Composting is a process of biodegradation, where micro-organisms break down materials into smaller parts. For a material to be compostable, it should generally be demonstrated that it can biodegrade under optimal conditions within six months, disintegrate into small particles, and be able to support plant growth.
As any home gardener will tell you, composting is a fine art. You need to have the right balance of carbon- and nitrogen-rich material, and it needs to be aerated regularly. The ambient temperature and moisture levels can also have a big impact on how quickly compost breaks down.
These conditions can be found in the natural environment. However, in many instances – including at landfills – these conditions are not available. Wastes disposed to landfill are generally contained, packed tightly and eventually sealed underground. Oxygen is low, temperature and moisture vary, and there is limited opportunity for mixing to obtain the necessary balance of nutrients.
A compostable material will not compost under these conditions. Instead, fermentation occurs. This produces gases, like methane, and liquid wastes. The gases can escape to the atmosphere, where they can contribute to air quality issues and climate change. The liquid wastes form leachate, and can move into the surrounding soil and waterways. (Many groups around the world are working to reduce the emissions of gases and leachate from landfills.)
For waste to be able to undergo composting, it must be disposed to either a household compost bin or an industrial composting facility. However, household composting systems can vary widely in their effectiveness (for example, due to temperature fluctuations), so a compostable material can still take a long time to break down under these conditions. Industrial composting – while often more effective – is generally not available to households. Some areas collect household wastes for industrial composting, but this is not a widespread practice.