Treatment of Metal Finishing Wastewaters in the Presence of Chelating Substances

Hill Air Force Base, Utah (Hill AFB) performs repair and maintenance on a variety of aircraft including the F-16, A-10 and C-130 as well as other weapon systems. Because of its breadth of industrial operations, much of the industrial wastewater generated at Hill AFB is subject to specific metal discharge concentration limits found in the national categorical pretreatment standards for metal finishing (i.e., 40 CFR Part 433) as well as electroplating (40 CFR Part 413). However, to ensure its continued ability to land apply its sludge, the local publicly owned treatment works has established pollutant discharge limits that are more stringent than those found in the national pretreatment standards. To comply with the local pretreatment standards, Hill AFB is evaluating a number of novel technologies for treating its industrial wastewater.A new electrocoagulation (EC) process design has demonstrated significant promise for chromium, cadmium and nickel removal from metal finishing wastewaters in both bench and pilot scale testing. By maintaining a direct current and voltage of 18 amperes and 75 volts, respectively, cadmium, chromium and nickel concentrations were reduced by over 96% in the presence of chelating substances. Other wastewater species whose presence can potentially affect heavy metals removal by EC included chlorine. However, it was determined in pilot scale testing that, by maintaining an influent wastewater pH of between 8.0 and 8.5, the presence of chlorine had a negligible effect on pollutant removal.Results of EC and ultrafiltration pilot scale testing have demonstrated their potential use in complying with Hill AFB's new pretreatment discharge standards. Documented advantages of employing and EC-ultrafiltration treatment include: 1) achievement of lower metals discharge concentrations, 2) smaller tank volumes, 3) reduced sludge volume and 4) effective metals removal in the presence of chelating compounds.