A productive transfer station design doesn't occur by accident. Careful strategies must be decided and business forecasts should be in place before operators review their equipment options.
No aspect of your operations is too basic to overlook. To start, you must know your facility's particulars. Transfer stations consist of two types:
*The open top system. Here, the waste is lifted from the tipping floor and dropped into a gondola-style transfer trailer. Some compaction occurs in the trailer as the operator uses the bucket to smash and level the waste for even loading and weight distribution. The loader operator's skill and the type of waste handled dictate the maximum weight loaded into the trailer. Often, this results in light transfer loads ranging between 18 and 22 tons. Here, waste is transferred by using either a bucket or clamshell loader.
*The compaction system. This system maximizes the weight transferred per load, since the waste is loaded into a compactor which compresses it into an enclosed trailer. Here, you have two choices: waste is compressed directly into a trailer or it is compacted into a separate chamber and extruded as a slug into the trailer. If you use the first method, the trailer must be structurally strong enough to withstand the compactor's force.
The trailer used with a pre compacted slug may be lighter, increasing the payload to as much as 35 tons per load. Since the waste is compacted into an attached chamber, it is possible to continue processing without the trailer present.
The selection of the type of transfer station operation seems to be largely a factor of geographic location. "Open top loading has been used more on the East Coast and Southern United States," explained Chris Brockway, project manager for Black and Veach, Overland Park, Kan.
"On the West Coast, there are more compactors due to the material's dryness. In areas like Florida, where it's relatively humid and the moisture content is high, it doesn't take much compaction in a trailer to get a maximum payload. However, farther West, it's dryer, and, therefore, the waste is less dense."
Other factors affecting the design - and ultimately the equipment type - include the:
*tons to be transferred;
*number of collection vehicles to be served;
*capital equipment budgets available;
*distances and geography of the route from the transfer station to the landfill site; and
*time it takes to load a trailer.
"Your throughput is a function of how quickly you can load and cycle trailers," Brockway said. "At some facilities, [it takes] somewhere between 10 and 15 minutes to cycle a trailer. A trailer load of 20 tons payload means about 80 tons per hour. Compactors vary by unit size; the old stationary compactors are up in the 70- to 80-ton-per-hour range, and the preload compactors have a maximum capacity of 125 tons per hour."
While most transfer stations are designed for current waste load configurations, experienced operators suggest that transfer stations should be designed for the higher capacity in the near future. For example, the South Utah Valley Solid Waste District's transfer station in Springville, Utah, experienced a dramatic increase in the waste generated due to population growth. In 1991, when the population was approximately 125,000 people, the transfer station anticipated the annual tonnage to be 75,000, said district manager Dale Stephenson. "That year, we did 82,000, and in our current fiscal year, it looks like we're going to finish somewhere between 110,000 and 115,000 tons processed. We've seen approximately a 40 percent increase in the six years we've been in operation."
While it is possible to design a simple transfer station in-house without technical help, use of engineering and design firms which have expertise in transfer station layout can avoid problems. Brockway suggested that the first step would be selecting an experienced consultant. While this may sound elementary, he explained, "there are people who claim that designing a transfer station isn't rocket science. That's true: It's not complex to design a transfer station, but the trick concerns all the little oddities that people never think about, and thus, end up making mistakes."
Also, make sure that your help consults with you during the process. "Engineers and consultants sometimes don't work with the facility operators," Brockway said. "Each transfer station depends on its operator's philosophy. Engineers should sit down with the operator and make sure that the operator is comfortable with the design that the engineer proposes."
Selecting Equipment Once the operating parameters have been determined, you can start identifying and developing the equipment specifications necessary for operation. The type of fixed and mobile equipment required must be considered during the overall design of the transfer facility to ensure proper orientations, physical arrangements and electrical and mechanical hook-ups. Generally, transfer station equipment can be boiled down into three functions:
*movement of the waste from the tipping floor;
*waste loading, placement, and compaction on the transfer vehicle; and
*the transfer vehicles.
Once the waste is delivered, it is dumped onto the tipping floor or pit area. Here, the primary equipment used include track or wheeled loaders, fixed or mobile clamshell hoist buckets or live floors. For smaller facilities, a tracked or wheeled loader, equipped with either a blade, bucket or integrated claw is useful. The wheeled loader can push material into either a drop pit, into an open-topped trailer or be used to feed a conveyor to a compactor.
The loader's additional benefit in facilities that do not use any compaction system is that it will help compress the waste by crushing it on the tipping floor prior to loading. When loading the transfer trailer, the bucket is used to level and pack waste into the trailer, increasing the payload.
Clamshell excavators also may be used to load waste into the transfer vehicle or compaction equipment. "The most common is the stationary clamshell which is bolted to the tipping floor, swings back and forth and compacts the load in the trailer," said Brockway.
