Based on some MEMS measurements made by Dr. Ernest "Chip" Blatchley, it does look like most of what is measured as Combined Chlorine (CC) is organic chloramine, presumably monochlorourea (his lab experiments showing volatile disinfection by-products from chlorination of urea showed DPD CC measurement with virtually no MEMS measurement of volatile inorganic chloramine). And yes, this comes from urea. There are other components of sweat and urine, but urea is the largest by far, followed by ammonia (see Table 4.1 in
this file).
Yes, you read the graph correctly, which implies a roughly 1:1 requirement of the amount of FC to get rid of CC -- it's somewhat less than that, but order of magnitude correct. The breakpoint graph shows what happens when you add smaller amounts of chlorine to a large amount of ammonia, and adding more and more chlorine. That is, initially chlorine combines with ammonia to from monochloramine and this occurs very quickly. After all of the ammonia has been converted to monochloramine, then additional chlorine starts the breakpoint reaction, moving monochloramine to dichloramine and dichloramine to nitrogen trichloride and then these to nitrogen gas (this latter combination is fast so nitrogen trichloride remains low in concentration). Since multiple reactions can occur at once, it's not completely a step-by-step process as the steps overlap as concentrations of various species come and go, but generally speaking it's as I described.
This post describes the core reactions of the breakpoint chlorination process.
The above is not exactly what happens in a pool because the amount of chlorine is generally much higher than the amount of ammonia (i.e. you are adding ammonia to an excess of chlorine, not the other way around) so in practice one is oxidizing ammonia to nitrogen gas somewhat continuously. There is some buildup of monochloramine if the bather load is high enough since the latter steps take a few hours to mostly complete, but in most residential pools this isn't even noticeable. If one has a pool party, then one can usually detect some jump in CC and, of course, if anyone pees in the pool then that increases the chlorine demand considerably and would register a lot more CC for a while.
Finally, the oxidation of urea is far slower, but it is very temperature dependent and it might be catalyzed by UV though this is pure speculation on my part. This would be consistent with why CCs are generally a lot higher in indoor pools than outdoor pools, though there are many different factors at play in addition to sunlight, such as air circulation and use of CYA.
Richard
: (Can't say I'm really following but it is interesting.)
