The city water with .6 ppm CC (you're lucky -- my water is about 1 ppm CC) is monochloramine since many municipal water districts are switching to using monochloramine instead of chlorine as their disinfectant residual. Fortunately, monochloramine is readily broken down through breakpoint, but if done with high chlorine levels it can result in nitrogen trichloride (which smells and is very volatile and irritating).
The 0.2 ppm CC requirement borders on insane (0.5 ppm is much more reasonable, though is still a challenge to achieve in most indoor pools). It's much more important to have a requirement minimizing specific contaminants such as nitrogen trichloride and chloroform, but such test equipment measuring air and water quality is expensive. Many types of CC are relatively innocuous and some are relatively persistent. This is why I think it's best to prevent their formation in the first place, either through use of MPS on a regular basis and/or by using a small amount of CYA in the water to cut down their formation rate, but the use of CYA hasn't been proven yet in a real indoor pool (it's obviously used all the time in outdoor pools).
Clearly measuring CC during or soon after high bather loads isn't fair since there's no way you can break down the ammonia/urea fast enough. Measuring it at the start of the day is more reasonable.
I wish someone would try the CYA and see if it indeed slows down CC formation by a factor of 10 or so as theory predicts (assuming similar FC levels). It will also slow down breakpoint itself but should still be fast enough to keep up with typical bather loads. Too much CYA, on the other hand, would slow down breakpoint too much and would lead to a buildup of monochloramine during high bather loads (which seemed to happen in some indoor pools that used Trichlor tabs in feeders and had 100+ ppm CYA -- see this link for an example). At the other extreme with no CYA, nitrogen trichloride would form in higher quantities (due to the higher hypochlorous acid concentration) and is more common at lower pH as described in this link.
Richard
The 0.2 ppm CC requirement borders on insane (0.5 ppm is much more reasonable, though is still a challenge to achieve in most indoor pools). It's much more important to have a requirement minimizing specific contaminants such as nitrogen trichloride and chloroform, but such test equipment measuring air and water quality is expensive. Many types of CC are relatively innocuous and some are relatively persistent. This is why I think it's best to prevent their formation in the first place, either through use of MPS on a regular basis and/or by using a small amount of CYA in the water to cut down their formation rate, but the use of CYA hasn't been proven yet in a real indoor pool (it's obviously used all the time in outdoor pools).
Clearly measuring CC during or soon after high bather loads isn't fair since there's no way you can break down the ammonia/urea fast enough. Measuring it at the start of the day is more reasonable.
I wish someone would try the CYA and see if it indeed slows down CC formation by a factor of 10 or so as theory predicts (assuming similar FC levels). It will also slow down breakpoint itself but should still be fast enough to keep up with typical bather loads. Too much CYA, on the other hand, would slow down breakpoint too much and would lead to a buildup of monochloramine during high bather loads (which seemed to happen in some indoor pools that used Trichlor tabs in feeders and had 100+ ppm CYA -- see this link for an example). At the other extreme with no CYA, nitrogen trichloride would form in higher quantities (due to the higher hypochlorous acid concentration) and is more common at lower pH as described in this link.
Richard