Recent studies highlight that compostable products may not be the best answer for the environment. The composting process requires oxygen and, for the most part, landfills are sealed, airtight — and therefore oxygen-free — containers within the ground. "Since landfill conditions are generally anaerobic [i.e., without oxygen], these materials do not 'compost'; instead they either persist or break down anaerobically," Peter Canepa, a life cycle assessment specialist with the Department of Environmental Quality in Oregon
However, what exactly happens to compostable materials in landfills is up for debate. For the plastic-like compostable material PLA (polylactic acid), which is made from corn, one study found that in landfills PLA breaks down anaerobically to release methane, a greenhouse gas that is about 30 times more potent than carbon dioxide, Canepa said.
But another study found that PLA doesn't break down at all in a landfill and therefore does not produce significant greenhouse gas emissions, Canepa said. LiveScience says that what's clear is that if a compostable item, such as a straw, winds up in the trash, it will not be composted. Canepa said the bigger question is how single-use items can be avoided. Read more here; https://www.livescience.com/63597-compost-trash-in-landfills.html