Pre-load compactors at transfer stations can have significant environmental and financial benefits. Trailers and rail-haul containers designed for pre-load compaction are lightweight and resistant to leaking during transit. The low tare weight of the vehicles and the ability of pre-load compactors to regulate the weight of the loaded waste make it possible to maximize payloads without exceeding legal limits.
Similarly, maximizing the load carried in each transfer trailer or container reduces the number of vehicles on the road or rails, with corresponding reductions in vehicle emissions, traffic congestion and noise. Significant long-term cost savings can be realized where the reduced transportation costs are sufficient to offset the capital, operating and maintenance costs of the compactor.
Despite these benefits, transfer station owners and operators considering the use of pre-load compaction need to consider waste throughput, transportation costs, the cost of fleet upgrades, unloading technology at the landfill and many other factors. This article focuses on some of the effects pre-load compaction technology has on the design of the transfer station itself.
How to Plan for a Pre-Load Compactor
Pre-load compactors compress solid waste into a bale in an internal chamber before ejecting the bale into a transfer vehicle. Because the trailer bodies or containers do not have to resist outward pressure from compacted loads, they can be much lighter than trailers or containers designed with reinforced walls for top loading or conventional direct-load compaction.
By weighing the waste in the compaction chamber, pre-load compactors allow the weight of the finished bale to be regulated so the gross weight of the transfer trailer is maximized without exceeding legal limits. Pre-load compactors also contribute to operational efficiency by allowing compaction to continue while loaded trailers or containers are being removed and replaced with empty units.
Pre-load compactors are available in a variety of configurations that can be matched to the size and arrangement of a particular transfer station, but they are all large, heavy pieces of equipment — generally 10 feet wide, 14 feet tall, 60 feet to 85 feet long, and up to 220,000 pounds in weight. Large pre-load compactors can handle up to 150 tons per hour.
How to Plan for a Pre-Load Compactor
Planning for the installation of a pre-load compactor requires a sustained collaborative effort on the part of the facility owner, the manufacturer and members of the design team, including the architect and engineers. Bring the contractor and the construction manager into the process as soon as possible and fully review issues related to systems integration, clearances and schedules. Finally, do not overlook the time needed for compactor start up, commissioning and training activities. This will occur at the end of site construction, when schedule delays often collide with plans to open the facility.
The following are some of the important considerations that should be kept in mind during the planning and design process.
Surge Capacity and Bypass Arrangements
Pre-load compactors have a proven track record of reliable operation, but like any piece of complex equipment, they occasionally break down. Large stations with more than one compactor can continue to operate in the event of a mechanical malfunction in one unit, but even these stations are subject to interruptions in electrical service. Pre-load compactors have high electrical loads — up to 320 motor horsepower — that generally make it impractical to power them with a standby electrical generator.
To minimize the need to close a site when pre-load compaction is interrupted, allow for sufficient temporary waste storage volume or "surge capacity." In high-capacity stations, this volume may be quite large, and surge capacity is likely to be the driving factor in sizing any facility.
The need for surge capacity may be reduced by providing alternative means of waste transfer, such as direct-drop into open-topped containers. This "bypass" loading option also provides a means of transferring materials that pre-load compactors cannot handle effectively, such as concrete, bulky items and wire cable, but it increases the complexity of the trailer/container fleet and further enlarges the transfer building.
Gravity feeding is the most common means of loading waste into a pre-load compactor. In this process, waste on the tipping floor is simply pushed into an opening above the compactor feed chamber. This arrangement is simple and reliable, but it requires the compactor bay to be located about 20 feet below the tipping floor, which can be very costly, particularly on flat sites.
Where lack of grade separation, limited site area, a high water table or other factors make gravity feed impractical, compactors can be fed by conveyor or grapple, but these methods have higher maintenance and operational costs than gravity feed. Compactors with the feed opening on the side rather than on top are available, but they are more expensive, less efficient, more difficult to arrange when more than one machine is needed, and more vulnerable to damage during the loading process.
