Modelling of biological Cr(VI) removal in draw-fill reactors using microorganisms in suspended and attached growth systems (original) (raw)
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Simultaneous phenol removal and biological reduction of hexavalent chromium in a packed-bed reactor
Journal of Chemical Technology and Biotechnology, 2008
BACKGROUND: Phenol and hexavalent chromium are considered industrial pollutants that pose severe threats to human health and the environment. The two pollutants can be found together in aquatic environments originating from mixed discharges of many industrial processes, or from a single industry discharge. The main objective of this work was to study the feasibility of using phenol as an electron donor for Cr(VI) reduction, thus achieving the simultaneous biological removal/reduction of the two pollutants in a packed-bed reactor.RESULTS: A pilot-scale packed-bed reactor was used to estimate phenol removal with simultaneous Cr(VI) reduction through biological mechanisms, using a new mixed bacterial culture originated from Cr(VI)-reducing and phenol-degrading bacteria, operated in draw–fill mode with recirculation. Experiments were performed for feed Cr(VI) concentration of about 5.5 mg L−1, while phenol concentration ranged from 350 to 1500 mg L−1. The maximum reduction/removal rates achieved were 0.062 g Cr(VI) L−1 d−1 and 3.574 g phenol L−1 d−1, for a phenol concentration of 500 mg L−1.CONCLUSION: Phenol removal with simultaneous biological Cr(VI) reduction is feasible in a packed-bed attached growth bioreactor. Phenol was found to inhibit Cr(VI) reduction, while phenol removal was rather unaffected by Cr(VI) concentration increase. However, the recorded removal rates of phenol and Cr(VI) were found to be much lower than those obtained from previous research, where the two pollutants were examined separately. Copyright © 2008 Society of Chemical Industry
Chromium species behaviour in the activated sludge process
Chemosphere, 2003
The purpose of this research was to compare trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) removal by activated sludge and to investigate whether Cr(VI) reduction and/or Cr(III) oxidation occurs in a wastewater treatment system. Chromium removal by sludge harvested from sequencing batch reactors, determined by a series of batch experiments, generally followed a Freundlich isotherm model. Almost 90% of Cr(III) was adsorbed on the suspended solids while the rest was precipitated at pH 7.0. On the contrary, removal of Cr(VI) was minor and did not exceed 15% in all experiments under the same conditions. Increase of sludge age reduces Cr(III) removal, possibly because of Cr(III) sorption on slime polymers. Moreover, the decrease of suspended solids concentration and the acclimatization of biomass to Cr(VI) reduced the removal efficiency of Cr(III). Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible and is affected mainly by the initial concentration of organic substrate, which acts as electron donor for Cr(VI) reduction. Initial organic substrate concentration equal to or higher than 1000 mg l À1 chemical oxygen demand permitted the nearly complete reduction of 5 mg l À1 Cr(VI) in a 24-h batch experiment. Moreover, higher Cr(VI) reduction rates were obtained with higher Cr(VI) initial concentrations, expressed in mg Cr(VI) g À1 VSS, while decrease of suspended solids concentration enhanced the specific Cr(VI) reduction rate.
Clean-soil Air Water, 2009
Soil samples extracted from a contaminated landfill with hexavalent chromium, Cr(VI), were treated in a packed-bed column reactor applying different reductant solutions under controlled flow conditions. In all cases, the kinetic study concerning removal of Cr(VI) from packed soil revealed the overall process (leaching and redox reaction) is described by pseudo-first order kinetics. The complexity of the process taking place in non-isotropic medium (packed soil) was evidenced by dependence of the overall pseudo-first order kinetic rate constant, k*, on treatment time, volumetric flow rate, and composition and concentration of the reductant solution. A phenomenological equation was proposed in order to represent k* in non-ideal conditions. The comparative study concerning the use of different reductants revealed application of ferrous sulphate, Fe(II), under flow conditions furnished the best results, since in this case the redox process reached a considerable extent and the reduced chromium, Cr(III), was immobilized in packed soil as an insoluble mixed compound.
