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EFFECTS ASSESSMENT
for ciliates are considered to be supplementary to the data for activated sludge or specific
bacteria, i.e. no correlation exists between activated sludge and ciliate test results, neither are
ciliates consistently more sensitive. The data from one ciliate species are representative for other
ciliates, i.e. test data from species not dominant or not present in STPs can serve as basis for the
PNEC-derivation. The function of the protozoa in STP is correlated to their growth. Therefore,
values from ciliate growth inhibition tests, preferably with Tetrahymena (cf. OECD, 1998a), are
relevant for the risk assessment for STPs. Tests using other characteristics (e.g. ciliary motion,
cell movement, etc.) should not serve as a basis for the PNEC-derivation.
Often information may also be present on individual bacterial species such as from tests with
Vibrio fischeri (used in the MICROTOX test), Pseudomonas putida, Pseudomonas fluorescens
and even Escherichia coli. These tests must be considered as less relevant. The tests with P.
fluorescence and E. coli (Bringmann and Kühn, 1960) cannot be used for determination of the
PNECmicroorganisms as they use glucose as a substrate. Likewise, the MICROTOX test cannot be
used as it uses a saltwater species. Results of the cell multiplication inhibition test with P. putida
(Bringmann and Kühn, 1980) should only be used for calculation of the PNECmicroorganisms in
cases where no other test results employing mixed inocula are available.
In general, the aim of the assessment is the protection of the degradation and nitrification
functions and process performance and efficiency of domestic and industrial STPs – as also
influenced by protozoan populations. The toxicity of a substance to microorganisms in a STP is
assessed by comparing the concentration of a substance in STP aeration tank with the microbial
effect concentration data for that substance (see also Section 2.3.7.1). If the substance under
consideration is relevant for industrial and municipal STPs the toxicity assessment should be
conducted for both kinds of STPs separately. A PNECmicroorganisms should be obtained as a first
step in the effects assessment for microorganisms in both domestic and industrial sewage
treatment plants. The PNECmicroorganisms is usually derived from results obtained in the most
sensitive test system available, regardless of whether this is a test with activated sludge, relevant
bacteria or ciliated protozoa:
• the PNECmicroorganisms is set equal to a NOEC from a test performed with ‘specific bacterial
populations’ like nitrifying bacteria or P. putida or from a growth inhibition test performed
with ciliated protozoa. An EC50 from this test is divided by an assessment factor of 10;
• a NOEC or EC10 from other test systems like the respiration inhibition test (EU Annex V
C.11; OECD 209, 1984f) is divided by an assessment factor of 10. An EC50 from this test is
divided by an assessment factor of 100;
• the lowest value is selected as the PNECmicroorganisms.
There may be cases in which the lowest PNECmicroorganisms does not correspond to the effect value
of the most sensitive test system because different AF (100 or 10) are applied to the different test
systems. In these cases expert judgement should be used to decide which effect value is
appropriate for the calculation of the PNECmicroorganisms. Usually the effect value of the most
sensitive test system should be used as a basis for the calculation of PNECmicroorganisms employing
the appropriate AF.
Table 17 provides a complete listing of the test systems mentioned above, effect concentrations
that are determined using them and the corresponding assessment factors.
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