Airports are under strict regulations that help protect the environment and quality of life. One of those regulatory programs is the National Pollutant Discharge Elimination System (NPDES), which controls water pollution. To meet these clean water requirements, airports need to manage the levels of biochemical oxygen demand (BOD5) in their stormwater runoff. That can be easier said than done. A major source of BOD5 is propylene glycol, which is used in aircraft deicing fluid, and it’s a big challenge to manage spent aircraft deicing fluid (SADF). There are a lot of variables to track in order to keep BOD5 levels in check — like the amount of rainfall, snow melt and the need to use deicing fluid. With so many factors in play, solutions are complex and unique to each facility. However, by developing a complex hydraulic and water balance model, airports can identify numerous ways to better control the collection, conveyance, storage and treatment of SADF and stormwater.
A Good Starting Point
To better understand options for managing SADF and stormwater, consider the procedures described in the Airport Cooperative Research Program (ACRP) Report 14. This report outlines the standard industry approach to developing management plans with the aim of reducing pollutant discharges. For an airport to maintain compliance, the report suggests a trial-and-error approach and recommends that performance be reviewed at the end of every season to address any deficiencies. If an airport’s management strategy is performing effectively, no more action is required. According to the report, the key to maintaining a successful management plan is to capture real data from the operating system and make it work alongside data from modelling tools. This approach allows more accurate and effective system management.
Site-Specific Management Options
There are numerous site-specific management options available to airports to manage stormwater runoff. They include deice pads, diversion valves, catch basins, pump stations, storage tanks, detention/retention ponds and treatment/recycling facilities. The most appropriate method should be chosen based on permitting requirements, off-site treatment options, capital improvement and operating costs. Given these factors, site-specific hydrologic and hydraulic models (which model weather and fluid movement) alongside mass balance and BOD5 loading models (which model pollution levels) can be effective tools when it comes to choosing an appropriate SADF management strategy.
Case Study: Management Options at a Large Hub Airport
Recently, our team conducted studies at a large airport to produce an SADF and storm sewer system management plan that aligns with the airport's overall strategy. These studies involved reviewing the airport's SADF recovery and treatment/recycling systems with help from specialized stormwater, wastewater, hydraulic and aviation engineers. Because every airport has different requirements, those working on the project had to develop plans to address each particular challenge. The existing SADF and storm sewer systems were complex and connected hydraulically; this created challenges for operations, maintenance and compliance. What’s more, the airport’s stormwater collection, conveyance and treatment systems weren’t operating at optimum conditions to maximize their efficiency and efficacy.
Predicting Factors That Can Change
When assessing how weather would affect the management plan, the design team realized it couldn’t use traditional rainfall return periods; a modified return period considered only the winter deicing months. The team also modified the intensity of the storm to match typical winter storm distribution levels. After determining the weather conditions, the team needed to predict the loss of glycol. Using historical flight patterns and glycol use levels, the system developed related the quantity of glycol used per aircraft to rainfall depths, which led to a water balance model that could be used with the hydraulic model. Once the team could predict how much glycol would be used, it needed a strategy to evaluate how it would be recovered. Some SADF would be captured by an existing glycol recovery and treatment system, but not all of it; some would be lost to the stormwater system through fugitive losses, which occur when fluid adheres to aircraft. When a plane is deiced, between 20 percent and 60 percent of the fluid stays on the plane, only to drop off the plane within an airport watershed that can cover thousands of acres. Testing at NPDES outfall locations can identify potentially affected areas, but the team needed something more comprehensive. An expanded sampling plan could identify fugitive loss hot spots, and future infrastructure opportunities could be identified to mitigate runoff in these additional locations.
Using Natural Treatment Systems to Comply with the Clean Water Act
The management strategy we created for this airport consisted of continued testing, evaluation and optimization of existing infrastructure. This is just one approach that was unique and appropriate for this airport; where additional infrastructure may be necessary, natural treatment systems could be used. Federal waters and wetlands exist on many airport facility grounds and, as airports develop, so do the amount of stormwater and SADF runoff. Consequently, this growth impacts the surrounding wetlands, which can require costly mitigation. There are natural treatment systems that are increasingly used for wetlands mitigation, like submerged bed wetlands, bioretention cells and wetland swales; these work in tandem with stormwater treatments. Any alternatives considered should be based on effectiveness, cost, operation and maintenance while taking into account their impact on wildlife. What do you make of these on-site strategies? Do you see them as effective in reducing deicing fluid and stormwater runoff?
Chris Hotop, PE, is a civil engineer and project manager at Burns & McDonnell. He works primarily with planning, design and design-build projects related to aviation and industrial facilities, including hangars, FBOs, fueling, airfield stormwater and deicing runoff management, and general civil projects.