Water Air and Soil Pollution, 2009
Chemical reduction of the hexavalent chromium, Cr(VI), present in contaminated soil and groundwater was carried out in a slurry reactor under dynamic conditions (120 rpm and 25°C) using different reductants [ferrous sulphate (Fe(II))], sodium bisulphite, sucrose, ascorbic acid and zerovalent iron (ZVI)] in order to evaluate the influence of the reductant on the redox process. Chemical analysis of the contaminated soil revealed a Cr(VI) concentration of 528 ± 31 mg kg−1. Batch studies under dynamic conditions (slurry reactor) using different [Cr(VI)]/[reductant] molar ratios revealed that only Fe(II) and ZVI species can promote both reduction of Cr(VI) and immobilisation of Cr(III) (formation of an insoluble hydroxide compound). It was verified that 1.0 g of ZVI is capable of converting 104 ± 5 mg of Cr(VI) in Cr(III). A kinetic redox study was carried out using ZVI in different conditions. In all cases, it was verified that Cr(VI) reduction follows a pseudo-first-order kinetic behaviour. The dependence of the pseudo-first-order kinetic rate constant, k obs, on [ZVI] indicates that the redox process taking place in the slurry reactor is rather complex. A phenomenological kinetic equation for the redox process taking place in the slurry reactor was presented in order to describe the behaviour of k obs under non-ideal conditions.
Reduction of Cr(6 + ) to Cr(3 + ) in a Packed‱Bed Bioreactor
Applied Biochemistry and Biotechnology, 1997
Hexavalent chromium, Cr(6+), is a common and toxic pollutant in soils and waters. Reduction of the mobile Cr(6+) to the less mobile and less toxic trivalent chromium, Cr(3+), can be achieved with conventional chemical reduction technologies. Alternatively, Cr(6+) can be biochemically reduced to Cr(3+) by anaerobic microbial consortia which appear to use Cr(6+) as a terminal electron acceptor. A bioprocess for Cr(6+) reduction has been demonstrated using a packed-bed bioreactor containing ceramic packing, and then compared to a similar bioreactor containing DuPont Bio-Sep beads. An increase in volumetric productivity (from 4 mg Cr(6+)/L/h to 260 mg Cr(6+)/L/h, probably due to an increase in biomass density, was obtained using Bio-Sep beads. The beads contain internal macropores which were shown by scanning electron microscopy to house dense concentrations of bacteria. Comparisons to conventional Cr(6+) treatment technologies indicate that a bioprocess has several economic and operational advantages.
Effects of combining biological treatment and activated carbon on hexavalent chromium reduction
Bioresource Technology, 2011
The objectives of the present work were: (a) to analyze the Cr(VI) removal by combining activated sludge (AS) with powdered activated carbon (PAC), (b) to analyze the effect of PAC and Cr(VI) on the growth kinetics of activated sludge, and (c) to determine if the combined method (AS-PAC) for Cr(VI) removal can be considered additive or synergistic with respect to the individual processes. Chromate removal was improved by increasing PAC concentrations in both PAC and AS-PAC systems. Cr(VI) removal using the AS-PAC system was higher than using AS or PAC. The increase of Cr(VI) caused longer lag phase and lower observed specific growth rate (l obs), biomass yield (Y X/S), and specific growth substrate consumption rate (q S) of activated sludge; additionally, PAC did not enhance the growth kinetic parameters (l obs , Y X/S , q S). Cr(VI) reduction in AS-PAC system was the result of the additive effect of each individual Cr(VI) removal process.
