Will Phosphates increase chlorine demand?

[benavidescj]

I am aware that maintaining the proper level of chlorine for CYA level will keep algae at bay. What I would like to know is does increasing the level of phosphates in the water increase the demand for chlorine. I am aware that phosphates do not directly cause a demand as they are stable in the presence of chlorine. However, do phosphates contribute indirectly through the increased organic activity? If someone has a report that has tested this hypothesis that would be great also. Otherwise, I would like to set up a test myself.

 

[anonapersona]

Since you are posting in The Deep End you will want a more detailed answer than this but I can help from the aquatic plant perspective.

Plant growth is limited by N (nitrogen), P (phosphorus), and (K) potassium - a deficit of any one of these will hinder growth. These are the major elements, there are also minor elements like iron and a few others that can be limiting. Then each plant has a range of other things that limit or encourage growth like sunlight, salinity, oxygen, water movement, etc. There are hundreds of species of algae, and we really do not know what would limit each as most within a category look pretty much the same to us though some are different colors (green, pink, yellowish, blue) or forms (free-floating, attached, rope-like, matted, sheet, slimy).

Increasing phosphates alone will not increase organic activity unless that is the only thing that was limited, for the plants can still be limited by other things; sunlight, nitrogen, potassium, chlorine, salt, etc. But, if there were sufficient other nutrients and conditions were right (light, salinity, flow, etc) and algae spores were present, then adding Phosphates could trigger an algae bloom. Any one thing that is missing or restricted that is important to that species of algae will limit that type of algae. The concern is how many other types of algae there might be that are not limited by that low or missing value.

When I was participating in an online planted tank forum there was a brilliant young man from California working with the state on natural ponds and aquatic life (nice guy, kind and funny, so very smart -- I met him once and he is totally gorgeous too -- yes, Tom I still have a secret crush on you) and his take on algae seemed to be that if you tried to limit something in the pond/aquarium to choke out one particular algae, another algae would come along that was not limited by that so it was a losing proposition. Plus, they were a bear to really identify, too many are too similar and no one has really bothered to work out the precise needs of one vs the other. So, in the fish tank, we added fertilizers to get the plants to out grow the algae and stocked algae eating critters like shrimp to keep the rest at bay.

Probably not the answer you wanted, but I thought I'd put in these two cents just for fun.

 

[JasonLion]

It depends on your FC level relative to the recommended level for your CYA level. At FC levels a little below the recommended level phosphates can affect chlorine consumption, because algae is always on the verge of getting going, there is enough chlorine to hold it in check but not enough to kill it all. That process consumes extra chlorine, and phosphates can stop that from happening. However, when FC levels are at or above recommended levels there is no algae and there is no difference in chlorine consumption with or without phosphates.

 

[benavidescj]

Thanks for the replies.

In another post I described how I had an issue with my SWG after doing a ascorbic acid treatment. The cell appeared to not be producing any chlorine after the treatment. I had Pentair come out and check the unit and they stated that the unit was producing chlorine just fine and that the only thing they could find wrong was that my phosphate levels where high. Being on this site for some time I dismissed his comment about the phosphates but had to lower them in order to get warranty work on my SWG.

Keep in mind that when I did the ascorbic acid treatment I did it exactly as stated in pool school and I added polyquat prior to lowering my FC level and only had the FC at 0 for 24 hours. I then increased my FC to that recommended for my CYA of 70 (FC 5 ppm) or above the entire time I was having this issue with the cell.

Low and behold when I added the phosphate remover all of a sudden the SWG started maintaining chlorine just fine. I did NOT shock the pool.

Having maintained my FC level the whole time (except for the 24 hours during the procedure) and the phosphate remover having such a dramatic impact I have to conclude that phosphates do increase chlorine demand, even if indirectly.

Jason, I know what you are saying, but as an engineer I want to see experimental proof before I am convinced that phosphates do not increase chlorine demand in certain environmental conditions.

I want to do a test but am fairly confident that I do not know what all the variables are. If anyone wants me to try an experiment I will need help with the test setup.

 

[anonapersona]

Sounds like there was a nascent algae bloom that had to be choked off and the phosphate reducer did it. I expect adding bleach would have done the same thing cheaper, just guessing.

 

[UnderWaterVanya]

In another post I described how I had an issue with my SWG after doing a ascorbic acid treatment. The cell appeared to not be producing any chlorine after the treatment. I had Pentair come out and check the unit and they stated that the unit was producing chlorine just fine and that the only thing they could find wrong was that my phosphate levels where high. Being on this site for some time I dismissed his comment about the phosphates but had to lower them in order to get warranty work on my SWG.

Keep in mind that when I did the ascorbic acid treatment I did it exactly as stated in pool school and I added polyquat prior to lowering my FC level and only had the FC at 0 for 24 hours. I then increased my FC to that recommended for my CYA of 70 (FC 5 ppm) or above the entire time I was having this issue with the cell.

