The Basics Of Equipping Transfer Stations

Erik E. Colville, Joseph Harrington and Nancy J. McFeron

Since purchasing transfer station equipment can be just as important as buying your first house, there are a few basics every operator should know.

Transfer station operations that deal directly with the movement of solid waste through the transfer station fall into four categories: compaction, weighing, conveying and processing.

Selecting equipment to suit the needs of your transfer station depends on the needs of the project and the project capital budget.

Compaction Equipment Carefully evaluate transportation costs when selecting transfer station equipment. To minimize costs, compaction equipment maximizes the amount of waste that can be loaded into a transfer trailer or shipping container. Compaction equipment includes tracked vehicles for maceration of waste, knuckleboom cranes, solid waste balers, solid waste extruders and preload compactors.

Maceration is achieved by driving a tracked vehicle, such as a bulldozer or loader, back and forth across the solid waste on the floor surface or tipping floor. This process breaks down large items such as cardboard boxes and wood crates and can increase solid waste density from approximately 300 pounds per cubic yard to as much as 900 pounds per cubic yard, although 500 pounds per cubic yard (on the tipping floor) is routinely achievable. Tracked loaders cost between $140,000 and several hundred thousand dollars and is available with specialized solid waste features such as trash guards and track cleaners.

Although it does little compacting, a knuckleboom crane is technically considered a form of compaction equipment. Knuckleboom cranes primarily are used to distribute loads more evenly within the transfer trailers so that trailer loads are maximized without exceeding legal axle weights. To increase waste density, the knuckleboom crane also can be used to break down large items. An increase in density from 300 to 375 pounds per cubic yard is routinely attainable with this method. Knuckleboom cranes can cost approximately $80,000 for a medium capacity unit.

Balers can be used to compact solid waste into roughly three-foot square by five-foot-long bales with a density approaching 1,300 pounds per cubic yard. The waste is formed into a bale inside a compression chamber and then tied with wire or twine so it will retain its shape and density during handling and hauling.

Solid waste balers are appropriate for small transfer stations that handle up to approximately 200-tons-of-waste per day and for stations preparing waste for a balefill. These balers are limited in use because the operating cost and labor associated with moving many small bales to a disposal site is fairly high. Their use is better justified when the waste disposal site is distant from the transfer station and when there is a need for recyclables to be baled. Solid waste balers range in cost from $170,000 to more than $300,000.

Solid waste extruders receive waste and push it into a chamber that has an "S"-shaped outlet path. The "S"-shaped path provides resistance to the movement of waste through the extruder and results in compaction. The waste exits the extruder in a continuous log which must immediately be placed in a transfer trailer or shipping container so it will maintain its shape and density. The transfer trailer or shipping container is typically subjected to the compaction force to attain maximum waste density. Typical density from using an extruder is 500 pounds per cubic yard. Extruders cost between $300,000 and $450,000.

A typical preload compactor receives 20- to 27-tons of garbage, compresses it within the compactor unit into a "loaf" approximately seven feet square by 40 feet long and then ejects the loaf into a semi-trailer or shipping container. Loaf densities of 500 to 600 pounds per cubic yard are considered typical. Because preload compactors help to maximize the transfer trailer/container payload and to minimize transport costs, they are used in nearly all waste export systems when the disposal site is more than 200 miles away. Preload compactors range in price from $325,000 to $600,000.

Weighing Equipment Weighing equipment measures the quantity of solid waste that is handled at a transfer station. It also gives transfer station operators feedback so that they can avoid overloading transfer vehicles and incurring road overload fines. Weighing equipment, which includes platform scales, axle scales and on-board trailer load cells, generally are located in the path of arriving and/or departing customer vehicles to record the vehicle's weight.

In small transfer stations, a single platform scale may be used for arriving and departing customers. However, larger transfer stations may use one or more scales each for incoming and outgoing customer vehicles. The number of scales needed at a particular transfer station depends on the number of vehicles that are handled per hour and the different customer vehicle types that must be weighed. Vehicles that have recorded tare weights and flat rate vehicles may not have to be weighed.

Platform scale lengths vary in length from 20 to 70 feet, depending on the type of customer vehicles expected. If the weight is needed for recording and not for the basis of disposal fees, a short scale can be used to split weigh axles on long vehicles.

There are two basic types of platform scales: pit and pitless. A pit scale generally is installed flush with the surface of the ground, with the working parts of the scale located in a concrete pit below grade. The working parts of a pitless scale are located above grade so vehicles must drive up a ramp to access the scale. Pitless scales can be relocated without leaving an expensive concrete pit behind and the pitless arrangement allows easier maintenance access.

Most platform scales are installed with computerized data management systems to automatically keep track of waste types, customer charges and other relevant information. The cost of a 35-foot-long scale is approximately $25,000 and a 70-foot scale is approximately $35,000; a computerized data management system costs an estimated $15,000. Installation costs account for the price difference between the two types. Installing a pitless scale, including the foundation, costs approximately $15,000; installing a pit scale, including the foundation, costs approximately $25,000.

