Phosphates.....are they worth removing??

[Saturn94]

Thanks for the update.

 

[treatmentnerd]

Okay, so I want to dive in here. I have not read all of the 300 posts in the thread so far, but I believe I take a different approach to this than most. Professionally, I'm an engineer and I design water and wastewater treatment facilities. Many wastewater treatment plants have to hit low phosphorous levels to discharge back to the river or receiving body. In cases where a WWTP cannot remove enough phosphorous biologically, they will dose chemicals, most commonly alum which precipitates phosphorous out of the flow stream. Alum is also the most commonly used flocculant at water treatment plants across the country.

Personally, I've dosed alum as floc to speed up an algae cleanup after a bloom (my filters alone would likely take 2+ weeks) and it has the side benefit of taking my phosphate levels down close to zero. I'm not sure why alum isn't ever discussed as a phosphate solution in the pool world. Like other floc chemicals, it's too cumbersome to use on a frequent basis, but it certainly works if you want to drop your phosphate levels down once a year or so. It isn't expensive, though I've only been able to find it on Leslie's website. Using it drops pH a couple tenths, so some adjustment is needed, but it's not that significant.

Also, as a side note, normal municipal sewage has total phosphorous levels in the range of 6,000 - 10,000 ppb. I'm not comfortable letting my pool water have a nutrient concentration significantly greater than what is typically seen in sewage, even if there aren't health impacts when properly sanitized with chlorine. That's just me though. I'm generally going to try to stay below 1,000.

 

[JoyfulNoise]

Okay, so I want to dive in here. I have not read all of the 300 posts in the thread so far, but I believe I take a different approach to this than most. Professionally, I'm an engineer and I design water and wastewater treatment facilities. Many wastewater treatment plants have to hit low phosphorous levels to discharge back to the river or receiving body. In cases where a WWTP cannot remove enough phosphorous biologically, they will dose chemicals, most commonly alum which precipitates phosphorous out of the flow stream. Alum is also the most commonly used flocculant at water treatment plants across the country.

Personally, I've dosed alum as floc to speed up an algae cleanup after a bloom (my filters alone would likely take 2+ weeks) and it has the side benefit of taking my phosphate levels down close to zero. I'm not sure why alum isn't ever discussed as a phosphate solution in the pool world. Like other floc chemicals, it's too cumbersome to use on a frequent basis, but it certainly works if you want to drop your phosphate levels down once a year or so. It isn't expensive, though I've only been able to find it on Leslie's website. Using it drops pH a couple tenths, so some adjustment is needed, but it's not that significant.

Also, as a side note, normal municipal sewage has total phosphorous levels in the range of 6,000 - 10,000 ppb. I'm not comfortable letting my pool water have a nutrient concentration significantly greater than what is typically seen in sewage, even if there aren't health impacts when properly sanitized with chlorine. That's just me though. I'm generally going to try to stay below 1,000.

Alum is hydrated potassium aluminum sulfate. It is not an optimal approach to phosphate removal in pools because the alum leaves behind sulfates in the water. By contrast, lanthanum chloride leaves the chloride anion in water when it reacts with phosphates. Chlorides are much less harmful to pools than sulfates. Sulfates can destroy SWGs, cause calcium sulfate precipitation, attack steel parts by enhancing sulfate corrosion mechanisms and, in high enough concentrations, damage concrete. This is also why TFP discourages the use of dry acid in favor of muriatic acid.

 

[treatmentnerd]

I doubt that an occasional use of alum would ever generate concerning sulfate levels (~300 mg/l from here 3 – Sulphate attack | ) like sodium bisulfate use would, but extremely frequent dosing certainly could. You could also acheive the same results with polyaluminum chloride floc without the sulfates.

 

[Swampwoman]

I actually considered alum but was concerned about my new swg since the mfg specifically advises against use of dry acid.

What I am curious about is the following, so please share if you know or point me to a resource:

1. What volume of ppm can alum remove per quart per 10,000 gallons if known?
2. If one uses alum as a true floc, shutting down the filtration and vacuuming to waste, do sulphates remain as a byproduct, or would they get caught in floc?
3. In your wastewater setting with your background in municipal water treatment, how are municipalities who use metal sequestrants managing accumulation in equipment? Or is it that unlike boilers, the polyphosphates are injected at a particular level then move on?