Low and behold when I added the phosphate remover all of a sudden the SWG started maintaining chlorine just fine. I did NOT shock the pool.

Having maintained my FC level the whole time (except for the 24 hours during the procedure) and the phosphate remover having such a dramatic impact I have to conclude that phosphates do increase chlorine demand, even if indirectly.

Jason, I know what you are saying, but as an engineer I want to see experimental proof before I am convinced that phosphates do not increase chlorine demand in certain environmental conditions.

I want to do a test but am fairly confident that I do not know what all the variables are. If anyone wants me to try an experiment I will need help with the test setup.

Another possible explanation: You may have had a low level algae outbreak just below the threshold - you couldn't see it, but it was there. The drop to 0 FC even with polyquat may have allowed this to happen. Bringing back to 5ppm but not shocking - could mean that whatever was there wasn't eliminated - just stalled. Then phosphate removal may have been just the needed kick over the knife edge that took the barely holding their own Algae crowd and made them give up.

Add some phosphates back to the pool now and see what happens. If the problem returns, shock this time and see if that fixes it. Then you will have two tests of the "same" conditions. It's possible that adding the phosphates back will not restart the problem if the algae has given up the ghost.

 

[anonapersona]

Add some phosphates back to the pool now and see what happens. If the problem returns, shock this time and see if that fixes it. Then you will have two tests of the "same" conditions. It's possible that adding the phosphates back will not restart the problem if the algae has given up the ghost.

Oooh, yes! A real experiment... ::

 

[UnderWaterVanya]

Add some phosphates back to the pool now and see what happens. If the problem returns, shock this time and see if that fixes it. Then you will have two tests of the "same" conditions. It's possible that adding the phosphates back will not restart the problem if the algae has given up the ghost.

Oooh, yes! A real experiment... ::

Any thoughts on how to get phosphates that are safe to add to the pool and cheap?

 

[benavidescj]

Add some phosphates back to the pool now and see what happens. If the problem returns, shock this time and see if that fixes it. Then you will have two tests of the "same" conditions. It's possible that adding the phosphates back will not restart the problem if the algae has given up the ghost.

Oooh, yes! A real experiment... ::

Any thoughts on how to get phosphates that are safe to add to the pool and cheap?

Yea, anyone know? I will do it go put this to bed.

 

[benavidescj]

In another post I described how I had an issue with my SWG after doing a ascorbic acid treatment. The cell appeared to not be producing any chlorine after the treatment. I had Pentair come out and check the unit and they stated that the unit was producing chlorine just fine and that the only thing they could find wrong was that my phosphate levels where high. Being on this site for some time I dismissed his comment about the phosphates but had to lower them in order to get warranty work on my SWG.

Keep in mind that when I did the ascorbic acid treatment I did it exactly as stated in pool school and I added polyquat prior to lowering my FC level and only had the FC at 0 for 24 hours. I then increased my FC to that recommended for my CYA of 70 (FC 5 ppm) or above the entire time I was having this issue with the cell.

Low and behold when I added the phosphate remover all of a sudden the SWG started maintaining chlorine just fine. I did NOT shock the pool.

Having maintained my FC level the whole time (except for the 24 hours during the procedure) and the phosphate remover having such a dramatic impact I have to conclude that phosphates do increase chlorine demand, even if indirectly.

Jason, I know what you are saying, but as an engineer I want to see experimental proof before I am convinced that phosphates do not increase chlorine demand in certain environmental conditions.

I want to do a test but am fairly confident that I do not know what all the variables are. If anyone wants me to try an experiment I will need help with the test setup.

Another possible explanation: You may have had a low level algae outbreak just below the threshold - you couldn't see it, but it was there. The drop to 0 FC even with polyquat may have allowed this to happen. Bringing back to 5ppm but not shocking - could mean that whatever was there wasn't eliminated - just stalled. Then phosphate removal may have been just the needed kick over the knife edge that took the barely holding their own Algae crowd and made them give up.

Add some phosphates back to the pool now and see what happens. If the problem returns, shock this time and see if that fixes it. Then you will have two tests of the "same" conditions. It's possible that adding the phosphates back will not restart the problem if the algae has given up the ghost.

I thought is was algae also. I would swear to it. That is why I did 3 separate overnight tests. All of them were negative. I do not see how I had any algae if all the overnight tests were always negative.

 

[JasonLion]

In another post I described how I had an issue with my SWG after doing a ascorbic acid treatment. The cell appeared to not be producing any chlorine after the treatment.

This is completely normal and expected after an ascorbic acid treatment. Chlorine and ascorbic acid destroy each other. It takes time to produce enough chlorine to finally burn off all of the ascorbic acid. During that time you will not have any chlorine in the pool. Then, when the ascorbic acid is used up, chlorine levels return to normal. Either, it just happened that the ascorbic acid got used up at about the same time you used phosphate remover, or you also had a low level algae problem at the same time (which is common after an AA treatment). Either way, there is nothing surprising there.

