There are fundamental differences between materials that are biodegradable and those that are compostable.

One main difference is that compostables can be integrated with all other organic materials and start breaking down within a few days.  Biodegradables take longer to begin to break down,  that is,  sometimes up to a few weeks but it does. Plastics takes hundreds sometimes thousands of years to disintegrate often releasing harmful chemicals and components into the env.  Switch to bio's and compo's to really make a change