Treatment of Electroplating Waste-Waters by Alkaline Ferrous Reduction of Chromium and Sulfide Precipitation

The Air Force operates a number of electroplating facilities in support of weapons systems maintenance. These range from small, three- to four-bath operations to very large, 40,000-square foot, shops. This laboratory and pilot scale research effort was undertaken to develop simple treatment operations applicable to the small Air Force plating shops and improve the waste treatment at the large facilities. Treatment of wastewaters containing chromium, nickel, and cadmium was investigated using both synthetic and actual plating baths. The major results of this effort are as follows Hexavalent chromium can be rapidly reduced and precipitated at alkaline pH levels, by stoichiometric, point-source additions of ferrous sulfate and sodium sulfide. Background sulfide lowers the solubility of cadmium and nickel in solution except when cyanides are present. Due to complexing, cyanides blocked nickel, removed completely, and reduced the effectiveness of chromium and cadmium removal. When dilute wastes were treated, the only unit processes required were in-line chemical addition for chromium reduction and upflow sandfiltration. For concentrated wastes, a clarification step was included to ease the solids loading on the filter.