technical guidance documents - Institute for Health and Consumer ...

ENVIRONMENTAL EXPOSURE ASSESSMENT
• they are only used for the water-dissolved fraction of the substance. Partitioning between
water and sludge phases should be calculated prior to the application of the rate constant;
• sufficiently valid data from internationally standardised tests are preferred;
Data from non-standardised tests and/or tests not performed according to the principles of GLP
may be used if expert judgement has confirmed them to be equivalent to results from the
standardised degradation tests on which the calculation models, e.g. SimpleTreat, are based. The
same applies to STP-measured data, i.e., in-situ influent/effluent measurements.
Table 6 Elimination in sewage treatment plants: Extrapolation from test results to rate constants in STP model (SimpleTreat)
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Test result Rate constant k . (h -1 )
Readily biodegradable a) 1
Readily, but failing 10-d window a) 0.3
Inherently biodegradable, fulfilling specific criteria b) 0.1
Inherently biodegradable, not fulfilling specific criteria b) 0
Not biodegradable 0
Notes to Table 6:
a) Ready biodegradability testing (28 d) (92/69/ EU Annex V C.4 A-F, or respectively, OECD 301A-F (1992f) or equivalent according to
expert judgement).
Ready biodegradability tests are screening tests for identifying substances that, based on general experience, are assumed to undergo
rapid and ultimate biodegradation in the aerobic environment. However, a negative result does not necessarily mean that the
substance will not be biodegraded in, e.g., a sewage treatment plant.
The degree of ultimate degradation may be followed by determination of the loss of dissolved organic carbon (DOC), the evolution of
carbon dioxide or the amount of oxygen consumed. It is generally accepted that a substance is considered to be readily biodegradable
if the substance fulfils the pass criteria of a test for ready biodegradability (cf. the Annex V methods or the OECD guidelines) which may
include the concept of the 10 days time window as a simple kinetic criterion. All percentage biodegradation results refer to true
biodegradation i.e. mineralisation excluding abiotic elimination processes (e.g. volatilisation, adsorption). This means that
corresponding data in adequate control vessels must be generated during biodegradation testing. The test may be continued beyond
28 days if biodegradation has started but does not reach the required pass criteria for final mineralisation: in this case however, the
substance would not be regarded as being readily biodegradable. If the substance reaches the biodegradation pass levels within 28
days but not within the 10-day time window, a biodegradation rate constant of 0.3 h-1 is assumed. In case that only old ready
biodegradation test results (i.e. tests executed prior to the introduction of the 10 days time window criterion and documenting only on
the pass level) are available a rate constant of 0.3 h-1 should be applied in case the pass level is reached. Based on weight of evidence
(e.g. several old test results) a rate constant of 1 h-1 may be justified by expert judgement.
If the substance is found to be not readily biodegradable, it is necessary to check whether it was inhibitory to microbial activity at the
concentration used in the biodegradability test. If the substance is inhibitory, it may be re-tested at low, non-inhibitory concentrations in
a test simulating the conditions in a sewage treatment plant (e.g. OECD guideline 303, 2001b; ISO 11733 or equivalent). If appropriate,
re-testing in another more suitable ready biodegradability test (e.g. Closed Bottle test) may be considered. Re-testing in a modified
ready biodegradability test at a much lower concentration (i.e. more than 10 times lower than prescribed) cannot generally be
recommended because suitable simulation test methods are available.
b) Inherent biodegradability testing (28d) (87/302/EEC, respectively, OECD 302B-C (1981d-1992g) or equivalent according to expert
judgement).
Inherent biodegradability tests are designed to assess whether the substance has any potential for biodegradation. A negative result
will normally mean that non-biodegradability (persistence) should be assumed. A positive result, on the other hand, indicates that the
substance will not persist indefinitely in the environment. In those cases where a more accurate prediction of degradation kinetics in
treatment plants is required, sewage treatment plant simulation tests (e.g. OECD guideline 303, 2001b; ISO 11733 or equivalent)
should be conducted.
In tests for inherent biodegradability, the test conditions are designed to be more favourable to the microorganisms in that the ratio of
substance to cells is lower than in the ready tests and there is no requirement for the (bio)degradation to follow a time pattern as in the
ready tests. Also, pre-exposure of the inoculum resulting in pre-adaptation of the microorganisms may be allowed. The time permitted
for the study is limited to 28 days, but it may be continued for much longer; 6 months has been suggested as the maximum duration for
the test. The results obtained in a test of more than 28 days are not comparable with those obtained in less than this period.
Usually, more than 70% (bio)degradation within 28 days indicates that the substance is inherently biodegradable. However,
extrapolation of the results of the inherent tests should be done with great caution because of the strongly favourable conditions for
biodegradation that are present in these tests. Therefore, a substance that passes an inherent test should in principle be given a rate