WEFTEC 2002 RECIPROCATING SUBSURFACE TREATMENT SYSTEM KEEPS AIRPORT OUT OF THE DEEP FREEZE

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1 RECIPROCATING SUBSURFACE TREATMENT SYSTEM KEEPS AIRPORT OUT OF THE DEEP FREEZE Tim Arendt Malcolm Pirnie, Inc Polaris Parkway Columbus, OH Mark Ervin, Malcolm Pirnie Don Florea, ABX Air, Inc. ABSTRACT 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 pilotscale study of the same technology. The reciprocating unit process consists of a pair of belowgrade 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 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. removal rates for the full-scale system have matched design levels, with influent 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. KEYWORDS Reciprocation, Deicing, Attached-Growth, Storm Water, Airport

2 INTRODUCTION At most cold weather airports, chemical deicers are applied to aircraft prior to takeoff to eliminate snow, ice, and frost on critical aircraft surfaces. The aircraft deicers are a blend of glycols and water, with small amounts of functional chemical additives. While the deicers are not considered a significant toxic threat to the environment, their discharge to adjacent surface waters can reduce in-stream dissolved oxygen concentrations, produce unpleasant odors, and promote growth of nuisance bacteria. These environmental effects led to concerted effort by the air transportation industry in the last decade to bring the deicer discharges into compliance with water quality standards. Compliance efforts at most airports have focused on managing discharge of the glycol-based aircraft deicing compounds through collection and disposal. Collection of deicers can be difficult because of the large land areas involved, large carry-off quantities, problems meshing of deicer collection with airport operations, and the potentially large volumes of storm water mixed with the deicers. Disposal of deicers can be problematic because of the large storm water volumes, low water temperatures, and seasonality of the discharges. As airports and airlines tackled management of deicers, it became readily apparent that sitespecific solutions for collection and disposal of deicer were required to adapt to the unique infrastructure, operational needs, and compliance requirements of individual airports. Numerous methods for collecting deicer have been developed, while disposal of deicer has evolved to include recycling, discharge to POTWs, and onsite treatment. Storm Water and Deicer Management at Airborne Express. Site constraints were the inspiration for a unique storm water and deicer management system developed at the private airport owned by ABX Air, Inc. in Wilmington, Ohio. ABX Air, Inc. (ABX) is the all-cargo express airline and main sort hub subsidiary of Airborne, Inc. ABX and Airborne, Inc., together with each of their subsidiaries, do business as Airborne Express, Inc. Airborne Express is a fullservice global transportation and logistics partner to businesses and government agencies worldwide. ABX operates over 115 Airborne Express aircraft from the Airborne Airpark hub. The 2,200-acre airpark has two runways and over 200 acres of concrete ramps for parking, loading, and deicing of aircraft. Typically, 200,000 to 300,000 gallons of pure propylene glycol based aircraft deicer are applied by ABX each year. A total of 15 outfalls at ABX discharge storm water to the airport s two receiving streams, Lytle Creek and Indian Run. Three of the fifteen outfalls drain the main deicing areas at the airport. These outfalls also discharge the majority of storm water flow from the site. Outfall flow rates vary widely, from less than 50 gallons per minute (gpm) in dry weather conditions to over 150,000 gpm in wet weather conditions. As shown in Figure 1, two of the major outfalls discharge into Lytle Creek, while the other major outfall discharges to Indian Run. In 1995, ABX began preliminary evaluation of alternatives to achieve compliance with conditions in its NPDES permit. ABX contracted with Malcolm Pirnie, Inc. for engineering services to develop a storm water and deicer management system. The initial work included

3 evaluation of storm water characteristics, existing deicing practices, assessment of deicer collection methods, and evaluation of deicer disposal alternatives. Figure 1 ABX Air Inc., Airborne Airpark Storm Water Drainage System Lytle Creek Outfalls Indian Run Outfall From the preliminary investigation, it was concluded that the most cost effective and technically feasible system was collection of deicer-laden storm water at selected airport outfalls and treatment of the collected storm water to biologically degrade the deicing compounds. After preliminary evaluation of conventional biological treatment processes, ABX pursued development of a subsurface, attached-growth treatment system. METHODOLOGY Applicable Regulatory Criteria. To determine the regulatory parameters governing collection and treatment of storm water at ABX, a modeling study was undertaken in 1998 to evaluate stream assimilation capacity for deicer discharges under dry and wet weather conditions. The model was used to develop a quantitative understanding of the effect of flow rate on meeting dissolved oxygen regulatory criteria for worst case deicer dischargers and to develop effluent limitations for the storm water discharges. The study used a dynamic hydraulic model integrated with a dynamic water quality model. In the model simulations, scenarios were developed for the three main outfalls in which all flows below certain threshold flow rates were collected and treated. Modeled peak flows above the threshold flow rates were discharged without treatment. The ability to discharge peak flows without treatment is significant because peak rainfall events can generate more than 60 million gallons per day of runoff. The modeling study led to development of the regulatory design criteria in Table 1.

