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ENVIRONMENTAL EXPOSURE ASSESSMENT
assumed that these two adverse effects compensate each other more or less with regard to
the pollution of the environment).
The degree of removal in a wastewater treatment plant is determined by the physico-chemical
and biological properties of the substance (biodegradation, adsorption onto sludge,
sedimentation of insoluble material, volatilisation) and the operating conditions of the plant. As
the type and amount of data available on degree of removal may vary, the following order of
preference should be considered:
Measured data in full scale STP
The percentage removal should preferably be based upon measured influent and effluent
concentrations. As with measured data from the environment, the measured data from STPs
should be assessed with respect to their adequacy and representativeness.
Consideration must be given to the fact that the effectiveness of elimination in treatment plants is
quite variable and depends on operational conditions, such as retention time in the aeration tank,
aeration intensity, influent concentration, age and adaptation of sludge, extent of utilisation,
rainwater retention capacity, etc. The data may be used provided that certain minimum criteria
have been met, e.g. the measurements have been carried out over a longer period of time.
Furthermore, consideration should be given to the fact that removal may be due to stripping or
adsorption (not degradation). In case no mass balance study has been performed, the percentage
of transport to air or sludge should be estimated, e.g. by scaling the fractions to air and sludge
from the tables in Appendix II to the measured removal.
Data from dedicated STPs should be used with caution. For example, when measured data are
available for highly adapted STPs on sites producing high volume site-limited intermediates,
these data should only be used for the assessment of this specific use category of the substance.
Simulation test data
Simulation testing is the examination of the potential of a substance to biodegrade in a laboratory
system designated to represent either the activated sludge-based aerobic treatment stage of a
wastewater treatment plant or other environmental situations, for example a river. The
wastewater treatment process can be studied in the laboratory by, e.g., the updated OECD
guideline on simulation testing of aerobic sewage treatment (OECD, 2001b) or the older
Coupled Units Test (OECD, 1981b). Removability is determined by monitoring the changes in
DOC (Dissolved Organic Carbon) and/or COD (Chemical Oxygen Demand). A number of
guidelines have recently been prepared, e.g. ISO/DIS 14952-1, draft OECD (2001d), OECD 307
(soil, 2000b), draft EU Annex V C.23, OECD 308 (sediment, 2000c), draft EU Annex V C.24.
The Coupled Units Test is not suitable for adsorptive, poorly water-soluble and volatile
substances because it is an open test and is only based on DOC analysis. Since, in addition, it is
possible that adsorptive or volatile metabolites may be formed during biological degradation,
this test cannot differentiate between biological degradation and other elimination processes.
Investigations with a closed vessel version of the Coupled Units Test using radioactively
labelled substances have been performed which would allow a determination of the complete
mass balance and would also be suitable for volatile or adsorptive substances. However, there is
no international standard method available for this modified test.
There is insufficient information available on the applicability of elimination data from the
laboratory test to the processes of a real sewage plant. The results can be extrapolated to
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