BIOLOGICAL GROUNDWATER TREATMENT FOR TOTAL AND HEXAVALENT CHROMIUM REMOVAL

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1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 BIOLOGICAL GROUNDWATER TREATMENT FOR TOTAL AND HEXAVALENT CHROMIUM REMOVAL MAMAIS D., NOUTSOPOULOS C., KAVALLARI, I., NYKTARI, E. and KALDIS A. Sanitary Engineering Laboratory, Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, Iroon Polytechniou 9, Zografou , Athens, Greece, EXTENDED ABSTRACT The natural reducing capacity of aquifers to facilitate Cr(VI) reduction and detoxification to Cr(III) is very often exceeded by the amount of Cr(VI) present in contaminated groundwater. In order to tackle the problem of high Cr(VI) concentrations in groundwater a variety of technologies have been developed that mainly incorporate physicochemical processes to achieve these transformations and remove the environmental hazard associated with Cr(VI) presence. These methods often present several disadvantages such as high capital and operational cost, production of chemical sludge, sludge disposal problems, etc. The objective of this work is to develop and evaluate biological groundwater treatment systems that will achieve hexavalent chromium reduction and total chromium removal. In order to evaluate the effect of electron acceptor on the mechanism of biological hexavalent chromium removal from groundwater three lab-scale units operated, as sequential batch reactors (SBRs), one as an anaerobic system, one as an anaerobicaerobic system and one as an anoxic-anaerobic system. All systems received groundwater with a Cr(VI) content of 200 μg/l. In order to support biological growth, groundwater was supplemented with milk to achieve a COD concentration of 200 mg/l. In addition the effectiveness of a sand filtration unit as a polishing step for the treatment of biologically treated groundwater was also evaluated. According to the results of the batch experiments Cr(VI) removal rates are highly dependant on redox potential. Cr(VI) removal can not be achieved in a fully aerated system whereas a fully anaerobic system dosed with an external organic carbon source at an influent COD concentration of 200 mg/l can lead to a practically complete Cr(VI) removal from groundwater. Significant Cr(VI) removal can be achieved in an anaerobic aerobic SBR system as well. However such a system cannot be cost efficient compared to the fully anaerobic system mainly due to the energy requirements for achieving aerobic conditions. More specifically anaerobic biological treatment systems were able to completely reduce 200 μg/l Cr(VI) to trivalent chromium CrIII. Sand filtration of biologicall treated groundwater results in average effluent total and dissolved chromium concentrations in the order of 5μg/l and 3 μg/l, respectively. KEYWORDS: biological groundwater treatment, hexavalent chromium removal, bioremediation 1. INTRODUCTION Several treatment technologies have been developed to remove chromium from water. The most often used methods are physicochemical techniques and most specifically: a) chemical oxidation (Barrera-Diaz et al., 2012; Singh et al., 2012a), b) ion exchange (Rengaraj et al., 2003; Ren et al., 2012), c) adsorption via activated carbon (Gupta et al.,2001; Babael et al., 2003; Singh et al., 2012b) and d) membrane separation (Ghosh et al., 2006;Mungray et al., 2012). Furthermore additional physicochemical methods have also be developed for chromium removal from water such as electrocoagulation

