In addition to use on industrial sites and for spill control, these devices are treating urban runoff.

By Carol Brzozowski

It's a simple principle even a child knows: Oil and water don't mix. But its impact on the environment is anything but elementary, particularly for government environmental agencies, which have enacted laws to protect water from the effects of oil as it runs off of everything from automobiles to airplanes. New regulations are a compliance concern to entities, such as industrial areas, whose activities involve oil.

Bernie Auld of B.A. Engineering in Nashville, TN, believes the issue is relevant to everyone. "Anytime you have a chance for a metals or oils mix— any type of impervious area like a parking lot, because you're also dealing with phosphates and things like that— oil/water separation is applicable," he says. "Your departments of transportation are going to be hit, and I think ultimately each 'mom and pop' shop will be hit. It's not just fire stations or somebody who has a potential for what's referred to as a larger leak or a larger use. Ultimately I think it's going to affect everyone."

The regulations are also of concern to manufacturers, many of whom have developed methods to separate oil from water. One such manufacturer, Oldcastle Precast in Tucker, GA, explains it this way: Oil, which is lighter than water, exists in a near-suspension state of tiny spherical droplets. Gravity separation is achieved by creating conditions in a flow chamber that cause oil droplets to reach a horizontal surface, where they are no longer carried along by moving water.

Typically, one 60-micron droplet takes half an hour to rise 1 ft. in still water. Unless a simple retention vessel is large enough or the water flow rate is kept low, the oil droplet is unlikely to be trapped on the vessel's surface.

Oldcastle cites studies showing that dividing a chamber vertically with a large number of horizontal plates—creating  a horizontal separation area—increases the performance of a gravity separation device. Coalescing plates are typically made with such durable plastics as polypropylene, which enable oil to adhere to them. They are corrugated and inclined to improve servicing and act as gravity separation devices. Water flowing through the separation chamber with coalescing plates is divided into many thin layers, and droplets are easily caught.

Oldcastle points out in its literature that most frequent hydrocarbon contamination of surface water results from small quantities of oil from regular fugitive sources, not from large accidental spills. An excess is considered equivalent to a teaspoon of oil in a 55-gal. drum of water, or levels as low as 5 mg/l. Some oil droplets are hardly detectable, but mixtures in runoff can range in size from one or two to several hundred microns. Traditional spill interceptors— retention tanks with baffles to trap floating oil on the surface— might be useful in a secondary role in a large accidental spill, but small droplets that separate very slowly from water are likely to pass through such a vessel.

Airport Expansions: Deicing and Fueling Areas

Throughout the United States, entities that generate oil runoff are addressing the problems it creates. Airport expansion presents significant environmental challenges because airports are among major sites where oil/water separation units are used. Smaller sites include parking lots, vehicle storage, repair and washdown areas, and industrial sites.

Underlying this effort is the prohibition of the discharge of any pollutant to United States waters from a point source unless the discharge is authorized under a National Pollutant Discharge Elimination System (NPDES) permit. The permitting program tracks point sources, monitors pollutant discharge from specific sources to surface waters, and requires the implementation of the controls necessary to minimize the discharge of pollutants. Airport deicing and fueling facilities are among industrial activities subject to the NPDES regulations.

When the Cincinnati/Northern Kentucky International Airport began a $100 million expansion program for the DHL Worldwide Express Cargo Facility, the challenge of treating stormwater runoff was a major concern. Engineers from the URS Corporation of Tampa, FL, selected the Highland Tank Model HTC EZ Access Oil/Water Separator. The oil/water separators were required to meet the new Underwriters Laboratories Inc. SU2215 approval for engineered oil/water separators rated at 10 ppm oil and grease.

Two 30,000-gal., one 40,000-gal., and two 50,000-gal. oil/water separators were installed. The double-walled devices were supplied with special leak detection and oil level sensors and alarm/control panels.

The model chosen by URS has large rectangular access chambers, allowing for unrestricted access from above to the removable parallel-corrugated plate and Petro-Screen coalescers within the separator for safe visual inspection, cleaning, and maintenance.

Because of the large area for collection of the deicing and transport fueling operations and the required sloping of the piping (typically 1/8 in./ft.), the separators had to be buried with the tops approximately 30 ft. below grade level. The protected steel separators were specially designed for the extreme pressures from the weight of the overburden, says Gregory G. Aymong, vice president of wastewater treatment systems for Highland Tank. Steel-reinforcing rings were welded to the steel shell for additional strength.

Highland Tank's oil/water separators include an underground stationary wastewater treatment vessel filled with water. Internal baffles and coalescers accelerate the oil/water separation process. Water accumulates within the separator while effluent is discharged by gravity. A diffusion baffle serves four functions: to dissipate the velocity head, to direct incoming flow downward and outward, to reduce flow turbulence and distribute the flow evenly over the separator's cross-sectional area, and to isolate inlet turbulence from the rest of the separator.

