Compostable vs Biodegradable – They Mean 2 Very, Very Different Things  

Increasingly products are being made with “compostable” and “biodegradable” on their labels (we should know ;-)).

But here’s the little known fact – the word “compostable” actually means something, and it’s definition is tightly controlled by International Standards… whereas the word “biodegradable” has a very loose, almost meaningless definition.

UPDATE (8/3/2010): Viv Video | “Biodegradable Packaging vs Compostable Packaging – DON’T get Greenwashed”

Let’s talk about it in the context of the new generation of eco-friendly plastics.

So, what is the difference between compostable and biodegradable plastic?

Well, the simplest way to identify a truly compostable product is to just look it up at the Biodegradable Products Institute (BPI) (the leading authority on compostable products).

As mentioned in our post on compostable cutlery that isn’t actually compostable, the long definition goes as follows:

Compostable Plastic“is capable of undergoing biological decomposition in a compost site as part of an available program, such that the plastic is not visually distinguishable and breaks down to carbon dioxide, water, inorganic compounds, and biomass, at a rate consistent with known compostable materials (e.g. cellulose), and leaves no toxic residue.” – as defined by the American Society for Testing and Measurement (ASTM-D6400 & ASTM-D6868).

According to ASTM D6400 and D6868 (and the European equivalent EN13432), compostable plastics must meet the following three criteria:

1. Biodegradability – Determined by measuring the amount of CO2 produced over a certain time period by the biodegrading plastic. The standards require 60% (90% in Europe) conversion of carbon into carbon dioxide within 180 days for resins made from single polymer and 90% conversion of carbon into carbon dioxide for co-polymers or polymer mixes.
2. Disintegration – Measured by sieving the material to determine the biodegraded size and that less than 10% remains on a 2mm screen within 120 days.
3. Eco-Toxicity – Measured by testing the concentrations of heavy metals to ensure that they are below the limits set by the standards and by testing plant growth by mixing the compost with soil in different concentrations and comparing plant growth in test and controlled compost samples.

So that’s the standard for “compostable plastics”.

Biodegradable Plastic, on the other hand, is plastic which will degrade from the action of naturally occurring microorganisms (e.g., bacteria, fungi) over a period of time. Note, however that there is no requirement for “eco-toxicity” and no time requirement for the biodegradation of biodegradable plastic.

The ASTM standard for compostability is applied to a range of products, but most typically is found on bags and food service ware, including: cups, cutlery, plastic coated paper items, and even bagasse and sugar cane products.

The rate of biodegradation for compostable products varies based on composition of the product, thickness, as well as composting conditions. Commercial composting systems, where compost piles are turned and typically reach high temperatures, often result in a much faster rate of biodegradation. (World Centric, a compostable food packaging manufacturer, actually has a fantastic chart showing high level estimates of biodegradation of their products in home composting vs commercial composting systems.)

In short, the term “biodegradable” is typically a term used to green wash products and make them seem good for the environment.

So, have you ever purchased “biodegradable” supplies and wondered if they actually break down? Tell us about an experience you’ve had.

And next time you’re making a purchase, make sure to purchase products that are certified compostable.

For more information, we suggest the following resources:
Biodegradable Products Institute
US Composting Council
ASTM
World Centric’s FAQ