PROCESS INTEGRATION IN BIOREMEDIATION OF MERCURY CONTAMINATED INDUSTRIAL WASTEWATER. Paweł Głuszcz1, Stanisław Ledakowicz1, Irene Wagner-Doebler2 1
Lodz Technical University, Bioprocess Engineering Department, Wolczanska 213, 90-924 Lodz, Poland, e-mail:
[email protected] 2
Helmholtz Zentrum fuer Infektionsforschung, HZI, Braunschweig, Germany
Abstract: The goal of this study was to improve the technology for bioremediation of mercury contaminated industrial wastewater, based on the enzymatic reduction of highly toxic Hg(II) to water-insoluble and relatively non-toxic Hg(0) by mercury resistant bacteria immobilized on porous carrier material in a fixed-bed bioreactor, originally developed at HZI, Germany, by using an activated carbon as a carrier for microorganisms and – at the same time – as an adsorbent for mercury. Such integration of the process should increase the technology efficiency. The experimental comparison of the performance of the original and the modified bioreactor was performed in laboratory scale and then the integrated process was applied in industrial scale in one of the Polish chemical companies. From the experimental results it may be concluded that the sorption/bioreduction processes integration enables improvement of the bioremediation technology.
Keywords: wastewater treatment, mercury bioreduction, bioremediation, activated carbon, process integration
1.
INTRODUCTION
Like in the case of all elements, the same amount of mercury has existed on the planet since the Earth was formed. However, the amount of mercury mobilized and released into the environment has increased since the beginning of the industrial age. Because of extensive use of mercury many areas in the world are contaminated by this element and/or its compounds, posing serious environmental problems. Mercury introduced to the natural environment, regardless of the form, can be relatively easily converted into highly toxic volatile forms, i.e. methyl- or ethylmercury chloride, far more bio-available and much more toxic than other forms of mercury. Furthermore, mercury is highly retained in living organisms, and therefore becomes biomagnified, mainly through aquatic food chains. (Boening, 2000). Today, the major sources of mercury emissions to the environment include the burning of coal to produce electricity, the incineration of waste and the chlor-alkali technology. The chlor-alkali industry produces chlorine and alkali, sodium hydroxide or potassium hydroxide, by electrolysis of brine. The main technologies applied for chlorine and alkali production are mercury-, diaphragm- and membrane-cell electrolysis. The mercury cell process has been in use, mainly in Europe, since 1892 and today still accounts for ca. 50% of total production of chlorine in Europe. In the EU and EFTA there are presently about 50 operating mercury-cell chlor-alkali plants, with a chlorine production capacity of over 5.8 million tons per year. In line with the commitment of Euro-Chlor members, these plants will be decommissioned or converted to an alternative mercury-free process by 2020, as will a number of mercury-cell chlor-alkali plants in the US and other countries. Considering only the European mercury-cell plants, this decommissioning activity will release ca. 12 000 tons of process mercury, and even if mercury-cell processes are phased out in the European Community they will be still operating in Eastern Europe and developing countries worldwide. Due to the amalgam chlor-alkali technology characteristics, mercury can be emitted to the environment through air, water, solid wastes and in the products. It must be underlined that mercury is a global pollutant due to atmospheric
transport throughout the world and accumulation in the food chain. Therefore removal of mercury from industrial emissions in all possible places is mandatory and should take into account the latest achievements in science and technology. Several chemical processes have been utilized for the removal of mercury from mercury contaminated chlor-alkali industrial wastewaters, but it is difficult to apply chemical processes for the purification of water containing low Hg concentrations, because they require great amounts of chemicals and can lead to a secondary pollution. Common treatment techniques for mercury removal from polluted wastewater of low mercury concentration are mostly based on the sorption onto different materials such as activated carbon or ion exchange resins. Although there are many advantages of an ion-exchange technique, such as high efficiency, selectivity, and insensitivity to concentration variability, high cost of using ion exchange resins is the main problem. For remediation of industrial wastewaters contaminated by mercury a unique biotechnological method based on the enzymatic reduction of Hg(II) compounds to water-insoluble Hg(0) by live mercury resistant bacteria has been developed in Helmholtz Centre for Infection Research, HZI, former German Research Centre for Biotechnology (GBF). (von Canstein et al., 2000). The microorganisms used in the process are natural, non-pathogenic soil bacteria (e.g. Pseudomonas), which possess a natural mercury resistance. They convert reactive ionic mercury to elemental mercury which remains in the packed bed of the bioreactor as water-insoluble metal and is no longer toxic for the bacteria. NADPH2 is the biologically active electron donor within the cell, which is provided by normal metabolism of the bacteria. The stoichiometric equation of this conversion is as follows:
Hg(II) NADPH 2
( merA) GP
Hg(0) NADPH 2H
(1)
The secreted metallic mercury which accumulates in form of small droplets (d < 12 µm) is retained in the packedbed bioreactor. This novel bioremediation technology was implemented in the chlor-alkali industry in a pilot-plant scale. (WagnerDoebler, 2003). The core of the technological process developed in HZI was a packed-bed bioreactor with pumice stones as a carrier material for microorganisms and an activated carbon adsorber used as a polishing filter after the bioreactor. The pilot plant installation was tested for 8 months at a German chlor-alkali plant and then operated at a Czech chlor-alkali electrolysis factory for more than two years. (von Canstein et al., 2001) It revealed high efficiency and stability against fluctuations of inflow parameters, inherent to the production process. The experience gained during operation of the installation led to the idea, that the process of bioremediation may be integrated with the adsorption of mercury from wastewater in a single bioreactor, by immobilization of the bacteria directly onto the activated carbon bed. Such an integration should increase efficiency of the technology and lower its costs. For this it was necessary to define several significant parameters of the activated carbon bed used in the bioreactor and of the adsorption process in the flow-through apparatus itself. These problems were solved in different investigations (Ledakowicz et. al., 1995; Głuszcz et. al., 2005; Głuszcz et. al., 2007). The goal of the presented study was to integrate the processes of bioreduction with adsorption of the ionic and metallic mercury by combining the bioreactor with immobilized microorganisms and an activated carbon filter in one piece of apparatus and to investigate the efficiency of such modified method in laboratory and industrial scale.