Axle scales are used in the trailer loading area of the transfer station. These scales are strategically placed so that the trailer axle can sit on the scales while the trailer is being loaded. The location of the axle scales allows the transfer station operator to read the display and distribute the waste evenly within the trailer during loading. The cost of axle scales is approximately $24,000, including the weight indicator display.

On-board trailer load cells are used for the same purpose as axle scales. They are installed on the trailer frame or axles and then connected through a pigtail to the transfer station weight indicator. Due to the abuse they experience while the trailer is on the road, on-board trailer load cells generally cannot be calibrated for use in collecting fees from customers.

Conveying Equipment Conveying equipment can be divided into two categories: mobile tractors, which include rubber-tired loaders, yard goats and tracked dozers/loaders; and mechanical conveyors, which include belt, apron, drag chain and live floor conveyors. For capital cost reasons and for long-term reality, most transfer stations only use a rubber tired loader or tracked dozer/loader for conveying solid waste. The non-uniform and abrasive nature of solid waste will subject conveying equipment to severe wear, which means that over the life of the transfer station, all forms of mechanical conveyors will require substantial maintenance to remain in service.

Tractors. Rubber-tired loaders are used primarily to push waste across a flat tipping floor and over the edge of a retaining wall into a transfer trailer. The loaders are sometimes used by small-volume stations to pick up waste and drop it into a transfer trailer.

Rubber-tired loaders also can remove a full transfer trailer from the transfer facility and move it to a holding area on the premises for a transfer vehicle to pick up. This requires the use of a fifth wheel dolly to support the front end of the loaded transfer trailer as it is being moved. The loader then moves an empty trailer into position to be loaded. Some varieties of rubber-tired loaders can be equipped with various interchangeable attachments such as light to heavy duty-buckets, utility forks, snow removal and salt spreader equipment, pavement sweepers, backhoe attachments and various rakes and blades. They also are available with specialized solid waste features such as trash guards and foam-filled tires to prevent flats. Rubber-tired loader costs, not including accessories, range from $85,000 to $150,000.

A small semi tractor yard goat, designed to maneuver full trailers around the yard of a facility, can be used to move loaded and empty transfer trailers on-site. Since this type of yard tractor can only be used for this purpose, it would be cost-effective only at a large facility whose operational procedures require loaded trailers to be moved frequently.

Tracked dozers/loaders are used to macerate waste in a pit prior to pushing it to the edge of a retaining wall and into a transfer trailer.

Mechanical Conveyors. Belt conveyors are frequently used to convey waste from a tipping floor to a loading hopper of a baler or preload compactor. The belts are made of rubber with a heavy-duty roller chain attached at the outer edges for driving the belt. Cleats protruding from the belt's surface help minimize waste slipping on the belt as it is conveyed up an incline. The belts are typically flat but can be shaped in a trough.

Apron conveyors are generally made of a continuous series of metal plates of steel or aluminum, with hinges between the plates so that they ride and return along a track. This type of conveyor is more expensive than a belt conveyor but typically will last longer, resulting in a competitive life cycle cost, especially when conveying abrasive waste.

Drag chain conveyors are made of a solid metal trough. Within the trough ride is a series of metal paddles attached to one or two chains. The chains run the length of the trough and pull the paddles. Waste in the trough is pulled along by the paddles. This type of conveyor is used when waste must be elevated on a steep incline, when waste temperature or chemical composition would damage a belt conveyor or when the consistency of the waste requires a leak resistant conveyor.

A live floor conveyor consists of parallel metal slats with the long axis oriented in the direction of desired waste flow. To move the waste forward, all of the slats slide a short distance in the direction of flow. The slats then are retracted a few at a time to the original position. While each slat is being retracted, the waste remains in place, moving only when all the slats again move forward at once. The process is repeated, moving the waste forward step-by-step.

Live floor conveyors have been commonly used in self-unloading transfer trailers for more than 10 years. This equipment has been shown to withstand the highly abrasive characteristics of waste and the impact loads common to solid waste operations while requiring less maintenance than other mechanical conveying methods. Generally, live floor conveyors cost more than the other conveying methods.

The cost of mechanical conveyors is a function of their length and width. A common baler-loading conveyor will cost about $50,000 for a 200 ton per day capacity while one for 60 tons per day will cost around $25,000. A common eight-foot-wide by 40-foot-long live floor conveyor will cost approximately $18,000, not including installation costs.

Solid waste processing at transfer stations is not a common practice; it is often done at materials recovery facilities or refuse-derived fuel production facilities.

Shredding or sorting is the main form of processing at transfer stations. Shredding can be the first step in a materials recovery or refuse-derived fuel processing system or a step prior to incineration since it makes the waste more uniform and allows the density to be maximized. Sorting, however, is usually done on the tipping floor where undesirable materials, such as bulky items, hazardous wastes or recyclable materials, such as cardboard, white goods or yard waste, are removed by hand.

Whether you are in the market for transfer station equipment or not, a basic understanding of the equipment will benefit your facility in the long run.