Phosphates and rapid Cl drop

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Naplespool

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Mar 26, 2017
101
Naples/FL
Three times this winter we've had a rapid loss of Chlorine as measured by the Taylor DPD kit. We maintain our pool to TFP standards. Mostly we have sparkling water, but ...

On Tuesday and for the past few weeks, our water has been sparkling and clear. (Very light bather load lately.) Here was the measured chemistry:

FC: 6.5
CC: 0
pH: 7.5 (looked between 7.4 and 7.6)
TA: 60
CH: 350
CYA: 70
Salt: 3800
Borate: 0
Temp: 84

Today, Thursday, the pool was ever so slightly cloudy,
FC: 3.0
pH: 7.6
all others as above.

SWG is 22.4 oz per day, SWG running at 50% for 10 hours, so adding 2ppm/day to our 17250 gallon pool. Pool inside a screen enclosure.

I've had the CYA double checked and we and our pool care get the same reading. Each time, we've had the problem, I've properly SLAMed the pool (brushed extensively, cleaned the filter, measured overnight Cl loss) and that seems to settle it until the next drop.

When we were dealing with a slight discoloration issue on the plaster finish, Haviland measured the water chemistry and found 10,600ppb phosphates in the pool and 260ppm in the autofill source water. My pool care guy who seems very good wants to treat the phosphates, says it's a problem here in Naples FL. (I know the TFP view on phosphates vs algae in the water.)

Is it possible that I need to keep FC in the 8-10 range as with a non-SWG pool? I'm wondering that because Cl usage seems to be 1.5-1.75 ppm per day until I hit 6-7 ppm.
 
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In very special cases phosphates could cause problems and consume a bit more CL. I would first look for potential algae hideouts, like light niches, skimmers, ladders if you have. Bring CL to slam levels and thoroughly clean all this parts. If after all this, CL keeps dropping like you mention its time for a phosphate treatment, which im no expert at.
 
10,600ppb = 10.6ppm. That’s a lot of phosphates.

0.26ppm in the source water sounds like the water supplier uses orthophosphates for scale and metal control (typical in old water supply systems with lead joints and copper pipes). That’s a fairly high phosphate level and should be treated. If your fill water is that high, then you should monitor your phosphate levels annually but you probably will only need to treat every few years or so. I wouldn’t be concerned until your phosphate level rises above 1500 to 2000 ppm.
 
Thanks everyone for your comments. Re: SLAMing, I've gotten good at it since I've done it three times now. These chlorine drop "events" have happened every 6 weeks or so. It's weird really, I can't pinpoint any real cause. I haven't taken the lights out yet but there are 2 large and 3 small so one could be a hiding place. I run all the features for an hour a day to keep algae from hiding out there.

Is 1500 to 2000 ppm the concern level or is it 1500 to 2000 ppb? I'll get a test kit.

I'm surprised by the high phosphates because we get 3-5' of rain here per summer and everything else gets washed out. I was advised to used Jack's Magic Purple because of the discoloration that I was seeing in the pool, could it be the source?
 
Phosphates are always measured in parts per billion (ppb). 1000ppb = 1ppm. 1500ppb to 3000ppb is where algae growth starts to accelerate due to nutrient availability (potassium and nitrogen are readily available in most pool water as well). Above 3000ppb, phosphate availability is at its maximum and algae now have ample nutrient resources to replicate at their most efficient rate. Staying below 1000ppb and as close to zero as you can manage starves algae of a vital resource making it harder to grow.

If you are using Jacks Magic products regularly than the phosphonate s ale inhibitor will breakdown slowly over time and form soluble phosphates. That is likely the major source of phosphates in your pool.

You either have to stop using the Jacks magic product and solve your staining issues another way (what is the source of the stain?) OR continue to use the scale inhibitor and keep your FC levels up at the high end of the scale in order to inhibit algae growth. There is no sense in treating phosphates if you’re constantly going to add them with the scale remover.
 
We (pool builder, pool maint. and I) felt the discoloration was metal but the stain test kits didn't confirm that. So for a year we've been treating to see if the discoloration comes down. It didn't so for the last month and a half I've been brushing the pool aggressively. Guess what? The discoloration is about 2/3 to 3/4 gone, to the point if it doesn't get better we can live with it. When I first started brushing every few days the pool kicked up a lot of dust, now much less.

As a result, I hired a new weekly service company that is doing a much more thorough job of the brushing.

These episodes of chlorine drop-out have lined up fairly well with the use of Jack's Purple. My plan is to discontinue the Jack's and run toward the upper end of the Cl range until summer rains have washed out the phosphate.
 
