Stormwater Control Takes Off

After recent expansions, Capital City Airport in Harrisburg, Pa., successfully upgrades its stormwater drainage system to accommodate increased flow rates

The Commonwealth of Pennsylvania constructed Capital City Airport in the 1930s as the Harrisburg regional commercial airport. Today it is one of the major general aviation airports for the city of Harrisburg, Pa. Averaging more than 67,000 corporate, charter, and aircraft operations every year, Capital City Airport has earned its place as the airport of choice for general aviation pilots and their passengers. The number of air operations continues to grow at a rate greater than the national average rate for general aviation airports. Capital City is also the home airport for the Pennsylvania Bureau of Aviation and the Pennsylvania State Police.

The engineering firm Buchart Horn Inc., located in York, Pa., was lead design engineer for the Capital City Airport expansion project and upgrades to the stormwater drainage system. The expansion of the airport runways, taxiways, and parking area made it necessary to update the stormwater drainage system to accommodate increased flow rates. The engineer specified two large stormwater treatment vessels to accommodate the increased flow rates. The engineering department at Highland Tank designed a two-vessel system as a treatment train. The first vessel, a double basin oil/interceptor was designed to accept all the stormwater runoff from an existing underground drainage piping system. The second vessel is a high performance oil/water separator treating the stormwater to satisfy the requirements of the airport's National Pollutant Discharge Elimination System (NPDES) permit.

NPDES Regulations
The U.S. Environmental Protection Agency (EPA) requires facilities that engage in regulated industrial activities to obtain NPDES permits before discharging to stormwater. Beginning in 1972 with the Clean Water Act (CWA), the U.S. Congress passed a series of laws prohibiting the discharge of pollutants into the waters of the United States from a point source unless the discharge is authorized by an NPDES permit. The CWA has been interpreted to cover all surface waters, including any waterway within the United States. Also included are normally dry creeks through which water may flow and ultimately end up in public waters. Some examples of public waters include a river, stream, or a tributary leading to a river or stream, lake, reservoir, bay, gulf, sea, or ocean within or adjacent to the United States. The CWA jurisdiction may also reach groundwater if it is directly connected hydrologically with surface waters.

In most cases, NPDES discharge regulations state that "any facility that discharges a harmful quantity of oil, or any petroleum product, and the oil enters a body of water in the United States, by whatever means, is liable for significant penalties for cleanup costs and ecological damage." A harmful quantity of oil by government definition is an oil discharge that can "cause a film or sheen upon, or a discoloration of, the surface of the water." It may also include a discharge that can cause a sludge or emulsion to be deposited beneath the surface of the water or upon adjoining shorelines. More specifically, an oil waste having an average oil content greater than 15 parts per million (ppm).

In 1987, Congress amended the CWA to direct EPA to establish NPDES requirements for stormwater runoff to include rainwater, snowmelt, and surface runoff and drainage. Up until that time, stormwater runoff had been defined as a "non-point-source" of pollution and had not been regulated under CWA. Under the NPDES program, targeted facilities like airports cannot discharge water into the stormwater runoff system without a permit. Such permits generally are conditioned on the water meeting specified pollutant-level requirements before being released.

Phase I of the NPDES regulates stormwater runoff discharges from certain industrial activities, construction sites of five acres or more, and cities within municipal "separate storm sewer systems" (MS4) that serve populations of 100,000 or more. The regulated industrial facilities under the NPDES program are identified within eleven major categories and then broken down into standard industrial classification (SIC) codes. Implementation of the NPDES Phase I rules focused on runoff characterization and effluent monitoring. Phase I stormwater permits must be obtained from EPA or EPA-authorized state agencies. Regulated industries can obtain individual permits or coverage under a general permit. Individual permits (covering only one facility) are usually required when site conditions warrant more stringent controls to prevent violations of water quality standards. General permits require the filing of a notice of intent for coverage. The notice of intent requires a stormwater pollution prevention plan that identifies specific pollutant risks and plans for preventing their exposure to wet weather runoff.

On December 10, 1999, EPA published the stormwater Phase II final rule, and the Phase II permit became mandatory on March 10, 2003. Phase II of the NPDES program extends stormwater permit requirements to include small municipal separate storm sewer systems (MS4) located within urban areas that have a total population of 50,000 or more and a population density of 1,000 persons per square mile. It also now includes construction activities of over an acre and many commercial and retail facilities that previously were exempt. The Phase II rule's focus is implementing "stormwater discharge management practices" to reduce contaminants in surface runoff, without specific quantitative effluent monitoring. Such practices are referred to as best management practices (BMPs). BMPs are a mix of both structural and non-structural practices designed to control stormwater pollution. Structural BMPs consist of physical devices and equipment to control and treat stormwater before releasing it into the receiving water (i.e., treatment devices may include sand/oil interceptors, high performance oil/water separators, filtration devices, catch basin inserts, and oil skimmers).