Other info I'd love to know is has your municipality studied the rate of breakdown of polyphosphates to orthophosphates?

Thanks in advance to your contributions

P.s. I also had been given to understand that alum is most effective at a ph of 8. However, those of us on well dread allowing ph to rise due to metal precipitation and staining. At the same time, I believe I was told alum floc can also reduce iron.

Any thoughts on this combo of needs ?

 

[JoyfulNoise]

I doubt that an occasional use of alum would ever generate concerning sulfate levels (~300 mg/l from here 3 – Sulphate attack | ) like sodium bisulfate use would, but extremely frequent dosing certainly could. You could also acheive the same results with polyaluminum chloride floc without the sulfates.

Of course the occasional or one-off use wouldn't be harmful. However, not all pools experience constant water exchange and sulfates do not go away without fresh water to remove them, so my thinking was more along the lines of pools that could suffer sulfate build up. Polyaluminum chlorine is certainly a better choice but it is not widely available as a pool chemical. Perhaps in the WWTP world it is very common, but for pool chemicals, not so much. I think HTH might be the only supplier of PAC flocculant.

I actually considered alum but was concerned about my new swg since the mfg specifically advises against use of dry acid.

What I am curious about is the following, so please share if you know or point me to a resource:

1. What volume of ppm can alum remove per quart per 10,000 gallons if known?
2. If one uses alum as a true floc, shutting down the filtration and vacuuming to waste, do sulphates remain as a byproduct, or would they get caught in floc?
3. In your wastewater setting with your background in municipal water treatment, how are municipalities who use metal sequestrants managing accumulation in equipment? Or is it that unlike boilers, the polyphosphates are injected at a particular level then move on?

Other info I'd love to know is has your municipality studied the rate of breakdown of polyphosphates to orthophosphates?

Thanks in advance to your contributions

P.s. I also had been given to understand that alum is most effective at a ph of 8. However, those of us on well dread allowing ph to rise due to metal precipitation and staining. At the same time, I believe I was told alum floc can also reduce iron.

Any thoughts on this combo of needs ?

Swampwoman,

Since we're having a splash-battle in The Deep End, let me see if I can at least answer questions #1 & #2 -

As for how much can be removed, let's look at the possible chemical reactions involved in removing phosphates. For potassium alum, a reaction would like this -

Al(SO₄)₂^- + PO₄^3- ----> AlPO_4(s) + 2•(SO₄)^2-

(I left out the spectator ions, like potassium (K) because they are of no consequence to the overall reaction)

For lanthanum chloride, it would look like -

LaCl₃ + PO₄^3- -----> LaPO_4(s) + 3•Cl^-

So, in both cases, 1 mol of either lanthanum chloride OR potassium alum will neutralize 1 mol of orthophosphate. Now, if you take, for example, pool water with 1ppm (1000ppb) orthophosphate (PO₄^3-) in it, then the concentration of orthophosphate is -

1ppm = 1 mg/L = 10.53 µM/L [PO₄^3-]

The molecular weights of our phosphate removers are -

MW_Alum = 258.205 gm/mol

MW_LaCl3 = 245.26 gm/mol

SO now you can find out the ppm's of remover needed to eliminate the 1ppm orthophosphate concentration -

Alum : 10.53 µM/L x 258.205 gm/mol = 2.7189 mg/L = 2.7189ppm Alum

LaCl3 : 10.53 µM/L x 245.26 gm/mol = 2.5826 mg/L = 2.5826ppm LaCl3

In a 10,000 gal (37,854 L) volume of water, you get the following amount by weight -

Alum : 102.9 gm

LaCl3 : 97.76 gm

I used the Sigma-Aldrich website for pricing and looked at technical grades of both chemicals. Lanthanum chloride costs ~ $1 per gram while Potassium Alum costs ~ $0.41 per gram. So, based on the small differences in weight needed and the price, alum would appear to be a better choice. The only problems I see with it are (1) potassium alum leaves sulfates behind in the water with use, not a lot, but some and (2) reaction efficiency. Lanthanum chloride specifically reacts with the carbonates and phosphates in pool water to produce the removal reaction and does not react with anything else in any significant way. Alum, on the other hand, is a flocculant that will react with lots of particulates in the water to help bind them up. So there's no way to predict how efficient the reaction with free orthophosphates will be. I suspect in practice one would have to use more alum than predicted by the chemistry alone.