2 (Emamjomech et al., 2009), electrodissolution of iron (Mukhopadhyay et al., 2007) and photocatalytic reduction (Rivero-Huguet et al., 2009; Sun et al., 2009). All these methods present several disadvantages such as high capital and operational cost, production of chemical sludge, sludge disposal problems, etc. Despite the extensive literature regarding the physicochemical methods for Cr(VI) removal from water, there is no literature on the biological treatment of groundwater for Cr(VI) removal. However the microbial reduction of Cr(VI) to Cr(III) has been extensively reported in the literature for the treatment of liquid wastes under both aerobic and anaerobic conditions (Ishibasi et al., 1990; Shen and Wang, 1993; Stasinakis et al., 2004). The main reasons that biological methods have not been implemented for the treatment of groundwater are: the absence of electron donor (organic substrate) to provide for the biochemical reduction of Cr(VI) to Cr(III) from the groundwater, the lack of experience on the operation of biological treatment systems for groundwater purification, the high operational cost due to aeration requirements in the case of aerobic treatment systems and the questionable effectiveness of anaerobic treatment systems at low water temperatures. The objective of this work was to develop and evaluate biological groundwater treatment systems that will achieve hexavalent chromium reduction and total chromium removal. In order to evaluate the effect of electron acceptor on the mechanism of biological hexavalent chromium removal from groundwater, three lab-scale units operated, as sequential batch reactors (SBRs), one as an anaerobic system, one as an anaerobicaerobic system and one as an anoxic-anaerobic system. All systems received groundwater with a Cr(VI) content of 200 μg/l, respectively. In order to support biological growth groundwater was supplemented with milk. Finally the effectiveness of a sand filtration unit as a polishing step for the treatment of biologically treated groundwater was also evaluated. 2. EXPERIMENTAL MATERIALS AND METHODS In order to evaluate the effect of several design parameters on the biological removal of hexavalent chromium three bench scale units were employed. All bench scale units were operated as sequential batch reactors (SBRs), one as an anaerobic system (ANAER), one as an anaerobic-aerobic system (ANAER-AER) and one as an anoxic-anaerobic system (ANOX-AER). All bench scale systems were fed with milk as the sole substrate at a COD concentration in the water of 200 mg/l. The concentration of Cr(VI) in the influent water, was equal to 200 μg/l and was achieved through everyday hexavalent chromium addition in the form of K 2Cr 2O 7 solution. Average mixed liquor temperature of the three bench scale units was equal to 17.5 ο C, while sludge age was equal to 3 days for systems ANAER-AER and ΑΝΟΧ-ΑΕR and 10 days for the pure anaerobic system (ΑΝΑΕR). All experimental systems were inoculated with mesophilic digested sludge from Psyttalia Wastewater Treatment Plant (PWTP). Untreated water supplemented with the substrate, a mixture of nutrients to support microbial growth and hexavalent chromium solution was being fed to the experimental units once a day. The nominal hydraulic residence time of the three systems were equal to 1.5 d for systems ANOX-AER and ANAER-AER and 1.7 d for system ANAER. To establish aerobic conditions, air was provided at a constant rate, whereas anoxic conditions were acquired through the addition of nitrate-nitrogen at an influent nitrate nitrogen concentration of 10 mg/l. In addition to biological treatment, the effectiveness of a sand filtration unit as a polishing step chromium removal was also evaluated. More specifically following biological treatment, effluent treated groundwater was transferred to a sand filter through a peristaltic pump at an average velocity of 6 m/h.

3 The performance of the bench scale units was assessed by routine daily measurements of total and soluble COD, TSS, VSS, ΝΗ 4-Ν, ΝΟ 3-Ν, DO, total and hexavalent chromium throughout the experimental period. Το measure total and hexavalent chromium removal, hourly samples from the three bench scale units were taken and analyzed for soluble COD, ΝΟ 3-Ν, DO, redox, total and hexavalent chromium for a period of 24 hours. All analyses were done in accordance with Standard Methods (APHA, 2005). 3. RESULTS AND DISCUSSION A series of bench scale SBR were operated for a period of 11 months in order to assess the optimal operating conditions for efficient hexavalent chromium removal. Based on the experimental protocol important design parameters which were evaluated were: type of electron donor, i.e. aerobic, anoxic or anaerobic conditions, tertiary treatment In the following paragraphs the results of all the aforementioned parameters are presented and discussed. Type of electron acceptor: Three experimental units were operated as sequential batch reactors (SBRs), one as an anaerobic system (ANAER), one as an anaerobic-aerobic system (ANAER-AER) and one as an anoxic-anaerobic system (ANOX-AER). The objective of the first phase was to evaluate the effect of the electron acceptor on the mechanism of biological hexavalent chromium removal from water. The results from the operation of the bench scale units are summarized in Table 1. As shown in Table 1 both ANAER and ANAER-AER systems that employed anaerobic conditions during their operation exhibited a much higher hexavalent removal efficiency compared to the anoxicaerobic system. Table 1: Results from the operation of the lab-scale units under different electron donor conditions ΑΝΑΕR ANAER-AER ANOX-AER MLSS (mg/l) MLVSS/MLSS (%) CODsol eff (mg/l) TSS eff (mg/l) CODsol rem (%) Cr(VI) eff (μg/l) Cr(VI) rem (%) In order to assess the effect of reductive conditions on hexavalent chromium removal rate, batch experiments were conducted. During the batch experiments hourly samples from the three bench scale units were taken and analyzed for soluble COD, ΝO 3-Ν, DO, redox, total and hexavalent chromium for a period of 24 hours. In addition to the three experimental systems hexavalent chromium removal was also monitored under fully aerobic conditions. The results of the batch experiments are presented in Figures 1, 2 and Table 2.

