Factors affecting the biological removal of hexavalent chromium using activated sludges.

CONTRERAS E.M., FERRO OROZCO A.M., ZARITZKY N.E.

Management of Hazardous Residues Containing Cr (VI)

Nova Science Publishers, Inc.

Lugar: New York; Año: 2009;

For many years, conventional Cr(VI) removal was achieved by chemical reduction, ion exchange or adsorption. Because the high costs associated to physical-chemical processes to remove Cr(VI), the search for new technologies has focused attention on the biotransformation of Cr(VI). Several researchers demonstrated that most of bacteria present in activated sludges are capable of reducing Cr(VI) to Cr(III). For this reason, removal of Cr(VI) using activated sludge is a promising technique. This chapter addresses the effect of important factors concerning the bioreduction of Cr(VI) by activated sludge in batch reactors, such as, biomass concentration, the presence, type, and concentration of carbonaceous substrates, the nitrogen source concentration, and Cr(VI) concentration. In addition, a mathematical model that describes the Cr(VI) bioreduction kinetics is also discussed.The reduction of Cr(VI) to Cr(III) in activated sludge batch reactors is a catalytic mediated process associated to the presence of biomass. Although the biomass itself could serve as the electron donor for reducing Cr(VI) to Cr(III), an external electron donor (e.g. carbonaceous substrate) increased the rate and extent of Cr(VI) reduction. High Cr(VI) reduction rates were obtained with fermentable substrates such as cheese whey, lactose, and glucose. On the contrary, with non-fermentable substrates such as citrate and acetate, Cr(VI) removal rates were slower. The low fraction of electrons of the carbonaceous substrate that reduce Cr(VI) to Cr(III) suggests that Cr(VI) reduction in activated sludge batch reactors is mainly via a cometabolic pathway.Besides the carbonaceous substrate, the presence of a nitrogen source is also necessary to obtain high Cr(VI) removal rates. In addition, when one of both carbonaceous or nitrogen source is depleted during the growth phase, Cr(VI) removal continues but at a lower rate. Thus, the bioreduction of Cr(VI) is the result of a fast Cr(VI) reduction process, associated to the biomass growth, and a slow one that does not depend on the presence of both a carbonaceous substrate and a nitrogen source. The second process may be1important under certain conditions, such as in continuous cultures in which substrates concentrations are normally low.In the present chapter, the effect of the initial Cr(VI) concentration on the activated sludge growth kinetics was studied. As the Cr(VI) concentration increased, the Lag phase also increased; in addition, a threshold inhibitory concentration of Cr(VI) within the range of 100 to 300 mg/L was observed. The biomass growth yield and the observed specific growth rate decreased as the Cr(VI) concentration increased; on the contrary, the substrate oxidation coefficient increased with higher Cr(VI) concentrations. These effects were attributed to the additional energy needed by microorganisms to maintain an active metabolism due to the presence of Cr(VI).