Reduction, coagulation and biotic filtration (RCbF) for Cr(VI) groundwater remediation – A case study

Remediation of hexavalent chromium (Cr(VI)) contaminated groundwater is imperative, as in this case study with concentrations of 55 μg L−1 Cr(VI). At pilot scale, the combined treatment process Reduction, Coagulation and biotic Filtration (RCbF) was investigated and optimized. Fe(II) was dosed into the influent to reduce Cr(VI) to insoluble Cr(III), which was removed together with the excess Fe(II) in the subsequent biotic deep-bed filter. First, the biotic iron removal was established after 10 days and a Fe(II) dosage of 3–4 mg L−1, afterwards Fe(II) dosages and different filtration velocities were tested, and lastly a second Fe(II) dosage before a second biotic filter was examined. Even for low Fe(II) dosages of 1.00 mg L−1, chromium was removed reliably to below 9.0 μg L−1 and higher Fe(II) doses resulted in increased chromium removals. High filtration velocities of 20 m h−1 (5.7 m3 h−1) were possible and a short contact time of 2.7 min between Cr(VI) and Fe(II) before filtration were sufficient. During two stage filtration (1.50 mg L−1 Fe(II) before filter one and 0.50 mg L−1 before filter two), effluent chromium concentrations of 2.0 μg L−1 were reliably reached. The backwash water production was minimized to 1.2 m3 (∼1 % of treated water).