Hexavalent chromium (Cr(VI)) can be biologically reduced to nontoxic and easily separable trivalent chromium (Cr(III)) without generating concentrated wastes.Using a 6-25 gpm pilot-scale two-stage, fixed-bed (FXB), biologically active carbon (BAC) treatment system, approximately 75 μg/L Cr(VI) was consistently removed to less than 7 μg/L with a 10-min empty-bed contact time. Potential Cr(VI)-reducing bacteria, including members from the Dechloromonas and Acinetobacter genera, were present in the system. The system was resilient, and 91% Cr removal was observed when the system was challenged with a 24-h phosphoric acid feed shutdown, a 3-day system shutdown, spiking Cr(VI) to 100 μg/L, or operating intermittently with regular shutdown periods of hours to days. The system recovered within 6 h after a 26-h acetic acid feed shutdown. Readily settling backwash wastewater was generated with characteristics similar to municipal wastewater. Overall, the results indicated that a two-stage, FXB BAC system can provide an effective and robust option for Cr(VI) removal. K E Y W O R D Sbackwash, backwash wastewater, biological, bioreactor, fixed-bed, hexavalent chromium, microbial community, robustness

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