Biodegradation refers to the process where micro-organisms break down chemicals into smaller parts. This provides them with the energy they need to survive. It is not that different to the way we eat and digest chemicals, such as carbohydrates, to get the energy we need to survive.
If a chemical is completely biodegraded, the leftovers are very simple chemical forms, such as water and carbon dioxide. These substances are then taken up by other animals and plants to make more chemicals, like carbohydrates, and the circle of life continues.
Biodegradation is a term that can be used very loosely, and in misleading ways. Almost every chemical will undergo at least some biodegradation if you give it enough time. The important factors are how quickly the biodegradation happens, and how complete it is. Because of this, scientists use specific terms when talking about biodegradation. These terms are also used on this website.
A chemical can be called readily biodegradable if a test has shown that the majority of it will be degraded quickly in the environment. Usually, it is safe the assume that the rest of the chemical will be degraded relatively quickly too. However, it is important to know that the leftovers might not break down further and could pose an environmental risk.
A chemical is ultimately biodegradable if all of it is degraded into the simplest chemical forms. This can also be called mineralisation. Knowing whether a chemical is ultimately biodegradable is very useful, but it is not as easy to measure as ready biodegradation. As a result it is not as common to have test data for this, but computer modelling can help to fill the gaps.