Reply #41: Actually, cities worry about getting bacteria in the water during transit

[AllyCat]

from their plant to your house. Colorado Springs used to add more chlorine to the water during summer when road construction crews were more likely to hit water mains and disrupt water supplies. They had to do it to make sure people didn't get giardia (actually a protozoan killed by chlorine). But yeah, it's added later in the process to keep people from getting sick from the local "bugs".