ATSM American Society for Testing and Materials had a standard for Bio-degradable. They have weight:
"that which is capable of undergoing biological decomposition in a compost site such that the material is not visually distinguishable and breaks down into carbon dioxide, water, inorganic compounds and biomass at a rate consistent with known compostable materials."
The problem with that standard was that it included inorganics. The standard got pulled.
So buy all the biodegradable plastics you want but if they don't biodegrade back into the natural food chain they are not alright. May as well pour hormones and motoroil into the water.