Most places don't have the recycling facilities for styrene, so if you put it in your general recycling, it will just get moved to the trash. Your best bet is to use sprues to make terrain and vehicles with, like this:

Orc Wagon:

WW2 entrenchment and barb wire fence:

I shaved some sprues into round "logs" and put the shavings into a jar of Tamiya plastic cement to make some sprue glue. This gets used for gap filling and making seamless transitions. The logs I used to make the wagon axles, parts of the wagon's overall structure, and livestock padding. Other logs were cut into various lengths to make posts for the barb wire or logs to reinforce the trench walls. What's especially nice about using them to make the trench is they fuse together to make really sturdy terrain.