As for your Question #2 about sulfates, the answer is no. Sulfates are highly soluble in water and will simply go into solution and not get floc'd out in any way. The only reaction I know of that removes sulfates is the standard quantitative chemistry method for measuring sulfate concentration. That is, you mix a solution of barium chloride (very soluble in water) with a water sample that contains sulfates and the barium cation will react with the sulfates and form an insoluble precipitate of barium sulfate which you can filter and dry out to weigh. Barium sulfate is used in radiology as a contrast medium for CT scans of the intestine. Barium sulfate is completely insoluble in water and is not absorbed by the human gut but, because the barium atom is extremely heavy, it give good contrast on a CT scan. Anyone that has ever had a "barium enema" will attest to it's unpleasantness

Matt

 

[treatmentnerd]

I need to review the math above for equivalence, but I know that in WW we expect to need about 3 lb Aluminum for every pound of phosphorous. This is approximately double what the pure chemistry would suggest, similar to what JoyfulNoise said above. I'm not sure what you cost numbers are from though. I can get an 8lb bucket of alum for $26, which is less than $0.01 per gram. I've never shopped for the other. I'll expand when I have a bit more time.

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[Swampwoman]

Treatmentnerd, earlier in this thread I used 2 gallons of Seaklear commercial phosphate remover (lanathum chloride) that retailed at about $125 per gallon. It was rated to remove 9,000 ppb per quart per 10,000 gallons.

At that concentration, it did form floc. This product has been extensively effective for others and I have no reason to believe it didn't work except that it now appears my po4 level continues to elevate.

We had problems reading, were diluting and using aquarium kit, no pool po4 test could seem to read, then switched and finally now using a Hanna meter that reads to 30,000 ppb (still have to dilute.) I'd specifically asked Hanna if it read total p or po4 only and they said po4 only.

However, in my case, for years HEDP has been added frequently to control iron from well. The average sequester level of HEDP that's effective is generally around 25 ppm (aka possible 25,000 ppb eventual po4 when spent, we're guessing.)

So at this point, our suspicion is that while phosphate removers may be effective treatment at lower levels, someone maintaining 20 - 30 ppm of HEDP for years would have an endless breakdown of poly to ortho.

In which case, do I continue to chase phosphates? Never had algae, pool is crystal, water is soft, my blood phosphate level is not elevated from swimming in it (I actually checked) ... And presently, m new swg is operating just fine without the theoretical phosphate scaling that was technically possible but not happening...yet anyway.

That's just to catch you up on why I'm sitting here at 51,000 ppb unclear as to whether I further adulterate this water

When I do a liner change (this one is 15 years old) in the next season or so, i will be starting fresh with $1,000 of trucked in water. Since I've switched to softened water for fill, at that time I hope to find a way to manage po4 via annual treatments to offset what I hope will be reduced sequestrant use

-- Matt, thanks for the math! The alum is tempting as it will also reduce iron...the sulphates, however, would be impossible to dilute without adding more iron hi ho. Always a Sophie's choice around here

 

[JoyfulNoise]

I need to review the math above for equivalence, but I know that in WW we expect to need about 3 lb Aluminum for every pound of phosphorous. This is approximately double what the pure chemistry would suggest, similar to what JoyfulNoise said above. I'm not sure what you cost numbers are from though. I can get an 8lb bucket of alum for $26, which is less than $0.01 per gram. I've never shopped for the other. I'll expand when I have a bit more time.

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I appreciate any additional info you might add. The above math is nothing more than a little bit of mass-balance chemistry which assumes that all the reactions are complete and one-way. This is certainly not a good assumption as one has to take into account the effects of pH and reaction equilibrium. Since I have no info on the equilibrium constants for the reaction, there is no way to include that in the math and there could be significant pH effects. One thing I do know is that at pool water pH, the lanthanum ion is very unstable and will react both quickly and completely to form carbonates, phosphates and oxides. Alum, on the other hand, may have a more complicated reaction equilibrium.

