Hexavalent chromium removal from municipal wastewater using ferrous salts
Hexavalent chromium is difficult to remove during conventional biological wastewater treatment. This is because the hexavalent form is dissolved and is only sparingly removed by adsorption onto biomass in conventional processes. Hexavalent chromium is of particular concern because of its aquatic toxicity, and an increasing number of wastewater works have effluent discharge limits to protect receiving water courses, some as low as 8 μg Cr L−1. A relatively simple improvement to the removal of chromium could be made by switching the aluminium or ferric solution dosed at most treatment works for the removal of solids, organic load and phosphorus, to a ferrous salt. Ferrous reduces hexavalent chromium to insoluble trivalent chromium, which can be readily settled out of waste streams as a particulate. In the present study, laboratory experiments using real wastewater and ferrous doses of 10 mg Fe L−1 achieve the chromium discharge consent of 8 μg L−1 from initial solution concentrations of up to 40 μg L−1. A ferrous chloride dosing system was subsequently installed at an operational sewage treatment works that has produced an average final effluent chromium concentration of 2.4 μg L−1 (with a maximum of 4.2 μg L−1), despite influent spikes >300 μg L−1.