To my understanding there is no recycling for SodaStream bottles...this means you throw them in the regular trash.
On the other hand and in the same line, SodaStream is not recycling friendly corporation. Once the 2 years guarantee is passed they don't repair machines. They ask you to buy a new one. I am just experiencing this after my 179CHF machine that i bought 4 years ago was not carbonating correctly. They told me as the machines are so cheap they do not repair the machines! :-0
Very eco-friendly approach!