How to reduce nitrates

[smallpooldad]

I need help on how to reduce nitrates, I believe, or am I worrying and over thinking this?

I will be draining the pool to remove nitrates which are around 40+ ppm and to do an iron stain removal service, which is normally done every year but I skipped it last year. By using phosphate remover I was able to keep any algae at bay, however if phosphates go above 1000 ppm green algae appears on the suction pool canister which is near the surface. The water and walls are otherwise algae free. I normally maintain 3 ppm of chlorine with my SWG and pump, running 4 hours per day in a 10,000 gallon gunite pool.

Unfortunately my water supply contains 20 ppm of nitrates, and with the near constant trade winds which blow red volcanic dirt and fertilizer from agricultural fields and occasional vog from Kilauea Volcano (not sure if the vog has anything to do with it) the nitrates increase slowly over time. Phosphates from the water supply register around 200 ppm. I think the reason the numbers are high, on delivery, might be due to volcanic soil and some supplies coming from over fertilized sugar cane and pineapple fields in the past with these chemicals percolating through the soil into the aquifers, although this is pure speculation on my part.

I believe I read that it is possible to reduce nitrates by raising the chlorine to twice the level recommended for Yel/MstrdShock. As my CYA is 30 ppm that would be 18.2 x 2, using the Chlorine/CYA chart, or 36.4, is this correct, and if so, how long would it take to get the nitrates close to zero?

I know that the preferred level for CYA at this site is 80, but I also have an orp controller, and prefer to keep it at 30, so please no insisting on 80 ppm. I am happy where I am, and the automatic acid feed helps control the pH, the acid usage is not that high.

The normal numbers I set the pool at are:

pH 7.6
Alk 80
Free Cl 3.0
Calcium 300
Salt 3300
Gallons 10,000
Temp 80 F - 83 F

No Borates

Any help would be appreciated, thank you.

 

[bk406]

As far as I know, there is not a lot you can do about nitrates except water replacement/dilution. But, it should not be too much of a concern, really. The phosphates are even less of a concern. You have algea because you dont have enough chlorine in your pool. Even for a CYA level of 30 (ok, ill say it, 30 isnt enough for your location and the SWG) your FC should be at 4-5 ppm. 3 ppm is barely enough. At some point, the FC dropped and got algea in the pool. You should raise the CYA to 40-50 if you like to keep it on the lower side (seriously, 30 is not enough in your situation) adn keep your FC targeted for 5-6 ppm. You should also shock your pool and hold it at shock level until you get </= 0.5 ppm FC loss overnight and your pool is clear.

IMO, 4 hours out of 24 for a filter pump run time is not enough. If I had to guess, you FC falls fairly low at some point after your pump and SWG get turned off. Thats when the algea starts to grow. The FC comes back up when your pump/swg kick back on for the 4 hours, but by then its too late.

 

[chem geek]

With the CYA truly at 30 ppm, the FC at 3 ppm is fine for preventing green algae and in fact is doing so except at the suction pool canister near the surface when the phosphate level is high. Is the circulation poor there? Perhaps with only 4 hours per day with the pump, the circulation isn't enough to make sure the chlorine is well distributed in that area. 4 hours per day isn't very much circulation. If you have a 2-speed or variable speed pump, it would be better to pump longer or at multiple times at a slower speed. Why don't you take a sample near the surface where algae sometimes forms and measure the FC there, especially after the pump has been off for a while (so measure it before the pump next turns on). If the FC is very low, then that's the primary problem; also measure it even when the pump is on, for comparison.

Chlorine, no matter how high you get it, isn't going to get rid of the nitrates. The nitrogen in nitrate is at its highest oxidation state (+5) and cannot be oxidized further by chlorine or anything else. Water dilution is the only way to reduce nitrates.

However, neither nitrates nor phosphates are going to be a problem if you not only keep the FC and CYA levels that you have, but also make sure your circulation is better so that this chlorine level can get to all areas of the pool reliably. If you really want supplemental help against algae, then there are other alternatives. The easiest would be to use 50 ppm Borates -- it's not a perfect algaecide, but it should take the edge off of algae growth somewhat like the phosphate remover has been doing for you or that weekly PolyQuat 60 dosing would do.

Do you get any rains? If so, then let those overflow the pool to dilute the water. Since your fill water is apparently high in both nitrates and phosphates, you can use a pool cover to prevent evaporation which should significantly cut down the increase in those levels.

 

[bk406]

With the CYA truly at 30 ppm, the FC at 3 ppm is fine for preventing green algae and in fact is doing so except at the suction pool canister near the surface when the phosphate level is high. .

While I do agree with this in theory, I still think the FC is too low as a practicle matter. It may be 3 ppm at some point during the day, but with only running the pump and SWG 4 hours per day, coupled with the lower CYA level in that tropical sun, I dont believe it's enough, even with a CYA or 30 ppm. Agree on the circulation issue as well. I still think, bottom line, there is not enough FC, on a regular basis, in the pool.

 

[smallpooldad]

Well,

Thank you one and all.