 

[chem geek]

If there was leftover ascorbic acid, then that would certainly create a chlorine demand, but it would be very fast so you wouldn't be able to add chlorine and pass the overnight chlorine loss test. Not having the chlorine lose much overnight also tends to rule out algae, at least a lot of algae. The loss you were seeing was only in the struggle of the chlorine generator to keep up during the day. We know that sunlight will break down chlorine during the day so maybe there was nascent algae growing but only just enough to prevent the chlorine generator from keeping up (along with the sunlight) but not enough to drop the chlorine significantly at night.

I think that had you shocked the pool with chlorine, you would have killed off anything just growing. We've seen that happen quite a few times. Nevertheless, with higher phosphates in the pool it will be more "reactive" so if the FC gets too low (or never got high enough to kill off the algae) then the chlorine demand will increase.

As for adding more phosphates, you'll have to add enough to overcome the phosphate remover you added since there is likely lanthanum leftover, but you'll know you've gone beyond that point when it no longer precipitates or clouds. You can get a phosphate buffer such as sodium phosphate dibasic or sodium phosphate monobasic or you can get Osmoprep tablets and crush them up or you can get a pure phosphate fertilizer (0-20-0 is fine). I don't know how high your phosphate level was, but to get 1000 ppb phosphate in 10,000 gallons you'll need to add 1-1/3 ounces weight of phosphate so somewhat more than this depending on the specific product you use.

 

[JamesW]

There are several pH buffer products sold for spas that use phosphates.

However, as you have a concrete pool, your calcium level is likely to be high enough to cause it to form a precipitate with any significant amount of phosphate.

http://piscines-apollo.com/bioguard/142210.pdf

http://www.rhtubs.com/MSDS/SpaEssentials_pH_Anchor.pdf

 

[anonapersona]

http://www.planetnatural.com/site/organic-potash.htmlOne choice, found online, not sure what is available locally.

Not really sure about rock phosphate vs potash, but you do not want bat guano for sure. Something that lists itself as 0-0-30 is a good start, no nitrogen or potassium included. The more mineral-like the better, I'd guess, though that does not assure no other metals or such are included. I'll keep looking.

[edit] - OK so I did really badly in Chemistry, it's true. Potash is NOT phosphate. And Phosphates can include various metals, not sure if this is in concentrations that will be bad for your pool. We clearly need someone better versed in this to help here.....

 

[anonapersona]

One good site about phosphates that I am still reviewing.
http://www.phosphatesfacts.org/what.asp

 

[anonapersona]

I don't guess anyone can find TSP anymore? trisodiumphosphate?

 

[techguy]

I saw TSP labeled products near the thinners at Lowes but I am not sure they are the same product as in the olden days... Lol

I found the MSDS from DAP for their TSP and it says weight is 30-60% sodium phosphate, tribasic and sodium carbonate, anhydrous 10-30%

 

[chem geek]

If you can find TSP then that's fine to use. What techguy found is TSP with extra sodium carbonate that won't be a problem. Remember that the quantities we are talking about here are small. I wouldn't worry about calcium phosphate precipitation either -- I had 3000 ppb phosphates in my pool with 300 ppm CH. It takes much higher phosphate or CH levels before calcium phosphate becomes a problem. For fertilizer, the three numbers are for Nitrogen - Phosphorous - Potassium so you want something like 0-20-0.

 

[benavidescj]

The loss you were seeing was only in the struggle of the chlorine generator to keep up during the day.

Wanted to clarify a few things after reading all of your replies.

1. When I say that the SWG appeared to not be producing any chlorine after the treatment, I am talking about not producing chlorine for three weeks. I am aware that the ascorbic acid treatment will cause a demand in chlorine for a short time while the chlorine destroys it.

2. The appearance of a lack of chlorine production was not only during the day but also at night.

3. The change from apparently not producing to producing chlorine was immediate following the addition of the phosphate remover.

Another possibility that I was thinking about last night is that the phosphate somehow coated the cell plates and would not allow normal chlorine production. I have read where phosphate is used in water treatment to coat metal pipes in order to protect them from corrosion. Maybe?

I think what I am going to do for my experiment is the same thing I did before with modification. I will first add the sequestrant, which contains the phosphate, and see if that has the same effect. I will let you know.

 

[JasonLion]

1) Don't assume it will be a short time, if you are not adding enough chlorine it can be a very long time. And don't assume that the SWG will ever catch up on it's own. You need to raise the FC level up to measurable levels by manually adding chlorine, how ever much chlorine that takes.

3) Then you had a low level algae problem, presumably started because you failed to raise FC levels quickly enough after the AA treatment. The period after an AA treatment is a balancing act. You can't add chlorine too quickly, nor can you add chlorine to slowly. You have to establish a minimal FC level by the second or third day or you are certain to get algae (which may or may not be visible). And once you get algae it is common for the SWG to be unable to deal with it on it's own.