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This study reports the feasibility of recovering metal precipitates from a synthetic acidic wastewater containing ethanol, Fe, Zn, and Cd at an organic loading rate of 2.5 g COD/L-day and a COD to sulfate ratio of 0.8 in a sulfate reducing down-flow fluidized bed reactor. The metals were added at increasing loading rates: Fe from 104 to 320 mg/L-day, Zn from 20 to 220 mg/L-day, and Cd from 5 to 20 mg/L-day. The maximum COD and sulfate removals attained were 54% and 41%, respectively. The biofilm reactor was operated at pH as low as 5.0 with stable performance, and no adverse effect over COD consumption or sulfide production was observed. The metals precipitation efficiencies obtained for Fe, Zn, and Cd exceeded 99.7%, 99.3%, and 99.4%, respectively. The total recovered precipitate was estimated to be 90% of the theoretical mass expected as metal sulfides. The precipitate was mainly recovered from the bottom of the reactor and the equalizer. The analysis of the precipitates showed the presence of pyrite (FeS2), sphalerite (ZnS) and greenockite (CdS); no metal hydroxides or carbonates in crystalline phases were identified. This study is the first in reporting the feasibility to recover metal sulfides separated from the biomass in a sulfate reducing process in one stage.

BACKGROUND: Sulfate-reducing fluidized bed reactors represent an alternative for the treatment of wastewaters that contain dissolved metals. However, the low acetate consumption achieved through sulfate reduction affects the organic matter removal efficiency. The aim of this present work was to develop a sulfidogenic biofilm able to consume acetate via sulfate reduction within a short start-up period (21 days). Three experiments were conducted in a down-flow fluidized bed reactor with different acetate/lactate proportions in the feed (50/50, 80/20 and 90/10). Reduction of the influent pH from 6.0 to 4.0 was also studied at the higher acetate content. RESULTS: Organic matter oxidation efficiency was similar in the three experiments (∼67%), nonetheless the sulfate reduction rate was higher in the experiments with 80 and 90% acetate in the feed (744 and 730 mg L −1 d −1 ). Acetate oxidation via sulfate reduction was highest (39% of inlet) at the ratio 90/10, at which the biofilm specific sulfate reducing activity with acetate was 4.3 times higher than that developed at 50/50. Influent pH reduction to 4.0 was not detrimental to acetate consumption via sulfate reduction, which was 56%. Analysis of the biofilm through DGGE found a similar community in the three experiments and the presence of acetotrophic microorganisms affiliated to Desulfobacca acetoxidans.CONCLUSIONS: Limiting the substrate (lactate) was an appropriate strategy to enrich acetate-consuming sulfate reducers and improve the low acetate removal efficiency that sulfate-reducing reactors face.