Water systems, most of which are "looped" can suck contaminates back into their system due to pressure fluctuations during the various usage loads across a days span. This is why over the last 15 years the bigger municipal water supplies have mandated back-flow devices on new water taps. Most fire sprinkler systems however don't have them due to the size of the line. Instead they have detector check or double check valves. Old water taps have not usally had this feature added.
Once contaminants are in the system past your local tap on the main, they circulate freely. Shut off any water tap branch, add contaminate upstream of the back-flow and its going to work its way back to the whole system diluted only by the volumn of water in the system in its local area.
This little guy won't remove CN from the water but it will make dirty water potable when the public systems cannot.
I'll be adding one to my cache soon.