Chromate is a highly soluble, toxic and carcinogenic oxyanion commonly found in groundwaters and soils. A 200 dm(3) pilot bioreactor was designed to treat typical Cr(VI)-polluted groundwater. A fixed bed column was filled with pozzolana, inoculated with a bacterial population containing the sulfate-reducing organism Desulfomicrobium norvegicum and fed with an H-2 + CO2 gas mixture. The pilot was operated under continuous feed conditions for 3 months, first with a synthetic solution and then with real polluted groundwater containing 15 mg dm(-3) Cr(VI). The residence time was decreased to 7 h, while the Cr(VI) concentration in the outflow remained lower than 0.2 mg dm(-3). The bacterial population did not need any organic carbon supply thanks to acetogenesis. The highest Cr(VI) reduction rate was 2.2 mg dm(-3) h(-1). The CO2 flow rate was regulated in order to maintain the pH in the interval 7.5-8.5. The CO2 consumption rate increased with feed flow rate and sulfate reduction activity. The process worked satisfactorily between 10 and 35 degrees C. (c) 2006 Society of Chemical Industry.