PROJECT DESCRIPTION
BACKGROUND
Mercury compounds are among the most toxic substances in the natural environment. Unlike other metals, which are required in tiny amounts by living organisms and only become toxic at higher doses, there is no concentration of mercury which is harmless. Mercury ions bind to the sulfhydryl groups which are present in all enzymes and proteins, and thereby inactivate them, resulting in complete disruption of cell metabolism. In the European Union, the discharge limit for industrial waste water is 50 µg/l. For drinking water, less than I µg/l are only allowed. The most important sources for mercury contaminated waste water are chlorine-alkali-electrolysis processes and the production of organomercurials, which have been used in the past as pesticides, e.g. for staining seeds, paints etc. Additionally, mercury is used in rather large quantities for the industrial mass production of catalysts or as an amendment (amalgam, mercury vapour lamps). Production and use of organomercurials have been stopped within the European Union because of their toxicity. Large areas of highly contaminated soils and sediments are remains of production processes. Moreover considerable mercury contamination is found in soils where organomercurials were stored, refilled or applied in large volumes. Chlorine-alkali-electrolysis on the basis of the amalgam process is still being used in large scale because of the purity of resulting sodium hydroxide. There are presently no suitable clean-up technologies available for mercury contaminated soils or sediments. Efforts to deal with polluted sites are directed towards the mechanical removal of contaminated material and its deposition elsewhere. Such processes are costly and often result in remobilization of toxic mercury compounds during the dredging process. The state of the art technology for cleaning mercury containing electrolysis wastewater are regenerable ion exchange resins, which are however very sensitive and expensive. Cleanup technologies are urgently needed which are capable of treating large amounts of soil, water or sediment contaminated with relatively low mercury amounts in a cost-effective way.
OBJECTIVES
The overall aim of the project was to demonstrate in pilot scale for the first time an efficient, cost-effective, environmentally friendly and sustainable microbiological process for removing mercury compounds from waste water. On this basis, marketable plants would be developed which are capable of treating mercury containing waste water (from chlorine-alkali-electrolysis processes, waste deposits, catalyst production etc.). This was to be demonstrated in a plant of pilot scale (up to 1m3) for mercury containing waste streams at a German electrolysis factory, ECI Elektrochemie Ibbenbüren. The demonstration plant was to consist of a mobile container housing fixed bed reactors, equipped with the necessary measurement and control devices as well as periphal instruments. The plant was to be constructed and built by Preussag Wasser und Rohrtechnik GmbH and operated together with the project partners German Research Centre for Biotechnology (GBF) and ECI. Based on conservative calculations from laboratory results it was expected that a fixed bed reactor of 1m3 volume with growing and mercury transforming biomass would decontamine 20.000 m3 of waste water with an average concentration of 2 ppm mercury (representing a mercury concentration factor of 20.000). The plant was to be operated in a joint effort between Preussag, GBF and ECI, with GBF being responsible for the analytical and microbiological monitoring of the plant under varying process conditions directly on site. To this end, a Mobile Laboratory available at the GBF was to be used. To exclude the formation of mercury compounds of higher toxicity (e.g. methylmercury) ESWE (Institut für Wasserforschung und Wassertechnologie GmbH) was to perform a detailed analysis of the mercury speciation process.
RESULTS
A new technology was demonstrated for the clean up of mercury contaminated wastewater at pilot scale, based on the ability of natural bacteria to detoxify mercury compounds. Mercury is retained with high efficiency as metallic mercury within a bioreactor because of an enzymatic reduction carried out by micro-organisms. An automated pilot plant was developed in cooperation with Preussag Wassertechnik which was able to clean 4 m3 wastewater per hour (50% of the wastewater at the German electrolysis factory ECI Elektrochemie Ibbenbüren). It was operated continuously for 8 months at the factory. The biological mercury removal process was extremely robust against fluctuations in inflow parameters and various stresses. The pilot plant removed 98% of inflow mercury over the testing period. The cleaned water had less than 50 µg of Hg per litre, complying with the European wastewater discharge limit for mercury, and thus could be discharged into a river. The costs of a biological mercury clean-up process are less than half of those for ion exchange columns and can be used for other mercury contaminated wastewater, e.g. water used for soil washing. Commercialisation of the process by Preussag Wassertechnik is underway. Future projects are directed towards the clean-up of other types of mercury contaminated material (soil wash water, water from gas pipes, vaccine production effluents). A new technology was demonstrated for the clean up of mercury contaminated wastewater at pilot scale, based on the ability of natural bacteria to detoxify mercury compounds. Mercury is retained with high efficiency as metallic mercury within a bioreactor because of an enzymatic reduction carried out by micro-organisms. An automated pilot plant was developed in cooperation with Preussag Wassertechnik which was able to clean 4 m3 wastewater per hour (50% of the wastewater at the German electrolysis factory ECI Elektrochemie Ibbenbüren). It was operated continuously for 8 months at the factory. The biological mercury removal process was extremely robust against fluctuations in inflow parameters and various stresses. The pilot plant removed 98% of inflow mercury over the testing period. The cleaned water had less than 50 µg of Hg per litre, complying with the European wastewater discharge limit for mercury, and thus could be discharged into a river. The costs of a biological mercury clean-up process are less than half of those for ion exchange columns and can be used for other mercury contaminated wastewater, e.g. water used for soil washing. Commercialisation of the process by Preussag Wassertechnik is underway. Future projects are directed towards the clean-up of other types of mercury contaminated material (soil wash water, water from gas pipes, vaccine production effluents).