"At larger transfer stations, you sometimes see mobile clamshells, which are rubber-tired excavators," he continued. "Their advantage is that if you have multiple hoppers, you can handle more than one with a single piece of equipment. It also gives the ability to bring in another piece of equipment if one goes down for maintenance. With the stationary unit, you don't have the ability to switch equipment easily. The downside of mobile units is that they're more expensive."
While live floors have been used primarily in material recovery facilities to move recyclables from the sorting line to the baler, their use is expanding to include moving waste from the tipping area to the open pit or compactor.
"[Live floors] can move as much garbage as a front end loader," said Don Hepperle of Keith Manufacturing, Madras, Ore. However, while it is possible to load transfer trailers with a front end loader in as little as two-and-a-half minutes, consistency is not possible.
On the other hand, live floors "don't have to depend on an operator to achieve consistency and the system can be automated. All the operators have to do is watch for the trailer to get full," he added.
Live floors can move a lot of material and can be sized according to the operation's flow needs. "We've moved 50 tons per hour or more," reported Russ Halvorsen of Hallco Manufacturing, Tillamook, Ore. "It just depends on your desired operation and floor size. We've built floors 20 feet wide and 100 feet long which will move at about 10 feet per minute, but most operations don't ever want to move that fast."
Scrutinizing Compactors While an open top loading system loads the waste directly into the trailer, compaction equipment increases the materials' density and the tons transported. Two types of systems are designed for transfer stations: the stationary and pre-load compactors.
With a stationary compactor, the waste is moved from the tipping floor into the unit, where it is compressed directly into the enclosed container or trailer. "The stationary compactor is basically a ram, and the actual compaction takes place inside the trailer," Brockway said. The pre-load compactor uses a separate chamber where the waste is compacted prior to loading.
Some of the pre-load compactors allow variable weights to be established to take advantage of local laws. "Certain states are more restrictive on their over-the-road legal hauling limits," said Gary McLeskey of SSI Shredding Systems, Wilsonville, Ore. McLeskey reported that a system with a variable density capability which can compress a bale can be helpful if a facility is loading compacted waste directly onto a rail car or if it is in a state which mandates a maximum of 80,000 pounds.
Another advantage is a reduction on trailer wear-and-tear. "You're not putting those impact loadings on the trailer's suspensions or on the trailer's live floor," said Kenny Japhet of Transpak, Salem, Ore. "[After compaction] when it's extruded into the trailer, there's no trailer damage and no wear within the trailer."
Whatever equipment you choose, don't underspec it, warned Christina Harris of Marathon Equipment, Vernon, Ala. "Also, the cycle time and machine capacity should exceed the estimated volume by 20 percent," she added.
Moving It Out Transfer vehicles are the final piece of major equipment to be considered. The particular trailer that is purchased will depend on the type of transfer station operation and on whether truck or rail will perform the transfer. For open top loading stations, transfer trailers which are designed to haul larger volumes of low density materials will be required. These trailers will be lightweight in order to maximize the payload.
For transfer stations which will be delivering waste by truck or rail, a solid, leak proof container should be purchased. If rail is used to transfer, then buy an intermodal container which meets all standards and can be handled by the rail system. If trucks are used, consider appropriate vehicle weights, dimensions and unloading characteristics.
While many transfer trailers use self-contained hydraulic lifts to raise the body for waste discharge, such trailers can roll-over because of the landfill's uneven ground. Also, if there is no self-contained unloading system in the trailer, such as a live floor, the site must have a mechanical tipper to unloaded the trailer.
Floor Care Transfer station floors are constructed with concrete of varying thickness and of either low, medium or high compressive strengths and with various additives. Since all concrete is brittle, it wears down and eventually must be repaired.
Floor failure occurs generally when the surface becomes worn down about two inches and the equipment starts getting snagged on the rebar in the concrete. Although the floor is an important part of operations, most facility owners and operators feel that this wear and tear is simply a cost of doing business.
The cost of the floor can be measured in the same way as other operating expenses, in cost per ton. "For a tipping floor processing from 800 to 1,000 tons a day, this cost can range from 8 to 12 cents per ton, or $23,000 to $43,000 per year," said Lee Smith of Master Builders, Cleveland.
An iron surface topping can be added to extend the floor's life. "The level of protection should be based on the size and scope of a facility's operation," said Smith. In addition, he suggests that you work with a specialty contractor with extensive experience in both the waste industry and with concrete surface preparation and bonding of specialty toppings.
Equipping a transfer station can be either simple or complex, depending on the amount of planning and design work accomplished prior to buying equipment. Understanding the entire waste transfer process and knowing the waste's destination is probably the most important aspect of the purchasing process.
"I often hear people say, I wish I knew about all this stuff before we designed our transfer station," said Brockway. "It's not the big items that really make a difference; it's the little things that come back to haunt you."