Newbie just beginning to understand "free" and "total" chlorine. . .

Hi all! I have a 21,000 gal inground, vinyl lined SWCG pool built last year (now in second season). All is going fairly well, but on my last trip to pool store, water test showed there was some combined chlorine (which they called "chlorine lock" - uh oh . . . ), and water was ever so slightly greenish - they prescribed "shock" a "non-chlorine" product that had an active ingredient of potassium monopersulfate (and was 55% "other ingredients" - yikes! what DID I put in my pool?) That seemed to clear it up for about 10 days, now slightly greenish again. My browsing on this site/forum has raised a red flag about blindly following their instructions.

We began the year with a failure of the salt sensor in the SWCG cell and had the unit replaced, so our chlorine was a bit low at the last test, and our salt is a bit high because the faulty unit kept saying to add more! The most recent water test results (6-1-12) follow:
FC .8
TC 1.1
pH 7.9
Hardness 234 ppm
Alkilinity 86 ppm
CA 40 ppm
Nitrate 26 ppm
Salt 4000 ppm

We fired up the new SWCG cell, lowered pH some and shocked and, as I said above, it was crystal clear for about a week. Current readings by my drop test about 3 ppm TC and 7.6 pH. Any advice anyone has on 1) combined chlorine and the best way to get rid of it; 2) recommendations for a reliable FC and TC test meters - digital - something that will give me a number not make me play "identify this shade of yellow"; or 3) causes of slightly greenish (but definately not cloudy and no sign of algae) water would be greatly appreciated.

Thank you!!

Combined chlorine is chlorine that is actively engaged in oxidizing goo from your water. The best remedy for it is ...more chlorine! It just means that the chlorine is doing its job. Usually if you have CC more than .5 ppm, we suggest shocking the pool until that CC level drops to zero. If it's less than .5, as in your case, just keep your chlorine levels where they need to be, keep the pool uncovered, and they'll take care of themselves. (By the way, there's no such thing as "chlorine lock", and the non-chlorine product they sold you was a really expensive way to do what you could have done with plain, generic bleach).

The FC and TC tests that we recommend are the DPD-FAS test kit (K-2006).(See the test kit link in my sig). It uses a scoop of powder to turn your sample pink, then you add drops of the reagent until it goes clear. No comparing yellows.... the complete kit is the K-2006, but if you have a drop-based kit already, the K-1515 is the standalone chlorine part of the kit. You can get them at a good price online using the Amazon link in my sig.

One of the main caues of slightly greenish but not cloudy water is the presence of metals. What is your source of fill water, and is there any known copper or iron in the water? Have you used anything containing copper (algaecides, trichlor tabs, any other type of "dual action" product) that contains copper?
!

Thanks for the speedy reply! Interesting that you mention metal. I left it out of my original post, but the last water test said 0ppm copper and .31 ppm iron. The pool store considers the limit for irom to be .30, so they gave me a "metal out" product which I used. Our fill water source is our well, which is generally good (has been professionally tested for every contaminant known to man, as well as minerals and metals), but occasionally contains a little iron (seasonally depaning on groundwater levels and was <.03 ppm for Iron in January of 2012). Also, water gets to pool through hose on frost free hydrant which has some parts that could leach. My husband just added water (for the first time this year) today with that hose! Could the addition of new water w/ metals be reacting to earlier administration of "metal out" product? Maybe I should just let the pump go and see what we've got in 24hrs?

I will plan on getting the chlorine test you recommend and shock with bleach. Does it matter what bleach you use - obivously not the funny ones with added lavender scent, etc - but is there really any difference between the "splash-less" and all the other permutations out there these days and regular old bleach? I don't want to add anything that might cause any problem with the SWCG cell.

Thanks again.

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Originally Posted by Hainsworth

I will plan on getting the chlorine test you recommend and shock with bleach. Does it matter what bleach you use - obivously not the funny ones with added lavender scent, etc - but is there really any difference between the "splash-less" and all the other permutations out there these days and regular old bleach? I don't want to add anything that might cause any problem with the SWCG cell.

