The biodegradability of substances and wastewater ingredients depends not only on the molecular structures of the test material, but also on important additional factors such as the
- aquatic or terrestrial test environment;
- aerobic or anaerobic test conditions;
- source and concentration of the microorganisms of the inoculum;
- acclimatisation and adaptation of the inoculum;
- concentration of the test material;
- availability of inorganic nutrients and additional organic material for co-metabolic processes;
- possible toxic effects of the test material under the test conditions;
- physical and chemical properties and bioavailability of the test material (e.g. volatility, water solubility, adsorption);
- conditions and: physical and chemical properties of the test system (volume of test vessels, static, dynamic, closed, CO2 removal, temperature, mixing, shaking, oxygen supply);
- test duration;
- analytical parameters used (summary parameters such as DOC, BOD, CO2 or substance specific analysis).
As so many factors can influence the test results it is not possible to define a "true" or "reference" method. The reproducibility of the test results using different methods or conditions or even using identical test methods can be low and differing test results can be obtained. Normally a test material which is either easily biodegradable or poorly biodegradable will produce similar test results. So-called "grey" substances, which are moderately biodegradable and need, for example, special bacteria or long adaptation periods, will often produce disparate results.
Biodegradability tests are used to establish the biodegradability of chemical structures and to predict the biodegradation behaviour of a test material in a natural or technical environment. The aim of each test method should be to simulate, to a certain degree, such an environment. Therefore, as so many factors can influence or even exclude certain methods, it is necessary to have a sufficient number of different standardized test methods to allow the choice of the best one for the specific purpose. It is also to be remembered that it was ISO's intention to standardize the biodegradation tests of the OECD Guidelines for Testing of Chemicals (see annex B).