There are many different test methods for testing biodegradability, but there are only two main principles: primary degradation and mineralization. The mineralization methods are often called “complete degradation” because the molecules of the sample can be broken down to the final product, carbon dioxide, which is an indicator of how much of the test sample is already degraded. But this does not mean that all molecules of the sample have been broken down to carbon dioxide, since a biodegradation rate of 60% is enough for the material to be considered biodegradable. In samples that contain only one type of molecules (pure substance), this interpretation may be right. But in mixtures, an interpretation of a positive test result (>60%) is impossible, since there could be components that are not at all biodegradable, and the term “complete degradation” is a misleading description. Methods for primary degradation are easier and much more precise. This is shown by the OECD round robin of 1988. In such test methods, the molecules of the sample are also broken down to the final product carbon dioxide but the indicator of the degradation rate is the concentration of the remaining sample molecules and their dissolved degradation products. A substance is considered biodegradable if the extent of biodegradation is higher than 80%.
In this chapter, we wish to compare different biodegradability test methods concerning their precision and quality of test results.