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

[Water_man]

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

This was an assumption. I'm accepting your convincing rebuttal of this assumption.

By the way, today I measured CC with the highest sensitivity (25 ml sample in Taylor's k-2006 kit) and found CC <0.2 for a FC 4.5 water sample. It's amazing that the pink color for CC <0.2 is detectable.

I still don't understand why the same shocking level needed to kill algae is also needed to destroy CC because I'm sure that one algae cell has more than one or two Cl receptors, so the high level of FC in the case of algae makes sense from a stoichiometric point of view, not only just for kinetics, but if you say that this is a well established empirical fact, I'll accept it by the same way I'm accepting your calculated values of FC needed for preventive maintenance and sanitizing.

As to the other issue of deceit etc. - please wait until I post it and see what I mean.

[chem geek]

I still don't understand why the same shocking level needed to kill algae is also needed to destroy CC because I'm sure that one algae cell has more than one or two Cl receptors, so the high level of FC in the case of algae makes sense from a stoichiometric point of view, not only just for kinetics, but if you say that this is a well established empirical fact, I'll accept it by the same way I'm accepting your calculated values of FC needed for preventive maintenance and sanitizing.

It's not a fact -- you do not need to use the shock table values to get rid of CC. It's just easier and convenient to refer to those values since anything above the minimum stochiometric amount will get rid of the CC -- it will just take longer at lower FC/CYA ratios. So how does one determine how quickly one wants to get rid of their CC? It's arbitrary (when the CC isn't increasing), hence the simple rule of just follow the shock table (if you don't want to wait for normal FC levels to handle it in most cases). Also, CC is often measured when fighting an algae bloom so tying the two concepts together is just easier to explain as one rule, especially in that case where the shock table sets a reasonable rate for clearing a pool of algae (chlorine kill rate faster than algae reproduction rate by an amount that clears the pool in a matter of days).

As for the stochiometric amount of FC needed to completely kill and oxidize algae, we don't list that anywhere since it varies considerably depending on the extent of the bloom. So instead, the shock table just determines a rate of kill and we tell people to MAINTAIN that shock FC level by adding more chlorine as frequently as you can -- several times a day. You keep that FC level up until three things occur: 1) the pool is crystal clear, 2) there is minimal (<= 1 ppm FC) drop in FC level overnight and 3) there is minimal (<= 0.5 ppm) CC. The total amount of chlorine to get to this point is usually fairly large and you are right that it is much larger than the amount of FC needed to get rid of a small amount of CC.

Nevertheless, there are some situations where an extraordinary amount of FC is needed even when there is no algae. This usually happens when opening the pool if the CYA in the pool has been converted over the winter to ammonia by soil bacteria (that got into the pool) and the ammonia hasn't dissipated. In this case, it takes a total FC that is 10x the amount of ammonia (measured as ppm Nitrogen) to get rid of it. I just saw a report on one forum of an 8 ppm ammonia measurement. That would take 80 ppm FC to get rid of! A bucket test is being done to verify that this is indeed the case as one doesn't want to over shoot by too much. Technically, you don't have to add all the chlorine at once and can do it in steps. Initially, chlorine added to ammonia will produce monochloramine which will register as CC. Once all the ammonia is converted, any additional chlorine will lead to breakpoint. I've read about problems with not using enough chlorine and getting "stuck" at dichloramine, but I've run through the models and this simply isn't true (though obviously without enough chlorine you can have leftover monochloramine). In fact, by adding too much of a concentration of chlorine to monochloramine, one can produce more di and trichloramine -- going slower is actually more effective with less by-products though obviously takes longer.

I look forward to your new post or thread regarding industry or health department info.

Richard

[Water_man]

This was a great thread. I feel that all my questions and speculations were properly answered and dealt with. Thank you for your patience and the keen desire and ability to share your valuable knowledge in such a thorough and time consuming way.