Compactor and Vehicle Clearances
Allow at least four feet at the sides, three feet above and 12 feet to the rear of the compactor for maintenance access. A 12-foot-wide vehicular lane along the full length of one side of the unit is also highly recommended.
A clear ceiling height of 16 feet in the parking and movement areas is common, but this height depends on how high the site's yard tractor lifts the trailer during normal operation.
At the loading end of the compactor, provide space for opening and closing the trailer doors. This area should be close to the compactor and long enough to give the operators some latitude in stopping the truck and trailer. Trailers and containers are fitted with either side-hinged double or single doors. Double doors require at least five feet on each side while single doors require at least nine feet on the side opposite the door hinge.
Hydraulic Power Unit Location
To minimize hydraulic fluid compressibility effects and hydraulic friction losses, place the hydraulic power unit (HPU) as close to the compactor as possible. Securely attach hydraulic pipes and hoses to the building structure or equipment frames to resist thrust loads generated by the fluid during compactor operation, and make sure these lines are easily accessible for replacement. Locating the HPU in an enclosed room to control noise and provide a clean environment for the electrical gear is beneficial, but provide means of removing waste heat from the confined space and allow sufficient access for maintenance.
The operating temperature of the hydraulic fluid is generally regulated by a fan-cooled air/oil heat exchanger placed on an exterior wall so cooling air will be discharged from the facility. Determine a location for the heat exchanger reasonably close to the HPU early in the design process.
Consider the costs and benefits of technology to improve the mechanical and thermodynamic efficiency of the HPU. For example, variable frequency drive (VFD) motor controllers can reduce the electrical consumption of the pre-load compaction system up to 50 percent depending on the throughput of the system, and oil/water heat exchangers can reduce building heating loads by capturing waste heat.
Waste Water Management
Depending on the water content of the solid waste and the extent to which water is used to clean the tipping floor, a significant amount of contaminated water may exit the compactor. Provide drains near the discharge points to collect this water and convey it to a sanitary sewer or other suitable disposal mechanism. The combination of small, light contaminants, like foam packing material, and dense grit or "fines" tends to clog drainage systems. Easily cleaned intake screens, large-diameter drainage pipes with straight runs, and multiple cleanouts with access for a vacuum truck can help to avoid chronic operational headaches.
Compactors are heavy machines, weighing up to 110 tons, and they produce significant thrust loads during operation that must be taken into consideration in the structural design of the building. Their size and weight also means that securing them in the event of an earthquake (if the transfer station is located in a seismically active zone) is not a trivial matter.
Compactors are generally supported on a pair of steel pedestals near each end of the unit. In lieu of independent footings for each pedestal, a single footing spanning between the pedestals will provide a more massive foundation capable of distributing impact loads and ensuring the compactor assembly moves as a unit. Tying the foundation to the surrounding concrete slab rather than isolating it may add stability.
Compactors are commonly secured to the foundation by cast-in-place steel base plates to which the compactor pedestals are welded, or by directly embedding the pedestals in the concrete footing.
Steel base plates do not require precise location during the concrete pour, but it may be difficult to place concrete under the plates, which can be up to four by eight feet in size. Direct embedment is structurally superior, but requires precise placement of the pedestals during the concrete pour; an inaccurately positioned pedestal will be very costly to move.
Compactors are generally placed in below grade tunnels or "bays." Compactor bays with more than one compactor can result in long ceiling beam spans of up to 40 feet. High floor loadings on the suspended slab over the compactor bay may require very deep, costly beams.
Pre-load compactors have the potential to significantly reduce the costs and environmental effects of solid waste transfer operations, but determining if these benefits justify the capital, operating and maintenance costs requires a detailed analysis of the transfer operation, from tipping floor to disposal site. When the decision has been made to incorporate pre-load compaction into the transfer building, accommodating the technology will likely become the single most significant and complex factor affecting the facility design.
Charlie Conway is an architectural project manager for KPG Inc. in Seattle. Richard Lippold, P.E., is a consultant for the solid waste practice of Seattle-based R. W. Beck, an SAIC Company. Dave Miller is a sales and project engineer for SSI Compaction Systems in Wilsonville, Ore.