Chlorine/CYA and Combined Chlorine

[chem geek]

On the other hand, you can also say that a properly maintained pool (FC according to charts, etc) shouldn't have an algae bloom, and still people get it.

That's not really true if you look at the large number of posts on multiple forums. For green algae, we have hardly ever seen such algae bloom in the presence of sufficient FC relative to CYA with the cutoff being an FC around 7.5% of the CYA level (unless the CYA is really low in which case a minimum FC of around 2 ppm is needed so that chlorine does not run out locally). With very high phosphate levels > 3000 ppb, the algae could grow faster than the chlorine can kill it, but that's a very high phosphate level and higher chlorine levels would work, but may not be worth it. Mustard/Yellow algae is the one that requires almost double the amount of chlorine to keep away -- about an FC that is 15% of the CYA level -- but it's less common and is something that seems one can completely get rid of, though it isn't easy since it hangs around in light niches, etc.

A lot of people let their pools go over the winter so I don't count opening a pool to green algae when you don't have chlorine in it as being maintained with chlorine.

Why do you say that pools get algae even with sufficient chlorine?

So suppose CC is >0.5 for a few days. What should be the level of FC needed in shocking, regardless of what the underlying arithmetics is based on - kinetics or stoichiometry?

Generally if you raise the FC to the shock levels in the table then that's what we normally recommend, but I've seen cases where the CC won't go away no matter what you throw at it -- clearly such CC isn't traditional chloramine. Fortunately, this is a rare occurrence and is most frequently seen in indoor pools.

However, in my own pool, if I have a bunch of people over and the CC goes up, I just wait a day since it will invariably go back down. In other words, I usually don't worry about it and it's almost always <= 0.2 ppm in my pool. Basically, if the CC is just monochloramine, it will break down over time so long as there is FC in the pool. Of course, high bather load pools such as commercial and public pools are a different matter and may require higher chlorine levels or periodic shocking to stay ahead of such higher demand (i.e. introduction of urea, ammonia and some amino acids from sweat and urine).

The China Shop isn't just about debate -- it's also where we put things that are too technical for most pool owners. We've talked about renaming this or having another section, but never got around to it. So go ahead and post such items in The China Shop (it's probably better to link to the info and just refer to short excerpts). I'm pretty aware of the lack of awareness of the chlorine/CYA relationship and am trying to change that. I don't think it's corruption, but rather deceit since it's not lying but rather intentionally withholding information that might produce a less than desirable economic result. The irony is, IMHO, that this deceit (for 34 years) has led to more pool owners having algae and other problems with their pools with the pool stores not being very effective. This more than anything else has driven pool owners to SWG pools, even when such pools were not as economical (i.e. when SWG cells were more expensive in the past). So the stabilized chlorine industry has shot itself in the foot where now 85% or more of new pools have an SWG. The trend towards SWG pools would have happened anyway just based on convenience alone, but probably not as quickly and perhaps to not as high a percentage.

The lack of detailed information about the chlorine/CYA relationship has had other side effects that I believe were unintentional. With an industry mantra that "CYA doesn't matter; only FC matters" and that "CYA is only needed to protect chlorine from breakdown from sunlight", this leads one to the conclusion that there is no reason to use CYA in indoor pools. However, the chemistry says that most indoor pools are over-chlorinated because even 1-2 ppm FC with no CYA is 10-20 times the level of disinfecting chlorine (hypochlorous acid) as is found in outdoor pools where the FC is around 10% of the CYA level. My wife experiences this difference every year when her swimsuits degrade after one winter of use in an indoor pool with no CYA while duplicate swimsuits in our own pool during the summer do not degrade. There are similar differences in flaky skin and frizzy hair as well. Furthermore, the most recent breakpoint chlorination model from Jafvert & Valentine (1992) shows that the higher disinfecting chlorine level in indoor pools leads to 10-20 times faster production of disinfection by-products and 10-20 times greater end-point concentration of nitrogen trichloride. What is not known is if this is also true of urea which is the primary component of sweat and urine (ammonia is present in smaller amounts). Interestingly, this patent assigned to one of the largest companies in the stabilized chlorine industry describes how CYA as well as a CYA-like compound called glycoluril reduces the rate of production of chlorine disinfection by-products (DBPs). I just don't think they put 2-and-2 together. I couldn't imagine how anyone could live with themselves given the number of incidents of asthma, respiratory problems and ocular issues there are with many indoor pools. Though some people blame lower air exchange due to higher energy efficient buildings (and I don't dispute that change), I think it's a lot easier to use a small amount of CYA in indoor pools (say, 20 ppm) to cut down DBP production by a factor of 10 or more -- that seems a lot easier than trying to exchange the air 10 times faster!

Richard