Absorption filter media is preferred over adsorption filter media because when the media is saturated, the fuel cannot be removed by squeezing and therefore they are suitable for landfilling or waste to energy incineration. This type of filter media typically removes more than 95 percent of the hydrocarbons in the inlet stream, whether free product, emulsified fuel, or dissolved fuel.
When the filter media becomes saturated, typically at 300 percent of the dry media weight at the time of installation, it must be replaced. In the past, the high cost and limited absorption capability of such filter media have relegated its use to final polishing of low volume streams where the hydrocarbon concentrations are very low.
One disadvantage of passive absorption systems is that they can be overpowered by a large spill of fuel — once they become saturated with absorbed fuel, additional fuel will pass directly through them. This can be prevented by the addition of an oil stop valve upstream of the absorption filter media.
Oil stop valves stop the flow of raw fuel by means of a ballasted float, which is designed to sink when submerged in fuel and float when submerged in water. They are relatively low cost and are available in sizes up to 12 inches in diameter with a rated flow of 1,400 gpm (3 cfs). An oil water separator consisting of a 12-inch oil stop valve followed by an absorption filter with a frontal area of 25 square feet (5x5 feet) would provide pre-treatment of 1,400 gpm of stormwater.
Pre-Treatment of Stormwater Discharges
Larger airports normally have stormwater systems which are very similar to municipal storm sewers. A series of concrete catch basins with high-strength metal grating are connected by concrete pipe and typically lead to a headwall where they discharge to surface waters or to excavated recharge basins for discharge to groundwater. Flow rates can be very high during major storm events, due to the large impervious watersheds that airport ramp fueling areas represent.
The first flush of stormwater from aircraft refueling areas can contain appreciable traces of fuel from accumulated minor drips or spills which cause stains on the fueling ramp. If a larger spill occurs during dry periods, it will normally be retained in the bottom of catch basins in the storm drain line and it will not reach the outfall.
However, in the event of a larger fuel spill on the refueling ramp during a storm event, the spilled fuel can be entrained in the stormwater flowing through the stormwater drainage system and reach the outfall, resulting in a reportable release to surface or groundwater.
Pre-treatment of such stormwater can take place in one of three locations;
- At the source (each catch basin in a fueling area);
- At or near the end of pipe (vault or filter box on outfall);
- Immediately beyond the end of pipe (containment boom on surface waters).
Pre-treatment at the source (catch basin) requires the insertion of a filter rack and filter media into each catch basin to process the stormwater as it enters the catch basin. For small airports with a limited number of catch basins in the fueling ramp, this approach may be the most cost-effective way to treat the first flush issue. However, filter maintenance can be problematic, as it has to be done at night between parked aircraft and parked ground support equipment in a high security area.
The photo below depicts a high-performance absorption filter catch basin insert in service at the bulk fuel facility at Newark Liberty International Airport, where Allied Aviation was proactive in meeting bulk fuel facility stormwater discharge requirements under NJDEP permit requirements. This filter strips hydrocarbons from stormwater in a curb inlet for stormwater, ensuring that the discharge is free of hydrocarbons.
The photograph below indicates absorption filters installed in an underground vault to treat stormwater flowing in a buried pipe prior to the point of discharge, in a residential storm sewer system in Babylon, NY.
The next photograph indicates absorption filters installed in an end of pipe discharge at Newark Liberty International Airport. The absorption filters are downstream of a conventional oil water separator and they are used as final treatment prior to discharge.
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