Compostable plastic a challenge to compost

Do those compostable, corn-based plastic products really biodegrade?

In a laboratory, sure. In your backyard compost pile? Not so fast, buster.

(Image: Corn-based plastic cup after five months in a backyard composter.)

Our experiment, with a corn-based cup marked “compostable” in green ink, has this preliminary result after five months in an Earth Machine composter: Not yet.
And it's not just me. I checked with some of the other compost-oriented folks on Kaua'i.

“The cups compost slowly in my pile. It is better to cut them in small pieces,” said Gordon LaBedz, chair of Zero Waste Kaua'i. “The forks and spoons hardly break down at all. At least they are not petroleum based.”

Katie Paul of Mālama Kaua'i said shredding is key. One presumes that's because it increases the surface area.

My compostable plastic cup, acquired at an earth-friendly event that touted zero waste, had cracked after five months in my compost pile, but it hadn't composted. Not at all.

A lot of folks are spending a lot of time feeling good about themselves for using compostable products at mass events. Is this stuff really as eco-friendly as everyone says?

And if it didn't compost in half a year in my Earth Machine, how does it compost?

It appears that the composting of compostable plastics requires fairly specific conditions. They need adequate moisture, high temperature, and they also need the microbes in compost to do a complete job.

In J.A. Brydson's 1999 book, “Plastics Materials,” Brydson says that biodegradables first require the moisture to break up the larger molecules in the plastic, and then the various life forms in compost to further break it down to the primary components of carbon dioxide and water.

High temperatures are also needed—and although elevated temperatures are readily achievable in a well-managed compost pile, the question is whether they're getting high enough.

I found detailed instructions at the site of a firm called NatureWorks PLA, a Cargill company. (http://www.natureworksllc.com/media/files/distributor_flyer_12_5_05rv.pdf)

The site says its plastic will break down in just 47 days into earth-friendly carbon dioxide, water and humus (decayed vegetable matter).

What's it need to have that happen? “Composting PLA requires approximately 140ºF and humidity between 80% and 90% for extended periods of time.”

The firm BASF has a compostable plastic it calls Ecoflex. In a laboratory, using one heat-loving compost-derived organism, they were able to obtain 99.9 percent biodegrading in 22 days at 130 degrees Fahrenheit.

And in an environmental assessment for the BASF product, there are these words: “While Ecoflex is compostable, this is not expected to be a significant means of disposal in the United States because composting is not currently a significant waste disposal method in this country.”

That, and the difficulty of composting the “compostable” plastics leads to this suggestion:

“I guess I just want to remind you to use reusable dishes and linens as much as possible. Please!” said Pam Lightfoot, emailing from Maine.

So what happened with our experiment with a compostable plastic cup?

The experiment took place through a dry summer, and other than kitchen scraps, little additional moisture was provided to the Earth Machine. It was in fact composting other organic materials, and providing good soil amendment, but did so slowly. Probably, it didn't have the requisite moisture or the requisite temperature to accomplish the breakdown of the plastic.

This experiment will continue, and perhaps the wet winter will assist the degradation. The point of this particular experiment is not to create perfect laboratory conditions—the plastic companies have done that and their results seem clear. The point is to determine whether this stuff breaks down in the standard backyard operation.

I turned to an expert on the topic, John Harder, who spent his career in solid waste management, and is now working with Zero Waste Kaua'i. He has a large biodegradable plastic composting experiment underway.

I turn it regularly, I try to balance the nitrogen, but I still only get about 120 degrees. The cups are turning milky white but they're not disappearing. You need to get it to 140 degrees for six to eight weeks and you really only get that in an active commercial operation,” he said.

One problem with leaving cups whole is that they tend to trap air, and the microbes can't get at the whole cup: “If you shred them with the green waste (in a composting operation), you get a lot of surface area.”

Harder said that since you can't have cups breaking down while you're drinking from them, they need to be quite durable. That makes their breakdown a little more challenging—but still possible.

“If you grind them up and get the temperatures up, they will break down,” Harder said.

©2008 Jan TenBruggencate