Breakpoint Chlorination

[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