As regards the timing of the pool and circulation the following might be of interest:

a) The pool uses a suction side vacuum which stays moving the entire pool pump running time.
b) It is most probably true that there is less circulation at, or near, the suction canister.
c) The green algae however only appears when the phosphates are above 1000 ppm, but the algae grows inside the canister near the top where there is water flowing at 60 gals approx per minute. The pool steps on the other side of the pool, which get slightly more sun and are 8 inches below the surface and the lower step where there is not much flow gets no algae whatsoever.
d) In the rest of the pool there is no algae, nor is it cloudy, and it has been that way more or less, except when I was a away for long periods of time, for over 10 years.
e) The pool is only 4 ft 6 in deep at it's maximum point.
d) I used to run chlorine at 5 ppm, then lowered it to 4 ppm, then to 3 ppm as it seemed to make no difference.

The timing of the pump is predicated on a one turn plus (10,000 gals divided by 55 gals per minute over 4 hours gives a turn of 1.36 times) , plus I swim in it daily, electricity is extremely expensive in Honolulu.

The reason I choose 3 ppm for chlorine is that according to the Chlorine/CYA chart the target free chlorine for a CYA of 30 ppm is 3.6 ppm and according to the chart I need less as I have a SWG, about 60% of that need for regular liquid chlorine or 0.60 x 3.6 = 2.16, so I raised it to 3.00 ppm. It seems to work. I am not sure why I would need to shock as the algae was only on the suction canister. This I removed with a one hour chlorine/water soak.

The other reason I choose 3 ppm, is that my step-son and his wife both of whom I really like are neurotic about chlorine affecting their children, my step-grandchildren; where this came from I do not know. The way around this is to state that at 3 ppm the chlorine is only slightly higher than seawater at 1.94 ppm. This seems to satisfy them, being only slightly higher. I know go figure, but he heads up a major computer company so I think, and maybe he is right, that he knows more than me. Explaining the relationship between CYA and chlorine does not register only the raw chlorine number works, even though the true Cl level is lower, I think.

I will measure the chlorine prior to the pool pump starting at 11am and if so adjust accordingly if the drop is more than 0.50 ppm. Also I will try two two hour sessions. I do have a pool cover and will use it more often. Thank you all for those great ideas.

Sadly as written I will just have to drain when the nitrates get too high, say over 40 ppm but then again I need to do this anyway to control iron stains, maybe once a year.

Aloha and thank you to you all for helping, and a note to chem geek I still love reading all your many comments after all these years.

 

[chem geek]

The other reason I choose 3 ppm, is that my step-son and his wife both of whom I really like are neurotic about chlorine affecting their children, my step-grandchildren; where this came from I do not know. The way around this is to state that at 3 ppm the chlorine is only slightly higher than seawater at 1.94 ppm. This seems to satisfy them, being only slightly higher. I know go figure, but he heads up a major computer company so I think, and maybe he is right, that he knows more than me. Explaining the relationship between CYA and chlorine does not register only the raw chlorine number works, even though the true Cl level is lower, I think.

Just to note that when I say 3 ppm FC is OK, I do mean as a target and that the minimum doesn't get much below that. I think the point that bk406 is making is that with the pump only running 4 hours a day, so SWG generation only during that time, that the chlorine can get significantly lower than 3 ppm given full sun and the low CYA level and he's right that this could be a problem. That's why I wanted you to measure the chlorine at the worst spot at the worst time.

On the other hand, you say you've also run at 5 ppm and still had this particular problem. It's odd that the step area without much circulation seems to do fine but that this one suction area that seems to get a lot of water flow doesn't. I have some algae in my skimmer, but it's above the water line which makes sense since it gets moisture from evaporation, but doesn't get direct exposure to chlorine.

Anyway, with regard to what you wrote that I quoted above, I don't know where you get that the chlorine level in seawater is 1.94 ppm. There is NO chlorine in seawater whatsoever. If there was, most smaller aquatic life would be dead -- no algae, no plankton, no small fish (depending on chlorine level), etc. There is, of course, a lot of chloride at around 35,000 ppm or thereabouts. And you are correct that the actual hypochlorous acid concentration in your pool is much, much lower due to the CYA in the water where roughly speaking it's the same as having 0.1 ppm FC with no CYA.

Well, at least you now know you can't get rid of nitrates which was your original concern. Sorry I can't find a way to keep the algae away from that one part of your pool without some sort of supplement, though at least you have some other options if you want to pursue them.

 

[smallpooldad]

Chem geek,

Thank you for the reply and clarification.

The reason I adjusted it down from 5 to 3 ppm of chlorine is that I had no problems whatsoever with algae at 5, 4, or 3 until only recently. I did not make this clear enough please accept my apology. I had not checked for either phosphates or nitrates in over two years and will be more diligent in the future. I will however check at the worst time of the day and just before running the pump to see if there are issues with the chlorines ppm. If so I will up the ppm until the issue is gone.

I will be doing the iron flush tomorrow, so it might be a few days before I could report back. Giving time for the balance to occur.

