The characteristics and crucial role of undesired biofilms (i.e., fouling and clogging layers) were studied with regards to the removal and fate of trace metals during the long-term operation of a pilot-scale membrane bioreactor for the treatment of wastewaters from a large industrial area. The results of the analyses of 24 metals showed contents well under 100 µg/L, which is often under the limit for urban wastewater. After one year of operation, the suspended and clogging sludges were analysed for both chemical-physical and bio-molecular characteristics. In spite of the low concentration of dissolved metals in the influent, we observed the selective microbial speciation of the phylum of Bacterioidetes, which is highly resistant to heavy metals. The biofilm layer was more effective than activated sludge in the biosorption of As>Zn>Ni>Cd>Sb>Fe>Se due to the synergic effects of extracellular polymeric compounds and metal-resistant bacteria. The desorption over the course of membrane maintenance cleaning was as high as 10-15% and was not directly related to the metal content. As for the removal mechanisms, the high pH effect allows us to conclude the importance of bio-sorption/desorption with respect to bio-precipitation/dissolution (more influenced by Eh).
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