate number is not necessarily the only thing to look at. Just out of curiosity, did your water report show the nitrate level? Of course, I have no idea what nitrate levels are good for algae growth (I'll have to find out more about that), but I suspect that nitrates are probably in most pool water as they are produced from side reactions of the oxidation process (normally, nitrogen gas is produced, but nitrates can be formed instead).

Anyway, thanks for the info. It is VERY enlightening and very interesting that the fill water in some areas is high in phosphates!

Richard
 
Waterbear has lots of experience with aquariums where the nitrates are more of a critical factor than phosphates. This link he sent to me a while ago implies blue-green algae at higher phosphate, lower nitrate levels which sounds like your situation and would imply blue-green algae. I'm not sure if it's that easy to tell the difference in a pool between blue-green algae and green algae nor if this "Redfield ratio" applies to pool water -- perhaps under a microscope. Other sources say that when the N : P ratio is 16 or more (this corresponds to a Nitrate:phosphate ratio of 12.5) and Silicon is available, then diatoms are formed, otherwise if Silicon is not available, green algae is formed (Redfield ratio of nitrate to phosphate says 25 or more is for green algae while 16 is ideal for no algae and under 10 is blue-green algae). If the N : P ratio is low, then blue-green algae is more likely. The reason is that blue-green algae can get their nitrogen from nitrogen gas (dissolved in the water), not just from nitrates. So the implication is that phosphorous may still be a required nutrient for all algae (it forms the backbone of DNA and RNA, is the key element in ATP that stores energy in cells -- ATP is produced during photosynthesis -- and phosphorous is in cell walls/membranes in the form of phospholipids), but will not be the limiting factor for green algae unless nitrates are [EDIT] high and not the limiting factor themselves [END-EDIT] (or ammonia, which is another source of nitrogen similar to nitrates -- but with ANY chlorine in the water, any ammonia will be converted to monochloramine).

So while looking at phosphates alone does not give the entire algae nutrient picture, I wonder if pools with very high phosphates that have algae blooms have blue-green algae instead of green algae. As for Ben's or my charts, it only makes sense to describe a phosphate "limit of applicability" to those charts when nitrates are not the limiting factor. This is why waterbear says it makes little sense to talk about phosphates without also looking at nitrates. Since the nitrate test is much more expensive than the phosphate test, it's just a lot easier to 1) try keeping away algae with the recommended FC/CYA levels in the table since that works for nearly all pools and 2) if that doesn't work, then and only then look at phosphate levels and consider reducing them or using an algaecide. This is no different than what we've been recommending all along, but I just wanted to get some rough quantifying of the table's possible limits (or adjusted higher FC levels for higher nutrient levels). Thanks again for your help.

Richard
 
Well, Natural Chemistry and Leslie's teamed up to promote their 5 Year Algae Free Guarantee. Essentially, their whole thing is about removing one essential element that helps promote algae, and keeping your pool operating under normal levels. The basic run down:
-Commit to (by registering) using PhosFree + Pool Perfect as a weekly maintainence (one capful per 8k, added through the skimmer)
-Keep pool balanced under normal ranges (2-4 ppm FC, 7.4-7.6, roughly -PH, etc. etc.,) and let Leslie's test your water 4 times per season. Normal circulation, etc.
-At start up and close down of your pool, add Pool Magic + PhosFree (I think it's about 1 liter per 25k)

Then you're guaranteed no algae for 5 years, and if you get it, Leslie's gives you the chemicals for free to fix it.

I may have some of the details a little off, but that's the basic run down. SO, I suppose they're very confident in that if you eliminate one source (i.e., phos, nit.), then you should have no problems.
 
These sorts of "algae-free guaranteed" programs are not new. Prior to the latest phosphate removal craze, similar programs used a weekly algaecide such as PolyQuat 60. We know that will also work. The key piece that isn't said in both of these programs is that the reason an algaecide or phosphate remover is needed at all to keep away algae is that the Free Chlorine (FC) to Cyanuric Acid (CYA) ratio is not maintained! So they don't tell you that chlorine alone will keep away the algae so long as you watch your CYA level and have enough FC accordingly.

At the recommended maintenance dose for both products -- PolyQuat 60 and PhosFree -- they are roughly comparable. The initial dose for PhosFree can be a killer if you've got any decent amount of phosphates (and most pools probably do). So the real big profit is with people who start this program. That's a much bigger one-time profit hit compared to the older PolyQuat 60 algaecide program.

