BIOLOGICAL TREATMENT OF GROUNDWATER WITH HIGH NITRATES CONTENT FOR HEXAVALENT CHROMIUM REMOVAL

摘要

Several methods have been developed in order to tackle the problem of high hexavalent chromium concentrations in groundwater. Among these, physicochemical methods such as chemical oxidation, ion exchange, adsorption through activated carbon, membrane separation, are very often adopted. However all these methods present several disadvantages such as high capital and operational cost, need for management of the chemical sludge, etc. Because of the need to develop cost - effective and more environmentally friendly techniques, biological removal of Cr(VI) have aroused great interest the last few years. Biological systems, such as bio-reduction, bio-accumulation or bio-sorption using living cells have been examined for their chromium removal abilities. The objective of the present study is to evaluate the effectiveness of anoxic-anaerobic biological treatment systems to provide for appreciable hexavalent chromium reduction from groundwater. According to the results of the batch experiments, Cr(VI) removal can be achieved in a fully anoxic-anaerobic system dosed with an external organic carbon source at an influent COD concentration of 200 mg/L. Furthermore the effect of the hydraulic residence time on process performance was assessed through the operation of a series of anoxic-anaerobic sequential batch reactors at different hydraulic residence times. According to the results of the experimental systems, Cr(VI) removal can be achieved in a fully anoxic-anaerobic system dosed with an external organic carbon source at an influent COD concentration of 200 mg/L and operated at a hydraulic residence time of 1.7 d. Under these conditions almost complete Cr(VI) removal can be achieved for initial Cr(VI) groundwater concentration as high as 3000 ug/L. For lower concentrations of external carbon source hexavalent chromium removal is lower and does not exceed 90%. Complete Cr(VI) removal can also be achieved at lower hydraulic residence times as low as 0.6 d, for initial Cr(VI) concentrations equal to 200 ug/L and mixed liquor temperatures greater than 20°C.