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Removal and Recovery of Heavy Metals from Waste Water by Sand Filters Inoculated with Metal Biosorbing or Bioprecipitating Bacteria

Exploitable results

It was already known that bacteria can have a high biosorption and bioprecipitation capacity for heavy metals. This was especially shown by some of the partners of this project in a former project (BRE2/0199/C) whose selected bacterial strains would be used in this technological development project. Conclusions: It was shown that the use of a moving bed sand filter inoculated with heavy metal biosorbing and bioprecipitating bacteria, forms a new reliable technology. The removal of Zn and Cu was between 95 and 100 %. Co removal was between 80 and 90 %. Fe was removed for 60 - 80 % and other metals e.g. Al, Ag, Cr, As and Se were removed at least for more than 80 %. In separate experiments removal of TOC (65 mg O2/l to 13 mg O2/l) was demonstrated in addition to the removal of metals like Tl (625 µg/l to 235 µg/l). In addition some COD, NO3-N and NH4-N were removed. The technology can therefore be used as a cheap polishing step. It allows metal using companies to reach the new environmental standards and hence to reduce ecotaxes on the discharged metals and nutrients. The moving bed sand filter technology allows a homogeneous treatment of waste water containing heavy metals. Due to the continuous washing system the filter bed resistance and flow rate are always stable and self regulating, providing a stable output and steady-state operation. The sludge released in this washing process could be thickened into a filter cake of 50% dry weight. This cake could be recycled in a shaft furnace and in that way reintroduced in the process. So no extra waste products are released from the process. The use of active bacteria in a biofilm allows the system to remove the heavy metals not only by biosorption but also by bioprecipitation processes. The bioprecipitation is induced by the presence of functional groups and crystallization foci together with the physico-chemical microenvironment created by the biofilm. The biofilm generates steep pH gradients at the cell surface that allow the crystallization to occur. The biofilm is further studied now by the use of molecular biology techniques. These include PCR and DGGE in order to identify the microbial population and the presence of the inoculated strains. After one year of operation about 9 different bacterial species could be observed in the biofilm. The use of different primers in the PCR allows to distinguish the bacteria from each other. This information can be combined with the results obtained from Biolog and metal resistance patterns. Complementary results are presented by Pümpel et al. [12]. The technology is now protected (patent pending) and will allow an economically acceptable treatment of large waste water flows containing heavy metals. These waste waters include waste water from metal processing and surface treatment companies, mine water, groundwater and landfill leachate. Experiments are indicating that the presence of the specific bacteria is of high importance for a successful metal removal. Acetate, ammonium nitrate and nitrate are dosed. Denitrification could be done and even some COD removal was obtained. Special attention had to be paid to high wash water flows and big sand washers as the sand washing and especially the separation between metal laden sludge and sand seemed to be very critical.