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

Primary Settler Aeration Tank
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ENVIRONMENTAL EXPOSURE ASSESSMENT
For the purpose of modelling a STP, the rate constants presented in Table 6 have been derived
from the biodegradation screening tests. The modelling results from SimpleTreat using these
first-order rate constants of 0, 0.1, 0.3 and 1 h-1 are tabulated in Appendix II. It contains relative
emission data pertaining to air, water, and sludge as a function of Henry's Law constant and log
Kow for the different biodegradation categories, according to Table 6. If no specific measured
biodegradation rate data are available for the particular substance, the tabulated values from
Appendix II should be used.
Typical characteristics of the standard sewage treatment plant are given in Table 9. The amount
of surplus sludge per person equivalent and the concentration of suspended matter in influent are
taken from SimpleTreat (run at low loading rate).
These values are the same as applied to derive the tables in Appendix II. At a higher tier in the
risk assessment process more specific information on the biodegradation behaviour of a
substance may be available. In order to take this information into account a modified version of
the SimpleTreat model may be used. In this version the following scenarios are optional:
• temperature dependence of the biodegradation process;
• degradation kinetics according to the Monod equation;
• degradation of the substance in the adsorbed phase;
• variation in the sludge retention time;
• not considering a primary settler.
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Surroundings
Air
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Solid/Liquid
Separator
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Advective Flow Dispersive Flow
Suspended solids Bottom sediment
biodegradation
Figure 6 Schematic design of the sewage treatment plant model Simple Treat
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