Back to chemistry class...

[chem geek]

If your CYA drop was due to bacteria consuming the CYA, then that produces ammonia so that will consume chlorine very quickly (and make combined chlorine go up very fast until breakpoint and eventual breakdown mostly from sunlight).

Every 1 ppm of CYA that gets broken down to ammonia (and carbon dioxide and carbonate) requires 2.5 ppm FC to fully break it down (it's not 10x due to the fact that the CYA is a much heavier molecule so the "ppm" represents much less substance than "ppm" of chlorine).

If all of your 40 ppm CYA loss was due to consumption by bacteria, then it would take 100 ppm FC cumulatively to clear your pool. I suspect that at least some of the CYA loss may have been due to dilution from winter rains so your total chlorine consumption wont' be 100, but it would be interesting to know what it ends up being. Be sure to let us know.

Richard

[Donna's Poolboy]

Richard-
I didn't keep track of how many gallons I drained for winter close, but it was between 12"-18". Factor in rain water and fresh water that I added after vacuuming to waste at spring open, and I've got a large volume of "new" water without any cya. Is there I test for the presence of ammonia (other than smell)? I went and counted and we've gone through almost 50 bottles of bleach as of today (1 jug of 6% raises the fc just shy of 2 ppm). So we've run almost 100 ppm FC. But it took less than 48 hours for the water to go from cloudy (couldn't see the drains in the 8' end) to crystal clear.

Do you have any opinion on the "pollen theory" and if it's playing a role in this?

Last year on a sunny day with light swim load (me and 2 potty-trained kids for an hour or two), I'd typically see the pool burn 2 ppm.
Cl consumption is slowing but still high. From 4:30 am until 4:30 pm I tested the water 4x and saw a 5 ppm drop. Yesterday for the same time period it was about 7ppm drop.

This is our 3rd pool season but first year opening with BBB and I'm assuming this isn't typical. I'm trying to identify if there's something I did wrong when opening, closing or somewhere else. I faithfully adopted Carl's "5 minutes a day" mantra last season with perfect results, so I feel like I'm letting him down!

You guys have all been a great help!
Thanks,
Bill

[chem geek]

Though it is possible for the chlorine to get consumed by pollen quickly, I would be surprised if it as fast as combining with ammonia or combining with algae, but I don't have experience with pollen so it's certainly possible. The rate of reaction is partly going to be related to the surface area to volume ratio so very small items will consume chlorine more quickly than larger ones, assuming the same total volume. Pollen is pretty small, but I would have thought it would have remained clumped (and I'm probably wrong about that). Of course, you've been dealing with this over many days so that's plenty of time for chlorine to combine with pollen, I would think.

As for detecting ammonia, probably the easiest thing to do is to measure the Combined Chlorine level very soon after adding the chlorine. Chlorine combines with ammonia VERY quickly so that in a matter of seconds it should be fully combined (it won't be at "breakpoint" -- that takes far longer). So, taking a sample of pool water upon opening and adding some chlorine to it (not too much -- say 3-5 ppm FC) and measuring CCs will tell you you've got ammonia. Even if it's after opening, if you do a "before" and "after" of FC and CC measurements then if the CC almost immediately develops (i.e. add FC gets nearly fully converted to CC), then that's almost certainly from ammonia. As for smell, you won't smell the ammonia that much, but after you've added chlorine you should smell chloramine and it won't be a "clean" smell like bleach.

Anyway, you are doing just fine and shouldn't blame yourself. If you have a high CYA level upon closing and the chlorine level gets to zero, then you may end up with a LOT of ammonia upon opening. So the only options are 1) not to have the CYA as high upon closing, 2) maintain chlorine levels through the winter (if possible), 3) add PolyQuat 60 upon closing (do this AFTER shocking with chlorine when the FC level drops back down a bit -- according to Buckman Labs). The PolyQuat will prevent algae from forming, but probably won't do much about CYA getting converted to ammonia from bacteria (unless the PolyQuat inhibits bacteria -- it just might hinder them through the same "clumping" and ion blocking mechanism that occurs with algae, it just won't be quick).

Richard