4 Table 1 - Regulatory Design Criteria for ABX Storm Water and Deicer Treatment System Threshold Collection Flow Rates Effluent Limits Lytle Creek Discharges 5,300 gpm 210 mg/l Indian Run Discharges 1,800 gpm 540 mg/l Design Loading Criteria. Both storm water flows and deicer loadings collected at the system outfalls can vary considerably. Collected flow rates ranged from less than 50 gpm up to the threshold flow rate values while concentrations range from less than 100 mg/l to greater than 20,000 mg/l. A treatment system designed to treat these ranges would be excessive in size. Therefore, design loading criteria were based on managing flow and deicer input into the treatment system through use of a flow and concentration equalization system. The equalization system incorporates a detention basin to temporarily detain peak flows, a pump station to limit maximum flow rate into the system, and an equalization basin to attenuate concentrations. The flow control system allows a maximum flow of 1,000 gpm per minute into the treatment system. Outfall flow rates during wet weather events can exceed the maximum flow rate entering the treatment system. Under these conditions, excess flows collected at the outfalls are temporarily held in a lined detention basin. Controlling flow rates and loadings into the treatment system allowed the treatment system to be designed and downsized to meet the loading criteria given in Table 2. Table 2 - Design Loading Criteria for ABX Storm Water and Deicer Treatment System Parameter Average Flow Conditions Maximum Flow Conditions Flow Rate (gpm) 250 1,000 Concentration (mg/l) 3,200 1,250 Loading (lb/day) 9,600 15,000 Although the equalization basin attenuates concentrations, the peak outfall concentrations can still exceed the design maximum (some deicing events produce concentrations in excess of 20,000 mg/l). If the treatment system capacity is exceeded and effluent concentrations exceed the regulatory limits, flows can be recirculated back to the detention basin. The storm water collected from the airport contains very low concentrations of the nutrients necessary to sustain biological growth. As a result, both phosphorus and nitrogen (in the form of ammonia) are added to the storm water upstream of the treatment system. Subsurface Reciprocation Treatment Technology. Treatment of deicer-laden storm water at ABX is achieved through application of subsurface reciprocation technology. The reciprocation technology was initially developed and patented by the Tennessee Valley Authority (TVA) for nutrient removal in subsurface treatment wetlands. The TVA reciprocation system design was modified and adapted to a large-scale basis for use in the reduction of high concentrations in the deicer laden storm water at ABX.

5 The reciprocating subsurface treatment system at ABX is an attached growth biological treatment process designed specifically to reduce through breakdown of the glycol into simple compounds in cold conditions. The basic unit of the reciprocating subsurface treatment system is the reciprocation cell pair. Schematics of the reciprocation cell pair are shown in Figure 2. The reciprocation cell pair consists of two subsurface treatment cells (labeled A and B cells) interconnected to a pump station that is used to transfer water between the two cells. Each treatment cell is a below-grade earthen trapezoidal basin lined with a synthetic fabric. The cells are filled with gravel. The gravel acts as the substrate for bacterial growth. The pump station is hydraulically connected to Cell A and Cell B through a perforated pipe penetrating the cell liners. The cells are both filled and drained through the perforated pipe. Water movement within each pump station compartment is accomplished with a propeller pump. Figure 2 Reciprocation Cell Pair in First and Second Stages of Reciprocation Cycle Pump Station A Cell B Cell First Stage of Cycle Second Stage of Cycle An individual cell pair operates as a cycling batch system. Initially, Cell A is filled with water and Cell B is nearly empty, as shown in Figure 2 (First Stage). In this configuration, bacteria

