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2.3.8.3 Calculation of PEClocal for the aquatic compartment
In this section, the following parameters are derived:
• local concentration in surface water during emission episode;
• annual average local concentration in surface water.
ENVIRONMENTAL EXPOSURE ASSESSMENT
The effluent of the sewage treatment plant is diluted into the surface water. Figure 9 shows the
most important fate processes of the aquatic compartment. For the calculations, the following
assumptions are made:
• complete mixing of the effluent
in surface water is assumed as a
representative exposure situation
for the aquatic eco-system;
• for the first approach in the local
assessments, volatilisation,
degradation, and sedimentation
are ignored because of the short
distance between the point of
effluent discharge and the
exposure location.
The calculation of the PEClocal for the
aquatic compartment involves several
sequential steps (see also Figure 9). It
includes the calculation of the
discharge concentration of a STP to a
water body, dilution effects and
removal from the aqueous medium by
adsorption to suspended matter.
Dilution in the receiving surface water and adsorption to suspended matter
The distance from the point of discharge where complete mixing may be assumed will vary
between different locations. A fixed dilution factor may be applied. Dilution factors are
dependent on flow rates and the industry specific discharge flow. Due to the different seasonal,
climatic and geographical conditions in the Member States, those dilution factors may vary over
wide ranges. They have been reported in a range from 1 (e.g. dry riverbeds in summer) up to
100,000 (de Greef and de Nijs, 1990). The dilution factor is generally linked to the release
scenario of the use category. For example, for consumer products an average dilution factor for
sewage from municipal treatment plants of 10 is recommended. This is also regarded as a default
dilution value for other types of substances if no specific data are available.
When a substance is released to surface water predominately as particles (e.g. as precipitates or
incorporated in small material pieces – see Section 2.3.3.5) this may lead to overestimation of
PECsurface water and underestimation of PECsediment. If this is expected to occur it should be
considered in the further evaluation (e.g. when comparing PEC with monitoring data and in the
risk characterisation).
In certain circumstances, it may be possible to identify specific emission points which would
allow the use of more precise information regarding the available distribution and fate processes.
volatilisation
STP
degradation
dilution
partitioning
sedimentation/
resuspension
Figure 9 Fate processes in surface water
suspended
matter
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