What chlorine level to shock -- now that half my CYA is gone

[JimK]

My appologies if this is the wrong place to post this. I just recently became a subscriber though I've been registered for over a year.

I took the cover off today to start opening the pool. I did a quick check on pH and CYA. pH is 7.2 and CYA is 30, which means I lost about 40ppm CYA over the winter. I know this means I'll probably need more chlorine to get rid of the byproducts from the critters that ate the CYA.

Does this mean I need to shock at a higher level, or should I just shock according to the Best Guess Chart (15ppm CL given 30ppm CYA)?

Thanks. :)

[PoolDoc]

Exactly the right place to post it!

CYA loss can have 3 end products: nitrogen gas (no problem!), nitrates (not necessarily a problem) or ammonia (the PROBLEM!). The easy way to see what you'll need is simply to shock it, and see what happens. Is your pool really 20K gal? That's small for a 22x40, unless you have no deep end.

If 20K is correct I'd add 5 gallons of 8% bleach tonight, and then test again in the AM. The pH should be raised, too. I'd start with 2 boxes of borax. Low pH makes the byproducts of chlorine oxidation nastier.

If you raise the pH, and add the bleach (about 15 - 20 ppm on a 20K pool) . . . and you have ammonia, you'll have little or no FC in the AM, but may have a high CC. If the pool is slimy but has no ammonia, you'll lose some of the chlorine, but should still have some, and not so much CC. If the pool is just a bit dirty, a lot of the chlorine should last overnight.

[JimK]

Thanks Ben.

The pool installer gave us the 20k gal figure (installed in March 2004). It might be a little bigger, but dosing based on the 20k figure gets me real close. The 22ft width figure is an average of the narrower deep end, 18.5ft, and the shallow end, 24ft I believe. The pool is shaped in what's called a "lazy L" (not a true L, kind of in between a straight rectangular shape an a true L). The shallow end is 3ft (actual water level) and the deep end 5.5ft (they called it a "non-diving pool). I can send you pics if you like.

One winter some years ago I lost all CYA and it took a lot of CL before it would hold. Well, it seemed like a lot but that was when I was operating by pool store rules; I earned here they had me adding too little CL at opening. It was sometime after that when I learned here that CYA could biodegrade and about appropriate CL levels. ;)

7.2 pH is pretty normal for my pool when I first open it. In the past I just let it drift up as the SWCG ran (another lesson I learned several years ago after the first cell died prematurely; only use the SWCG to maintain CL levels, NOT to shock!). I've not noticed any problems in the past doing it this way. Given this is it ok to proceed without the borax (I don't have any borax on hand, just boric acid [I maintain 50-60ppm boric acid in the pool])?

So, I guess the answer to my question above is to shoot for normal shock levels? I'll add the bleach tonight (I bought 10 jugs (121oz) of 8.25% bleach to have on hand for opening) and see how much I lose overnight.

:)

[JimK]

I added five 121 oz jugs of 8.5% bleach around 7:30pm (it's been raining today so no sun) and brushed the entire pool. I just checked CL level (9pm) and it barely registered! This is what it measured:

FC < .5
CC < .5

Strange......... The water is cold (about 60); does this impact the DPD Powder test?

Does this mean I'm fighting ammonia? Should I go ahead and add the other 5 jugs tonight, or wait until morning?

PS -Just found this in a Google search:

"...11. When a DPD chlorine test flashes a little pink then turns crystal clear, your water has almost no chlorine in it.
The opposite is true! More spas are fried and bathing suits bleached because of this error than just about any other. As you know, the universally accepted DPD test for chlorine turns progressively more pink as higher residuals are encountered in the sample… that is until the indicator is itself reverts to a clear resultant. This phenomenon occurs somewhere near 20 ppm chlorine, the actual value varying with different pH or total alkalinity conditions. Many, many untrained operators or “helpers” have dumped excessive amounts of chlorine in their water (most often in hot whirlpools or spas), thinking all along that for some crazy reason they just hadn’t yet put enough in to get a reading! In a small body of water this testing error has resulted in a few hundred parts per million chlorine residual, creating some very unsafe, damaging or at least unpleasant conditions..."

