Chlorine Demand and Non-chlorine shock

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thejunkbox

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Jul 8, 2009
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I bought a house w/ a pool at the end of last summer and hired a local free lance guy to do my pool closing, as I was going to be out of town for several weeks around the time I wanted the pool closed. In hindsight this was mistake, when I opened the pool this summer, it was green, had a bunch of leaves rotting in the bottom etc.

So I did my best, shocked shocked shocked, and floc'ed and had the water looking pretty nice, but it wouldn't hold chlorine. I checked the CYA and it was zero. ph was 7.2. I discussed it w/ the local pool supply store, who thought it may have been chlorine demand (which after reading about it, sounded very plausible). They tested my water and said that it would take 60 gallons (yikes!) of liquid shock to overcome the demand. When I asked them about the possibility of using non-chlorine shock/mps, they said no, it wasn't what their distributor recommends.

But after reading this article, especially the last two lines which state you can use a 1:1 ratio w/ non-chlorine shock rather than the 10:1 ratio of chlorine, I was interested in the possibility of doing so.
http://www.poolspa.com/publications/sin/stories/shocktrt.htm

Anyone have any thoughts? I saw another post from chem geek that stated its not cost effective to use mps rather than chlorine to rid the ammonia, but the above article seems to say doing so allows you to skip the ammonia -> monochloramine -> dichloramine -> trichloramine -> N2, HCl, H2O pathway and go straight from ammonia -> N2, HCl, H2O.

I just want to make sure I can't simply buy 1/10th the amount of MPS before I pickup 60 bottles of chlorine. Thanks.
 
I can't speak to the chemistry of adding MPS as a "short-cut" to faster oxidization as the article states, but he does make a lot of generalizations in the article.. and it's also funny to me that he spends paragraph after paragraph explaining how chlorine works, using math calculations, etc, and then in a couple sentences says to use MPS.. with no further chemistry, math or backup provided.

What I can speak to is that on this forum, we've seen countless stories of folks with algae or other water problems... when they follow the procedures laid out in Pool School, using liquid chlorine, they always return and say "That worked great, thanks!".

I would be concerned about the longer term effects of MPS in the water... will getting rid of the MPS cause a bigger chlorine demand later?

Posting the specs on your pool would be helpful, particularly how many gallons. You'll get a lot of feedback regarding the amount of chlorine needed and when to use it.

Good luck!
 
Welcome to Trouble Free Pool! The article you linked to has several inaccuracies. In a low-to-medium bather-load pool, there is continuous "breakpoint" of ammonia and nitrogenous organics going on. The traditional "breakpoint" graphs show what happens when you add chlorine to water that has ammonia in it, not the other way around. However, that's not exactly the situation in a pool since a small amount of ammonia and urea from sweat and urine are introduced into water that already has chlorine in it (unless you've got a high bather load of children all peeing in the pool). If you have sufficient Free Chlorine (FC) in the pool so that it does not get to zero getting used up by the ammonia/urea, then you'll never see such a breakpoint curve effect. Instead, you might see a very small blip in Combined Chlorine (CC) that over an hour or so goes away. In fact, in most residential pools you won't even see the CC blip unless you've got more people in the pool.

The article implies that things get stuck if you don't superchlorinate or otherwise shock the pool, but that simply isn't true. The worst thing that would happen would be a buildup of slow-to-oxidize chemicals that get up to some level where their rate of introduction (from bathers, leaves, etc.) equals their rate of breakdown. Shocking can temporarily accelerate this breakdown so lower this overall level, but in practice in a residential pool this is usually irrelevant. Also, in most outdoor residential pools, the Combined Chlorine (CC) level is typically very low -- often <= 0.2 ppm which is the lowest measurable in our FAS-DPD chlorine test kits (using a 25 ml sample) and certainly <= 0.5 ppm which is a very reasonable level to tolerate for technical reasons I won't get into here.

MPS is more expensive to use than chlorine. As I noted in the thread that reebok linked to, 207 fluid ounces of 6% bleach is the same as 67 ounces weight of MPS where both are equivalent to 10 ppm FC in 10,000 gallons. However, the bleach will cost under $3 while the MPS (which is at least $4 per pound) would cost over $16. The side effect of using MPS non-chlorine shock is an increase in sulfates. For this same 10 ppm FC in 10,000 gallon equivalent, MPS adds 31 ppm to sulfates. We don't know at what level sulfates become a problem, but we generally avoid adding unnecessary chemicals to the pool water and sulfates have more effects than chloride (sulfate increases conductivity more; magnesium sulfate is most damaging to stone/cement; high sulfates can hurt SWG cells).

Getting back to your specific problem, it sounds like you may have had the Free Chlorine (FC) level in your pool drop for some time and that the Cyanuric Acid (CYA) level used to be higher and is now zero. This sounds like bacteria in the pool converted the CYA into ammonia. You should get an inexpensive ammonia test kit at a pet/fish/aquarium store to see how much ammonia you've got. Also, you can do a bucket test to see how much chlorine it will take before it holds -- 1/4 teaspoon of 6% bleach in 2 gallons is 10 ppm FC. You can then decide whether it's better for you to dilute the water with partial drain/refill to reduce the amount of chlorine that will be needed to get rid of the ammonia. For clearing ammonia, there is no good reason to use non-chlorine shock -- chlorinating liquid or bleach will work just fine and any volatile disinfection by-products that are formed will dissipate away if this is an outdoor pool (for an indoor pool, strong air circulation should be used during this process).

You can read about how this situation happened to me in this thread (a summary of the amount of chlorine that it took is in this post).

Richard
 
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