How Long Does It Take for Plastic to Decompose?

Wood, grass and food scraps undergo a process known as biodegradation when they're buried, which is a fancy way of saying they're transformed by bacteria in the soil into other useful compounds.

But bacteria turn up their noses at plastic waste. Load their dinner plates with some plastic bags and bottles, and the one-celled gluttons will skip the meal entirely.

Based on this logic, it's safe to argue that plastic will never biodegrade. Of course, that's not the end of the story. In 2008, Daniel Burd, a student at Waterloo Collegiate Institute, demonstrated that certain types of bacteria can break down plastic. His research earned the top prize at the Canada-wide Science Fair, earning him $10,000 cash and a $20,000 scholarship.

Researchers have confirmed Burd's findings and identified several other plastic-eating bacteria, but none have been effective in practical applications. Until waste treatment plants can implement any new processes, the only real way to break down plastic is through photodegradation.

Photodegradation is a type of of decomposition that requires sunlight, not bacteria. When UV rays strike plastic, they break the bonds holding the long molecular chain together. Over time, this can turn a big piece of plastic into lots of little pieces.

Of course, plastic buried in a landfill rarely sees the light of day. But in the ocean, which is where a lot of discarded grocery bags, soft drink bottles and six-pack rings end up, plastic bathes in as much light as water.

In 2009, researchers from Nihon University in Chiba, Japan, found that plastic in warm ocean water can degrade in as little as a year. This doesn't sound so bad until you realize those small bits of plastic are toxic chemicals such as bisphenol A (BPA) and PS oligomer. These end up in the guts of animals or wash up on shorelines, where humans are most likely to come into direct contact with the toxins.

One solution to the environmental disaster of plastic pollution is so-called biodegradable plastic. There are two types currently on the market: plant-based hydro-biodegradable plastic and petroleum-based oxo-biodegradable plastic.

In the former category, polylactic acid (PLA), a plastic made from corn, tops the list as the most talked-about alternative. PLA decomposes into water and carbon dioxide in 47 to 90 days — four times faster than a PET-based bag floating in the ocean.

But conditions have to be just right to achieve these kinds of results. PLA breaks down most efficiently in commercial composting facilities at high temperatures. When buried in a landfill, a plastic bag made from corn may remain intact just as long as a plastic bag made from oil or natural gas.