I didn't notice it "flash pink", but I'm wondering if the CL level is over 20 causing a reading error? Out of curiosity, I did another sample and added several scoops of powder (maybe 4 to a 10mil sample?) but that didn't seem to change anything.

Thoughts?

PPS - Found on Taylor site where FAS DPD test can measure up to 20ppm. Hmmmm.... I have a hard time imagining all that chlorine was gobbled up in 1.5 hrs....I'm thinking CL may be higher than 20ppm so I'm not getting a good reading. I'll hold off adding more CL tonight and check it again in the morning.

[PoolDoc]

The 'pink then clear' transition occurs with the DPD tablets; not the with the DPD FAS test.

But . . . while cleaning this up, it might be better to use an OTO drops kit -- OTO is not too accurate, but it's bomb-proof, in the sense of no false-positives OR false-negatives.

OK. Do this:

1. Use borax to push your pH to 7.8 or so.
2. Tomorrow PM late, add (10) gallons of plain bleach.
3. Test with OTO & phenol red in the AM. Add borax as needed to maintain the 7.6+ pH. Remember that phenol red will not return accurate results if the chlorine is very high.
4. Repeat this cycle till you can sustain a strong chlorine reading overnight.
5. At that point, you should resume using the K2006 testing.

It may take a LOT of chlorine to recover. Unfortunately, with a vinyl pool, draining and refilling is not really an option. If you'd like, you can probably drain to 1 foot above the highest horizontal surface, and then refill. (Ie. you can't drain a pool with vinyl covered steps!).

However those are your options, so you just have to suck it up, vow to NOT let 'slime' (CYA-eating bacterial and algal biofilms) grow next winter, and dose with chlorine till you're done, no matter how much it takes.

It's possible to figure out approximately how much total chlorine you'll need. But other then scratching a mental itch, that information has no value: you do NOT want to do an all-at-once dose; there's too much chance you'd bleach your liner.

So, dose, check, adjust pH, repeat . . . till it's all done.

Here's hoping it won't take much.

[BigDave]

I hate to sound like I'm contradicting PoolDoc, but, I do see a value (beyond my itchy brain-scalp-very itchy) to figuring how much chlorine will be needed and that's how much to buy. The logistics of buying, moving, and storing enough chlorine for an algae / ammonia clan-up may be well worth considering; especially for us weekend warriors who can only collect the materials on Saturdays.
My two cents

[JimK]

Thanks Ben. :)

Looks like I've got a few more items to pick up.

The info I found on the Taylor site referred to the FAS-DPD test using the powder and mention a max test limit of 20ppm:

https://www.taylortechnologies.com/ChemistryTopicsCM.ASP?ContentID=11


.The latest trend among service professionals and a significant number of health officials is testing chlorine with FAS-DPD. This variation of the traditional DPD method allows users to measure both free and combined chlorine levels as low as 0.2 ppm—the maximum allowable level for combined chlorine according to most health authorities as well as the Association of Pool & Spa Professionals (ANSI/APSP)—and as high as 20 ppm. By contrast, the color comparators used with the standard DPD test generally allow readings at the low end of 0.5 and 1 ppm and at the high end of 5 or maybe 10 ppm.

(Bold added)

I'll test again tomorrow morning and see what I get.

Btw, I understand high CL will give false pH readings, so how do I accurately measure pH while shocking? Also you said, vow not to let CYA eating slime grow next winter.... How do I prevent it? Seems like whether or not I lose CYA over the winter has been a crapshoot in the past.....most years no/little loss, a couple times big loss. I don't know what I may or may not be doing that causes this. :confused:

Btw, I took a look at what I did at opening last year (I keep records of my pool maintenance) and I added the same amount of bleach, but I didn't test CL until the next day (I got a reading of 16.5). I'm hoping the issue this time is I'm not getting a good reading because CL is over the FAS-DPD test limit, and not that there is so much ammonia that I'm just pouring bleach into a black hole! I should have an answer for this tomorrow.

