What is the difference between compostable plastic and biodegradable plastic?

“Biodegradable” means that an object is capable of disintegrating by biological means.  Technically, these items could be composed of almost any material since with enough time microorganisms can decompose almost anything.  For example, aluminum cans will biodegrade in the ocean in about 200 years and hard plastic bottle caps will biodegrade in about 400 years.

“Compostable” means that a product breaks down to carbon dioxide, water, inorganic compounds, and biomass which leaves no toxic residue.  According to D6400 composting standards set by the American Society for Testing and Materials (ASTM), compostable products must break down at a rate similar to paper or disintegrate into small pieces within 90 days so that the original product is not visually distinguishable in the compost.