WHEN A CAR RECENTLY BACKED OVER THE SAFETY RAILS at a push-pit style transfer station in Snohomish County, Wash., no one was surprised, and fortunately no one was hurt. Those kinds of accidents have been all too common over the years at one of the county's three push-pit transfer facilities.

In fact, customers and employees have been falling into pits at transfer stations for years. Spokane, Wash., Solid Waste Management Director Dennis Hein recalls an incident at a facility he managed in another state in which a homeowner purposely backed a pickup truck over the transfer station pit, set the brake and began pushing trash out of the back of the truck with a broom. At one point the broom flew out of his hands, and when he reached for it, he lost his balance and dropped into the pit. He was embarrassed but otherwise fine. On another occasion at the same transfer station, a woman wasn't so lucky, Hein says. When she lost her balance and fell into the pit, a shard of steel speared her through the back and punctured a lung. Fortunately, she survived.

“I was always a dyed-in-the-wool pit man,” Hein says. But his experiences with flat-floor transfer stations in Spokane have changed his mind. “We've been operating [flat-floor] stations since 1991 and never had a serious accident. I don't know of anyone that has decided to go back to a pit station after working in a flat-floor facility.”

What is a floor-floor facility?

The definition of a flat-floor transfer station varies; however, by any definition, it is a facility without a push pit. For some, a flat-floor transfer station is simply one in which trucks and transfer trailers use the same floor level. “There are one-level, flat-floor facilities,” says Keith Gordon, a principal with Albuquerque, N.M.-based Gordon Environmental. “Most are retrofits, where they tried to use an existing building with a big floor. To me, a flat-floor transfer station is an oddity.”

Other engineers define flat-floor transfer stations as two-level facilities in which arriving commercial and residential trucks dump directly onto an expansive flat floor, instead of into a surge pit. A wheeled loader then pushes the trash into an opening cut through the floor where it falls into a transfer trailer positioned in a tunnel under the opening. Gordon calls that “a tunnel station, not a flat-floor station.” Whatever you call it, “that design is the trend,” he says. “It is by far the most common design for [new] transfer stations.”

However, Karl Hufnagel defines the two-level design as a flat-floor transfer station. As senior director with Seattle-based R.W. Beck, Hufnagel has been involved in several flat-floor transfer facilities, including two in Snohomish County. Hufnagel also works with King County, Wash., which has two flat-floor stations currently scheduled for construction.

In contrast to a more conventional transfer station, a flat-floor facility as defined by Hufnagel lacks a third level. A traditional facility features a tipping floor where trucks dump solid waste into a pit, in which a tracked loader macerates the material and pushes it into another opening, through which the waste drops into a transfer trailer. Whether the facility is called a flat-floor or tunnel-style transfer station, the design eliminates the pit level, but not the tunnel level. The two grade changes in a conventional transfer station raise costs significantly, Hufnagel says.

Safety, Efficiency

Neil Fujii, managing engineer in King County, says the flat-floor design is the safest and most efficient. “Whenever you tip into a pit, the elevation difference creates a fall hazard,” he says. “When you have a fall hazard, you have to design some kind of fall restraint. More often than not, fall restraints obstruct dumping and slow down the trucks.”

Flat-floor designs also work better with rail transport systems, Fujii says. “The more tons you get into a rail car, the lower your costs. With [our] older style transfer station, the waste goes directly from the packer truck into the trailer, and you can't compact the load,” he says. King County operates four older stations that will not work when the county begins using rail transport in about six years. “The sites are small, too, so we can't expand. So we're building new flat-floor facilities.”

Hein says large flat tipping floors offer advantages, such as making it easier for workers to screen waste. One of Spokane's three flat-floor transfer stations feeds its waste to a waste-to-energy plant. To ensure the quality of the fuel, non-combustible waste must be removed at the transfer station. “We segment the floor for different kinds of waste,” Hein says. “Green waste is dumped at one end. White goods have their own area. Long, flat floors make it very easy to segregate elements of the waste stream.”

Pit-Style Advantages

Hufnagel points out that old, pit-style transfer stations have their advantages, too, particularly the space to store waste. A pit can store refuse for weeks when levels surge and cannot all be loaded into transfer trailers. “The number one advantage of a pit is storage volume,” Hufnagel says. “You will probably have to clean up a flat floor at the end of the day.” In addition, because a pit-style station is designed to store surge waste, “the overall footprint of the building with a flat floor has to be larger,” Hufnagel says.

Tracked equipment also can be used down in the pit, where heavy metal tracks compact larger loads for the trailers. Of course, the rubber-wheeled equipment used on flat tipping floors does not macerate the waste.

And, while people fall into pits, pits offer safety advantages that flat floors do not. “On a flat floor, trucks, heavy equipment and people often occupy the same space,” Hufnagel says. “A pit at least separates people from the yellow iron.”

What Kind of Pit Was It?

Small pit-style facilities offer few advantages. Snohomish County recently replaced two of its three push-pit transfer stations with flat-floor facilities. The old facilities' pit was 15 feet deep, 12 feet wide and 100 feet long. “We could only store about eight truck loads of garbage in that pit,” says Leo Kypuros, county operations manager. “Our new flat floor can store several days worth of garbage. We can also dump multiple trucks at once. Once the pit is full, the next truck has to wait until the rams clear out the waste.”

Snohomish County's two new flat floor transfer stations are about 200 feet by 200 feet, or 40,000 square feet (sq. ft.) of space. Engineers say that 200 feet is the maximum width of a flat floor station, because wider structures require columns for stability. While 200 feet is the maximum width, flat-floor transfer stations can stretch out. “You increase capacity by increasing the length of a flat-floor station,” Hufnagel says.

The 40,000 sq. ft. facilities were large enough for Snohomish County, according to Kypuros. He says the county's facility in Everett, Wash., is completely enclosed by small doors to facilitate bird control. The design for the Everett transfer station also had to address stormwater issues. “[At $26 million], it turned out to be a pretty expensive installation,” Kypuros says.

The second station, built into the side of a hill, required an unusual amount of earth movement, and its tab eventually reached $24 million. “I think that the numbers really represent the high end of cost for these kinds of facilities,” Kypuros says. “You can build a flat floor transfer station for about $16 million, if you have an accommodating site and do not run into objections from homeowners and businesses in the surrounding neighborhood.”

Nevertheless, when Snohomish County replaced its small surge pit facilities with two-level transfer stations featuring large expansive flat floors, they enhanced both the safety and productivity for customers and operators alike. While the majority of existing transfer stations still remain three level facilities, those operating in Washington may serve as good examples of the flat-floor facility's long term benefits.

Michael Fickes is Waste Age's business editor.