Do Biodegradable Bags Make Disposable Ones OK?

Late last week, San Francisco Mayor Gavin Newsom signed a plastic shopping-bag ban into law. Within the next six months, large supermarkets must stop using standard plastic bags. Large chain pharmacies must follow suit within the year. Instead, they can use recycled paper bags or biodegradable “bioplastic” ones.

But one question remains: Are biodegradable plastics really as eco-friendly as they seem?

Regular supermarket grocery bags are made from petroleum-based plastics. They’re usually used once and thrown out. Some facilities collect bags for recycling, but since these bags are made from a mix of materials and often contaminated by food, their yield is very low.

Biodegradable plastics seem like a great solution. They are usually made from starch and come from starch-rich plants like corn. Unlike fossil fuels, plants are renewable resources. On top of that, any carbon dioxide belched out as the bioplastic composts should have come from the atmosphere as the corn grew, leading to a net effect of no emissions. It seems almost too good to be true. 

But engineers Tillman Gerngross from Darmouth University and Steven Slater from Arizona State University are long-time critics of biodegradable plastics. A few years ago they did the math and found that some bioplastics production emits more carbon dioxide than the production of normal bags made of polyethylene terephthalate (PET). This is especially true for the biopolymer polyhydroxyalkonate (PHA).

The culprit is the large amount of energy that goes into farming, fertilizing and fermenting corn. Gerngross and Slater voiced their concern in a 2003 letter to Science: “Biodegradable polymers can convert a solid waste disposal problem into an air pollution problem, an approach that is clearly at odds with current international efforts to reduce greenhouse gas emissions.”

There is another catch to the bio-bags: they only degrade if microorganisms can get at them. Bags in landfills don’t degrade well at all, whether they’re biodegradable or not. Biodegradable bags can take 10–45 days to degrade in a controlled composting environment, but then they release even more CO². Then again, if we wind up littering the wild with biodegradable bags (i.e., stuck in trees or floating in lakes, where they can take 8–12 months to decay) they will at least break down and reduce the risk of injury to animals or clogged sewer pipes.

So what is the best solution to the grocery bag waste problem? Not using them in the first place. Reusable bags are far more eco-friendly (and pretty) than disposable bags of any kind.