6 film on the gravel in Cell A is exposed to the organic compounds (i.e., deicers and their breakdown products) and nutrients (nitrogen, phosphorus) present in the storm water. At the same time, the bacterial film on the gravel in Cell B is exposed to atmospheric oxygen. As a cycle starts, the propeller pump for Cell A transfers water from the Cell A pump station into the adjacent Cell B to reach the second stage of the cycle (Figure 2). In the second stage, Cell B contains the water while Cell A is exposed to the atmosphere. The cell pair maintains these water levels for a predetermined period of time, after which the Cell B pump starts and the process is reversed. The typical total reciprocation cycle time is four hours, but can range from two to 24 hours. Reciprocating Subsurface Treatment System Configuration. The reciprocating subsurface cell pair is the unit process for the ABX treatment system. Treatment of the entire deicer load from the airport outfalls required multiple cells pair units to bring the storm water concentrations into compliance with effluent limitations. Sizing of the system was performed using data acquired from a three-year pilot-scale study at ABX in which degradation rates were assessed under a variety of conditions. The pilot study work led to development of the ABX full-scale system design in which the unit cell pairs were arranged in a parallel and series configuration (Figure 3). In this configuration, the unit cell pairs act as a series of batch reactors. Figure 3 - Aerial View of ABX Subsurface Treatment System for Indian Run

7 Two separate treatment systems were constructed at the airport because of the split in storm water runoff between Lytle Creek and Indian Run. The systems are located on opposite sides of the airports, outside of the runways. The treatment systems are essentially identical, except that the Indian Run system (Figure 3) contains three sets of reciprocation cell pairs per treatment train compared to the four sets of cell pairs per treatment train on the Lytle Creek system. Operation of Treatment System. At the influent to the treatment system, the flow is split in two, sending storm water to the left and right treatment trains. Each treatment trains consists of several reciprocation cell pairs in series. Water is transferred from one cell pair to the next by gravity as excess water builds up from the influent flow. Operation of the reciprocating subsurface treatment system is based on manipulating system parameters to achieve sufficient degradation of pollutants while minimizing operation and maintenance costs. In general, degradation rates increase (more pounds of deicer broken down in a given time) when a larger volume of the gravel substrate is exposed to the atmosphere. Degradation rates also increase when the total reciprocation cycle time is reduced (shorter time between pump starts). The control system was designed to give the operator the ability to change both the portion of the cell volume pumped out in a given cycle and the total reciprocation cycle time. Typically, the control variables for the system (reciprocated cell volume and reciprocation cycle time) are only modified during significant changes in influent concentration. Little dayto-day modification of main treatment system operational parameters is required. The system operator is required to monitor effluent concentrations and manage recirculation of the effluent to the detention basins when influent concentrations exceed the design concentrations. Although some short-term storage is available in the detention basin during the winter months, the system is designed to operate under all weather conditions. This differs from other cold water treatment systems that collect deicer-laden storm water in the winter and treat the storm water when water temperatures increase. Summer operations are designed to maintain an aerobic bacterial population and prevent the cells from going septic. Typically, only background concentrations are expected in the treatment system influent from June through October. This lack of food supply in summer would be expected to result in a decrease in the bacterial population, although no studies have been performed to specifically demonstrate this change. Longer cycle times are used in the summer to reduce operational costs. Because of the lack of food during the warm weather periods, the biological treatment system must essentially startup each year once deicing resumes. RESULTS The ABX storm water collection and treatment system started operations in January Operational data collection was limited to a three to four week period that winter because of a lack of deicing during the mid-to-late winter. However, degradation was observed within three weeks of startup. Average influent concentrations in the Lytle Creek system during this period were approximately 1,200 mg/l. Removal rates as high as 90% from influent to effluent were observed on several occasions.

8 In the deicing season, a warm early winter resulted in no significant input until the second week of January At the time of the first deicer inputs, storm water temperatures were 35 F to 39 F. During this startup period, concentrations averaged 1,100 mg/l for the Lytle Creek system. As seen in Figure 4, removal rates were initially low, but increased quickly once the nutrient loading was rebalanced and the biological population had a chance to develop. Within two weeks of the initial deicer loading, removal rates reached 70%. Within three to four weeks, the removal rates reached 90%, where they stayed during the remainder of the winter (the slight decrease shown in mid-march was related to mixing and dilution effects caused by a spike deicer loading, rather than a decrease in actual degradation rates). Effluent concentrations were well below effluent limitations for most of the winter. Figure 4 - CBOD5 Removal Rates for ABX Lytle Creek Subsurface Treatment System 100% Removal of COD as Percent of Influent 90% 80% 70% 60% 50% 40% 30% 20% 10% 5-day Average 0% 01/01/ /15/ /29/ /12/ /26/ /12/ /26/ /09/ /23/2002 Peak influent concentrations for the Lytle Creek system during 2002 were 3,300 mg/l, slightly higher than the maximum design capacity of 3,200 mg/l. Effluent concentrations under peak loading conditions were approximately 270 mg/l, slightly higher than the effluent limitation of 210 mg/l. Under this condition, storm water effluent from the treatment system that didn t meet effluent limitations was recirculated back to the system detention basin and metered back into the treatment system per design operational guidelines. Similar results were observed on the Indian Run system. Ammonia loads required for degradation were determined based on the ammonia concentration reduction in each of the cell pairs. The ammonia to ratios were most consistent in the

