Breakpoint Chlorination

[waterbear]

[EDIT] Here's an interesting link that summarizes both existing literature on breakpoint chlorination and gives some real-world measurements, albeit with no CYA (since it's for sewage treatment). I'll see if I can work with the rate and equilibrium constants to come up with some meaningful conclusions. [END-EDIT]

Richard

Richard, Funny you should post that link. That is one of the sources that I was looking over before posting my comments! I am continuing to research this also. There really does not seem to be any definative answers from what I have been able to find and there are a lot of 'real world' variables that need to be taken into consideration. My main interstest would be if a slow rise in chlorine over a, say 24 hour period, would be as effective as a fast addtion of chlorine all at once to reach breakpoint since this the main difference between shocking with a SWG and shocking with the manual addition of chlorine. It might be interesting to also try and determine what difference shocking at night vs. shocking during the day might produce since from what I have been able to research does indicate that sunlight is effective in breaking down the chlorine-ammonia bond (at least for the lower, non-organic chloramines). It does seem that most of the research on the subject is from the field of sewage treatment and not specifially pools.

Edit: I was able to find some info that indicates that incomplete oxidation of organic ammonia compounds by chlorine will lead to the formation of trihalomethanes (chloroform seems to be the most commen one formed in pools) which are very difficult to break down by normal means. Not sure what effect sunlight would have on them.

[chem geek]

Evan,

Here is another interesting source of information giving rate constants for a variety of reactions. I verified that these constants are consistent with those in the earlier link (note the constants are in "per hour" rather than "per second"). Unfortunately, some of the data in this table is incomplete and some seems just plain wrong as there is no path to breakpoint that occurs in a reasonable time. The original paper I read on CYA equilibria was in a book that also had breakpoint data that pretty much had breakpoint occurring in around 30 minutes or so for 0.1 - 1.0 ppm ranges of chlorine and ammonia. If you find anything that makes sense, let me know. At least I've got logical reaction times even with CYA for monochloramine (seconds) and dichloramine (minutes) formation. I just need the rest of the breakpoint times so I can figure out dominant species.

As for the THMs, these appear to occur when organics are present and happen more when the pH is [EDIT] higher [END-EDIT]. It isn't so much an incomplete breakpoint that makes them happen as just having chlorine available to combine with such organics -- if there's enough chlorine, then THMs can form. The newer approach to water treatment using chloramines avoids the THMs because the chlorine is all consumed with the fast reaction to form chloramines and doesn't combine with organics. That's not what we do in pools as we usually have enough chlorine for continual breakpoint (hopefully).

At higher pH, including those of typical pools, there are [EDIT] more [END-EDIT] THMs formed, but [EDIT] fewer [END-EDIT] VOC (general volatile organic compounds) so there doesn't seem to be any way around getting something unless you use KMPS non-chlorine shock to get the organics oxidized before chlorine can combine with them. Fortunately, the rate of production for these nasties is low, especially with the use of CYA that makes effective chlorine levels very low, so whatever gets produced gets swept away by a light breeze quite readily. The same cannot be said for indoor pools that have no CYA and therefore high chlorine levels and generally poor air circulation. This is why I propose either using KMPS in indoor pools (as a preventative weekly oxidizer) and/or use a small amount of CYA (< 10 ppm) to keep the chlorine levels lower (while still being able to maintain a large FC residual). This use of CYA would also make swimsuits last longer which I know my wife would appreciate (the rubber in her suits deteriorates in the winter when she uses an indoor pool -- never had a problem when using our outdoor pool in the summer).

Richard