CYA / Pool Pilot Question

[chem geek]

I would be somewhat skeptical of the claims of superchlorination with SWG systems. Yes it's true that the concentration of chlorine generated in the cell is higher than the average you end up with in the pool, but if you do a little math you can see that the chlorine levels in the cell are not as high as implied by the manufacturers.

First of all, Ben's chart refers to chlorine levels that must be maintained throughout your pool in order to disinfect and prevent algae. Now any superchlorination done at the cell (more on that below) may disinfect more, but unless your pool turnover is very fast, it is likely you won't "disinfect" your entire pool in less than several hours. Also, both bacteria and algae can stick to and grow on pool surfaces (in what are known as biofilms) where the superchlorination in the cell does absolutely no good at all.

If your normal 60% operation setting means that the cell is turned on 60% of the time and you maintain a 2.0 ppm chlorine level at this setting, then if we assume a turnover rate for your pool (i.e. how long it takes for your entire pool's volume of water to go through the pump and chlorinator) of 4 hours and a chlorine loss rate of 1 ppm per day, and that your pump (and chlorinator) is on 8 hours/day, then your chlorinator is introducing 1 ppm of chlorine over 24 hours which is 8/4 = 2 turnovers so that is 1 ppm in 2 pool volumes or 0.5 ppm actually being introduced -- divided by 0.6 for the time the cell is on gives 0.83 ppm.

Yes, this means that your "superchlorination" is simply having 2 ppm chlorine enter your chlorinator and 2.83 ppm leave it. That's not very much.

If you think about it, any chlorinator that is turned on 60% of the time couldn't be introducing huge amounts of chlorine into the system because if it did, you would end up with that large amount of concentration in your pool in about the turnover time (ignoring the chlorine loss over that time which isn't huge in your case).

Now in the above discussion I have assumed that all of the pool water goes through the chlorinator. I don't have a chlorinator so I don't know if they are installed in a "bypass" fashion such that only a portion of the pool water goes through the chlorinator. If that is the case, then the chlorine levels would be higher by the inverse fraction of the amount of water that goes through the chlorinator, but then it would take even longer for the entire pool's volume of water to get "exposed" to such superchlorination (and as I said before, that doesn't help bacteria and algae stuck to pool surfaces anyway).

Now, back to your CYA issue. Use Ben's table. The fact that you are using an SWG system is not relevant. You still need to use Ben's table. Since you are not losing a lot of chlorine, you can use a lower level of CYA so that your 2 ppm of constant chlorine will work. That means using about 10-20 ppm of CYA though my calculations show that you would be fine at 20 ppm of CYA. Of course, most test kits only measure down to 30 ppm CYA (a few kits measure down to 20), but since you don't have extreme sunlight you can probably get away with less CYA that even your test kit can't measure (just make sure it looks cloudy even though you can still see the black dot) and make sure that your chlorine levels are maintained, especially during the peak of the sunlight part of the day (near noon?).

[EDIT]A little CYA goes a long way as shown in the following graph Halflife.gif
You don't save that much chlorine by adding more CYA beyond a relatively low level, but you significantly increase the need for higher Free Chlorine (FC) to maintain disinfection and algae prevention. Another way to look at it is with the following graph ChlorineLoss.gif which shows that at constant disinfecting chlorine (HOCl) levels, lower CYA has a lower absolute rate of chlorine loss and therefore a lower absolute rate of chlorine generation need from your chlorinator. Ironically, you optimize the use of your chlorine generator if you use absolutely no CYA at all. Yes, this is counter-intuitive but is true because the chlorine tied up by CYA still gets broken down, though at a much slower rate, and yet it does virtually nothing for disinfection or algae prevention. The chlorine tied up by CYA is simply a reserve of chlorine -- nothing more. So by using CYA, you are simply having an additional, and rather large, reservoir of chlorine that must be maintained and in fact is where most of the chlorine loss comes from (except at very low levels of CYA). This does not mean, however, that you should not use any CYA at all because the graph assumed that with no CYA you would maintain throughout your pool a chlorine level of, say, 0.05 ppm which is not realistic. From a practical point of view, you would have at least 1 ppm of chlorine in your pool so that implies a CYA level no greater than about 10 ppm. Since you are already at 2 ppm of chlorine, you should not have your CYA level higher than about 20 ppm. Again, use Ben's table, but my advice to maximize the life of your cell is to use the lowest level of CYA that is practical for measurement and for having a decent minimum FC level in your pool. An FC of 2 ppm is reasonable since this gives a decent buffer to handle areas of "less than ideal" circulation plus localized introduction of ammonia and organics from bather load (sweat and...). The reason Ben promotes a high-CYA/high-FC combination is to have a sufficient buffer of chlorine to ensure that you never, ever get to dangerous levels in manually-dosed pools. In a pool with an automatic chlorinator, this is far less a concern, though you should make sure that your FC never drops below the minimum level (say, 2.0) even during times of the day or night when your chlorinator is not operating (i.e. set your chlorinator to a higher FC level when it is on so that the worst-case when it is off is 2.0 FC).[END-EDIT]

Richard