Cr(III) in Activated Sludge

A mathematical model (CRAS) has been developed to evaluate the behavior of trivalent chromium [Cr(III)] in the activated‐sludge process. The validity of the model was tested by comparing its prediction with experimental data. Laboratory‐scale continuous‐flow activated‐sludge reactors receiving a continuous Cr(III) dosage provided the data. The independent experimental parameters were sludge age $\left({\theta }_x\right)$ and $\mathrm{pH}:{\theta }_x=5\text{,}10\text{,}\mathrm{and}15\mathrm{days}$; $\mathrm{pH}=6\text{,}7\text{,}\mathrm{and}8$. Comparison of the experimental results with the predictions revealed that: (1) More Cr(III) was intracellularly transported into the solid phase than Cr(III) adsorbed on activated sludge; and (2) the solid‐bound Cr(III) per unit weight of suspended solids in the effluent was considerably greater and more varied than that in the mixed liquor. By incorporating these two findings, the CRAS model was found capable of predicting quantitatively the percentage removal of Cr(III) (the average relative error was 4.5%) and the total Cr(III) concentration in the mixed liquor (14.9%) as a function of pH and ${\theta }_x$.