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Landfill operations are unique compared with other industrial or construction operations in that they require a relatively large up-front capital cost and subsequently a relatively small per-unit operating cost. The capital costs involved in the construction of a landfill include engineering (planning, design, and permitting), liner construction (excavation, structural fill, compacted clay, geomembrane, and leachate controls), quality control (surveying, material testing, and project management), and eventual final cover installation (cover soil, compacted soil cap, vegetation, landfill-gas systems, and surface-water controls). Compared to this multitude of construction-related activities, landfill operations consist of only three things: environmental monitoring (sampling and testing of groundwater, surface water, landfill gas, leachate, and air emissions), overhead (billings, accounting, and management), and last, but certainly not least, waste disposal itself (waste receipt, waste spreading, and waste compaction). Waste receipt is a function of the waste-hauling trucks carrying waste into the landfill, getting weighed at the entrance truck scales to measure the loads, and dumping the waste at a location on the current workface designated by a spotter. Only the last two tasks, waste spreading and waste compaction, are performed by the landfill’s fleet of heavy equipment. It’s these tasks, along with the equipment and operators that perform them, that will be the focus of this article. Specifically, this article will examine those factors that affect the efficiency and productivity of the waste-management equipment. The Required Fleet
MSW requires efficient spreading and compaction operations that should reduce the volume of the incoming waste by at least 50%. For the most part, modern landfills utilize the area method of waste disposal: Waste is deposited in an area called the workface that is large enough to allow for efficient compaction and the integrated choreography of incoming waste-hauling trucks and waste-disposal equipment. Old-style disposal methods utilizing pits, trenches, or ramps have been largely abandoned since the widespread adoption decades ago of Subtitle D regulations mandating the use of engineered landfills with complex liner-containment and leachate-removal systems. The deposited and compacted waste will then need cover placed at the end of each workday to prevent the spread of disease vectors, odors, or blowing debris. This cover can be plain soil (usually a layer of 6 inches spread over the current workface) or some alternate material, such as plastic sheathing or spray foam. The type of cover used (or if it is to be reused as in the case of soil cover being stripped and stockpiled for further use) will determine a significant portion of the landfill’s equipment fleet. All other factors being equal, the minimum waste-management equipment fleet required by a landfill varies with the average daily incoming waste receipts. A typical scheme for categorizing the size of the daily waste receipts utilizes intervals of 250 tons of waste per day. There are small-scale landfills (0–250 tons per day), medium-scale landfills (250–500 tons per day), and large-scale landfills (500 tons per day or more). Each has its appropriate range of waste-handling equipment:
Caterpillar has a proven series of waste-management compaction and handling equipment. Caterpillar’s machines spread and compact the waste efficiently. For small-scale landfills, these include track-type loaders or track-type tractors. For medium-scale landfills, Caterpillar recommends a combination of tractors or track loaders operating in coordination with a waste compactor. Finally, for large-scale landfills, Caterpillar offers larger compactors equipped with wheels big enough to meet the landfill’s compaction needs. Its landfill compactors range from the 52,634-pound, 253-horsepower model 816F to the 81,498-pound, 401-horsepower 826H to its largest model the 118,349-pound, 554-horsepower model 826H. Its machines come standard equipped with protective features that minimize debris buildup and increase cooling capacity. For waste-handling tractor dozers, Caterpillar starts with its D6. The D6, whose diesel engines meet the emissions requirements for EPA Tier 3, and the tractor dozer can come equipped as a low-ground-pressure version with extra-wide track shoes for better movement on unstable surfaces. The D6R and D6N are this machine’s waste-management configurations with a special guarding fixture to protect critical machine components and minimize debris damage, as well as cooling systems specifically designed for a harsh landfill environment. At the high end of the waste-dozer product line is its 310-horsepower, 81,150-pound model D8T WH and the 410-horsepower, 109,180-pound model D9T WH. Bomag produces the BC772RB, a mid-range waste compactor (81,000 pounds, 442 horsepower), and the BC1172RB (120,000 pounds, 511 horsepower), a heavy waste compactor. They are somewhat fleet of foot and can operate at speeds up to 7 miles per hour. To protect their components and provide maximum comfort to their operators, Bomag’s waste compactors have been modified for operation in a difficult landfill environment. First and foremost, these modifications include dust protection by fully sealing and enclosing the frame. Maximum compaction power is delivered to the waste by oscillating joints driven by a hydrostatic power system that utilizes four