Capital City Airport's System
The Capital City Airport deicing and fueling facilities are specifically identified as industrial activities subject to NPDES regulations. The engineers selected the Highland Tank model HTC 30,000-gallon EZ-Access™ oil/water separator as the primary treatment vessel, designed for the treatment of contaminated stormwater runoff at a rate of 0 to 3000 gallons per minute, discharging with a certified effluent quality of 10 ppm of free oil and grease.

The oil/water separator is a stationary underground or aboveground wastewater treatment vessel that is filled with water. It utilizes internal baffles and coalescers to accelerate the oil/water separation process. Waste accumulates within the separator while effluent is discharged by gravity. The system can be designed for access from above for observation, maintenance, and cleaning. Highland separators meet the new Underwriter's Laboratories Inc. (UL) SU2215 design, construction, and performance standards for engineered oil/water separators rated at 10 ppm for oil and grease. Currently, code enforcement officials consider UL-SU2215 certification as the preeminent national consensus standard for oil/water separator construction and performance. The other design standard is American Petroleum Institute (API) 421 for sizing surface area for the oil/water separator plates and coalescers. The proper surface area for the internals is critical to the performance of the unit. Highland Tank uses the technology listed in the API manual as "parallel plate design."

The Capital City Airport Project was a large underground installation, so the oil/water separator received flow by gravity from the interceptor tank upstream. The flow enters the inlet pipe of the oil/water separator and hits the diffusion baffle. The velocity head diffusion baffle, located near the inlet is designed to serve four basic functions:

  • To dissipate the velocity head, thereby improving the overall hydraulic characteristics of the separator.
  • To direct the incoming flow downward and outward maximizing the use of the separator volume.
  • To reduce flow turbulence and to distribute the flow evenly over the separator's cross-sectional area.
  • To isolate inlet turbulence from the rest of the separator.

When the flow passes through the diffusion baffle area, it moves into the sediment chamber. The sediment chamber allows heavy solids to settle out and concentrated oil slugs to rise to the surface. The flow is directed through the inclined parallel plate coalescer that is sloped toward the sediment chamber. The parallel plate is designed to intercept a 60-micron diameter or larger oil droplet from the waste stream. It directs the flow of the separated oils to the surface of the tank and separated solids to the bottom. As the oily water passes through the parallel corrugated plate coalescer, the oil rises and coalesces into large globules on the underside of each plate. The oil then creeps up the plate surface and breaks loose at the top in the form of large sheets of oil. These sheets of oil then rise rapidly to the surface of the separation chamber where the separated oil accumulates. For further treatment of the wastewater, a sectionalized removable Petro-ScreenTM (polypropylene coalescer) designed to intercept oil droplets of 20 microns in diameter or larger is located in the effluent end of the oil/water separator tank.

The effluent flows downward to the outlet downcomer pipe where it is discharged by gravity from the lower regions of the separator. The oil/water separator has an oil level monitoring system for efficient operation and maintenance. The oil level float sensor will sound an (audio/visual) alarm at high oil levels so waste can be removed from the separator tank. The monitoring system also has a leak detection probe between the two walls of steel to further protect the environment from any discharges.

Upstream of the oil/water separator, the engineers from Buchart Horn Inc. specified the inclusion of an additional treatment vessel, Highland Tank's model HT-DB 15,000-gallon interceptor tank. The double-basin interceptor is designed to intercept and collect sand, grit, grease, and some free oils before entering the primary treatment tank. The primary function of the tank is to retrofit with an existing 30-inch concrete stormwater pipe at Capital City Airport. The double-basin interceptor was equipped with an overflow bypass to direct the target treatment rate of 3,000 gallons per minute to the oil/water separator and prevent overloading of the oil/water separator in the event of a major storm.

With the use of Highland Tank's treatment vessels, the design engineers from Buchart Horn Inc. have made every effort to ensure that stormwater discharge from the expansions at Capital City Airport will offer no threat to water quality. Fabricated with Highland Tanks' HighGuardTM Corrosion Control System, the vessels are designed with double-wall type-1 construction (360 degree) steel secondary containment and have a 30-year warranty. The inner steel tanks are completely contained within the outer tanks and equipped with a leak detection system on both vessels. The separator equipment supplied exceeds all current federal, state, and local stormwater regulations.

Since their installation, the winning combination of inceptor and oil/water separator has effectively captured and treated the increased volume of stormwater runoff discharges at the Capital City Airport. The two-step process is successfully meeting the water quality requirements of the site.

This article originally appeared in the 01/01/2005 issue of Environmental Protection.

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