Hexavalent chromium reduction in a sulfur reducing packed-bed bioreactor
Journal of Hazardous Materials, 2012
The most commonly used approach for the detoxification of hazardous industrial effluents and wastewaters containing Cr(VI) is its reduction to the much less toxic and immobile form of Cr(III). This study investigates the cleanup of Cr(VI) containing wastewaters using elemental sulfur as electron acceptor, for the production of hydrogen sulfide that induces Cr(VI) reduction. An elemental sulfur reducing packed-bed bioreactor was operated at 28-30 • C for more than 250 days under varying influent Cr(VI) concentrations (5.0-50.0 mg/L) and hydraulic retention times (HRTs, 0.36-1.0 day). Ethanol or acetate (1000 mg/L COD) was used as carbon source and electron donor. The degree of COD oxidation varied between 30% and 85%, depending on the operating conditions and the type of organic carbon source. The oxidation of organic matter was coupled with the production of hydrogen sulfide, which reached a maximum concentration of 750 mg/L. The biologically produced hydrogen sulfide reduced Cr(VI) chemically to Cr(III) that precipitated in the reactor. Reduction of Cr(VI) and removal efficiency of total chromium always exceeded 97% and 85%, respectively, implying that the reduced chromium was retained in the bioreactor. This study showed that sulfur can be used as an electron acceptor to produce hydrogen sulfide that induces efficient reduction and immobilization of Cr(VI), thus enabling decontamination of Cr(VI) polluted wastewaters.
Biological Reduction of Hexavalent Chromium—A Field Study
Water Environment Research, 2005
Enterobacter sp. HT1, Cr (VI) resistant bacterial strain was isolated from the wastewater sample of the tannery in Mongolia. Batch experiments on hexavalent chromium removal was carried out at 10, 20, and 30 mg/L of Cr (VI) added as potassium dichromate (K 2 Cr 2 O 7), at pH 7 and temperature of 30 °C using pure culture of Enterobacter sp. HT1 as inoculum. The isolated HT1 is capable of reduction nearly 100% of Cr (VI) resulting in the decrease of Cr (VI) from 10 to 0.2 mg/L within 20 hours. When the concentration of Cr (VI) increased to 20 and 30mg/L, almost complete reduction of Cr (VI) could achieve after 72 and 96 hours, respectively.
Investigation of Cr(VI) reduction in continuous-flow activated sludge systems
Chemosphere, 2004
The aim of this research was to investigate hexavalent chromium, Cr(VI), reduction by activated sludge and to evaluate the use of continuous-flow activated sludge systems for the treatment of Cr(VI)-containing wastewater. Three series of experiments were conducted using two parallel lab-scale activated sludge systems. During the first experiment, one system was used as a control, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5 mg l À1 . For all concentrations added, approximately 40% of the added Cr(VI) was removed during the activated sludge process. Determination of chromium species in the dissolved and particulate phase revealed that the removed Cr(VI) was sorbed by the activated sludge flocs mainly as trivalent chromium, Cr(III), while the residual chromium in the dissolved phase was mainly detected as Cr(VI). Activated sludge ability to reduce Cr(VI) was independent of the acclimatization of biomass to Cr(VI) and it was not affected by the toxic effect of Cr(VI) on autotrophic and heterotrophic microorganisms. During the second experiment, both systems were operated under two different hydraulic residence time (h equal to 20 and 28 h) and three different initial organic substrate concentration (COD equal to 300, 150 and 0 mg l À1 ). Cr(VI) reduction was favored by an increase of h, while it was limited by influent COD concentration. Finally, at the last experiment the effect of anoxic and anaerobic reactors on Cr(VI) reduction was investigated. It was observed that the use of an anoxic zone or an anaerobic-anoxic zone ahead of the aerobic reactor favored Cr(VI) reduction, increasing mean percentage Cr(VI) reduction to almost 80%.
Experiments on the removal of chromium (VI) from its aqueous solution in batch reactors
Journal of Powder Technology and Advanced Functional Materials
The adsorption of chromium (VI) onto activated carbon experimented in a batch reactor under two different conditions, namely, initial metal ion concentration and adsorbent dosages. For the five different initial metal ion concentrations such as 500, 600, 800, 900, 1000 mg/L, the steady-state values of chromium removal efficiency were 64, 92, 83, 71 and 66 %, respectively, using 5 grams of activated carbon under shaking at the end of 8th hour. The equilibrium of the process was found to fit into the two well-known adsorption models, Freundlich and Langmuir. It was also observed that the experimental kinetic data followed the first order rate expression.