Role and characteristics of problematic biofilms within the removal and mobility of trace metals in a pilot-scale membrane bioreactor

Abstract

The characteristics of problematic biofilms (i.e., fouling and clogging layers) were studied with regards to the removal and fate of trace metals (contents well under 100μg/L) during the long-term operation of a pilot-scale membrane bioreactor for the treatment of real wastewaters from a large industrial area.Results showed that clogging layer was more effective than suspended 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. In fact the selective microbial speciation of the phylum of Bacteroidetes, which is highly resistant to heavy metals, was observed in the clogging sludge in spite of the very low concentration of dissolved metals in the bioreactor.Compared to the suspended activated sludge, the clogging layer enhanced the biosorption of very toxic substances such as As, Cd and Ni. In fact, the metal contents were respectively: 7.9–7.4 vs. 690–840μgAs/kgTS; 1.5–2.2 vs. 149–219μgCd/kgTS; 58.8–71.7 vs. 227–298μgNi/kgTS. Then, the potential desorption of metals during the membrane acid cleanings was estimated as relevant as 10–15% of the metals associated to the clogging sludge. The combined effects of pH and the selected microbial community, and the minor effect of the redox potential, let us conclude on the major importance of bio-sorption/desorption mechanisms with respect to bio-precipitation/dissolution.