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This paper analyses how sludge age, the most important design parameter for activated sludge processes, affects effluent concentrations of those micropollutants which are removable only by adsorption on activated sludge flocs. A mathematical model has been developed for sequencing batch reactors (SBRs) treating readily biodegradable substrates and micropollutants, in order to calculate the effect of sludge age on liquid- and solid-phase concentrations of micropollutants, both during the startup and at steady state. It was shown that the increase in sludge age, at fixed values of the other parameters, causes an increase of the concentration of micropollutants both in the liquid and in the solid phase. At any given sludge age, the effect of process parameters (e.g., biomass kinetic and stoichiometric parameters, influent concentration of readily biodegradable substrate) on the removal of the micropollutant was discussed. Experimental data were collected from two parallel laboratory-scale SBRs operated under identical conditions and feed composition, the only difference being sludge age (4 vs 26 days). Removal of cadmium and lead in the two reactors was compared. Effluent cadmium concentration was significantly higher at higher sludge age (25% of the influent concentration vs 12% at the lower sludge age). Lead removal, on the ther hand, was not significantly affected by sludge age and other removal mechanisms, as precipitation, likely occurred. Other literature data on removal of micropollutants in activated sludge processes were critically analyzed in light of the findings obtained in this study.

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