You will want to have Nalgene bottles to fill with warm water to bring up Kala Patthar to avoid from it freezing.
Having a bladder is essential for 95% of the trip.  This is the best way to get in enough water each day.  However, when you are going up Kala Patthar, your bladder will freeze, therefore you can not bring it.  You will want to have Nalgene bottles to fill with warm water to bring up Kala Patthar to avoid from it freezing.  Also, you can fill your Nalgene bottle each night with boiling water and put into your sleeping bag.  This will act as a hot water bottle to keep you warm at night.  However, this does cost extra and ranges from $1 – $4 to fill your Nalgene with boiled water, depending on where you are.