Chloramines and CYA

[Whatrymes]

I'm a bit curious. We know that our levels of chlorine are determined by the amount of cyanuric acid in the pool and this also effects the amount of chlorine to use for an effective shock treatment. But when it comes to straight use of chlorine to break down chloramines is CYA a factor. If I wanted to add CL just to deal with chloramines to I need to be concerned with how high my CYA level is. Again, just curious. Can't think of a real scenario were I would want to deal with chloramines only.

Thanks

 

[BoDarville]

Good Morning Whatrymes:

Shocking your pool by raising the FC levels to shock levels is what helps break down CC. The target shock FC is dependent on the CYA level. Shocking is simply the raising of the active chlorine level to a higher level to make all chlorine chemical reactions go faster. Most CC in outdoor residential pools gets handled at normal FC levels relative to the pool’s CYA level. However if you have CC > 0.5, that is an early warning sign that there is some organic matter in the pool that the chlorine is breaking down.

If you do need to shock, here's the process: http://www.troublefreepool.com/pool-school/shocking_your_pool

 

[Smykowski]

But when it comes to straight use of chlorine to break down chloramines is CYA a factor. If I wanted to add CL just to deal with chloramines to I need to be concerned with how high my CYA level is.

Theoretically, yes, CYA effects how chloramines are produced. Chem Geek posted a great explanation here. About half to a third of the way down he gives the 0 vs. 30 CYA example.

Practically, though, no. Shock the pool based on the Cl/CYA chart, and you will solve the problem, whether you have CCs or not.

 

[chem geek]

But when it comes to straight use of chlorine to break down chloramines is CYA a factor.

Yes it is. Look at the "Chlorine/CYA Relationship" section in the Certified Pool Operator (CPO) training -- What is not taught post where you can see that it is the active chlorine (hypochlorous acid) level, not the FC level, that determines the rate of disinfection/killing of bacteria, inactivation of viruses and protozoan oocysts, oxidation of ammonia and organics, and we know it is what kills algae. It isn't that chlorine bound to CYA has zero activity, but it's much much slower than that of chlorine unbound to CYA -- on the order of 150-200 times slower in the case of oxidizing monochlorodimedone and about 200 times slower in the case of bacteria.

So for practical purposes you can consider the chlorine bound to CYA to just represent the chlorine that is in reserve and that the chlorine unbound to CYA is what is active. For the purposes of non-disinfecting chemical reactions, it may be hypochlorous acid or it may be hypochlorite ion that is the primary reactant and therefore there is often a pH dependence to the reaction rate. In the case of disinfection, it is primarily hypochlorous acid that kills more quickly because it is a neutral molecule that can more easily pass through cell membranes (it looks like water) while hypochlorite ion is negatively charged and has a harder time getting through the net negative surface charge of most cells so overall it kills at least 20 times more slowly.

Though CYA significantly reduces the active chlorine level and therefore all disinfection and reaction rates associated with active chlorine, it fortunately takes a very low level of active chlorine to disinfect quickly enough for most pathogens and to oxidize some bather waste. With an FC that is around 10% of the CYA level, at 77ºF ammonia gets 95% oxidized by chlorine in about 4 hours. For outdoor residential pools, the breakdown of chlorine in sunlight produces hydroxyl radicals that may aid in the rate of oxidation, especially for organics such as urea that would otherwise be very slow to oxidize by chlorine.