Oh....forgot to ask.....is it ok to leave CYA at 30 until CL holds? Is this high enough to prevent sunshine from completely depleting CYA?

[JimK]

I hate to sound like I'm contradicting PoolDoc, but, I do see a value (beyond my itchy brain-scalp-very itchy) to figuring how much chlorine will be needed and that's how much to buy. The logistics of buying, moving, and storing enough chlorine for an algae / ammonia clan-up may be well worth considering; especially for us weekend warriors who can only collect the materials on Saturdays.
My two cents

Thanks Dave. :)

I'm also limited to when I can buy supplies and try to have everything on hand in advance (I figure what I need based in last seasons).

[PoolDoc]

Losing CYA is a bio-degrative process: no "bio"; no CYA loss.

Covering a pool stops solar loss of chlorine, and it mostly stops algae, but it does not stop bacteria, especially once the chlorine is gone. Maintaining minimal chlorine levels under the cover, will almost certainly stop the process. BUT, one class of the operative organisms is Pseudomonas genus, a group notorious for forming sanitizer resistant biofilms.

Following this process of winterization should do the trick:

1. Stop using the pool at least 1 week before closing (to allow all human added organics, etc to be oxidized and/or filtered).
2. Physically clean the pool at this time, removing all leaves and debris, and brushing thoroughly to break up any forming biofilms.
3. Run the filter 24/7 during the final week. (Same reason). Clean the filter 1/2 of the way through the week, to remove all goop from the filter.
4. Raise the pool to maximum chlorine levels at the beginning of this period. Allow it to drop to high-normal levels by week's end.

5a. Winterize the pool OR
5b. Cover the pool, but do not winterize that that time.

6a. Check weekly while water temps are > 60 degrees to maintain chlorine levels OR
6b. As above, but circulate the water for several hours (or for 1 hour per day every day)

7a. --
7b. Winterize the system, once freezing temps are imminent, or water temp is below 50.

8. Continue to maintain chlorine levels all winter, with monthly checking.

9a. Once water temps rise above 60 degrees, check chlorine weekly.
9b. As above, but also open the circulation system, while leaving the pool covered.

10. Open the pool.

[PoolDoc]

Dave has a point about accumulating chlorine.

If it's not convenient to purchase bleach every other day or so, it may be worthwhile to do a chlorine demand test: http://pool9.net/bucket-demand

[JimK]

I need to print out that post about closing for future reference. :)

I'll check out that link also.

Thanks. :)

I'll report again tomorrow.

[JimK]

Day 2...

Hi Ben. I got the items you suggested (borax, OTO/Phenol Red test kit, 8.5% bleach - I have 19 jugs on hand now (121oz each).

I used the OTO kit to check CL and pH to see how it matched up with the Taylor kit.

FC - 0
pH - 7.2 (before adding borax; I'll test again in about an hour)

So it matches the Taylor kit results.

This evening I'll add the bleach as you recommended.

Is it OK to let the pool go all day with 0 CL???

[PoolDoc]

Probably be best to add 1/2 gallon or so.

[JimK]

Probably be best to add 1/2 gallon or so.

I'll do that right now.

[JimK]

I'm curious, with the initial dose of CL eaten up so quick, should I expect to see high CC? (I'm only seeing .5 max). Or does CC get eaten up some how as well?

[JimK]

.....
OK. Do this:

1. Use borax to push your pH to 7.8 or so.
2. Tomorrow PM late, add (10) gallons of plain bleach.
3. Test with OTO & phenol red in the AM. Add borax as needed to maintain the 7.6+ pH. Remember that phenol red will not return accurate results if the chlorine is very high.
4. Repeat this cycle till you can sustain a strong chlorine reading overnight.
5. At that point, you should resume using the K2006 testing.

