I mentioned contact time in my post about chlorine. I know in ontario, the standard contact time for water at the treatment plant to be exposed to chlorine is 20 minutes. granted, no doubt they are extra conservative.
for wastewater (my side) it used to be 12-15 minutes if I recall. the creek is not as particular about the level of faecal coliform bacteria in the water as humans are when they drink it
the water operators have whats called a multiple barrier concept set up.
remember how I said that chlorine is a surface disinfectant? well, if you have a bug hidden inside a dirt particle, the chlorine cannot get to it.
this is why tap water must be very clear (low suspended solids) because suspended solids can trap bugs and sheild them from the chlorine. the way around this is filtration prior to disinfection, hence the multiple barrier concept.
a bit off topic I know, but if we are talking about chlorine in tap water, I think it helps to have some background info as to how it is used at the treatment plant.
(when I was studying to become an operator, I didn't know where I would end up, so I got certified in both water and wastewater, but ended up in wastewater as the job offer came up first and I was poor LOL)