According to the ElektroG (German Electronics Law), UHF-Transponders are defined as electronics.
To save resources these should no longer be disposed of in the regular household trash. You can deposit your used transponders at any electronics recycle collection center. These are usually located at waste management and recycling centers as well as big electronics stores, where all electronics will be recycled and converted for reuse.
Please follow your country's local government laws for recycling and disposal of all electronics.