....

Ok...here we go...did my homework for today. ;)

Added borax this morning to raise pH
Per post above, added some bleach to carry it through the day since this morning's test showed 0 CL
Added 10 jugs of bleach about 8:30pm.

I'll give it about 1/2hr to mix then test to see if any CL shows up this time. I'll get up early tomorrow and test again.

Here's hoping.......

EDIT: Just checked CL using the OTO kit; sample turned dark orange....a good sign! We'll see what it looks like in the morning. Keep your fingers crossed.......

[JimK]

7am this morning (no sun on the pool yet) OTO test was still dark orange....yea!

Since I was still getting a strong CL indication with the OTO test, I also checked it with the Taylor kit and got this;

FC 25.5 (Taylor site says FAS-DPD can measure up to 20...the test behaved normally, so can I count on this result being accurate?)
CC 0
pH 7.6

So CL looks good, yes? I know high CL can cause inaccurate pH results, but this is real close to what I got when I checked pH before adding bleach (FC was at 0 at that point). Any chance this pH reading is good?

I'll check CL again after sunset using the Taylor kit, and if it's still at shock level I'll hold off adding anymore bleach and recheck early in the morning to see how much is lost overnight. Am I correct that if the loss is 1ppm or less I'm good to go?

At this point with the high CL level is it OK for me to test and start addressing TA, CYA, borates, and salt? Or do I need to wait until CL and pH are balanced first?

Sorry for all the questions. I realize this is a very busy time for y'all, but I'd really like to understand all this so I don't have to bug y'all again should the same situation occur again. With this knowledge I can contribute more/answer questions instead of asking questions all the time. :D

Thanks! :)

[PoolDoc]

Those chlorine levels indicate your chlorine demand is gone. Whether you have chlorine this PM will depend on how much sun you get today, and how much CYA is actually in your pool. BUT . . . you're on the home stretch here.

Take this opportunity (while chlorine is high) to brush your pool carefully in any spots where there might be slime, debris, or algae. You want to knock the goo loose, break it up, and then let the high chlorine wipe it out.

Once levels drop back to 10 ppm or so, do a full set of pool tests (FC, CC, pH, TA, CH, CYA) so you can see what 'tweaking' is needed.

[JimK]

Thanks Ben. :)

That's good news! So it ended taking 16 jugs of bleach total (about $50). Not too bad....I had fears it would be much worse!

Btw, this whole time since removing the cover Saturday, the water has been clear, and after vacuuming yesterday morning the pool looks great.

[Watermom]

Good job!

[PoolDoc]

Not too bad....I had fears it would be much worse!

Me, too! I'm glad we were both wrong.

Happy to hear it's looking good.

[JimK]

Good job!

:)


Me, too! I'm glad we were both wrong.

Happy to hear it's looking good.

It looks real inviting....except for being 68 degrees! Brrrrrrr..... :D

Tomorrow I'll post some more numbers.

[JimK]

Just checked FC...dropped to 16 (from 25.5 early this morning). Today was sunny all day. Hopefully it will hold overnight. If not, should I bump it back up to 20?

[PoolDoc]

Nah. Unless you have algae or slime, let it drop to levels appropriate for your current CYA level

[JimK]

Thanks. No algae or slime, so I'll let it drop to normal. 5-6 seems to be the sweet spot for my pool given CYA 60-80 recommended by my SWCG manufacturer (Goldline Aqua Rite, now Hayward). :)

[PoolDoc]

Sounds good!

[JimK]

7:30am this morning:

FC 13
CC 0

So it dropped 3ppm overnight. I expect by this evening it should be at the target range (hopefully not under!) and should be able to measure/tweak everything else as needed (pH, salt, CYA, TA, and borates). :)

[PoolDoc]

Sounds good.

