The OECD in their guidelines distinguish 6 forms of biodegradation, as follows (OECD, 1981b, 1991, 1992a, 1992b, 2001, 2002, 2004a, 2008).
- Ultimate biodegradation (mineralisation): The level of degradation achieved when the test compound is totally utilised by micro-organisms resulting in the production of carbon dioxide, water, mineral salts and new microbial cellular constituents (biomass).
- Primary biodegradation (biotransformation): The alteration in the chemical structure of a substance, brought about by biological action, resulting in the loss of a specific property of that substance.
- Readily biodegradable: An arbitrary classification of chemicals which have passed certain specified screening tests for ultimate biodegradability; these tests are so stringent that it is assumed that such compounds will rapidly and completely biodegrade in aquatic environments under aerobic conditions.
- Inherent biodegradable: A classification of chemicals for which there is unequivocal evidence of biodegradation (primary or ultimate) in any test of biodegradability.
- Half-life (t0.5): The time taken for 50% transformation of a test substance when the transformation can be described by first-order kinetics; it is independent of the initial concentration.
- Disappearance time 50 (DT50): The time within which the initial concentration of the test substance is reduced by 50 percent.