In the internal sediment chamber, heavy solids settle out and concentrated oil slugs rise to the surface. As the oily water passes through the parallel corrugated plate coalescer, the oil rises and coalesces into sheets 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 globules, which rise rapidly to the surface of the separation chamber where the separated oil accumulates. The effluent flows downward and is discharged by gravity displacement from the lower regions of the separator.

For enhanced oil removal efficiency, a Petro-Screen polypropylene coalescer (a bundle of oil-attracting fibers layered from coarse to fine and encased within a solid framework) intercepts droplets of oil too small to be removed by the parallel corrugated plate collector.

Oil-level sensors can sound an alarm at high oil levels. Double-wall separators can be furnished with a leak detection system. Highland Tank also designs systems with single basins, double basins, or triple basins and aboveground systems.

California Fire Stations

The challenge for the City of Palo Alto, CA, was to upgrade and maintain its oil/water separation process at two of its fire stations. The existing fueling and wash pads had been discharging directly into the sanitary sewer system, and the city's waste program required pretreatment.

In the past, the industrial nonpoint-source discharges had been treated through large-volume modified septic tanks. The city chose ecoSep oil/water separator units, with city officials hoping to realize a return on the investment within three to four years.

The ecoSep's below-grade oil/water separation unit has a grit chamber that removes solids from the influent. The grit trap is the first of two concrete tanks in the design. The grit chamber compensates for influent temperature fluctuations and oil concentration influxes and initiates the separation of light fluids. A perforated 90º outlet tube prevents solids from entering the separation chamber.

The water then goes to a gravity separator through a float-activated shutoff valve in the inlet, and the oil floats on the surface; ecoSep separates light liquids that have a gravity below 0.95.

How They Work

In residual oil media, fine droplets too small to be separated solely by gravity accumulate into larger drops that rise to the surface. The coalescing media is made of reticular foam. A venting pipe provides an effluent sampling port. The separated water has a residual contamination of free petroleum content of less than 5 ppm.

Most oil/separation units work in a similar fashion, though each manufacturer has a variation on the common principles.

According to PS International in Sioux Falls, SD, the location of inlet and outlet piping in its separators ensures that the device remains full of liquid to use the total volume of the vessel. Incoming flow is directed toward a corrugated plate placed perpendicular to the inlet pipe, which reduces the velocity of incoming flow and spreads it out. The flow patterns created by the corrugations in the plate separate solids and cause oil droplets to coalesce. Additional corrugated steel plates— parallel to one another, slanted to prevent solids from accumulating, and spaced far enough apart to prevent debris from becoming trapped between them— are used in the second stage of separation.

As the stream moves through the separator, solids collect in front of a sludge baffle, which is situated below the access way to allow the sludge to be easily removed. The baffle directs flow upward toward the top of the vessel, encouraging small oil particles to float in the water stream.

At the opposite end of the separator, an outlet pipe draws water from the least turbulent section of the vessel; the effluent contains a free oil and grease discharge of no more than 15 ppm in some models and no more than 10 ppm in others.

Some oil/water separators offer special options. The PSI units, for example, can include an additional polypropylene coalescer, consisting of a matrix of polypropylene fibers designed to coalesce oil droplets down to 20 microns.

The company also offers a line of electrical accessories, including high-oil-level alarms, leak detection systems, automatic oil and water pump-out systems, automatic inlet shutoff valves, and freeze protection. In addition, the company can custom design a separator to include solids compartments when high sludge loading is anticipated, water compartments with simplex or duplex pumped discharge systems, or three-phase separation systems.

Royal Environmental offers an automatic oil draw-off device to remove accumulated light fluids from the water surface and store them in an oil receiver.

With new environmental regulations coming into play, engineering firms that have to consider oil/water separation units are just beginning to learn what makes them tick, how each manufacturer's unit differs from others, and how to quantify the results.

"Each individual case can mean something different, and it's important for engineers to study up and learn about the products that are out there," says B.A. Engineering's Auld. "There is no one product that can always be used, because each product has a different application and a different case where it can be used for different types of applications."

Performance Testing in Tennessee

In Franklin, TN, various models are being tested. Don Green, stormwater coordinator for Franklin, says the municipality has installed one oil/water separation unit in a municipal parking lot and had planned the installation of four at a municipal center. Those four are different types and will be monitored to determine which one works best, Green relates.

City officials determined that the amount of oil, grease, and gas runoff from the sites necessitate the installation of the units. "We're right here on the beautiful Harpeth River, and we want to protect it," Green says.

Engineers consider space an issue when determining whether oil/water separation units will be used solo or as part of a group of best management practices (BMPs). In differentiating oil/water separators from hydrodynamics, Auld notes the biggest difference is when higher velocity is involved because there is no natural separation.

"In a lot of these areas, what you're looking at is the first flush— trying to separate that from the other. With hydrodynamics, you've got to get your lowest velocities; otherwise you get a mix where you can't separate your oil out. When you have something that's standing still, some type of pondage, you can use anything from a skimmer to what I refer to as a sock, something like that that absorbs the oil."