[JimK]

I just wanted to say thank you for the help dealing with the byproducts of CYA biodegradation. I won't bore you with the latest numbers other than to say I just need to tweak a few levels before I start up the SWCG.

Water warmed up to 76 today, so we officially kicked off the 2014 season by getting in! :D

Thanks again. :)

PS - I don't care that some years ago my (ex!) pool person at the pool store said y'all are just some nuts on the internet and don't know what you're talking about, y'all are A+ in my book. :)

[PoolDoc]

PS - I don't care that some years ago my (ex!) pool person at the pool store said y'all are just some nuts on the internet and don't know what you're talking about, y'all are A+ in my book. :)

When I first started PoolSolutions and the PoolSolutions forum (later, PoolForum), I used to get regular 'nasty-grams' from dealers and pool guys. But other than complaints from some of the shysters we've exposed, that no longer seems to happen. We've actually had pool guys tell people to use our site to figure out pool chemistry.

[JimK]

I must admit the first time I came across your site some years ago, I wasn't sure what to think of it. After all, my go to person at the pool store was an "expert" with many years experience (to his credit he was never pushy about selling chemicals).

However, it became apparent over time that their recommendations sometimes did not work and only resulted in dumping more money into the pool. The last straw was when I discovered that the reason my pool was staining was from all the copper algaecide they had me use (I kept getting the beginnings of an algae outbreak (slick liner) so they had me start using the algaecide; it did work though).

After spending $$$ to deal with the stains and copper (from info I found on the internet :D ), I started really reading your site and giving your methods a try........best thing I ever did as a pool owner! :). My pool is now much easier and much cheaper to maintain with FAR fewer problems. :)

[PoolDoc]

Thanks.

[Watermom]

PS - I don't care that some years ago my (ex!) pool person at the pool store said y'all are just some nuts on the internet and don't know what you're talking about, y'all are A+ in my book. :)

But, I have to say, some of us might actually be a little "nuts!" :p:p;)

[JimK]

Lol!

[JimK]

Well, I thought it would be smooth sailing from here, but this morning's readings threw me off.

Last night:

FC 9
CC 0

There was a 4ppm loss of FC during the day yesterday (seems normal since I hadn't started the SWCG yet).

This morning:

FC 2
CC .5

I certainly didn't expect a FC drop of 7ppm overnight! :eek:

Last night I did take a full set of measurements (I can post those if needed), and added the following:

1 box borax (pH was reading 7.2 so I threw in the last box of borax I had)
3.25 lbs of stabilizer to bring it up to the recommended 70-80ppm
100 lbs of salt
2 quarts of Jack's Magic Purple Stuff (if I don't use this I get staining...I'll address this in another thread at another time)
1 quart of polyquat (60%); I do this every spring for insurance while getting CL levels stabilized.

Could anything I added last night explain the big FC drop? :confused:

This morning I started up the SWCG and added 2 jugs (121 oz each) of 8.25% bleach. Also, when starting up for spring I always run the pump 24/7 until everything is balanced and stable....don't know if this matters in this case.

[PoolDoc]

I've pretty much given up explaining everything I see happen in pools. Sometimes, years later, I find out what was going on, but mostly not. Weird stuff happens from time to time in pools, and unless your goal in life is to be an amateur chemical detective, it's better to let it go UNLESS it's causing a real problem. Who knows, if you have a dog, maybe this:
http://hqdesktop.net/wallpapers/m/95/funny_dog_peeing_in_the_pool_m94487.jpg
is the explanation. :)

If you are regularly adding Jacks Magic, you may want to get and use some CuLator packs. They are not effective for quick clean-up, but they do continuously remove metals from the water, unlike products like Jacks Magic which work temporarily to keep it soluble. If you do get some, do NOT throw them away monthly, like the company encourages. They are not 'used up' till badly discolored OR fouled with goop. Keep one in your skimmer, but use a skimmer sock on top to protect the pack from 'goo'. You can also just chuck them in the pool. The more you have, the more effective they'll be. Just don't put them where they will get ruined with greasy dirt. (So, don't put them in a chlorine floater, where lotion and suntan oil can foul them.)


