ChristiaanJ, your idea about sinking composite is not a bad one. But burning is one of the most environmental-friendly way to get rid of non decomposing materials. We here in Switzerland, famous for maybe the most rigid ecological rules, are incinerating nearly all the rubbish - except the recycled one (glas return ratio: nearly 100%!).
The problem with burning is that you have to do it in a controlled fashion, i.e. very high temperature, in a plant, controlled, and exhausts filtered.
Dani