279 BIOFILM REACTORS IN MINING AND METALLURGICAL EFFLUENT TREATMENT: BIOSORPTION, BIOPRECIPITATION, BIOREDUCTION PROCESSES M. Tsezos, A. Hatzikioseyian, E. Remoudaki National Technical University of Athens, School of Mining and Metallurgical Engineering, Laboratory of Environmental Science and Engineering, Heroon Polytechniou 9, Zografou 15780, Athens, Greece. Tel +30 210 772 2271 Fax +30 210 772 2173, e-mail: tsezos@metal.ntua.gr ABSTRACT During the last decade, considerable volume of work has been carried out by Institutes, Universities and private corporations on the development of novel biotechnological processes for the advanced treatment of metallurgical and mining liquid effluents. Reactors that can support the development and maintenance of biofilms, such as, static beds, fluidized beds, moving bed sand filters and rotating biological contactors (RBC), provide a variety of liquid effluent advanced treatment options for the above categories of effluents. Biofilm reactors, both in pilot and industrial scale, have been tested and the available results on pollutant sequestering are very promising. The successful development of biofilm reactors has been based both on the research advances on the characteristics of microbial biomass species and on the elucidation of the likely mechanisms of interaction between the microbial biomass and the targeted pollutants which include toxic metals, radionuclides and metalloid oxyanions. Microbial biomass- metals interactions occur through a variety of mechanisms. These mechanisms can be classified in two principal categories: the metabolically active (biomineralization, biotransformation, bioprecipiation) and metabollically passive (biosorptive) processes. Metabolic interactions between microbial biomass and metals or oxyanions in liquid effluent solutions may lead to metals precipitation (bioprecipitation), metals reduction (bioreduction) etc. Passive microbial biomass-metal/metalloids interactions include extracellular, intracellular sequestering, chemical bonding, complexation and ion exchange mechanisms. This variety of metals sequestering mechanisms by microbial biomass, together with the low amounts of energy and materials required for their scale up and application, make biotechnological processes promising for efficient metal