Ecotoxicological assessment of bauxite residue (red mud) overflow treated by dissolved air flotation (DAF)

Abstract

In this study, the effluent from the Bayer process of transforming bauxite into alumina, followed by dissolved air flotation (DAF) treatment, was evaluated to identify the best experimental conditions that generate less toxic or nontoxic effluent for discharge. Two freshwater organisms, the Chlorophyceae microalga Raphidocelis subcapitata (96-h algal growth inhibition test), and the microcrustacean Daphnia similis (48-h acute immobility test), were used to analyze the efficiency of effluent treatment by detecting and comparing the toxicity of the treated effluent. Experimental factorial planning used different concentrations of the coagulant ferric chloride and the flocculant Nalco® N-99-005B, and different recycle ratios. The highest recycle ratio (30%) and highest flocculant concentration (150 mg/L) had the best solids removal efficiency, above 90%. Probably the higher concentration of flocculant led to the formation of a more stable froth, promoting an increase in the hydrophobic characteristics of the flocs. Two operating conditions were satisfactory according to acute and chronic bioassays, with removal efficiencies of total suspended solids and turbidity higher than 98%, for both variables. Producers, R. subcapitata microalga, were more sensitive than the primary consumers, D. similis microcrustaceans, indicating the importance of performing tests with different bioindicators.