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EFFECTS ASSESSMENT
3.3.2 Effects assessment for substances with intermittent release
For substances subject to intermittent release (see Section 2.3.3.4 for the definition of
intermittent release), a single exposure event may be of only short duration. At least for dynamic
systems such as rivers, the likelihood of long-term effects arising from such exposure is low, the
principal risk being that of short-term toxic effects. Thus, the risk assessment should be based on
a no-effect-concentration for intermittent release. In extrapolating to such a PNECwater,
intermittent, therefore, generally only short-term effects need to be considered. It is therefore
proposed that, to derive a PNECwater, intermittent for such situations, an assessment factor of
100 be normally applied to the lowest L(E)C50 of at least three short-term tests from three
trophic levels. The assessment factor is designed to take account of the uncertainty that exists in
extrapolating from the results of short-term laboratory toxicity tests to short-term effects that can
be anticipated in the ecosystems.
In undertaking such an extrapolation, due account is taken of the biological variables of intraand
inter-species toxicity, as well as the general uncertainties in predicting ecosystem effects
from laboratory data. This extrapolation should be carried out with care. Some substances may
be taken up rapidly by aquatic organisms and this can lead to delayed effects even after exposure
has ceased. This will generally be taken into account by the assessment factor of 100 but there
may be occasions when a higher or lower factor would be appropriate. For substances with a
potential to bioaccumulate the lowered assessment factor of 100 may not always be sufficient to
provide adequate protection. For substances with a known non-specific mode of action, interspecies
variations may be low. In such cases, a lower factor may be appropriate. In no case
should a factor lower than 10 be applied to a short-term L(E)C50 value.
3.4 EFFECTS ASSESSMENT FOR MICROORGANISMS IN SEWAGE
TREATMENT PLANTS (STP)
Since chemicals may cause adverse effects on microbial activity in STPs it is necessary to derive
a PNECmicroorganisms (see Section 2.3.7). The PNECmicroorganisms will be used for the calculation of
the PEC/PNEC ratio concerning microbial activity in STPs. Current test systems for measuring
the effect of chemicals on microbial activity have different endpoints and different levels of
sensitivity. A number of internationally accepted test systems exist (cf. table below). Available
data (e.g. UBA, 1993; Reynolds et al., 1987) suggest the following order of increasing
sensitivities among particular test systems: respiration inhibition test (EU Annex V C.11; OECD
209, 1984f) < inhibition control in base-set tests < growth inhibition test with P. putida <
inhibition of nitrification.
In general, short-term measurements in the order of hours (e.g. 10 h) are preferred, in accordance
with the retention time in a STP. Information available on the toxicity for microorganisms has
also to be relevant for the endpoint considered, i.e. microbial degradation activity in a STP. Test
systems such as the respiration inhibition test and the nitrification inhibition test can be used.
Respiration tests using a mixed inoculum are considered more relevant than respiration
inhibition tests using a single-species inoculum.
The assumption that the substance under investigation is not inhibitory to the microorganisms
when dosed in the test system is implicit in ready biodegradability testing (i.e., EU Annex V
C.4A-F, OECD 301A-F, 1992f). Reynolds et al. (1987) report that microbial EC50 values
determined for test substances using a variety of tests (Annex V C.11, OECD 209, 1984f, Annex
V C.4F, Closed Bottle Test, Growth Inhibition) were found to be inhibitory in ready
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