Pilot scale hexavalent chromium removal with reduction, coagulation, filtration and biological iron oxidation

[Display omitted] •Cr(VI) removal below 0.5 µg L−1 with RCF combined with biological iron removal (RCbF).•RCbF is robust against variations of pH, filtration velocity or Cr(VI) concentration.•Fe(II) dosage is the key parameter for low target Cr(VI) concentrations.•Chromium is strongly immobilized as co-precipitated Cr(III) solid solution.•The technical effort for RCbF is considerably decreased compared to conventional RCF. Cr(VI) is identified as highly toxic, therefore a far-reaching limitation of total chromium or Cr(VI) in drinking water was proposed by the Germany Environment Agency. There is a lack in efficient treatment processes to reach Cr(VI) concentrations below 1 µgL−1. In this study, the combination of chemical reduction, coagulation and filtration (RCF) was further developed by adding biological iron removal as filtration step (RCbF). The aim of this enhancement was to reach lower effluent concentrations and a higher robustness regarding process parameters. The effectiveness of Cr(VI) removal was investigated using two-stage pilot-scale waterworks. RCbF reaches Cr(VI) effluent concentrations below 0.5 µgL−1 despite variations of pH, filtration velocity, or Cr(VI) influent concentrations. Fe(II) dosage and hence molar excess of Fe(II) over Cr(VI) was identified as the key parameter for Cr(VI) removal. Low oxygen dosage for biological iron removal improved the efficiency of RCbF compared to RCF. The co-precipitation of Cr(III) and Fe(III) as solid solution in the supernatant of the filter bed was promoted by low oxygen concentrations making Cr(VI) the preferred oxidant. RCbF was shown to be a suitable treatment process for reaching a low limit value for total chromium or Cr(VI) concerning technical feasibility.