1 ppm Culator (http://www.amazon.com/exec/obidos/ASIN/B003C5PNUW/scouscho-20)
1.5 ppm Culator, sold by Amazon (http://www.amazon.com/exec/obidos/ASIN/B004Y6RK3Q/scouscho-20)
4 ppm Culator, sold by Amazon (http://www.amazon.com/exec/obidos/ASIN/B007AHDMTM/scouscho-20)
Poolmaster 16242 Poolmaster Skimmer Basket Liner (http://www.amazon.com/exec/obidos/ASIN/B004XUIRQW/scouscho-20)

[Watermom]

I bet it was the polyquat that caused the drop in FC.

[JimK]

I've pretty much given up explaining everything I see happen in pools. Sometimes, years later, I find out what was going on, but mostly not. Weird stuff happens from time to time in pools, and unless your goal in life is to be an amateur chemical detective, it's better to let it go UNLESS it's causing a real problem. Who knows, if you have a dog, maybe this:
http://hqdesktop.net/wallpapers/m/95/funny_dog_peeing_in_the_pool_m94487.jpg
is the explanation. :)

If you are regularly adding Jacks Magic, you may want to get and use some CuLator packs. They are not effective for quick clean-up, but they do continuously remove metals from the water, unlike products like Jacks Magic which work temporarily to keep it soluble. If you do get some, do NOT throw them away monthly, like the company encourages. They are not 'used up' till badly discolored OR fouled with goop. Keep one in your skimmer, but use a skimmer sock on top to protect the pack from 'goo'. You can also just chuck them in the pool. The more you have, the more effective they'll be. Just don't put them where they will get ruined with greasy dirt. (So, don't put them in a chlorine floater, where lotion and suntan oil can foul them.)


1 ppm Culator (http://www.amazon.com/exec/obidos/ASIN/B003C5PNUW/scouscho-20)
1.5 ppm Culator, sold by Amazon (http://www.amazon.com/exec/obidos/ASIN/B004Y6RK3Q/scouscho-20)
4 ppm Culator, sold by Amazon (http://www.amazon.com/exec/obidos/ASIN/B007AHDMTM/scouscho-20)
Poolmaster 16242 Poolmaster Skimmer Basket Liner (http://www.amazon.com/exec/obidos/ASIN/B004XUIRQW/scouscho-20)

Thanks Ben. There's no sign of algae or anything, so as you suggest I'll just write it off as a quirk and move on. :)

LOL.....no dog here. ;) 99.99% of the time it's just the two of us using the pool.

Once I get CL levels stabilized and the SWCG set, I'll look at addressing the metal issue. BTW, last year you had me do the bucket test for metals. Despite the results being negative for metals, the stairs starting staining again when I stopped adding sequest (6 oz a week after the initial does of 2 qts does the trick).

My hesitation with the Culator is cost. At least if used as recommended, it costs more to use the Culator than the sequest, not to mention that for some time I'd have to use the sequest and the Culator together, increasing cost further. Then add the cost of the skimmer socks, which would likely get clogged quickly and require frequent replacement due to all the nearby trees. So it seems that going with the Culator would cost more and add more maintenance than just adding the sequest once a week. I would like to persue this discussion further, but I'll start another thread for that at a later date when I'm ready to tackle it. :)


I bet it was the polyquat that caused the drop in FC.

Now that you mention it, I do think I remember reading that polyquat increases CL demand? If so, I think that may be it. Thanks for pointing that out. :)

[Watermom]

Sequestrants keep metal in suspension, the Culator will remove the metals. Also, skimmer socks can be rinsed out and reused over and over again. Just a little info.

