Abstract
The impact of chemical speciation on heavy metals toxicity and behavior during the activated sludge process was investigated in this study. For the first purpose, experiments were conducted in the presence of the most significant chemical species of arsenic (As), mercury (Hg), tin (Sn) and in the presence of hexavalent chromium (Cr(VI)). For the second purpose, experiments were conducted in the presence of Cr(III) and Cr(VI). Evaluation of As, Fig and Sn chemical species toxicity on the respiration rate of activated sludge heterotrophic microorganisms showed that As(III) and CH₃Hg were much more toxic to activated sludge than As(V) and Hg(II), respectively. Moreover, TBT (tributyl-Sn), DBT (dibutyl-Sn) and TPhT (triphenyl-Sn) were more toxic than MBT (monobutyl-Sn). An increase of sludge age or the concentration of suspended solids reduces the observed inhibition. Determination of maximum specific growth rate, μm and biomass yield, YH, of heterotrophic biomass showed that Cr(VI) concen ...
The impact of chemical speciation on heavy metals toxicity and behavior during the activated sludge process was investigated in this study. For the first purpose, experiments were conducted in the presence of the most significant chemical species of arsenic (As), mercury (Hg), tin (Sn) and in the presence of hexavalent chromium (Cr(VI)). For the second purpose, experiments were conducted in the presence of Cr(III) and Cr(VI). Evaluation of As, Fig and Sn chemical species toxicity on the respiration rate of activated sludge heterotrophic microorganisms showed that As(III) and CH₃Hg were much more toxic to activated sludge than As(V) and Hg(II), respectively. Moreover, TBT (tributyl-Sn), DBT (dibutyl-Sn) and TPhT (triphenyl-Sn) were more toxic than MBT (monobutyl-Sn). An increase of sludge age or the concentration of suspended solids reduces the observed inhibition. Determination of maximum specific growth rate, μm and biomass yield, YH, of heterotrophic biomass showed that 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). Determination of in batch reactors with high initial substrate to biomass ratio (S₀/X₀) favor the bacterial species growing faster. Therefore kinetic constants determined at high S₀/Χ₀ reflect the characteristics of the culture developed at the end of the batch assay and less the original culture. On the other hand, at low S₀/X₀ ratio, storage phenomena prevail and the obtained kinetic constants are more representative of the original culture. Oxygen uptake rate measurements provide information on oxygen consumption and substrate oxidation by bacteria but does not always determine how the cell utilizes this energy. Under certain conditions (high S₀/X₀ and low θc), significant uncoupling between catabolism and anabolism may occur during a batch assay and therefore oxygen consumption may not be directly correlated to bacterial growth rate μm. The use of continuous flow activated sludge plants showed that Cr(VI) concentrations of 0.5 mg l⁻¹ caused significant inhibition of the nitrification process. On the contrary, the effect of Cr(VI) on organic substrate removal was minor for concentrations up to 5 mg l⁻¹, indicating that heterotrophic microorganisms are less sensitive to Cr(VI) than nitrifiers. Concentrations equal to or higher than 1.0 mg l⁻¹ Cr(VI) reduced gradually the presence of rotifers and protozoa and the abundance of filamentous microorganisms, causing the appearance of pin-point floes and free-dispersed bacteria. To investigate the effect of chromium speciation on its behaviour in activated sludge systems, at first an analytical procedure was developed for the determination of chromium species on the dissolved and particulate phase of the wastewater. Then, experiments were realised in batch reactors and continuous-flow activated sludge plants. According to these experiments, almost 90% of Cr(III) was adsorbed on the suspended solids, while the rest was precipitated. On the contrary, removal of Cr(VI) was minor and did not exceed 15%. Increase of sludge age and decrease of suspended solids concentration reduced Cr(III) removal, while Cr(VI) removal was not affected. Cr(III) can not be oxidised to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible. The reduction of Cr(VI) to Cr(III) reduces the toxicity of wastewater, because Cr(VI) is very toxic, carcinogenic and mutagenic, while Cr(III) presents much lower toxicity. Moreover, it was observed that, the reduced part of Cr(VI) was adsorbed as Cr(III) on the suspended solids, while the residual Cr(VI) remained mainly in the dissolved phase. Cr(VI) reduction was favored by the presence of organic substrate, the increase of hydraulic retention time in the aerobic reactor and the presence of anoxic tank.
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