Factors affecting the biological removal of hexavalent chromium using activated sludges

Abstract

For many years, conventional Cr(VI) removal was achieved by chemical reduction, ion exchange or adsorption. Recently, researchers have focused attention on bioremediation of hexavalent chromium. In contrast to the conventional methods, bioremediation is cost-effective. A great number of bacterial genera and also consortium cultures, such as activated sludge, were described as capable of reducing Cr(VI) to Cr(III). The aim of this chapter is to analyze some factors involved in the removal of hexavalent chromium using activated sludge such as the presence and type of carbonaceous substrate, the initial hexavalent chromium concentration, and the ratio between nitrogen and carbonaceous sources of the wastewater.Obtained results showed that in the absence of cells (abiotic controls), no measurable reduction of Cr(VI) occurred over a period of several days. On the contrary, high Cr(VI) removal efficiencies were obtained in the presence of carbonaceous substrates. In addition, chromate removal using oxidizable substrates (lactose, glucose, cheese whey) was higher than the obtained with non-oxidizable ones (acetate and citrate). In all cases, the maximum Cr(VI) removal rates occurred when there was no limitation on carbonaceous or nitrogen sources.The effect of Cr(VI) concentration (10, 25, 50, 100, and 300 mg/L) on the growth kinetics of activated sludge in batch systems was evaluated. No metabolic activity was observed when the Cr(VI) concentration was 300 mgCr(VI)/L, indicating that the Cr(VI) threshold inhibitory concentration was within the range 100-300 mg/L. The increase of Cr(VI) concentration from 0 (control experiment) to 100 mgCr(VI)/L caused an increment on the lag phase of activated sludges. After that, biomass concentration and oxygen uptake rate increased, and carbonaceous substrate concentration decreased as a function of time. As the initial Cr(VI) concentration increased, the biomass yield (YX/S) and the maximum growth rate (m) decreased, while the carbonaceous substrate oxidation coefficient (YO/S) increased.The effect of the ratio between nitrogen and carbonaceous sources on the chromate removal efficiency was also studied. As the nitrogen source and carbonaceous substrate ratio increased, Cr(VI) removal by activated sludge increased up to a certain value and then remained constant because the depletion of the carbonaceous source. Implications of these results on the amendment of the wastewater composition to optimize the removal of Cr(VI) by activated sludge batch systems are discussed.