[JimK]

Sequestrants keep metal in suspension, the Culator will remove the metals. Also, skimmer socks can be rinsed out and reused over and over again. Just a little info.

Thanks. I'll keep that in mind when I'm ready to tackle that. :)

[PoolDoc]

@Watermom: good catch; thanks!

[JimK]

Ugh...still losing too much FC overnight (about 6ppm).

Although there is no sign of algae, my concern is if I set my SWCG to maintain 5-6ppm during the day like I normally do, then when the system shuts off in the evening (I normally run the pump about 10hrs a day) all the FC will be gone by morning.

So this evening I hit it with another 7 jugs of CL bringing FC back up to a little over 30ppm. Hopefully this will knock out whatever is eating up the FC. I'll keep checking CL levels twice a day (morning and evening) to see if this does the trick.

[PoolDoc]

I think we need to do a better job of explaining to pool owners the options for avoiding any bacterial growth over the winter! It doesn't effect everyone equally, but for many it causes some painful start-up problems.

Hopefully, we can help you avoid this next year!

[JimK]

I think we need to do a better job of explaining to pool owners the options for avoiding any bacterial growth over the winter! It doesn't effect everyone equally, but for many it causes some painful start-up problems.

Hopefully, we can help you avoid this next year!

Most years it hasn't been a problem. But now and then the issue pops up.

A quick update:

Last night after checking FC level I shut down the pump (up until now it's been running 24/7). This morning's test showed no FC loss overnight.....yea! I don't know if that last shock treatment did the trick or turning the pump off last night had something to do with it. Does aggitation cause FC loss?? I know in another thread some time back there was a discussion about whether using one of those magnetic stirrers when testing CL altered the results as apposed to just swirling by hand. Was there any conclusion on that?

Btw, the first night after the last shock it lost 11ppm overnight.....however we did get some heavy rains, so that may have contributed to the loss.

So once the FC drops to normal levels, I'll start up the SWCG again and see if FC stays steady, day and night.

[PoolDoc]

Does aggitation cause FC loss?? I know in another thread some time back there was a discussion about whether using one of those magnetic stirrers when testing CL altered the results as opposed to just swirling by hand. Was there any conclusion on that?

Agitation does not cause chlorine loss, in and of itself.

But, apropos of your situation, filters can accumulate massive bacterial growth. Judging from the odors I've encountered when opening filters in spring, I would guess that what's happening is anaerobic decomposition of oils and films on the sand - which is a good thing, I suppose.

However, the residual bacterial and decomp products could definitely create a chlorine demand.

-- This is a new idea for me. I've known about this, but never considered it. I have encountered 'greasy' sand on multiple occasions, and have considered that using the skimmers to add chlorine prevents this. But I'd never thought about 'cleaning' filters with bacterial digestion -- frankly, I'd never gotten past 'gagging' at the odor and black slime! --

[JimK]

Agitation does not cause chlorine loss, in and of itself.

But, apropos of your situation, filters can accumulate massive bacterial growth. Judging from the odors I've encountered when opening filters in spring, I would guess that what's happening is anaerobic decomposition of oils and films on the sand - which is a good thing, I suppose.

However, the residual bacterial and decomp products could definitely create a chlorine demand.

-- This is a new idea for me. I've known about this, but never considered it. I have encountered 'greasy' sand on multiple occasions, and have considered that using the skimmers to add chlorine prevents this. But I'd never thought about 'cleaning' filters with bacterial digestion -- frankly, I'd never gotten past 'gagging' at the odor and black slime! --

Now that the pool itself seems fine would you suggest that I backwash the DE filter and recharge to get the "spring cleaning gunk" out of it? When we vaccumed it we did it through the filter and not to waste (only raised pressure 1-2 pounds) because vaccuming to waste uses up so much water.

[PoolDoc]

Actually, I'd recommend you OPEN the filter, and wash it out. Backwashing never fully cleans a DE filter.