I stopped the Jack’s Magic, my pool guy added some phosphate remover (not sure what), my chemistry as below, and my pool has been crystal clear for 3 weeks.

Because of the phosphate I have held the CL in the range of a non-SWG Pool with 80ppm CYA, which is 9-11 ppm CL.

CL: 9-10
CC: 0
PH: 7.4-7.8
TA: 70-80
CA: 325
CYA: 70-80

SWG is set to make 1.8 ppm CL per day (45% for 10 hours). The CL stays steady at around 9.0.

When if ever should I venture down to 6ppm on the CL?
 
Lower your pump run time by perhaps an hour each time and see where your FC levels off at. You can lower the run time and then wait a week and see what happens. Run time has the biggest impact on the amount of FC produced. From the perspective of particulate filtration and cleanliness, most pools don’t need more than 2 hours or so of pump run time so the majority of your pump ON time is really dedicated to FC production. So don’t go lower than 4-5 hours before adjusting the SWG output. I’m going to bet you can probably shave 2 hours or so off your pump run time to get near the FC set point you’re looking for and keep algae at bay.
 

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JoyfulNoise said:
Phosphates are always measured in parts per billion (ppb). 1000ppb = 1ppm. 1500ppb to 3000ppb is where algae growth starts to accelerate due to nutrient availability (potassium and nitrogen are readily available in most pool water as well). Above 3000ppb, phosphate availability is at its maximum and algae now have ample nutrient resources to replicate at their most efficient rate. Staying below 1000ppb and as close to zero as you can manage starves algae of a vital resource making it harder to grow.

If you are using Jacks Magic products regularly than the phosphonate s ale inhibitor will breakdown slowly over time and form soluble phosphates. That is likely the major source of phosphates in your pool.

You either have to stop using the Jacks magic product and solve your staining issues another way (what is the source of the stain?) OR continue to use the scale inhibitor and keep your FC levels up at the high end of the scale in order to inhibit algae growth. There is no sense in treating phosphates if you’re constantly going to add them with the scale remover.
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@JoyfulNoise -> you mentioned above "If you are using Jacks Magic products regularly than the phosphonate s ale inhibitor will breakdown slowly over time and form soluble phosphates. That is likely the major source of phosphates in your pool."
In looking at jacks magic website for it's magenta product ( which is what I use as part of weekly maint ) written right on the label of the bottle that it "contains Phos-Stop Plus: will not yield phosphates, reduces existing phosphates in pool water ". I'm not seeing anything regarding phosphates for what the OP is using... the Purple stuff. It's odd because my water has been a TFP pool for years without issues and now my pl1400 jandy SWG seems like it's not producing chlorine and the first thing my pool builder said to me was to check my phosphate levels and I'm at 1900 ppb. It caught my eye that you said Jack's magic might contribute to this and when I double checked it seems it shouldn't add any phosphates, in fact, it claims to lower them. Can you please clarify? All other issues w/ SWG ruled out besides being defective ( no scale buildup on SWG plates, proper voltage is being measured at cell, pool chemistry near perfect w/ CSI close to 0. ). Thanks!
 
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johnxxx said:
@JoyfulNoise -> you mentioned above "If you are using Jacks Magic products regularly than the phosphonate s ale inhibitor will breakdown slowly over time and form soluble phosphates. That is likely the major source of phosphates in your pool."
In looking at jacks magic website for it's magenta product ( which is what I use as part of weekly maint ) written right on the label of the bottle that it "contains Phos-Stop Plus: will not yield phosphates, reduces existing phosphates in pool water ". I'm not seeing anything regarding phosphates for what the OP is using... the Purple stuff. It's odd because my water has been a TFP pool for years without issues and now my pl1400 jandy SWG seems like it's not producing chlorine and the first thing my pool builder said to me was to check my phosphate levels and I'm at 1900 ppb. It caught my eye that you said Jack's magic might contribute to this and when I double checked it seems it shouldn't add any phosphates, in fact, it claims to lower them. Can you please clarify? All other issues w/ SWG ruled out besides being defective ( no scale buildup on SWG plates, proper voltage is being measured at cell, pool chemistry near perfect w/ CSI close to 0. ). Thanks!
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@JoyfulNoise I was browsing and found a post of yours from 2016 that answered my same question to you!
Aug 29, 2016
  • The magenta stuff is PAA - polyacrylic acid. It will not yield phosphates on breakdown.

    So it is ok to use.

 
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I like how their marketing guys think -> “Phos-stop Technology” ? It makes their product sound so sophisticated...
 
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