Treating the water is only half the job. A perfectly disinfected batch of water leaving the plant with a strong chlorine residual can still make someone sick three miles and eighteen hours later if that residual doesn't survive the trip -- and keeping it alive for that entire journey, through every dead-end street and undersized main in the system, is a core part of a Distribution Operator's daily responsibility.
Why the Job Doesn't End at the Plant
It is tempting to think of disinfection as something that happens once, at the treatment plant, and then the water is "done." That is not how it works. Drinking water regulations require a detectable disinfectant residual to be maintained essentially everywhere in the distribution system, not just at the point of entry. This is a distinct requirement from the CT (concentration x time) inactivation credit earned at the plant, which is