As for prices, I simply looked up technical grade chemicals on Sigma-Aldrich (a major supplier of reagent grade chemicals). There could easily be other, cheaper sources of these materials where the pricing is very different and bulk orders may be quite a bit cheaper. Lanthanum is one of the most abundant rare-earth metals but it is still quite scarce and so I would expect $1/gm to be close enough to the mark.

-- Matt, thanks for the math! The alum is tempting as it will also reduce iron...the sulphates, however, would be impossible to dilute without adding more iron hi ho. Always a Sophie's choice around here

Yes Kate, unfortunately there is no magic silver bullet when it comes to pool chemistry, just trade-offs....

 

[treatmentnerd]

From the Water Enviroment Federation's Manual of Practice 8, (which is one of the most common references used by engineers in designing WWTPs) the chemical equation for phosphorous precipitation with alum is:

Al₂(SO₄)₃*(14 H₂O) + 2 H₂PO₄^- + 4 HCO₃ ---> 2 AlPO₄ + 4 CO₂ + 3 SO₄^2- + 18 H₂O

Note that they assume a hydrous form of alum (which I believe is what is supplied when you buy the bucket from Leslie's), and that 2 moles of alkalinity are concumed with the bicarbonate to CO₂ transformation. The theoretical aluminum to P ratio is 1:1 on a molar basis, and 0.87:1 on a mass basis, given that aluminum is lighter than phosphorus. The manual also notes that side reactions with alkalinity and organic matter push the ratio up from 0.87:1 to 3:1. Since your pool does not have any appreciable organic matter compared to wastewater, this ratio is probably higher than you need, and 1.5:1 - 2:1 is probably more appropriate in a pool.

In WW: the following ratios are given:

P removal	Al mole ratio	Weight ratio	Hydrous Alum Weight Ratio
75%	1.38:1	1.2:1	13:1
85%	1.72:1	1.5:1	16:1
95%	2.3:1	2.0:1	22:1

I think that we can add a column for an alum : phosphate ratio since P is 31 g/mol and PO₄^3- is 95 g/mol.

P Removal	hydrous alum : phosphate weight ratio
75%	4.3:1
85%	5.2:1
95%	7.2:1

I'm not sure if the phosphate tests that you have gotten results for reflect the ppb in phosphate itself or in ppb of phosphorous. That will have a pretty significant impact on how much alum you need.

Much more information is available in this summary chapter from WEF in a different treatment manual that has an example problem to determine the amount of aluminum that needs to be added to drop out the phosphorous.

http://www.wefnet.org/mopnew/BNR/Chapter 08 Final Draft.pdf

Also, since you seem to have a never ending P problem, you might consider polyaluminum chloride as an alternative. It won't drop the pH as much, and it won't generate sulfates. I found it online below. Note that polyaluminum chloride is discussed in the WEF chapter linked above as an alternate means of phosphorous precipitation with aluminum.

(6 Pack) HTH Flocculant Liquid Chloride Drops 32 OZ 61201

 

[treatmentnerd]

I actually considered alum but was concerned about my new swg since the mfg specifically advises against use of dry acid.

What I am curious about is the following, so please share if you know or point me to a resource:

1. What volume of ppm can alum remove per quart per 10,000 gallons if known?
2. If one uses alum as a true floc, shutting down the filtration and vacuuming to waste, do sulphates remain as a byproduct, or would they get caught in floc?
3. In your wastewater setting with your background in municipal water treatment, how are municipalities who use metal sequestrants managing accumulation in equipment? Or is it that unlike boilers, the polyphosphates are injected at a particular level then move on?

Other info I'd love to know is has your municipality studied the rate of breakdown of polyphosphates to orthophosphates?

Thanks in advance to your contributions

P.s. I also had been given to understand that alum is most effective at a ph of 8. However, those of us on well dread allowing ph to rise due to metal precipitation and staining. At the same time, I believe I was told alum floc can also reduce iron.

Any thoughts on this combo of needs ?

