Investigation of Cr(III) removal from wastewater with the use of MBR combined with low-cost additives

This work investigated the removal of Cr(III) from wastewater with the use of a combined system of a membrane bioreactor (MBR) together with low-cost mineral additives. The additives employed were zeolite, bentonite and vermiculite. These minerals have high adsorption and ion-exchange capacity. Sludge from an MBR was enriched with an initial chromium concentration of 320 ppm and the pH value was adjusted in order to minimize chemical precipitation. Batch filtration tests were conducted using ultrafiltration membranes having a nominal pore size of 0.04 μm. The results show that ultrafiltration membranes with no mineral addition could remove significant Cr(III) from the final effluent with removal efficiencies ranging between 43.2 and 69.2%. The addition of specific mineral concentration could further increase the chromium removed from the final effluent, reaching very high removal efficiencies. The ion-exchange capacity of minerals followed the order bentonite > vermiculite > zeolite. The combined system of MBR with 10 g/l bentonite or 10 g/l vermiculite at pH 8.0 and MLSS = 4.5 g/l was able to produce a final effluent with Cr(III) concentrations of 0.003 and 0.010 ppm respectively which meets the recommended reuse guidelines of US EPA of 0.1 ppm required for long-term irrigation. At pH 5.5 the addition of 10 g/l of zeolite, vermiculite and bentonite achieved removal efficiencies of 81.5, 95.2 and 99.5% respectively with the latter condition resulting in a final effluent of 1.76 ppm.Furthermore, it was found that high pH and MLSS values and elevated temperatures favour the Cr(III) removal in this system. Sludge MLSS values above 6.0 g/l at pH 5.5 resulted in Cr(III) removal >68%, while pH values above 7 for MLSS = 4.5 g/l resulted in Cr(III) removal efficiencies >95%.The impact of the mineral addition on membrane fouling was also investigated. It was found that all minerals mitigate fouling and it follows the order zeolite > bentonite > vermiculite. However, the high initial Cr(III) concentrations in sludge resulted in increased membrane fouling.The results show that this system of MBR combined with low-cost minerals can effectively remove chromium and can therefore be used to treat industrial wastewater to a level that can be lower than the US EPA limit of 0.1 ppm required for reuse applications, provided that certain conditions are met.