RECIPROCATING SUBSURFACE TREATMENT SYSTEM KEEPS AIRPORT OUT OF THE DEEP FREEZE

ABX Air, Inc., the Airborne Express airline (ABX), operates a shipping service in Wilmington, Ohio. During winter operations, chemical deicers are used to remove frozen materials from critical aircraft surfaces. Following application, the deicers mix with storm water and are conveyed to the airport's storm sewer system. If the deicer-laden storm water is discharged to the local receiving streams, in-stream dissolved oxygen concentrations can decrease below regulatory water quality standards. In January 2001, construction was completed on a storm water collection and treatment system designed to treat the collected storm water using reciprocating subsurface treatment cells. The system design was based upon a three-year pilot-scale study of the same technology. The reciprocating unit process consists of a pair of below grade lined cells containing gravel. A bacterial population forms on the gravel substrate, resulting in an attached-growth biological treatment system. The deicer food source, nutrients, and oxygen are supplied to the bacterial films through the reciprocation process. In the reciprocation process, water is exchanged between the subsurface treatment cells every two to four hours, creating alternating states of submergence of the gravel in wastewater and exposure of the gravel to atmospheric oxygen.The full-scale treatment system at ABX consists of a series of the reciprocating subsurface treatment cell pairs in parallel trains. Influent flows and CBOD5 concentrations, which can each vary by two orders of magnitude, are managed through a detention and equalization system. The system started up in January 2001 and has operated for two deicing seasons. CBOD5 removal rates for the full-scale system have matched design levels, with influent CBOD5 concentrations reduced by an average of 90% from influent concentrations as high as 3,500 mg/l. Effluent suspended solids concentrations have been minimal.The process has been proven to operate well at cold-water temperatures (as low as 34°F), absorb shock loads well, maintain consistent degradation rates, produce low solids concentrations, and operate without significant odor. The system has applications for any high strength wastewater and can easily be designed to fit any size application.