User Guide to Thresholds and Classification - Environmental ...

363User Guide for Thresholds and ClassificationsThe screening test for biodegradability in seawater (OECD Test Guideline 306) may be seen as seawaterparallel to the ready biodegradability tests. Substances that reach the pass level in OECD Test Guideline306 (that is, > 70% DOC removal or > 60 theoretical oxygen demand) may be regarded as readilybiodegradable, since the degradation potential is normally lower in seawater than in the freshwaterdegradation tests.Inherent biodegradabilityTests for inherent biodegradability are designed to assess whether a substance has any potential forbiodegradation. Examples of such tests are the OECD Test Guidelines 302A-C tests, the EU C.9 and C.12tests, and the ASTM E 1625-94 test.The basic test conditions favouring an assessment of the inherent biodegradation potential are a:prolonged exposure of the test substance to the inoculum allowing adaptation within the test period;high concentration of micro-organisms; andfavourable substance/biomass ratio.A positive result in an inherent test indicates that the test substance will not persist indefinitely in theenvironment, however a rapid and complete biodegradation can not be assumed. A result demonstratingmore than 70% mineralisation indicates a potential for ultimate biodegradation, a degradation of more than20% indicates inherent, primary biodegradation, and a result of less than 20% indicates that the substance ispersistent. Thus, a negative result means that non-biodegradability (persistence) should be assumed.In many inherent biodegradability tests only the disappearance of the test substance is measured. Such aresult only demonstrates a primary biodegradability and not a total mineralisation. Thus, more or lesspersistent degradation products may have been formed. Primary biodegradation of a substance is noindication of ultimate degradability in the environment.The OECD inherent biodegradation tests are very different in their approach and especially, the MITI II test(OECD Test Guideline 302C) employs a concentration of inoculum that is only three times higher than in thecorresponding MITI I ready biodegradability test (OECD Test Guideline 301C). Also the Zahn-Wellens test(OECD Test Guideline 302B) is a relatively ‗weak‘ inherent test. However, although the degradation potentialin these tests is not very much stronger than in the ready biodegradability tests, the results can not beextrapolated to conditions in the ready biodegradability tests and in the aquatic environment.Aquatic simulation testsA simulation test attempts to simulate biodegradation in a specific aquatic environment. As examples of astandard test for simulation of degradation in the aquatic environment may be mentioned the ISO/DS14592Shake flask batch test with surface water or surface water/sediment suspensions (Nyholm and Toräng,1999), the ASTM E 1279-89(95) test on biodegradation by a shake-flask die-away method and the similarOPPTS 835.3170 test. Such test methods are often referred to as river die-away tests.The features of the tests that ensure simulation of the conditions in the aquatic environment are:use of a natural water (and sediment) sample as inoculum; andJanuary 2012 EPA0109