2. EXPERIMENTAL
2.1 Apparatus. In the laboratory experiments four glass bioreactors (columns) of internal diameter of 10 mm and height of 50 mm were used. The columns were filled with a carrier material and sterilized by autoclaving. Sterile, neutralized, oxygensaturated and Hg containing model wastewater was enriched with nutrient medium and pumped into the bioreactors using a peristaltic pump. The total flow rate through the column was changed in the range of 0.5 – 2 bed volume/hour. All processes with microorganisms were operated at room temperature.
2.2 Strain. The strain used in experiments, Pseudomonas putida Spi3, was originally isolated from sediments of the Spittelwasser River, a tributary of the Elbe River (Germany). The Spittelwasser River was subjected to massive industrial pollution, including pollution with inorganic and organic mercury compounds, up to 1989. The strain was identified by the German Culture Collection of Microorganisms and Cell Cultures, DSMZ, as P. putida strain on the basis of 16S ribosomal DNA sequencing data, physiological tests and ribotyping analysis. (Wagner-Doebler et. al., 2000).
2.3 Carrier materials for microorganisms Two different porous materials were used as a fixed-bed in the experimental bioreactors: - pumice stones (Raab GmbH, Luckenau, Germany); grain diameter 1 to 3 mm (as a reference material which was used in previous investigations), - activated carbon DG, (Carbotech Aktivkohlen GmbH, Germany) as a carrier and adsorbent in the modified bioreactor. Activated carbon used was selected as the most suitable for the technology of mercury bioremediation out of eight different types of activated carbon, in other investigations (Głuszcz et. al., 2004). Total mercury was determined by flameless cold vapor absorption spectroscopy using a flow injection system (FIAS 200, Perkin-Elmer, Uberlingen, Germany) that was linked to the atomic absorption spectrophotometer (AAS 2100, Perkin-Elmer).
3. RESULTS
3.1 Mercury bioreduction efficiency in laboratory bioreactors with different carrier materials. In the described experiments the optimized enriched yeast extract medium was applied for feeding the microorganisms. Being the only variable parameter the mercury concentration was changed every 100 hours of the process, covering the range from 1.5 to 18 mg L-1 of Hg2+. In all the columns used (4 columns at the same time, parallel feeding with the same solution) other operating parameters were identical for 5 weeks. Comparison of the performance of bioreactors with different carrier/adsorbent materials is presented in Fig. 1. It shows that up to the inlet Hg concentration of 8 mg L-1 both bioreactors, with pumice and with activated carbon, give very similar results. After a step change of Hg concentration the efficiency of bioremediation process decreased for some time (there was a transient increase of the outflow Hg concentration visible). Even then the bioreactor with activated carbon worked better: its reaction was lower and lasted shorter time in comparison to the bioreactor with pumice stones. And when inlet concentration of mercury was risen up to 18 mg/dm3 the breakthrough of mercury occurred in pumice-stones reactor whilest only small, non-significant and short-lasting increase of Hg outlet concentration was observed in the modified bioreactor. From the experiments in the laboratory scale the following results were obtained: - the removal efficiency of ionic mercury in the column filled with pumice stones was about 90 - 95% and the outflow concentration varied between 100 and 300 µg L-1; the mercury breakthrough in the outflow stream from this column was observed for inlet Hg concentrations higher than 10 mg L-1; - the bioreactor with microorganisms immobilized on activated carbon showed higher mercury bioreduction efficiency; the Hg concentration in the outflow stream decreased to 50 - 70% as compared to pumice-stones-bed bioreactor, in the range of inlet Hg concentrations up to 10 mg L-1; - in the case of shock loading (sudden increase of the mercury concentration in the inflow from 10 to 18 mg L-1), the outflow concentration of mercury from the modified bioreactor did not increase significantly and remained below 0.5 mg L-1; - the integrated bioreactor with activated carbon bed has not only significantly higher bioremediation efficiency than the non-modified one but is more robust and flexible if fluctuations of Hg concentration occur. In
such a case activated carbon plays a role of a buffer repository for the excess of mercury, which may be treated by microorganisms in later time when the concentration in solution goes down again.