With mechanical units that require maintenance, he notes, "there is so much expense involved that you try to do something that's much lower maintenance and deals more with just gravity. If you can afford the area or the land, you can use skimmers and pull the oil off that way. But you have to have the very low velocities to be able to allow the oil to rise to the top."

In rural areas such as those that surround Nashville, land is available with which to use a variety of means. "We've even combined detention areas with wetlands as an oil-type removal," Auld says. "That was the cheapest, plus we were able to get wetland credits. I've done it on two different projects in both Tennessee and Arkansas."

In urban areas, Auld says, the smaller the mechanical means, the less expensive. He also favors treating in smaller areas, such as spraying airplanes in one area of an airport and recapturing the oil/water mix there.

"When you're dealing with stormwater, you have rapid velocities, so you don't have the luxury of going through a baffle system because you have so much mixture, you have to get through a filter-type system. If you can afford to settle it out on open land, that's going to always be your least cost," Auld points out.

Jared Gray concurs. He is with the commercial development division of Ragan Smith Associates, also in Nashville. Gray says his firm sometimes specs out oil/water separators as an alternative to stormwater filtration, depending on the application.

"Particularly we've found them useful whenever the application of a water-quality sedimentation detention pond is not available to us due to constraints on lot size or any other factors involved," Gray says. "They seem to do a fine job relative to cleaning the stormwater as stated; otherwise we couldn't get them through the application process with the cities."

One of the advantages of oil/water separation units is the sedimentation time— the time when the stormwater actually falls into the unit— as well as the design of the tangential inlet pipe and the outlet invert being above the inlet invert, Gray says. "That helps to eliminate some of the resuspension that potentially could occur if you have a rather large rain event come into the system."

In comparing the effectiveness of oil/water separators in capturing oil, grease, and hydrocarbons with that of catch basin inserts containing absorbent filter media, Gray says the biggest difference is that a catch basin insert can filter only about 1-2 ft.³/sec. "Another issue with catch basin inserts is, as they fill up with sedimentation, they lose surface area," says Gray, adding that oil/water separators by design can treat more flow.

The cost of oil/water mechanical separator units compared to other BMPs runs the gamut, Gray says. "It's not the cheapest alternative. The cheapest alternative by far is to put it in some type of detention pond. However, whenever you're looking at vault systems, the larger vaults have filter cartridges that have to be replaced, then these units are actually cheaper. We found them to be more cost-efficient for both the contractor and the client."

Gray says he stays away from baffle-type systems. "They do an adequate job, but the problem is if you don't clean them, it really doesn't do as good of a job in eliminating the total suspended solids from being revamped into the system in a main rain event, so the resuspension aspect on a baffle system is more likely in that case."

How common the use of oil/water separation units is as a preventative measure in places with the potential for industrial or chemical spills depends on the application. Auld notes that industrial spills require total containment, including precautionary double tank systems and leak detection. "On that, the industry is hit with a different standard," Auld says. "Preventative maintenance is dependent upon the individual city's NPDES requirements. In terms of a metropolitan area, it deals with water-quality standards."

Water/oil separation is only one part of the BMP and clean-water processes, Auld states. "You have grit, small particles suspended, and even phosphates," he adds.

Green points out that the units work in combination with other BMPs, including swales and detention ponds. "We have all three of those working in what we call a 'treatment train,'" Green says. "On smaller sites, like our parking garage, it's so small that we don't have a combination; we don't have a detention pond or any kind of bowed detention like a swale or anything, so they work better probably in a treatment train. But if you have small sites, sometimes it's all you have. They are good in small areas where you've tried to retrofit in something that will solve part of a problem."

Maintenance schedules depend on a number of factors, Auld says. With stormwater, it depends on the frequency of rainfall events. In the Nashville metropolitan area, for example, monthly checks are the norm.

"Once you do your check, then you get into maintenance issues, and that may be anywhere from three months to six months, depending on the type of process you have," he says.

As for quantifying results, engineers and entities using oil/water separators are relying on manufacturers' research and development studies to compare results. Some government entities, such as Franklin, are choosing a variety of units in order to determine effectiveness.

Another issue to consider is how to treat the oil. Auld treats the mix of oils and metals as a hazardous waste but adds that it's up to each public entity to determine how to deal with it. "I know some cities are saying, 'It's not our problem,'"Auld notes. "Because of the solids in it, I would tend to treat it as a hazardous waste, but there are different classes. It's considered a very low class of hazardous waste."

Green also isn't sure how the oil will be treated. "We are going to be a Phase II compliant city, so we've got to go by the Tennessee Department of Conservation standards about how to treat this, and I assume it's going to be treated as hazardous or semihazardous waste," he relates. "We'll have to dispose of it in some other way than a landfill, but right now I'm not sure how we're going to dispose of it."

Carol Brzozowski is a journalist in Coral Springs, FL.

SW - May/June 2003

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