User Guide to Thresholds and Classification - Environmental ...

328User Guide for Thresholds and ClassificationsThe present standard methods for investigating degradability of chemical substances are developed forreadily soluble test compounds. However, many organic substances are only slightly soluble in water. As thestandard tests require 2–100 mg/L of the test substance, sufficient availability may not be reached forsubstances with a low water solubility. Tests with continuous mixing and/or an increased exposure time, ortests with a special design where concentrations of the test substance lower than the water solubility havebeen employed, may be available on slightly soluble compounds.Test duration less than 28 daysSometimes degradation is reported for tests terminated before the 28-day period specified in the standards(for example, the MITI, 1992). These data are of course directly applicable when a degradation greater thanor equal to the pass level is obtained. When a lower degradation level is reached, the results need to beinterpreted with caution. One possibility is that the duration of the test was too short and that the chemicalstructure would probably have been degraded in a 28-day biodegradability test. If substantial degradationoccurs within a short time period, the situation may be compared with the criterion BOD 5 /COD > 0.5 or withthe requirements on degradation within the 10-days time window. In these cases, a substance may beconsidered readily degradable (and hence rapidly degradable), if:the ultimate biodegradability exceeds 50% within 5 days; orthe ultimate degradation rate constant in this period is greater than 0.1 day-1 corresponding to a half-lifeof 7 days.These criteria are proposed in order to ensure that rapid mineralisation did occur, although the test wasended before 28 days and before the pass level was attained. Interpretation of test data that do not complywith the prescribed pass levels must be made with great caution. It is mandatory to consider whetherbiodegradability below the pass level was due to a partial degradation of the substance and not a completemineralisation. If partial degradation is the probable explanation for the observed biodegradability, thesubstance should be considered not readily biodegradable.Primary biodegradationIn some tests, only the disappearance of the parent compound (that is, primary degradation) is determinedfor example by following the degradation by specific or group specific chemical analyses of the testsubstance. Data on primary biodegradability may be used for demonstrating rapid degradability only when itcan be satisfactorily demonstrated that the degradation products formed do not fulfil the criteria forclassification as hazardous to the aquatic environment.Conflicting results from screening testsThe situation where more degradation data are available for the same substance introduces the possibility ofconflicting results. In general, conflicting results for a substance that has been tested several times with anappropriate biodegradability test could be interpreted by a ‗weight-of-evidence approach‘. This implies that ifboth positive (that is, higher degradation than the pass level) and negative results have been obtained for asubstance in ready biodegradability tests, then the data of the highest quality and the best documentationshould be used for determining the ready biodegradability of the substance. However, positive results inJanuary 2012 EPA0109