I just looked up the manual for that Hayward filter. It's a big filter case, and they are somewhat tricky to re-assemble. You have to be careful to get the O-ring clean and aligned, the O-ring groove clean, and the case in place, without bumping the O-ring. Then you have to install the band and torque it into place.

It works better if you use a machine hammer to tap the ring as you tighten:
1. Snug the bolt up.
2. Starting in the position 180 degrees from the bolt, use a machine hammer to tap the ring lightly, moving from the opposite position evenly to the bolt. Do this on both sides.
3. Then tighten the bolt some more, and 'tap' again.
4. Once you are nearly fully torqued, tap the ring again, and then fully torque the bolt.

If you don't have a torque wrench, here's a well-rated, but inexpensive one. Please note that 150 inch-pounds is specified, but that that is equal to 12.5 foot pounds. (Which actually seems light to me -- but I have enough experience to go by 'feel', and adjust that way.)

Anyhow:
Hayward Pro Grid Manual (https://www.hayward-pool.com/pdf/manuals/ProGrid-DExx20.pdf)
Neiko 3/8-Inch 10-80 Foot-Pounds Automatic Torque Wrench (http://www.amazon.com/exec/obidos/ASIN/B000MB53ZE/scouscho-20/)

[chem geek]

You can see from my post (http://www.troublefreepool.com/threads/10519-It-Can-Happen-to-Anyone-Zero-Chlorine-CYA-gt-Ammonia?p=86129&viewfull=1#post86129) showing the log of chlorine addition to get rid of the ammonia and other products after CYA got degraded by bacteria in my pool. There are several things to note though some of them may be due to my catching the problem early before the degradation was complete. First is that adding chlorine resulted in it mostly getting consumed so that the FC stayed close to 0, but that the CC did not just keep climbing up. If the water simply had ammonia, then adding chlorine would normally result in a rise in CC until nearly all the chlorine combined with ammonia to form monochloramine that registers in chlorine tests as CC. Second is that the rate of chlorine loss starts out high for a while, but then drops off but not to normal right away.

So when this bacterial degradation of CYA occurs there may be several factors that lead to the chlorine demand. There is ammonia which combines very quickly with chlorine to form monochloramine. This happens in about a second with no CYA in the water and less than a minute when CYA is present (i.e. active chlorine level lower). It is possible for the degradation pathway to be interrupted and for intermediate products to accumulate such as the partially degraded CYA chemicals biuret and allophanate. These partially degraded CYA chemicals get oxidized by chlorine, but take longer than getting rid of ammonia (I don't know exactly how long, but given my situation I suspect it's a few days to a week). Finally, the bacteria themselves especially those in biofilms also are slow to oxidize by chlorine.

Regardless of the details of what is going on, the basic approach to handling it is the same. You keep adding chlorine until you get a consistent FC reading that holds. Initially, you won't get any FC holding for very long, but eventually its rate of drop will lessen. At that point you can consider physically removing the source of some of the demand and as Ben has noted the filter is a good place to start since it may have a lot of bacteria in it. This is especially true if your water is now clear. You can tell if your water is basically OK and your filter is not by doing an overnight chlorine loss test with the filter OFF either by bypassing the filter or turning off circulation (depending on what you can do with your particular setup) during the overnight test. If the chlorine loss is only substantial when the filter is in the loop, then you know your problem is primarily in the filter.

[JimK]

Thanks guys for the great info.

I break down and clean the filter every fall at closing, so I'm familiar with the process.....and yes, it's a PIA. ;)

I think I'll try the test chem geek suggested to see if the filter is placing much demand on CL levels before I resort to taking it apart.

The info about what happens to CL when breaking down ammonia is interesting. Through this whole process CC never got above .5ppm which makes me wonder if I had that much ammonia to deal with.

[PoolDoc]

Ammonia would react almost instantly with the chlorine.

Richard was mostly (I think) referring to other compounds which react much more slowly.