I just throw mine out on the garden...are you sure it is made of a kind of polystyrene? It's treated with acid, would have thought it would have dissolved??
I'll top up some of the pots, sprinkle it around like its a manure!
Quite how health & safety say it should be disposed of I've no idea.
According to them, your trad bucket of water is not allowed to be thrown down the nearest drain in the street, let alone be chucked out on the nearest road or pavement! Cos that is considered industrial waste too!!!
Utterly potty.
Ian