As regards seawater's chlorine level I obtained my information at the four sites below, am I reading this incorrectly, at my age most probably and certainly chemistry is not my strong point? They appear not to agree on the ppm, but I took Wiki's, is Wiki wrong to use the the word chlorine, if so it is a great mistake as it satifies ny stepson and daughter-in-law. But is this chlorine gas that is being produced or simply an ion measurement and does the ppm in any way relate to chlorine levels in a pool. I just thought of that after your reply. On thinking this thing through further I believe you are correct, are you?

http://www.seafriends.org.nz/oceano/seawater.htm

http://en.wikipedia.org/wiki/Seawater

http://www.marinebio.net/marinescience/02ocean/swcomposition.htm

http://www.physicalgeography.net/fundamentals/8p.html

Additionally this paper might be of interest to you, I certainly cannot understand it:

http://www.mdpi.com/1422-0067/9/4/542/pdf

Aloha and thank you.

 

[chem geek]

The first link refers to "Chlorine NaCl" with atomic weight 35.453 and ppm of 19,400. The second link refers to the element of chlorine at 1.94 percent. The third link does not refer to chlorine, but rather to chloride at 19.345 ppt. The fourth link says, "The chlorine ion makes up 55% of the salt in seawater. Calculations of seawater salinity are made of the parts per 1000 of the chlorine ion present in one kilogram of seawater. Typically, seawater has a salinity of 35 parts per thousand."

The 19,400 ppm refers to chloride ion whereas 35,000 ppm refers mostly to sodium chloride salt and these two are rougly the same except for units (and some minor ion contributors). The links refer to chlorine as an atom, but this is not the same as chlorine with chlorine gas or hypochlorous acid in water. The oxidation state is different and that makes the reactivity completely different. The chlorine atom in seawater is as chloride ion with a -1 oxidation state and is relatively inert in terms of chemical reactions though is obviously important in its role as an ion in biological systems. The chlorine atom in chlorine gas, hypochlorous acid, and hypochlorite ion (and even as chlorine bound to CYA) is in a +1 oxidation state that is much more reactive though relatively speaking the chlorine bound to CYA is far less reactive than hypochlorous acid.

Also note that the ppm of chloride ion in seawater is 19,400 ppm and not 1.94 ppm.

The PDF file (last link you posted) is talking about addition of chlorine to prevent biofouling in seawater used to cool thermal or nuclear power plants. The paper is mostly about estimating the Total Residual Oxidant (TRO), which is essentially the chlorine that remains after oxidizing organics in seawater, so that the water can use enough chlorine for treatment, but not so much as to result in too much chlorine in the discharge water. The paper sums up this nicely by saying "The results from this study provide a baseline for the operator of power plant to balance the tradeoff between antifouling requirements and environmental protection." They basically concluded that the iodine colorimetric method is suitable for determining TRO and was better than using DPD because it was more stable so would last longer between taking a sample and getting it measured.

 

[smallpooldad]

Chem geek,

Thank you for your quick reply.

If I am reading you correctly the chlorine in a pool is completely different due to its associations, well at least I now know why I will never fully understand chemistry, at least at your level. I will not bother to explain this to my step-son, as I myself, despite your concise and clear explanation, am at lost as to exactly understand these processes.

One last question, how accurate is the Taylor standard for chlorine is it off by say giving a measurement of 30% more free chlorine than is really there or is it very close; in other words is a reading of 3 ppm really 2.1 ppm? I read somewhere that it is was not too accurate and somewhat overstated, assuming fresh chemicals, but that could be a sales ploy on the part of the opposing method. And I myself find that hard to be true as Taylor has such an excellent reputation.

Aloha and thank you.

 

[chem geek]

I think the easiest way to understand it is that the chlorine in pool/spa chlorine products, including chlorinating liquid and bleach, and the chlorine you measure as FC or CC in pool water is "activated" or at a higher potential or has more chemical energy than that of the chlorine in salt such as sodium chloride found in salt water. In fact, when this activated chlorine gets used up, it becomes the relatively inactive chloride (i.e. salt). A saltwater chlorine generator (SWG) system essentially reactivates the chloride salt back to chlorine (hypochlorous acid) which is why it takes energy to do so. Think of it as a chemical boost.

The Taylor FAS-DPD test is very accurate, within one drop in the test which means that for a 10 ml sample size it's accurate to within 0.5 ppm while for a 25 ml sample size it's accurate to within 0.2 ppm. Of course, there is variability based on user-error since you need to accurately measure the sample size so in practice you might introduce a 10% error if you aren't careful. It's unlikely to get an error of 30% unless the reagents are very old or the sample collection and measurement is extremely sloppy.

There is also the error associated with the dropper tip size. The Taylor dropper tips are designed to be 24 drops/ml within one drop so an error of +/- 4%.

 

[smallpooldad]

Chem geek,

Thank you for a very clear analogy, I think I understand clearly. You should really write a pool book.

And thanks to one and all I think that I can get as close as possible to "Pool Chemical Balance Nirvana" once I sort out the correct balance between runtime and chlorine ppm.

I have taken all the contributors thoughts to heart and with an open mind on the issues will test to see where I come out, reporting back in a week or so as to the optimal runtime and chlorine ppm for our particular pool and situation.

Aloha to you all and have a nice mother's day, and now to start the cooking.