Helium tanks require proper disposal. It is not correct procedure to simply put a helium tank out with the trash when you are finished with it, but proper disposal is not too complicated.
Either use all of the helium in the tank or release all of the helium so that the tank is completely empty.
Roll the helium tank outdoors, if it is not outdoors already, and close the valve as tightly as possible.
Unscrew the nozzle on the tank and remove it completely. You can do this by hand or with a ¾-inch wrench. Once you do this, any residual helium left in the tank can be released by opening the valve.
Write EMPTY on the tank so that it can be distinguished from tanks that still contain helium.
Either take the tank to a steel recycling centre or place it with your other recyclables for pickup. Also check with the place you purchased the helium tank from, as sometimes they will accept the empty tanks.