9 first and second cells where the greatest reductions were occurring. The ammonia to ratio was approximately pounds of ammonia per pound of. Phosphorous loads required for degradation were less consistent than ammonia. Phosphorous uptake was high initially, but decreased near the end of the season as a phosphorus built up in the system. A summary of results from the deicing season is given in Table 3. Table 3 - Results for the ABX Deicer Treatment Systems for Deicing Season Indian Run System Lytle Creek System Average Influent Flow Rate 277 gpm 568 gpm Average Water Temperature 39 F 39.4 F Influent Concentration Average Peak Effluent Concentration Average Peak 401 mg/l 2,220 mg/l 373 mg/l 3,300 mg/l 43 mg/l 42 mg/l 303 mg/l 270 mg/l Total Influent Load 124,000 lbs 145,000 lbs Total Effluent Load 10,000 lbs 15,000 lbs Total Load Removed 114,000 lbs 130,000 lbs Average Effluent TSS < 2 mg/l < 2 mg/l DISCUSSION The full-scale reciprocating subsurface treatment system used at ABX was selected specifically because the pilot-scale system demonstrated the ability to produce significant degradation of glycol compounds in cold water temperatures while producing a minimal amount of solids. During the first year and half of operation, the full-scale system has matched the pilot-scale results. During most of the winter operating periods, water temperatures have been less than 40 F. Effective treatment has been observed at temperatures as low as 34 F. Effluent concentrations have not exceed regulatory limits and on many occasions been well below those limits. Removal rates have consistently average near 90%. Nutrient uptake ratios have been lower than expected, indicating that bacterial cells require a lower ratio of carbon to nitrogen and carbon to phosphorus than is required with other biological treatment systems. This has a positive impact on operational costs for the facility. Despite the large loadings being treated, total suspended solids concentrations have been below detection limits in the effluent discharge with insignificant accumulation of solids in the cells. No large scale sloughing of bacterial solids from the gravel substrate has been observed.

10 Other factors that were initially a potential concern have proved not to be significant. Odors from the treatment cells, while noticeable near the pump stations, are not obvious over the treatment cells. In the two years of operation, the system has not attracted large birds to any greater degree than other areas of the airport, which was a concern because of the close proximity of the treatment system to the runway. Startup of the system after a summer without a food supply takes two to three weeks at temperatures less than 40 F. At warmer temperatures, the startup time would be expected to be shorter. During the startup period, storm water is recirculated through the system s detention basin until degradation rates allow effluent limitations to be met. Nutrient balancing at startup appears to be a key factor in decreasing startup times. No laboratory studies to identify the type of microorganisms prevalent in the system have been attempted to date. Measurements of dissolved oxygen are very low in the bulk water, but sharp decreases in degradation rates that been observed when the cells are not reciprocated, cutting off the oxygen supply, suggest the primary degradation pathway is aerobic. CONCLUSIONS The Storm Water and Deicer Management System use at ABX Air featuring reciprocating subsurface treatment technology has provided excellent degradation of high storm water. Treatment under very cold water temperatures has been demonstrated. The large surface area available for attached growth organisms is likely to be a factor in the robustness of the system. From empirical measurements of degradation rates, however, it does appear that the unique method of alternating exposure to oxygen and exposure to the food supply produces a bacterial population with low solids production, the ability to work effectively at low temperatures, lower than expected nutrient uptake needs, and an ability to degrade glycols over a large influent concentration range. The reciprocation technology could provide an interesting treatment or pretreatment alternative for industrial wastewaters with high and variable organic loadings. ACKNOWLEDGEMENTS The authors from Malcolm Pirnie would like to thank Don Florea, the system operator at ABX, for his hard work, perseverance in data collection, and insight into the system operation. We also would like to thank ABX Air, especially the Environmental Compliance Department managed by Robert Hentrich, Jr., for their support from the initial stages of this project. Finally, we would like to acknowledge the early and ongoing work on reciprocation technology by Dr. Les Behrends and his staff at the Tennessee Valley Authority facility in Mussel Shoals, Alabama.