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Yes, there is!!! You want plain unscented, un-splashless chlorine bleach! All the others have additives that can cause foaming or worse and are not considered to be sanitizers.

What make of SWCG do you have. CYA of 40 ppm seems on the low end and it is a good rule of thumb to run the CYA at the manufacturers high end which is usually 75 to 100 ppm depending on the make.

Thanks Waterbear!

That's what I thought about the bleach, but I want to make sure I ask all the questions (even dumb ones :) ) so that I get this right! My SWCG is a Pentair Intellichlor Model IC 40. I checked the manual and it does say that optimum CYA levels for the unit are 50-80 ppm. Why should I aim for the manufacturer's high end? I appreciate the education.

Having the CYA at the high end of the manufacturer's recommendation will help the cell last longer.

Oh--and there are not dumb questions, when it comes to pool care! :)

No only will it help the cell live longer, it will indirectly give you better pH stability. It's a lengthy explanation that I won't go into now but once things slow down in the fall I will be glad to explain it!

Also, as a rule of thumb you want to keep your FC at 10% of your CYA level (as a minimum) or a bit higher for a margin of safety.
Order the K-2006 and do your own testing. Pool store testing is often suspect (I know this first hand, having worked in pool stores!)

Iron in your water can cause the water to turn yellow and can casue yellow to orange stains...combine that with a blue liner and you have your greenish color. Using a metal remover on a regular basis is the right thing to do but you want to use one that is HEDP. Many of the 'metal remover' products are blends or ineffective chamicals such as EDTA that break down quidkly in the presense of chlorine so they do not do much good at all. Exactly what brand of metal remover did they sell you?
If you prefer you can get this one from Amazon, it IS HEDP:

KemTek 20% HEDP @ Amazon

You want to use the initial dose of 1 oz per 1000 gallons (for your pool 2 quarts) and then a maintenance dose of 1/4 that amount ever week or two of HEDP.

Thanks a lot to aylad and Waterbear! Waterbear, you may be on to something with the iron/yellow water over blue liner idea, but so far the green is very faint and only really noticible on the (white) stairs and swim out. If the water hadn't been perfectly clear before I probably wouldn't even say that the water was green. The Metal product they sold me was SunGuard Metal-Out Plus, which I just found lists NO ingredients on the bottle (or the SunGuard website). . . hmmm. They had me use one quart for my 21,000 pool.

I think the "green" is less noticible today. pH is back up to 7.9 (tested w/ newly calibrated digital meter I had to get for household testing because we needed acid neutralizer for house - well is 4.9!!) which is what it was before last pH correction. After reading up on pH here on the forum, I've decided not to worry about that right now. Even with the (untreated) well water as fill last year, we were always on the high end 7.6-7.8 and had our lowest reading ever on opening this year - 7.5, so maybe that's just what my pool "wants" to be. I'm on my way to shock (bleach) and when my new super duper test kit gets here (K-2006) I'll check CYA, ALK, and FC/CC and adjust accordingly. Then I'll see where we are on pH. Thanks everyone!!

Update: My initial "green water" issue resolved itself after several days filtration. I did shock with bleach and deicded to use a borate buffer as is recommended on so many posts regarding SWCG pools. Hoping I can get to a nice equilibrium at 7.7 or 7.8 and avoid the drift with less carbonate alk.