4 30 (a) C r (VI) (μg /l) C ODs (mg /l) Time (min) C r(v I) C ODs 10 (b) C r(vi) (μg /l) Time (min) Figure 1: Results from the batch experiments for systems: a) AER and b) ANAER (c)

5 C r (VI) (μg /l) R edox (mv) Time (min) C r(v I) R edox (d) 16 12, C r (VI) (μg /l) ,00 6,00 4,00 2,00 NO3-N (mg /l) Time (min) C r(v I) NO3-N Figure 2: Results from the batch experiments for systems: c) ANAER-AER and d) ANOX- AER

6 Table 2: Results of the batch experiments of the first phase of the experiments Experimental system Hexavalent removal rate (μgcr(vi)/mgvss/h) ANAER-AER ANOX-AER ANAER AER (completely aerated) 0 The experimental data illustrate that the operation of a fully anaerobic system, dosed with an external organic carbon source at an influent COD concentration of 200 mg/l can lead to a practically complete hexavalent chromium removal from water. Appreciable hexavalent chromium removal can be achieved in an anaerobic aerobic SBR system as well. However such a system cannot be cost efficient compared to the fully anaerobic system mainly due to the energy requirements for achieving aerobic conditions. Furthermore hexavalent chromium removal rates are highly dependent on redox potential. Therefore hexavalent chromium removal can not be achieved in a fully aerated system, whereas hexavalent chromium removal rates under anoxic conditions are almost half of those at pure anaerobic conditions. Post treatment of biologically treated groundwater: Based on the results of the lab scale systems the efficiency of anaerobic or anaerobic aerobic or anoxic aerobic SBR systems to provide for compete Cr(VI) reduction has been illustrated. However, despite the high Cr(VI) removal efficiency, total chromium effluent concentrations in lab scale systems were not minimal. For example for the anaerobic system average total chromium effluent concentration was equal to 105 μg/l. Based on analyses performed in effluent water samples, trivalent chromium accounts for more than 99% of the total chromium. Trivalent chromium in treated water was mainly present as dissolved chromium (mostly in colloidal form), whereas particulate trivalent chromium constitutes 37% of the total chromium (Figure 3). Cr(VI) 1% Cr (III) colloidal 54% Cr(III) particulate 45% Figure 3: Distribution of chromium forms in biologically treated groundwater

