What is the difference between ‘biodegrading’ and ‘composting’?

The first difference is that composting provides the Earth with nutrients once the material has disintegrated into compost, also known as humus. Eventually the humus is then biodegraded by bacteria, and fungi into the base elements, CO₂, and mineral nutrients for supporting the growth of biomass. Humus also ensures that oxygen and water can easily move through the loose soil, reaching the roots and supporting plant growth. [i]

A second difference is that in the testing protocols for composting it is included to perform an ecotoxicity analyses, whereas in a biodegradability this is not always obliged. So when a article is labelled compostable, it is also verified to be free of potentially toxic matters after breakdown.

[i] Humus | National Geographic Society

Biodegradability or the better said, the process of biodegradation passes through 3 stages:

• Biodeterioration (e.g., black spots on bananas – surface breakdown)
• Biodisintegration (e.g., the banana becomes soft due to bacterial breakdown, material breaks into smaller pieces)
• Bioassimilation (e.g., the banana is broken into compost mass, taken into biomass – also called ultimate biodegradability).

Biodegradability as a general term typically reflects all the stages, but is mainly tested for the last stage of the process, the bioassimilation by bacteria. Therefore, a material may be biodegraded by bacteria but might still not be useful for a good quality compost[i].

Compostability is a specific case of a biodegradation process and mainly covers the first two stages of biodegradation as seen in the figure. For a product to be called ‘Compostable’ it needs to be biodegradable and need to fulfill other requirements such as disintegration within defined time, no harmful substances or ecotoxicity. [ii]

[i] OK compost & Seedling (tuv-at.be)

[ii] NEN-EN 13432:2000 en