1. A 8lb bucket of dry alum should contain around 12 moles of alum at ~300 g/mol. On a pure molar basis, this would remove 12 mol of P, or 372 g P, pr 1.14 kg PO₄^3-. A 10,000 gal pool is 37,800 liters. 372g is 372,000 mg. 372,000/37,800 = 9.8 mg/l or ppm or 9,800 ppb of phosphorous. This number triples if the test you are getting records your ppb in phosphate rather than phosphate as phosphorous. This uses the theoretical ratios, though I believe you will be close to that if you are only adding one or two buckets due to your current concentration being sky-high. Adding more will involve a departure from the theoetical ratios as seen in my post above.

2. The sulfates will remain in solution.

3. Municiplaties don't really see accumulation because the processes are flow-through. The water goes out to the river or ocean, and the solids are either landfilled or land applied. They don't have anything like a swimming pool where concentrations can creep up indefinitely.

On pH - alum may floc better at a pH of 8, but minimum solubility f aluminum phosphate occurs between a pH of 5.5 and 6.5. You will actually want to drop pH a bit if you are trying to primarily precipitate phosphate. You likely won't need to add aicd unless your pH is up close to eight though, the alum will probably take care of the pH drop by itself.

 

[treatmentnerd]

I found these test instructions for the Taylor K-1106 test kit. It's clear that the test is for phosphate, not phosphorous, which is the unit we typically use in WW. In that case, my calculation of 9,800 ppb of phosphorous removed per 8lb bucket converts to ~ 30,000 ppb phosphate. You'll lose some effectiveness, so I'd expect to see 20,000 - 25,000 ppb of phosphate removed per bucket. You'll add sulfates at around 1.5 times the phosphorous removed. So if you removed all 50,000 ppb, you'll add around 75,000 (75 ppm) sulfate. This is around 1/4 of the way up to the 300 ppm concern level for sulfate concentrations.

> PHOSPHATE
> (0-1000 ppb)

COMPONENTS:

1 x 4025 Test Cell, Calibrated 5 mL, plastic
1 x 5434 Instruction
1 x R-0007 Thiosulfate N/10
1 x R-0980 Phosphate Reagent #1
1 x R-0981 Phosphate Reagent #2

TO ORDER REPLACEMENT PARTS AND REAGENTS CALL TOLL-FREE
800-TEST KIT (800-837-8548).

PROCEDURE: CAREFULLY READ AND FOLLOW PRECAUTIONS ON REAGENT
LABELS. KEEP REAGENTS AWAY FROM CHILDREN. THIS KIT IS FOR
COMMERCIAL USE ONLY.

1. Rinse and fill test cell to 5 mL mark with water to be
tested.

2. WARNING: R-0980 Phosphate Reagent #1 contains a strong
acid. Avoid contact with skin, eyes, or clothing.

Add 5 drops R-0980 Phosphate Reagent #1 to sample.
Cap and mix thoroughly. Wait 2 minutes.

3. Carefully remove cap. Add 1 drop R-0981 Phosphate
Reagent #2 and 1 drop R-0007 Thiosulfate N/10. Cap and
mix thoroughly. Wait exactly 1 minute. Carefully remove cap.

4. Within the next minute match color in test cell with closest
color on color chart*. Do this by positioning color chart
flat and holding test cell upright to right of printed
colors. Look down from top through solution. Hold test cell
by the flanges (to prevent fingers from shadowing color
development) and move test cell back and forth to find
match. Record as parts per billion (ppb) phosphate.

NOTE: To inhibit algae growth, water treatment is recommended
at phosphate concentrations above 125 ppb.

*To match colors see printed color chart.

 

[Swampwoman]

Thanks! I will need to process this a bit

At the moment, I'm making a case for a liner change this year, and ergo fresh truckload of water...but I'm up against an observatory h is determined to build

 

[treatmentnerd]

I understand. If a fresh truckload is about to come anyways and you don't have present issues, simply waiting it out might be better than the money and hassle that would be involved in using alum, polyaluminum chloride or some other P remover.

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[JoyfulNoise]

I understand. If a fresh truckload is about to come anyways and you don't have present issues, simply waiting it out might be better than the money and hassle that would be involved in using alum, polyaluminum chloride or some other P remover.