Todays numbers from my Taylor K-2006:
FC 6
CC 0
pH 7.8
Hard 280
Alk (carb) 90
CYA 75-80

But the plot thickens!! After your earlier posts, I tested both the pool and my fill water (hydrant on well) for copper and iron (because those are the test kits I have :) ) and came up nada on both. I thought I was good to go . . . until I had to put the hose in the pool again this morning and now have (drumroll) slightly greenish water! The goos news, I think I know what it might be - I just need to know what you all think. My well is acidic (about 4.9) so we installed an acid neutralizer for the house to save our fixtures and keep my from constantly scrubbing off the copper stains! These us Calcite (calcium carbonate) in a tank just like a water softner, but the goal is to raise the pH. However, because our pH was so low, we needed a mixed media, 90% calcite and 10% corosex which is magnesium oxide. Could that Magnesium be reacting with the pool water (chlorine) and causing the green tint? I can bypass the neutralizer (what a pain), and plan to in the future for adding water, so I will see what happens next time. What do you think?

Thanks again. Your site had been invaluable to me and I am so pleased to have such a beautiful (albeit currently slight green) pool that I now truly understand!!

Yup, magnesium reacts with the chlorine in the pool water, just like iron and copper do. Good work!!

Janet

I have never known magensium to turn water green but wonder if you might have any copper or brass plumbing on your well? That could explain the green water (acidic water going through the metal pipes or fittings).
If you have used a 'metal out' product (seqeustrant) then it is entirely possible that your water would test negative for metal even when they are present (but sequestered).

Thanks Janet and Waterbear,

There is no copper on the well line or on the way our to the hydrant, there are probably two brass fittings between the well and the fill hydrant, but certainly not enough to cause discoloration in fill water, especially now that the incoming ph has been corrected to darn near 7.0.

The neutralizer media is 10% magnesium oxide which combines with water to form magnesium hydroxide (a base) thus raising the pH of my incoming acidic water. My research says that magnesium hydroxide combines with chlorine to form magnesium chloride (a salt) which I think shouldn't be a problem in the pool, but also isn't doing any work for me either :). Unless my SWGC can break it down too, but I have no idea. . . .

The (again VERY slight) coloration to the water goes away with 2 to 3 days normal filter operation after a fill. It really only got my attention because we didn't have anything like it last year (first year for pool) . . . but that was BEFORE we got the neutralizer up and running. So, ironically, last year we were much more likely to have been adding some trace metals since we were adding pH 4.9 fill water that was surely leaching stuff from my hydrant, etc.! I'm thinking that the slight color cast after a fill is the effect of the magnesium compounds going through their changes when they hit the chlorine, and that after 2-3 days they're all magnesium chloride and the color cast is gone. I plan on bypassing the neutralizer the next time I need to fill and see what happens, if nothing, I'll assume I'm right.

In the meantime, I've added a borate buffer and am lowering my carbonate alk (per your instructions on the forum) to try and stabilize my pH - it was always creeping up as I (now) understand is normal for a SWCG system.

I do know why it happens, but I still can't believe that a pool company would sell you a system and not teach you the hows and whys of it's operation. That makes me nuts! So grateful to have found your forum so early in my days of pool ownership . . .Thanks all!

If you do bypass and add pH 4.9 water to your pool, be sure to have plenty of Borax on hand.

Thanks BigDave!

Will do. Intereseting to note that last year, when all fill was 4.9, we never had a pH reading below 7.7. I guess that a little bit of 4.9 made up for the SWCG pH creep. I'm only using "fill water" for incedental loss due to splash out or evaporation when it's dry so it's not too much (fow now). Didn't need any fill at opening because mother nature did if for me over the winter :), and it opened at 7.5!

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Originally Posted by Hainsworth

I do know why it happens, but I still can't believe that a pool company would sell you a system and not teach you the hows and whys of it's operation. That makes me nuts! So grateful to have found your forum so early in my days of pool ownership . . .Thanks all!

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I find that most folks that work in a lot of the pool companies (at least around here) don't actually own pools, and if you've never actually used a product yourself, you don't know the hows and whys, either. Just about anybody can read a set of instructions for a product and tell you what it's "supposed" to do, but having folks who actually have practiced what they preach makes all the difference. I'm still waiting for ANYBODY to report that a salesman who sold them an Intex pool warned them that metals from well water could be an issue for them!!

Janet