I spent a day this week catching up with the latest and greatest in biomaterials science at Wageningen-UR, a principal Netherlands center for research in this field. I saw algae making ethanol and a plant making electricity just by growing in very wet soil, and a lot of other cool stuff. One line of work the hosts were especially proud of was the array of plant- rather than petroleum-based plastics and other chemicals, and an advantage claimed for many of these is their biodegradability. If you use a potato-starch bag or food container, not only was the carbon in it taken from the air, where we want less, you can just discard it in your food-waste stream for composting instead of sending it to a landfill where it will last forever.
I’m not so sure about the last part; as usual, things are complicated. As global warming has come to the fore as the most imminent and scary environmental crisis, it has properly displaced to secondary status matters like energy security and depletion of resources like oil, landfill space, and the like. The whole carbon cycle of organic materials (especially plastics) actually has two almost completely separable subparts, and biodegradation of that bioplastic cup may not be its proper future.
The first part of the cycle has to do with where the material came from. With abundant oil, we learned to make all sorts of useful stuff starting with compounds from oil and coal that were almost entirely carbon and hydrogen. Now, we’re getting good at making competing products starting with celluloses and sugars from plants, different from petroleum mainly in having a lot of oxygen in their molecules. For example, you can make nylon fabric from oil, or rayon from wood. Unless the starting materials are harvested irresponsibly (clearcutting forests rather than harvesting a sustainable yield), biomaterials are a way to take carbon dioxide out of the air (what plants do for a living) and putting it in solid, stable form. This is an unalloyed Good Thing (unless the material in question requires very large amounts of fossil fuel to make) and will be until we get on top of global warming, perhaps in my children’s lifetime.
The second part has to do with what happens to the materials after use. For a piece of organic material (plastic, wood, fabric, etc.):, five futures are of interest:
-1.nothing (sitting forever in that landfill);
-2a. burning for usable energy (you can put lots of plastic into a boiler with other fuels directly, or process them into liquid fuel);
-2b. recycling (for example, turning old soft drink bottles into polyester fabric);
-3a.aerobic decay to CO2 and water, which is what happens to that potato-starch bag in the compost; and
-3b.decomposition and decay in anaerobic conditions, which generates methane, an especially nasty greenhouse gas (if you catch the methane and burn it for fuel, this is about equal to 2a). This is what happens in a landfill that’s airtight but isn’t kept dry inside.
Which is most green? If you care about carbon in the atmosphere, 1 is the winner, 2a and 2b are next, though if doing 1 instead of 2a just means more coal goes in the boiler, there’s no real gain, and if 2b requires a lot of fossil energy it may be even worse than 3a. So at least to a first approximation, simply “saving” organic materials indefinitely, which may be a very long time, perhaps until global temperatures are stabilized, is the best option, and this is best done by putting them into a dry, well-constructed, landfill.
Notice (my main point) that this ranking does not depend on how the object in question was made. Though biomaterial may be more green in creation than petromaterial, it doesn’t have any special claim to fate 3a. Biodegradability is better than having plastic junk floating around the ocean or sitting in the woods forever, but it’s a litter and wildlife and aesthetic strategy, and inferior to good housekeeping and landfilling.