Effect of chromium(VI) on bacterial kinetics of heterotrophic biomass of activated sludge

The effect of hexavalent chromium, Cr(VI), on the maximum specific growth rate, μm and biomass yield, YH, of heterotrophic biomass was studied in batch tests conducted under high (=10) and low (=1.5) substrate-to-biomass ratios (S0/X0). The effects of sludge age and biomass acclimatization to Cr(VI) on the bacterial kinetics were also studied. The μm values were determined by measuring oxygen uptake rate (OUR) and volatile suspended solids (VSS) increase. Cr(VI) concentrations equal or greater than 10 mg l−1 inhibited the growth of unacclimatized activated sludge and caused a significant decrease in μm and YH values. The acclimatization of biomass and the selection of a high operating sludge age reduced the inhibitory effect of Cr(VI). At a sludge age of 20 days, Cr(VI) concentrations of <10 mg l−1 stimulated bacterial growth as evidenced by an increase in both the μm and YH values. Determining μm values by OUR and VSS methods, revealed that the presence of Cr(VI) in unacclimatized biomass caused an inhibitory effect mostly on substrate oxidation, while in acclimatized biomass, anabolic pathways were inhibited more.