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

[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.

[PoolDoc]

thx.

[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]

Great stuff.

So I'd say in my case ammonia wasn't the biggest CL consumer since CC never got above .5ppm. Btw, I estimate I lost about 40ppm stabilizer over the winter (still had about 30ppm when I stated the pool up last weekend).

Too bad there's not a way to seal off the pool in the winter. While my solid cover does keep dirt and debris out, the worms and bugs love to get under there. Scooping out bunches of dead worms in the spring is so gross...lol. I'm sure all those critters introduce all kinds of bacteria to the water.

[JimK]

Tonight's test showed I lost 6ppm FC during the day (SWCG is off). It was partly cloudy today. I'll let the pump run overnight with the filter on to see how much I lose overnight.

If there is a loss, I think we can safely say something in the filter is consuming CL since last night with the pump off there was no loss. If the loss isn't that much I think I'll just let CL do it's job over time rather than tearing down the filter (or maybe shock it one more time). Tomorrow morning's test should tell us more.

[JimK]

This morning's CL test showed I only lost .5ppm FC (CC is still 0) overnight with the filter running. So I'm guessing that whatever was consuming CL overnight was knocked out by that last shock.

The good news is I can stop shocking and don't have to tear down the filter. The bad news is we don't know if the overnight loss was caused by gunk in the filter, in the pool, or both.

So once FC levels drop to normal, 5-6ppm, I can work on getting the SWCG set properly and get to a normal maintenance routine (which usually doesn't involve much with our pool). FC was 12.5ppm this morning so I expect by this evening it will be close to the normal maintenance level.

[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!

[PoolDoc]

Now where's the heat!

Hopefully arriving in early June! Thanks for the recap.