Chlorine/CYA and Combined Chlorine

[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