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Thanks for the additional information, that source info on WW phosphate treatment is great.

I haven't looked, but I wonder if the alum floc sold by the pool industry is the aluminum sulfate type or a potassium alum. Obviously the hydrated aluminum sulfate is better.

I also noticed that the treatment document doesn't cover the use of lanthanum for P removal as it is likely a cost-prohibitive strategy.

 

[treatmentnerd]

It likely is cost prohibitive. All of the primary chemicals discussed in the chapter can be ordered inexpensively in bulk. I'd be surprised if utilities pay more than 10% of the cost Leslie's is selling alum at. The other chemicals (lime, ferric chloride, etc.) are also cheap, though near impossible to use in a pool. I don't even want to think about trying to use lime in a swimming pool. It'd be a disaster. I'm sure ferric chloride would work, but I don't think anyone wants to put iron in their pool on purpose. That seems like a bad idea. The aluminum based flocculants seem to be the only ones that carry over.

Also, since I haven't used lanthanum, I'm not sure how is removed. In WW, the goal is to settle a precipitate out in a large settling tank. If lanthanum produces a compound that is predominately filtered out, it would be undesirable for the increased filter backwash requirements, which are much more significant in a flow through process than when treating a single volume of water like a pool.

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[treatmentnerd]

Also, I believe that the alum I have at home is aluminum sulfate hydrate. I can check just to be sure, but I think that it is.

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[JoyfulNoise]

Lanthanum definitely forms a fine, suspended precipitate of LaPO4. Typically a clarifier is used if you have a sand or cartridge filter. My DE filter was more than capable of removing it so I did not need a clarifier. Swampwoman's pool developed a very thick precipitate that settled out. Lanthanum reacts with both carbonate and phosphate but the phosphate has a very low solubility constant (Ksp) so the lanthanum carbonate that is formed slowly transforms into the phosphate. This is why one typically wants to wait 48 hours before doing any backwashing so that the lanthanum has enough time to react with the phosphate in the water.

I was at ~1000ppb [PO4] in late March and I dosed for 1,500ppb. My [PO4] dropped to <100ppb and has stayed down since then. I'll be running my pool at a lower FC/CYA ratio this summer to see how nutrient removal works.

 

[Swampwoman]

Yes, I had a good chuckle imagining adding ferric chloride and lime to my nutritious pool

Speaking of which, just having come from a dip, I must say my skin feels nourished and 10 years younger. Perchance I've stumbled upon the fountain of youth

Joking aside, the lanthanum flocced good and plenty at 2 gal per 23k gal. I vacuumed to waste for a good few consecutive days! The Seaklear rep had prepared me for this.

My lobby for the new liner may well fail this year...I already got my swg and slate floors and in fairness, I'd said hubby's observatory platform and dome came next with a roof redo. But we're still waiting for the construction bid, which might be sky high, if you'll excuse the pun

So I may have lots of time to experiment!

To that end, since its so cheap, I might try my own "jar test" with the pool water in question over the next few weekends with alum just to see what kind of results I get.

I could approximate a filter effect with sand in an old coffee filter, I think, and then measure the resulting filtered water for po4.

Btw, I own that Taylor pool kit, but since it only measures to 1,000 I wasn't trusting my 50x dilution. That's why I got the Hanna meter, that goes to 30,000. But I'm going to double check with Hanna that said meter is not really measuring more than just orthophosphate, since it uses amino acid method, which makes me wonder if there's not some digestion involved.

 

[Swampwoman]

Re: phosphorous versus ortho testing...

Hanna instructions include this reference:
Adaptation of the Standard Methods for the Examination of Water and Wastewater, 18Th edition, Heteropoly- molybdenum Blue method. The reaction between orthophosphate (reactive phosphorus) and the reagent causes a blue tint in the sample.

The Standard itself notes this:

Phosphates that respond to colorimetric tests without prelim-
inary hydrolysis or oxidative digestion of the sample are termed
"reactive phosphorus." While reactive phosphorus is largely a
measure of orthophosphate, a small fraction of any condensed
phosphate present usually is hydrolyzed unavoidably in the pro-
cedure. Reactive phosphorus occurs in both dissolved and sus-
pended forms.