Split off of Is shocking necessary.......... JasonLion
Urea is the primary bather waste. And, although urea can form chlorinated derivatives such as monochlorourea, dichlorourea, trichlorourea and tetrachlorourea, I don't think that these derivatives contribute significantly to the combined chlorine level. The oxidation process for urea is much slower than that for ammonia and creates dichloramine and trichloramine as intermediates in the process. Therefore, urea can create persistent levels of combined chlorine as it breaks down. Urea may take a day or two to fully break down.
The ammonia based chloramines absorb UV light and will be about 2/3 oxidized by the UV alone even if there is no free chlorine.
3NH2Cl + 3(uv) --> N2 + NH4+ + 2H+ + 3Cl-
monochloramine + uv --> Nitrogen + ammonium ion + hydrogen ions + chloride ions.
Indoor pools with heavy bather loads and persistent chloramines should consider installing a proper ozone system or UV.
The oxidation of ammonia by chlorine produces about 90 % nitrogen and about 10 % nitrate. If the only product were nitrogen, then the molar ratio would be 1.5 chlorine to ammonia. The formation of nitrate increases the needed ratio to about 1.75. Since chlorine is about 5.07 times the mass per mole of nitrogen, the needed ratio of ppm chlorine to ppm ammonia nitrogen is about 8.9.
Since combined chlorine is reported in units of chlorine, the total chlorine needs to be 1.75 times the combined chlorine, assuming that all of the ammonia is combined. As long as the total chlorine is at least 1.75 times the combined chlorine, then all of the chloramines can be broken down.
For example, if the CC were 2.0, then the TC would need to be at least 3.5 and the FC would need to be 3.5 - 2.0 = 1.5 ppm. Using more chlorine will make the process go faster. Using the minimum will oxidize the ammonia in about 1 hour. Using a higher chlorine level can oxidize the ammonia in about 15 minutes.
To calculate the best level of chlorine to add, you should determine the maximum level you want to swim in (say 20 % of the cyanuric acid) and add 1.75 times the CC level, and then increase the TC to that number.
FC = 1.0
CC = 2.0
TC = 3.0
CYA = 60
.20 x 60 + 2 x 1.75 = 15.5
15.5 (target TC) - 3.0 (existing TC)= 12.5 ppm of chlorine to add.
High levels of combined chlorine (from ammonia), especially after heavy bather loads, indicate that there are probably also high levels of urea (which won't show up in the CC test). As noted above, urea takes time to break down and produces ammonia chloramines in the process. This can make CC levels seem to take extended amounts of time to eliminate.