Such fuel-stained surfaces have the potential to impact stormwater quality and when the stormwater discharges to a surface water body, a Storm Water Pollution Prevention Plan (SWPPP) must be prepared and implemented. The SWPPP defines a series of Best Management Practices (BMPs) that are used to minimize the discharge of contaminated stormwater and required annual inspections and/or testing.
If testing indicates that hydrocarbon-contaminated stormwater is being discharged, then the impacted stormwater must be pre-treated before discharge to ensure compliance with State Pollutant Discharge Elimination System (SPDES) requirements. Such discharges are illegal and when discovered by either the state environmental agency or Coast Guard, legal action will be instituted against the airport operator and/or fueling system operator.
The federal SPDES regulations govern only stormwater discharges from regulated industries to surface waters, and stormwater discharges to groundwater would appear to be unregulated. However, many state SPDES programs address discharges to groundwater where drinking water aquifers are potentially impacted, and federal programs related to re-injection wells may preclude the discharge of petroleum contaminated stormwater to groundwater.
Conventional Oil/Water Separators
Most bulk fuel facilities utilize buried API gravity type oil water separators to process stormwater captured by secondary containment areas.
Depending upon specific design factors, these are passive devices that cause small fuel droplets entrained in the stormwater flow to coalesce and float to the surface of the separator, where they are retained by a diversion baffle.
These devices typically reduce total petroleum hydrocarbon (TPH) levels in the effluent stream to between 10 and 20 parts per million (ppm), provided they are properly maintained and cleaned out on a regular basis to remove sludge and slime build-up. One disadvantage of such systems is that they do not remove dissolved or emulsified fuels, and therefore can result in a stormwater discharge exceeding the limits for dissolved or total hydrocarbon contaminants under a (SPDES) permit. This situation necessitates the installation of an active pre-treatment system downstream of the oil water separator. Such systems are expensive to install and operate.
Oil/water separators are typically sized either to handle specific flow rates or to contain a specified volume of scavenged fuel. Oil water separators are typically designed to store captured fuel up to 50 percent of the working capacity of the separator. The largest transportable oil water separator is 60,000 gallons, which has a fuel storage capacity of approximately 30,000 gallons and can handle a flow rate of 5,000 gallons per minute (11 cubic feet per second) which corresponds to a 24-inch diameter storm drain pipe. The purchase price of a 25,000 gallon oil water separator is approximately $125,000 and the installed cost can exceed $200,000 under high groundwater level conditions.
For larger stormwater flow rates corresponding to major aircraft refueling ramps, several of these large units must be installed to handle the peak flow rates.
Passive stormwater filtration systems
Filter media designed to absorb hydrocarbons from water have been used in the past for stormwater pretreatment (ie: activated charcoal or Imbiber Beads) but they were plagued with high cost and flow resistance which precluded their use in high-flow stormwater installations typical of airports.
Recent developments in polymers have led to low-cost, high-flow rate absorption filter media. These absorption filters can be used not just to polish the effluent from an oil water separator, but in cases where the influent is relatively clean, they can replace the oil water separator, thereby producing a superior effluent quality at a much lower capital cost.
Typically, tank dike stormwater is sufficiently clean to be processed by absorption filters without oil water separators, as is stormwater from aircraft refueling areas and refueler staging areas. Truck loading and unloading racks tend to have more contaminated stormwater runoff and are more likely candidates for oil water separators.
These passive stormwater absorption filtration systems are starting to find widespread application in municipal stormwater systems in California and New York. The filters are used at the source (catch basin inserts), mid-stream (in buried vault systems), and at end of pipe (at the point of discharge).
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