Richard
 
I'm a little late to the party, but I think I can add to this discussion.
Natural Chemistry used to make a phosphate test kit that tested up to 5,000 or 10,000, I can't remember. Early in the year we would get people whose pools were full of decayed leaves from the winter whose phosphates would test at over 5,000 ppb. These pools would be a strange cloudy color, not green or any other color. One of my service men could look at a pool and decide if it were 'regular' cloudy or 'phosphate' cloudy. He was always correct upon testing. This cloudiness normally happened at phosphate levels above 4,000 or so, and it was impossible to get a chlorine reading. I'm talking adding 40 litres of 12% in one shot to a 24' above ground and 3 hours later having a reading of 0ppm after sunset. A dose of phosfloc later, the phosphates would be closer to 1,000 and the pool would be clear and you could easily get a chlorine residual. We rarely prescribed the other Natural Chemistry phosphate products that would lower the levels below 1,000.
I rarely saw a problem after the beginning of the season, outside of the customers that I considered the type to let the pool get away from them. Basically, if the pool didn't start the season with the problem and the customer kept their chlorine in line, they never had a problem and we never had a reason to test for phosphates. We recommend that most of our customers skip the algaecide and use liquid chlorine alone, or pucks and liquid chlorine weekly.
We now use the Aquatrends kit and I find that you can sometimes see a darker blue than the one they show as 2500 ppb; I consider those pools to be 3,000 plus.
 
That's interesting about the chlorine consumption when the phosphates were high (above 4000 ppb). The cloudiness was very likely to be algae forming since that looks cloudy before it turns into a full-fledged bloom that is green. Such a pending bloom would consume a lot of chlorine, especially if the CYA were high so that the needed amount of chlorine to stay ahead of the algae growth was higher. Nevertheless, a 24' above ground that is 4' deep is around 13,500 gallons while 40 liters of 12% chlorinating liquid is 10.6 gallons and would raise the FC by 94 ppm!

Do you know which chlorine test was used for measurement? OTO (yellow color), DPD (red color), or FAS-DPD (count the drop FC or so. It obviously wasn't the DPD test since that would bleach out and show no chlorine above around 10 ppm FC. So was the chlorine test OTO or FAS-DPD?

Orenda at another forum here also describes some sort of reaction of chlorine with phosphates though we've never figured out what it could be except possibly what I described above in terms of rapidly growing algae. However, with the numbers you described, I can't imagine the sort of algae growth that would consume chlorine that quickly. An algae bloom does consume a lot of chlorine, especially initially, but your numbers seem pretty high nevertheless even if the CYA level were high (at least up to 200 ppm). The thing to keep in mind, however, is that unless the phosphates somehow act as a catalyst for chlorine self-degradation, they can't participate directly in a chlorine-consuming reaction as there simply isn't enough (i.e. it would get used up).

If phosphates directly catalyzed the breakdown of chlorine, then that should be rather easy to determine by adding some fertilizer to chlorinated water and seeing if the chlorine goes away rather quickly (at levels you are describing -- above 4000 ppb for phosphates and high levels of chlorine > 10 ppm FC but still reasonably measurable initially).

Another possibility is that the chlorine test itself may have interference from high phosphates, though that isn't something we've heard of before. I'll ask Taylor about that and repost here.

Again, thanks for the info.

Richard
 
I got a reply from Taylor and there is no phosphate test interference. They said, however, that the chlorine tests will bleach out at high chlorine levels (something we already knew). Even the FAS-DPD test will bleach out, though the solution there is just to add another scoop of powder, though Taylor only says the test goes up to 50 ppm. The amount of chlorine you said you added would be over 90 ppm so could very well have bleached out the chlorine test. I'm not sure how high the OTO test could register -- it doesn't bleach out at more normal "high" chlorine levels -- but 90 ppm is very high.

If I get to the high phosphate fertilizer test, then I suppose we can get to the bottom of this in that I'll try and recreate the cloudy water situation using high phosphates and nitrates from fertilizer and then see what happens when chlorine gets added -- all done in a bucket or tub of pool water (I'm not about to use my pool for such an experiment :shock: ).

Richard
 

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