Treatment of Metal Finishing Wastewater from Aircraft Maintenance Operations Using an Electrocoagulation Treatment Process

Distribution A: Approved for public release; distribution is unlimited.(Ref. # 09-04-22-75C-008)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 military transport 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). To comply with the local pretreatment standards, Hill AFB is evaluating a number of novel technologies for its industrial wastewater treatment.Electro-coagulation (EC) has been touted as an industrial wastewater treatment process with the potential to reliably achieve metal discharge standards. Preliminary results from 1.5 gallon per minute Electro-coagulation (EC) unit had demonstrated that the process design could remove chromium, cadmium and nickel from industrial wastewater to below compliance limits by maintaining a direct current and pH. Results from the current testing of a 30 gallon per minute pilot scale EC unit indicated that consistently achieving regulatory compliance limits remains a technological challenge. While operating at a pH of 7.2, the EC unit was found to reduce the influent cadmium and chromium concentrations from 0.13 and 19.0 ppm to well below their regulatory limits of 0.04 and 1.0 ppm, respectively. However, when the pH was lowered to 6.9 or raised to 8.0, the cadmium effluent concentrations exceeded the current regulatory limits. Moreover, in the presence of chelating substances (in this case, the paint stripper B&B 5095), EC was found to be unable to treat the influent wastewater to regulatory compliance limits.Although increasing power application to EC was found to improve metals removal, it came at the expense of increasing foam generation. Moreover, in the presence of the industrial paint stripping solution, the rate of foam production and its physical stability increased. High pressure water spray was found to be the only effective method to limit foam production.Configuring the EC unit to utilize aluminum rather than iron plates demonstrated some potential advantages. Initial results indicated that EC operation with aluminum plates was effective in removing targeted heavy metals if the influent wastewater was maintained at acidic conditions. Under neutral or alkaline conditions, an aluminum scale rapidly formed that led to shortcircuiting and decreased system performance.