7 Therefore in order to further decrease total chromium concentrations in water at levels well below 100 μg/l further treatment is necessary. In view of the above the effectiveness of an integrated system comprising of an anaerobic SBR followed by a sand filter was evaluated. The objectives of this system was firstly to achieve complete Cr(VI) removal in the SBR system and then to remove Cr(III) in sand filter due to filtration. Based on a cumulative distribution analysis of the concentration of total and dissolved chromium at the effluent of the sand filter it is concluded that sand filtration of biologically treated groundwater results in total and dissolved chromium effluent concentrations in the order of 5μg/l and 3 μg/l, for 90% of the samples. 4. CONCLUSIONS This study investigated the efficiency of biological groundwater treatment for total and hexavalent chromium removal. Based on the results from the operation of bench scale units the following conclusions can be drawn: Biological treatment systems in the form of SBRs are suitable for complete microbial Cr(VI) reduction in groundwater provided that sufficiently reductive conditions are established A pure anaerobic activated sludge system provides for higher Cr(VI) removal rates compared to an anaerobic aerobic or anoxic aerobic system. An anaerobic SBR reactor operating at a sludge age of 10 d and a hydraulic residence time of less than 1 d provides for complete hexavalent chromium reduction to trivalent chromium for a groundwater with hexavalent chromium content of 200 μg/l. Anaerobic systems can lead to an appreciable chromium removal even for waters with a high NO 3-N content. Sand filtration of biologically treated groundwater can achieve total chromium effluent concentrations in the order of 5 μg/l. REFERENCES 1. American Public Health Association (2005), Standard Methods for Examination of Waters and Wastewaters, 21 st Ed., Washington D.C. 2. Babel S., Kurniawan T.A., (2003). Low cost adsorbents for heavy metals uptake from contaminated water: a review, J. Hazard. Mater. B 97, Barrera-Diaz C.E., Lugo-Lugo V., Bilyeu B. (2012). A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction. Journal of Hazardous Materials, , Emamjomech M.M., Sivakumar M., (2009). Review of pollutants removed by electrocoagulation and electrocoagulation/flotation processes, J. Environ. Manage. 90, Ghosh G., Bhattacharya P.K., (2006). Hexavalent chromium ion removal through micellar enhanced ultrafiltration, Chem. Eng. J. 119 (1), Gupta V.K., Gupta M., Sharma S., (2001). Process development for the removal of lead and chromium from aqueous solutions using red mud an aluminium industry waste, Water Res. 35 (5), Ishibashi Y. Cervantes, C., Silver, S., (1990) Chromium Reduction in Pseudomonas putida. Appl. Environ. Microbiol. 56, Mukhopadhyay B., Sundquist J., Schmitz R.J., (2007). Removal of Cr(VI) from Crcontaminated groundwater through electrochemical addition of Fe (II), J. Environ. Manage. 82,

8 9. Mungray A.A.,Murthy Z.V.P. (2012). Comparative performance study of four nanofiltration membranes in the separation of mercury and chromium. Ionics, 18 (8), Ren J., Li N., Zhao L. (2012). Adsorptive removal of Cr(VI) from water by anion exchanger based nanosized ferric oxyhydroxide hybrid adsorbent. Chemical and Biochemical Engineering Quarterly, 26 (2), Rengaraj S., Joo C.K., Kim Y., Yi J., (2003). Kinetics of removal of chromium from water and electronic process wastewater by ion exchange resins:1200h, 1500H and IRN97H, J. Hazard. Mater. 102 (2/3), Rivero-Huguet M., Marshall W.D.,(2009). Influence of various organicmolecules on the reductionofhexavalent chromiummediated by zero-valentiron, Chemosphere 76, Singh R., Misra V., Singh R.P. (2012a). Removal of hexavalent chromium from contaminated groundwater using zero-valent iron nanoparticles. Environmental Monitoring and Assessment, 184 (6), Singh S.R., Singh A.P. (2012b). Treatment of water containing chromium (VI) using rice husk carbon as a newlow cost adsorbent. International Journal of Environmental Research, 6 (4), Shen, H., Wang, Y.T. (1993). Characterization of enzymatic reduction of hexavalent chromium by Escheria coli, Appl. Environ. Microbiol., 59, Stasinakis A.S., Thomaidis N.S., Mamais D., Lekkas T.D., (2004) Investigation of Cr(VI) reduction in continuous-flow activated sludge systems, Chemosphere 57, Sun J., Maob J.-D., Gonga H., Lan Y., (2009). Fe(III) photocatalytic reduction of Cr(VI) by low-molecular-weight organic acids with OH, J. Hazard. Mater. 168,