Biofilm characteristics of a RBC reactor in presence and absence of heavy metals

The industrial effluents containing heavy metals cause toxic effects on the natural ecosystems, as well as efficiency problems in biological treatment plants. In the present work, the biofilm characteristics of a three stage RBC reactor in presence and absence of heavy metals (Cd 5 mg/L and Cr 20 mg/L) were compared, under aerobic-mesophilic conditions and laboratory scale. The biofilm was formed from an aerobic sludge obtained of a local wastewater treatment plant. The reactor was fed at continuous flow with synthetic industrial effluent containing an organic load of 5.81 gCOD/d.m(2). In the final effluent of each stage the physicochemical parameters were monitored: pH, total alkalinity, dissolved oxygen, redox potential, chemical oxygen demand (COD), total suspended solids, suspended volatile solids, Cd and Cr concentration. The evolution of the biofilm was recorded photographically, also determining: thickness, biomass index, heterotrophic count and characterization of the main bacterial groups. The content of organic matter (COD) in the treated effluent was significantly lower (18.0 %) in the absence of metals, while the removals of Cd and Cr were 96.7 +/- 0.2 and 57.2 +/- 1.3 %, respectively. The final concentrations of Cr did not meet the discharge limit established in the national regulations (2.0 mg/L). The biofilm had a lower thickness and abundance of heterotrophs in the presence of metals, as a consequence of their toxic effect. The most abundant bacteria during the treatment of the effluent belonged to the genera: Paenibacillus, Bacillus and Lysinibacillus.