different circuits maximizing drive torque on the compaction wheels. Al-jon also provides a range of waste compactors suitable for small-scale, medium-scale, and large-scale landfill operations. These are (from lightest to heaviest) its Advantage model series 500, 525, and 600. At 126,000 pounds, the Advantage 600 is the heaviest waste compactor available in the North American market. Its drive train is powered by a proven 600-horsepower Caterpillar engine. Since it utilizes an all-wheel hydrostatic drive, it does not need potentially vulnerable machine components, such as a torque converter, a clutch, differentials, or an axle shaft. Cleaner bars are not needed given the arrangement of its wheels’ extensive diameter (55 inches) and cleats (10 inches). This machine combines thoughtful design with power and weight. In all cases, the actual waste-handling equipment needs to be modified to work in such an extreme operating environment. Dozers should be equipped with trash racks on their blades. A semi-universal dozer blade equipped with a trash rack, for example, increases dozer capacity (it can move more waste per sweep). By catching more waste, the trash rack prevents most waste objects from damaging the dozer’s radiator. Also useful for improving waste-dozer performance are front and rear striker bars (to prevent waste from being carried over the tractor treads), rear counterweights to ensure balance while operating over variable slopes, heavy-duty crankcase guards, and add-on screening packages to provide protection from small objects. Waste-handling track loaders should be protected with add-on guard packages (to protect moving parts—such as idlers, pivot shafts, and drive seals—from thrown debris). Waste compactors are predesigned with protection from thrown debris and impact with large objects already in mind. In general, and where applicable, all landfill operating equipment should be modified as needed (and as the equipment’s configuration allows) to operate on the workface. Engine carburetors should be protected by precleaners that prevent airborne debris from clogging intake ports. Radiators should be protected by heavy-duty guards or shields that also allow quick and easy access for cleaning the front of the radiator. Though a guard will protect the relatively vulnerable radiator from direct-impact damage, small pieces of flung debris always seem to find their way to the front of the radiator, where they can accumulate and prevent the airflow necessary for reducing the temperature of the liquid coolant. Striker bars are also useful for wheeled equipment to prevent waste from being carried over the wheels and wrapping around an axle. In addition to the primary waste-handling equipment (dozers, loaders, and compactors), a well-run landfill is supported by an auxiliary fleet of equipment. This includes road graders (to spread cover material or remove accumulated muck from access roadways during inclement weather), backhoe excavator/loaders (for small repair excavations and loading gravel or other construction material onto trucks for hauling to the work site), water trucks equipped with spray arms (to water down dry areas and control dust), portable water pumps (to remove ponded water or accumulated leachate from outbreak locations or for when a sump pump fails), pickup trucks (to haul equipment, tools, and personnel around the landfill site), and articulated dump trucks (for site construction tasks and the hauling of cover material). Dozer Production Overall productivity depends on the machine’s baseline maximum potential production rate under ideal conditions and the field factors that decrease this value. These include the type of material being dozed (from easily dozed loose material to stiff or sticky material). Poor visibility, due to weather conditions as well as blown debris or the variable terrain on a landfill workface, can further reduce dozer operations. Usually, a landfill dozer works by itself, so tandem dozer operations usually don’t play a part in landfill dozer productivity. Dozer production increases with a reduced operating slope, with downhill dozing being far more productive than uphill dozing. So unlike compactors, which work best operating against an uphill slope, dozers work best in the opposite direction. Loader Production Overall though, loader operations are maximized by minimizing the machine’s operating cycle. This cycle consists of load time, maneuver time, travel time, and dump time—each measured in seconds. Load time depends on the material being loaded. Waste is highly differentiated in size, shape, and weight. Waste also tends to clump and stick together, making loading inherently difficult compared to well-graded soils but not as difficult as loading large chunks of concrete or cemented masonry. Maneuver time depends on how crowded the workface is and how variable the working slopes are. In other words, the more obstacles (stationary or moving), the longer the maneuver time. So the maximization of loader operations also depends on the work environment. Travel time depends on the hauling distance of the loader carrying the waste. This is typically very small for most workfaces and active disposal cells. A typical 10-acre cell could have dimensions of 600–700 feet, but most loader movements would be confined to a small section associated with the workface (1 to 4 acres in extent). Lastly, dump time depends on the location and height of the dump, which in turn often depends on the size of the dump truck accepting the waste.
