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

[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
Neiko 3/8-Inch 10-80 Foot-Pounds Automatic Torque Wrench

[chem geek]

You can see from my post 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.

[JimK]

Ammonia would react almost instantly with the chlorine.

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

That probably explains where all the CL went after the first shock; I put in 5 gallons of 8% and when I tested the water about an hour later FC was 0 and CC was .5

I'll post how the filter CL demand test goes.

[chem geek]

Yes, that's correct. In my situation, given the eventual chlorine demand and the amount of CYA loss I should have seen (if ammonia were present) very high CC levels over 30 ppm before it would have started to drop again, but in fact I only saw it rise to no more than 1.6 ppm CC. So the bulk of what was in the water was something other than ammonia, but that the chlorine reacts with albeit more slowly. I guessed that it was the intermediate breakdown products of CYA -- basically the bacteria weren't finished eating it when I got to them. We've seen other reports where the CC is high after adding chlorine and gets reduced rather easily and quickly, albeit needing lots of chlorine. For CYA degradation, it basically goes through the following steps so depending on where it stops you can end up with a mixture of different chemicals that will behave in different ways and you can get a mixture of any of the following:

CYA ---> Biuret ---> Allophanate ---> Ammonia ---> Nitrite ---> Nitrate
.................................................. ......... | .............. | .............. |
.................................................. ......... | ....... Nitric Oxide ... Nitrite
.................................................. ......... | .............. | .............. |
.................................................. ......... | ..... Nitrous Oxide ...... |
.................................................. ......... | .............. | .............. |
.................................................. ......... `------------`-------------`---> Nitrogen Gas

The following table shows the speed of chlorine demand for each chemical and whether it forms significant CC:

Chemical . Chlorine Demand . Forms CC
CYA .............. Very Slow .............. No
Biuret ............... Slow ................. No
Allophanate ....... Slow ................. No
Ammonia ....... Very Fast .............. Yes
Nitrite ............... Fast ................. No
Nitrate .............. None ................ No
Nitric Oxide ....... Fast? ................ Yes?
Nitrous Oxide ..... None? .............. No?
Nitrogen Gas ..... None ................ No

[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! --

This post made me think of something. When closing for the winter I drain and take the DE filter completely apart and thoroughly clean the grids, tank, etc., so I don't get any bacteria/gunk buildup over the winter.

I've always done this because I was told if you let the DE sit over the winter and dry out, it can get like concrete and be very difficult to clean out. It didn't occur to me that another issue would be increased CL demand from all the bacteria/gunk buildup that would occur.

Thanks for bring that up.

[JimK]

Just a final (hopefully) recap;

Lost about 40ppm CYA over the winter.
Went through 25 gallons of 8.25% bleach before it would hold FC level overnight.

Now where's the heat!