Chlorine Depletion Rate????

Assuming two identical pool with identical chemistry and identical weather conditions, both with no combined chlorine, will the chlorine be depleted at the same rate regardless of the chlorine level?

Example:

Pool A =
0.0 CC
6.0 TC
6.0 FC
50 CYA

Pool B =
0.0 CC
20.0 TC
20.0 FC
50 CYA

I will not be able to attend to my pool for several days and am considering shocking while I am away. If my pool normally consumes about 2.0ppm of chlorine per day, should a chlorine level of 20 be reduced to approx 10ppm in 5 days?

As usual, thanks for you help,

There are two rather different ways that chlorine gets depleted and these have very different rates.

1) Chlorine gets used up via the oxidation (breakpoint) of ammonia, the oxidation of organics, and the disinfection of bacteria and viruses. This sort of usage is wholly dependent on the amount of ammonia, organics and pathogens in the water and is therefore independent of the amount of chlorine you have (assuming you have enough to not get to zero). So this rate is often quoted as ppm chlorine per day.

2) Chlorine also gets used up through the breakdown from sunlight (UV) and some through breakdown from temperature and a very small amount outgassing as chlorine gas (except in salt pools without CYA where the amount is more significant). This sort of usage has a rate that is directly proportional to the amount of chlorine in the pool, so the rate is often quoted as a half-life or the time it takes to use up half of the chlorine.

You say that your pool normally consumes around 2 ppm of chlorine per day but unfortunately this does not tell us how much of this is due to (1) vs. (2) above. My guess is that without an opaque pool cover, an outdoor pool will generally have most of its chlorine loss due to (2) so let's just say for sake of argument that you lose 1.5 ppm per day due to the breakdown from sunlight and 0.5 ppm due to bather load and other organics and bugs. The other assumption to make is that the total chlorine level when you lose 2 ppm per day is 6 ppm.

Chlorine Loss per Day = 0.5 + (1.5/6.0) * 6.0 = 2.0

Now, with a chlorine level of 20, your loss would be greater as follows:

Chlorine Loss per Day = 0.5 + (1.5/6.0) * 20 = 5.5

Now I made some pretty bold assumptions, but the basic principle still holds that as long as some of the chlorine loss is due to processes that are proportional to the amount of chlorine in your pool (e.g. breakdown from sunlight), then higher levels of chlorine in your pool mean a greater absolute loss. [EDIT] Also note that the "5.5" number is an instantaneous rate and that this rate will decrease as the chlorine level drops. I also didn't take that into account in the original "2.0" calculation, but the point is still true that higher chlorine levels will have a greater absolute chlorine loss. [END-EDIT]

I wasn't sure if I should just give you the answer or give you the details on this. I chose the latter and I hope it wasn't too much info.

Richard

I have to ask, why are you considering shocking your pool if your CC is 0? Unless your CC is significant (> 0.5) or you have algae, you probably don't need to shock your pool. Of course, you still have the issue of not attending to your pool for several days so you will need to add more chlorine to ensure it doesn't get used up. If you put in 20 ppm into your pool, then it might get used up as follows if my 0.5/1.5 assumption was correct:

Start: 20 ppm
1 Day: 14.5 ppm
2 Days: 10.4 ppm
3 Days: 7.3 ppm
4 Days: 5.0 ppm
5 Days: 3.3 ppm

So you can see how you can get down to much lower chlorine levels than your initial assumption. The easiest way to prevent this it to stop the primary source of the loss which is probably the breakdown from sunlight, but that means putting on some sort of opaque cover over your pool -- even a solar cover is better than nothing as it probably cuts down much of the transmission of UV (I'm speculating here). [EDIT] You can even cover just part of your pool, say half to 3/4ths, and keep your pool pump on its normal cycle to get the combined benefit of reduced chlorine loss while still having some of the pool water exposed to get rid of disinfection byproducts (including the main ones of nitrogen and carbon dioxide gases as well as the bad ones of volatile THMs). Of course, keeping such a pool cover "in-place" with circulating water is an engineering challenge.;) [END-EDIT]

Richard

Let me also refer you to the "Can my pool stay home alone for 10 days?" thread since that seems to address your more immediate problem of how to keep the chlorine levels up during your extended absence. The two main ways that were suggested were to use Tri-Chlor tablets in a slow release feeder (plus having the TA and pH higher to start with) or to use liquid chlorine or bleach jugs with small holes punched in them.

Thanks Richard,

I appreciate all of your effort, and I really appreciate the detailed explaination as opposed to the answer only. I am of the curious type and like knowing the logic behind the computations. It gives me a better idea of the workings of the components of my pool chemistry.

I have learned so much in the time I have been a member of this board. This knowledge has resulted in perfectly clear water all summer, with the least amount of time, effort and $$$.

Thanks. I'm glad this was helpful. By the way, if we assume that your lack of using the pool eliminates the chlorine usage for ammonia/organics/pathogens, then that's 0.5/day less loss so you would end up with 5.8 ppm instead of 3.3 ppm. So you can see how a relatively small change in an assumption can change the end result quite a bit. Of course, with an open pool there is probably junk getting blown into it so the "organics" demand on the chlorine probably doesn't really go to zero.

The main thing is to find a way to get chlorine into your pool while you are gone and then enjoy your vacation! I just got back from vacation myself, but we had someone stay at our house to not only add some chlorine to the pool a couple of times during the week, but also to feed and care for our two cats. The pool is actually a positive draw for someone to stay in our home and its someone we know and trust.

Richard