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The abiotic and biotic fate in activated sludge plants of a mixture of aromatic pollutants including polycyclic aromatic hydrocarbons, polychlorinated biphenyls, chlorinated and non-chlorinated benzenes, phenols and surfactants was studied. To this aim, an activated sludge collected from a municipal plant was artificially spiked with the target compounds. Under aerobic conditions, adsorption and biodegradation were distinguished as possible mechanisms of target compound removal through comparison of tests with and without oxygen. With the exception of phenol, which was completely biodegraded (at an average rate of 2.6 mg phenol g −1 volatile suspended solids h −1 ), the pollutants were completely or significantly removed from the liquid phase through adsorption. In the tested conditions the amount of substance removed through adsorption ranged from 75-80% for 4-dodecylbenzenesulfonic acid and pentachlorophenol to more than 90% for the other compounds. Kinetic and equilibrium parameters describing the adsorption of the different xenobiotics to the sludge were also determined. The role of anaerobic digestion in the final decontamination of the xenobiotic-containing sludge was evaluated under mesophilic and thermophilic conditions. A significant average depletion of the chlorinated pollutants was observed in mesophilic conditions (24.6%). The addition of yeast extract stimulated the process by allowing a higher chlorinated pollutant average depletion (49.7%) and complete disinfection of the sludge.

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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Four experiments were performed in sequencing batch reactors, two by two in parallel, to study the effect of micropollutants (organic xenobiotics and heavy metals) on the performance of activated sludge processes. The reactors were operated for long times (at least 11 months each) and in a wide range of sludge ages (5-30 days). It was observed that production of biological solids, COD removal, and settling properties were not affected by the presence of micropollutants significantly. On the other hand, ammonia removal was much lower in the reactors fed with micropollutants (29-37% removal) than in the reactor without micropollutants (82% removal). Batch tests allowed the measurement of maximal activities of heterotrophic and autotrophic biomass. The activity of nitrifying microorganisms grown without micropollutants was greatly reduced (about 50%) by the addition of the micropollutants, but the residual micropollutants after treatment did not exert any inhibiting effect. Nitrate balances on the four runs confirmed that the fraction of nitrifying microorganisms was much higher in the reactor without micropollutants (4.5% of overall VSS) than in the reactors fed with micropollutants (maximum 0.5% of VSS). In spite of the fact that in the reactors fed with micropollutants the fraction of nitrifying microorganisms increased at increasing sludge age, a satisfactory nitrogen removal was not achieved in the whole tested range of experimental conditions, even after the 11-month acclimation. With regard to nitrogen removal in processes operated with micropollutants, the results obtained in this study seem to indicate the effectiveness both of processes with separate nitrification after a first treatment stage and of single-sludge processes carried out at very high sludge ages, as acheivable, e.g., by the use of membranes or of attached growth systems.