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

ENVIRONMENTAL EXPOSURE ASSESSMENT
The remarks in the section on soil biodegradation regarding use of half-lives derived in surface
water simulation tests may also apply for sediments.
2.3.6.6 Overall rate constant for degradation in surface water
In surface water, the substance may be transformed through photolysis, hydrolysis, and
biodegradation. For calculation of the PECregional, the rate constants for these processes can be
summed into one, overall degradation rate constant. It should be noted that different types of
degradation (primary and ultimate) are added. This is done for modelling purposes only. It
should also be noted that measurements on one degradation process might in fact already include
the effects of other processes. For example, hydrolysis can occur under the conditions of a
biodegradation test or a test of photodegradation, and so may already be comprised by the
measured rate from these tests. In order to add the rates of different processes, it should be
determined that the processes occur in parallel and that their effects are not already included in
the rates for other processes. If exclusion of hydrolysis from the other degradation rates cannot
be confirmed its rate constant should be set to zero. The equation below relates to primary
degradation. If the primary degradation is not the rate-limiting step in the total degradation
sequence and degradation products accumulate, then also the degradation product(s) formed in
the particular process (e.g. hydrolysis) should be assessed. If this cannot be done or is not
practical, the rate constant for the process should be set to zero.
Explanation of symbols
kdegwater = khydrwater + kphotowater + kbiowater (31)
khydrwater first order rate constant for hydrolysis in surface water [d -1 ] eq. (26)
kphotowater first order rate constant for photolysis in surface water [d -1 ] eq. (27)
kbiowater first order rate constant for biodegradation in surface water [d -1 ] Table 7
kdegwater total first order rate constant for degradation in surface water [d -1 ]
2.3.7 Elimination processes prior to the release to the environment
2.3.7.1 Wastewater treatment
In this section, the following parameters are derived:
• emission from a sewage treatmentplant to air;
• concentration in sewage sludge;
• concentration in effluent of a sewage treatment plant;
• PEC for microorganisms in a sewage treatment plant.
Elimination refers to the reduction in the concentration of substances in gaseous or aqueous
discharges prior to their release to the environment. Elimination from the water phase may occur
by physical as well as chemical or biochemical processes. In a sewage treatment plant (STP),
one of the main physical processes is settling of suspended matter which will also remove
adsorbed material. Physical processes do not degrade a substance but transfer it from one phase
to another e.g. from liquid to solid. In the case of volatile substances, the aeration process will
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