I believe in a larger setup you need to use 2 chillers chained together, one to prechill and the second to finish it off due to the sheer volume of wort etc. Might want to look into a second therminator or an IC set into a big bucket of icewater.
This has been working well for me for up to 30 gallon boils. I use a couple different configurations. ChillZilla with city water then through a MoreBeer heat exchanger in a bucket of ice with a thermometer and ball valve on the out port works pretty well. If I'm making a lager in the heat of summer, instead of the heat exchanger, i'll put a therminator on an icewater loop as the second stage. This requires a second pump but you can drop the wort to lager pitch temps very quickly.
To sanitize, regardless of the chilling set-up I try to recirculate boiling wort through the cooling equipment during the last ten minutes of the boil. This generally works but sometimes the pump pulls too much gas from the boil action and the pump doest work well. When this happens, I try to calm the boil a bit or just start the recirculation immediately after flame out. (prior to turning on the water flow) The wort is plenty hot to sanitize the chillers and hoses for several minutes. Just make sure to purge the first bit of flow into a waste bucket before you return the flow to the kettle. You don't want flush water from the last batch that didn't drain out of the chiller in your fresh wort.