The objective of this work is to evaluate biological groundwater treatment systems that will achieve hexavalent chromium removal from groundwater at hexavalent chromium (Cr(VI)) groundwater concentrations in the 0-200 μg/L range under anoxic conditions. The effect of type of organic substrate added as feed to the groundwater treatment system (milk, sugar and cheese whey), the effect of different concentrations of chemical oxygen demand added in the feed (100, 150 and 200 mg/L) and the effect of different hydraulic residence time (1.7, 0.9 and 0.7 d) on process performance were evaluated through the operation of a series of sequential batch reactors under anoxic conditions. Biomass receiving Cr(VI) contaminated groundwater with a low nitrates content exhibited similar Cr(VI) removal efficiency under reductive conditions, with biomass receiving Cr(VI) contaminated groundwater with a high nitrates content. The concentration of organic substrate was crucial for the microbial reduction of Cr(VI). The different hydraulic residence time of the reactors and the different types of organic substrates added did not affect the efficiency of hexavalent chromium removal which was complete. This study demonstrates that biological systems operating under reductive conditions can efficiently treat groundwater containing low or high nitrates concentration and can provide complete hexavalent chromium removal at initial Cr(VI) concentrations of 200 μg/L.
BACKGROUND: Microbial reduction of Cr(VI) to the much less toxic Cr(III), although practiced for the treatment of liquid wastes has not been implemented for the treatment of groundwater due to the absence of electron donors (organic substrates) and the lack of data on inhibition of biological Cr(VI) removal. The objective of this work is to evaluate biological groundwater treatment systems that will achieve high Cr(VI) removal with addition of a low dose of organic substrate and to estimate inhibition of microbial Cr(VI) reduction under anaerobic and anoxic conditions.
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