Compactor Production The compaction points utilized by waste-compaction machinery differ somewhat from those used by their soil-compacting cousins. Not particularly long (6–6.5 inches being common), compactor tips, teeth, or chopper blades are designed to shred waste as well as compact it. Most models have teeth fixed to the wheel drum by means of an easily removed cotter pin or similar fastener. This allows for quick replacement of worn teeth, which tend to wear out faster than those working in softer soils. Waste starts out at the curbside with a highly heterogeneous density of 200–400 pounds per cubic yard (equivalent to 7.5–15 pounds per cubic foot). The waste collection trucks perform an initial compaction during collection operations as the trucks’ hydraulic gates compress the waste to a density of 400 pounds per cubic yard to 800 pounds per cubic yard, a 50% volume reduction. Once the waste is hauled to the landfill and deposited on the workface, the goal of the waste-handling equipment operations is to reduce the waste volume by another 50%, achieving in-place densities of 800 pounds per cubic yard to 1,600 pounds per cubic yard. If waste-management operations are effectively carried out, the volume of waste you put out on the curb should be reduced in size by a factor of four. So how is this fourfold reduction in volume achieved? Begin with the thickness of the layer of waste that is spread across the workface after it is deposited by the hauling trucks. Studies have shown that the thinner the waste layer, the higher the in-place densities that can be achieved with each pass by a compactor. However, there is a limit as to how thin a layer of waste can be consistently spread given its heterogeneous nature. Seldom can even the best dozer operator spread waste to a consistent thinness less than 1 foot. A thickness of 1–2 feet appears to be the optimum layer thickness that allows for both maximum compaction and consistency of placement. At this thickness the goal density of 800–1,600 pounds per cubic yard can be achieved with most types of waste compactors. After a 2-foot thickness, the achievable compacted densities drop off precipitously. At a thickness of 3–4 feet, achievable densities hover around 800 pounds per cubic yard and 600 pounds per cubic yard, respectively. After 4 feet in thickness, the achievable density levels off to only 400 pounds per cubic yard, the low end of the waste coming out of the back of the truck. In other words, no significant compaction is possible at these layer thicknesses. So how many times must an appropriately sized compactor pass over and compact the waste spread into an optimum layer thickness? Again, studies have shown there is an optimal number associated with compaction. In this case, most waste (assuming it has been spread to an appropriate thickness) can achieve the goal density with three or four passes. The density increases as a straight-line function with one to three passes, but it levels off after four or five passes. This places an upper limit on the amount of time that should be spent working the waste with a compactor. After four or five passes, no more significant increases in density can be achieved no matter how many times the compactor hits the waste. How it hits the waste is vitally important. This is due to the angle of the workface and the orientation of the compactor to this slope angle. Studies have again shown there is an optimum slope angle. A slope ratio of 3 horizontal to 1 vertical (a grade of almost 18.5 degrees to the horizontal) has been found to be the best, since it allows the machine to impact the waste partially horizontally as well as vertically as it rams into the slope. In addition, it gives the tips and blades on the drum wheels an opportunity to effectively shred the underlying waste. The impact component is a function of simple geometry. The horizontal forward movement of the machine (the mass times its operating speed) is resolved into forces acting directly perpendicular to the waste. These forces are in addition to the dead weight of the machine. Shallower than the optimum slope, and the impact is lessened. Steeper than this, and the compactor has trouble climbing Finally there is the moisture content of the waste. A moisture content of 50% by weight has been shown to maximize compaction densities as the water weakens key structural elements in the waste (such as large pieces of cardboard) and lubricates the remaining waste objects to allow greater movement in response to compaction impacts. This can be highly variable given the season, the type of waste, recent rains, or even the application of water to reduce dust. Some wastes received by an MSW landfill (such as organic or industrial sludges that pass the paint-filter test and have acceptable leaching characteristics) have inherently high moisture contents. But these cannot be counted on to consistently increase the moisture content so as to achieve maximum density. And while the increase in moisture resulting from the application of dust-control water by a spray truck is a nice side benefit, few states allow the deliberate addition of large quantities of water for fear of increasing the amount of leachate produced by the landfill. So while moisture content is an important factor in increasing the productivity of the waste compactors, it is mostly outside the control of the landfill operator.
The Human Element Operator efficiency is easily the largest single factor affecting machine productivity. The preferred landfill equipment operator should be a jack of all trades and a master of more than a few. There is always a need for operational flexibility and associated personnel assignments on the workface. A rigidly pigeonholed and overly specialized workforce is an inflexible workforce. Change-outs are often required as a regular equipment operator needs to be reassigned to manage emergencies, cover for a vacationing colleague, perform cleanup, or perform daily cover duties. There are so many different tasks that need performing even on a typical workday that to have one person specifically assigned and exclusively trained to perform only one of them would require prohibitively expensive staffing. Conclusions Given equally capable equipment operators, the following factors affect overall waste-handling efficiencies of the landfill’s equipment fleet (which has been structurally modified to handle waste and be protected from damage by the waste):
Writer Daniel P. Duffy, P.E., is an environmental engineer in Cincinnati, OH.
MSW - July/August 2007
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