User Guide to Thresholds and Classification - Environmental ...

325User Guide for Thresholds and Classificationsshould only be used as supporting evidence for demonstration of either persistence in the aquaticenvironment or a rapid degradation.Inherent biodegradability testsSubstances that are degraded more than 70% in tests for inherent biodegradability (OECD Test Guidelines302) have the potential for ultimate biodegradation. However, because of the optimum conditions in thesetests, the rapid biodegradability of inherently biodegradable substances in the environment cannot beassumed. The optimum conditions in inherent biodegradability tests stimulate adaptation of the microorganisms,thus increasing the biodegradation potential, compared to natural environments. Therefore,positive results in general should not be interpreted as evidence for rapid degradation in the environment.The inherent biodegradability tests concerned are the Zahn Wellens test (OECD TG 302 B) and the MITI IItest (OECD TG 302 C). The conditions for use in this regard are:the methods must not employ pre-exposed (pre-adapted) micro-organisms;the time for adaptation within each test should be limited, the test endpoint should refer to themineralisation only and the pass level and time for reaching these should be, respectively:MITI II pass level > 60% within 14 days; andZahn Wellens Test > 70% within 7 days.Sewage treatment plant simulation testsResults from tests simulating the conditions in a sewage treatment plant (STP) (for example, the OECD TestGuideline 303) cannot be used for assessing the degradation in the aquatic environment. The main reasonsfor this are that the microbial biomass in a STP is significantly different from the biomass in the environment,that there is a considerably different composition of substrates, and that the presence of rapidly mineralisedorganic matter in waste water facilitates degradation of the test substance by cometabolism.Soil and sediment degradation dataIt has been argued that for many non-sorptive (non-lipophilic) substances, more or less the samedegradation rates are found in soil and in surface water. For lipophilic substances, a lower degradation ratemay generally be expected in soil than in water due to partial immobilisation caused by sorption. Thus, whena substance has been shown to be degraded rapidly in a soil simulation study, it is most likely also rapidlydegradable in the aquatic environment. It is therefore proposed that an experimentally determined rapiddegradation in soil is sufficient documentation for a rapid degradation in surface waters when:no pre-exposure (pre-adaptation) of the soil micro-organisms has taken place; andan environmentally realistic concentration of substance is tested; andthe substance is ultimately degraded within 28 days with a half-life < 16 days corresponding to adegradation rate > 0.043 day-1.The same argument is considered valid for data on degradation in sediment under aerobic conditions.January 2012 EPA0109