20 18 16
C / mg dm
-3
14 12 10 8 6
2
1
4
3
2 0 0
200
400
600
800
1000
t/h
Fig. 1. Comparison of mercury removal efficiency (outlet Hg concentration, C) in the non-modified and the integrated bioreactors in the range of high inlet Hg concentration; 1 – changes of the inlet Hg2+ concentration, 2 – non-modified bioreactor with pumice stones, 3 – integrated bioreactor with activated carbon
3.2 Bioremediation of real wastewaters in the industrial-scale integrated bioreactor. The modified industrial-scale wastewater treatment plant, based on the BIOMER installation described elsewhere (Wagner-Doebler, 2003) was applied in one of the Polish chemical companies in Tarnów, Poland. The plant essentially consists of a 1 m³ fix-bed bioreactor filled with selected activated carbon used as carrier material for the microorganisms. Due to the wastewater conditions in the Tarnow factory a pre-treatment of the mercury contaminated wastewater from chlor-alkali electrolysis have been applied, consisting of pH adjustment, oxidation of total mercury to Hg(II), removal of chlorine and sand filtration. The main task of the pre-treatment unit is to oxidise the total mercury in the wastewater (consisting of particle bound and complexes of Hg) to Hg(II). In addition, the remaining chlorine in the wastewater must be reduced to the level below 1 mg L-1. To obtain this the raw wastewater pH is adjusted in a first step to pH 3. In a second step the mercury is oxidised by adding sodium hypochlorite (NaOCl, 100 mg chlorine L-1). The excess of chlorine is removed by adding sodium hydrogen sulphite (NaHSO3) controlled by a redox potential of 40 mV. The mercury free solid fines are removed by filtration. After such a pre-treatment mercury contaminated wastewater can be cleaned by microorganisms with a high efficiency. In the reported time of 10 month the installation was operated continuously with a raw wastewater flow ranging from 1 to 2 m³ h-1. This was efficient enough to clean the whole mercury contaminated wastewater of the chlor-alkali plant. The wastewater treatment installation was able to deal with fluctuations of the mercury concentration in the range of 1 – 8 mg L-1 and a stable operation could be guaranteed also at high salt loads. The efficiency of the mercury removal in an integrated bioreactor with activated-carbon bed was 96-99%, i.e. higher than in the nonmodified installation ( von Canstein et al., 2001). The lower mercury outflow concentration (130 – 250 mg L-1) was reached without using an activated carbon filter after the bioreactor. As it was observed in the laboratory scale the
integrated bioreactor was able to deal with higher fluctuations, working stable even if the mercury concentration increased rapidly over 10 mg L-1 , e.g. due to the disturbances in the electrolysis hall. 4. FINAL REMARKS AND CONCLUSIONS In the presented study living mercury resistant bacteria were used to catalyze the reduction of ionic mercury to water insoluble metallic mercury in order to remove toxic form of mercury from the solution. To increase the efficiency of this process an activated carbon was used as a support for microorganisms and, at the same time, as an adsorbent for ionic mercury present in wastewaters and metallic (elemental) mercury formed during the process of bioreduction. The most of the reduced mercury was mechanically captured in the form of mercury droplets gathering in the bioreactor and the rest was adsorbed by activated carbon. One of the main advantages of the integrated system is higher mercury removal efficiency in comparison to originally developed technique. The laboratory-scale bioreactors with both investigated carrier materials, pumice stones and activated carbon, were able to treat different mercury concentrations. Pumice-bed bioreactor worked with high mercury retention efficiency (94 - 98%) if the inflow mercury concentration was up to 8 mg L-1. However, the toxicity of the stream of higher mercury concentration (above 10 mg L-1) resulted in reduced activity of the microbial community and increased mercury outflow concentration, finally leading to the mercury breakthrough. The bioreactor with activated carbon bed assured higher mercury reduction efficiency (97-99%) in a wider range of inflow concentration, up to 18 mg L-1. This bioreactor worked more steadily and responded moderately to the changes of mercury concentration in a wider range. The results from ten-month operation of the industrial-scale integrated bioreactor show that this process was scaled up from laboratory tests to an industrial plant without serious engineering problems and the integrated bioreactor was also very efficient in bioremediation of wastewaters in real industrial conditions. Hence, finally it may be concluded that integration of the bioremediation and sorption processes in one piece of apparatus by using the activated carbon as a carrier for biocatalyst and at the same time as an adsorbent for different forms of mercury leads to higher efficiency of the technology and to higher robustness and flexibility of the biocatalyst in variable industrial conditions.
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