Advice from Scottish Fire and Rescue Service:
"After your display used fireworks should be disposed of by soaking for a least a few hours in water before placing in a plastic bag then disposing of the soaked, bagged fireworks in your outdoor general refuse bin. If you have any fireworks which did not work, do not return to them till after your display has finished. At the end of the display the faulty fireworks should also be soaked, bagged and binned."