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ENVIRONMENTAL EXPOSURE ASSESSMENT<br />

Explanation of symbols<br />

Elocalwater local emission rate to water during episode [kg . d -1 ] eq. (5)<br />

Fstpsludge fraction of emission directed to sludge by STP [-] App. II<br />

SLUDGERATE rate of sewage sludge production [kg . d -1 ] eq. (37)<br />

Csludge concentration in dry sewage sludge [mg . kg -1 ]<br />

The rate of sewage sludge production can be estimated from the outflows of primary <strong>and</strong><br />

secondary sludge as follows:<br />

2<br />

SLUDGERATE = • SUSPCONCinf<br />

• EFFLUENT stp + SURPLUSsludge<br />

• CAPACITY stp<br />

3<br />

Explanation of symbols<br />

SUSPCONCinf concentration of suspended matter in STP influent [kg . m -3 ] Table 9<br />

EFFLUENTstp effluent discharge rate of STP [m 3. d -1 ] eq. (34)<br />

SURPLUSsludge surplus sludge per inhabitant equivalent [kg . d -1. eq -1 ] Table 9<br />

CAPACITYstp capacity of the STP [eq] Table 9<br />

SLUDGERATE rate of sewage sludge production [kg . d -1 ]<br />

Anaerobic degradation may lead to a reduction of the substance concentration in sewage sludge<br />

during digestion. This is not yet taken into account.<br />

Calculation of the STP concentration <strong>for</strong> evaluation of inhibition to microorganisms<br />

As explained above in the section on STP modeling, the removal of a chemical in the STP is<br />

computed from a simple mass balance. For the aeration tank this implies that the inflow of<br />

sewage (raw or settled, depending on the equipment with a primary sedimentation tank) is<br />

balanced by the following removal processes: degradation, volatilization <strong>and</strong> outflow of<br />

activated sludge into the secondary settler. Activated sludge flowing out of the aeration tank<br />

contains the chemical at a concentration similar to the aeration tank, which is the consequence of<br />

complete mixing. It consists of two phases: water, which is virtually equal to effluent flowing<br />

out of the solids-liquid separator (this is called the effluent of the STP), <strong>and</strong> suspended particles,<br />

which largely settle to be recycled into the aeration tank. Assuming steady state <strong>and</strong> complete<br />

mixing in all tanks (also the aeration tank), the effluent concentration approximates the really<br />

dissolved concentration in activated sludge. It is assumed that only the dissolved concentration is<br />

bioavailable, i.e. the actual concentration to which the microorganisms in activated sludge are<br />

exposed. For the risk characterisation of a substance upon microorganisms in the STP, it can<br />

there<strong>for</strong>e be assumed that homogeneous mixing in the aeration tank occurs which implies that<br />

the dissolved concentration of a substance is equal to the effluent concentration:<br />

Explanation of symbols<br />

64<br />

PECstp = Clocaleff<br />

Clocaleff total concentration of substance in STP effluent [mg . l -1 ] eq. (33)<br />

PECstp PEC <strong>for</strong> microorganisms in the STP [mg . l -1 ]<br />

(37)<br />

(38)

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