The dynamic behavior of trivalent chromium (Cr(III)) in the activated sludge process receiving a square-wave input of Cr(III) was studied. A laboratory-scale activated sludge reactor was operated under steady-state conditions with respect to the process parameters at pH 7 and a sludge age of 5 days. The CRAS model, an quasi-equilibrium model developed by Imai (1988), was further extended to predict the behavior of Cr(III) as a function of operational time. A comparison between the experimental results and the model predictions revealed that the CRAS model was capable of predicting quantitatively the total chromium (TCr) and dissolved chromium (DCr) concentrations when the TCr concentration was on the increase (average relative error: 5.46% for DCr and 7.45% for TCr), but underestimated the TCr and overestimated the DCr as the TCr was on the decrease. Intracellularly transported Cr(III) seemed to be trapped inside the cell and not released back into solution. Iron(III) coprecipitation treatment indicated that about 35.6% of dissolved Cr(III) was organically complexed, and the CRAS model predicted 35.3%. This agreement supported the validity of the CRAS model for Cr(III) speciation in the solution phase.

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