Biodegradable plastics have been advertised as one solution to the plastic pollution problem bedeviling the world, but today's "compostable" plastic bags, utensils and cup lids don't break down during typical composting and contaminate other recyclable plastics, creating headaches for recyclers. Most compostable plastics, made primarily of the polyester known as polylactic acid, or PLA, end up in landfills and last as long as forever plastics.