Reply #4: I always have a helium tank...

[deek]

I put it in curbside recycling bin.

Look here:
http://www.balloontime.com/BT_Faq.htm

Balloon Time tanks are fully recyclable. After removing the easy-fill nozzle from the empty cylinder, take the tank to a steel recycling center or place with curbside recycling pickup. Instructions for preparing the cylinder for disposal or recycling are located on the Balloon Time package and helium tank. You may also download the complete instructions here. However, please note that recycling programs for this product may not exist in your area. Contact your local solid waste authority for more details.

and

Balloon Time tanks cannot be refilled. These tanks are designed to be non-refillable. This design provides consumers with the convenience of being lightweight and easy to handle.

Do not attempt to refill. Please refer to warnings on the tank and carton. Attempting to refill can cause injury or death.