No Compaction
If not making a choice is itself a choice, then not compacting is a compaction strategy. In this option, waste is deposited at the working face and spread but not repeatedly run over with compactors. Such uncontrolled landfilling used to be the norm prior to Subtitle D. Since the passing of these regulations governing landfill construction and operation, waste compaction has become a financial necessity. With the mandating of expensive liner and cap systems constructed of low-permeability clay and impermeable geomembrane, a landfill operator has to cram as much waste tonnage as possible to maximize revenues and offset these costs. By pushing in-place density of waste to the limits, a landfill operator is reducing the area and volume needed for waste disposal operations and the associated impact on the environment. No doubt this was an intended side effect of the regulations.

In rare cases, however, noncompaction of waste may make financial sense (even if it does no good for the environment). Any remaining landfills that have been operating under pre-Subtitle D rules (or whose expansions have been “grandfathered” under the old rules) do not have to incur the heavy up-front capital costs of construction. Furthermore, for landfills in isolated areas without regional competition from other landfills, minimizing airspace is less of a concern. In this situation, an operator may decide to forgo or minimize compaction operations and the associated operating costs. Unfortunately, no compaction remains the rule in many Third World countries, as many poverty-stricken people make their livings picking through the waste for salvageable items. In this instance, compaction would actually hinder salvaging, which is, of course, banned under Subtitle D for reasons of personal safety and environmental protection.
However, if left to itself, waste over time will achieve some degree of consolidation and volume reduction without initial compaction caused by self-weight and biochemical decomposition. Landfill waste experiences an initial, short-term (1–4 months after disposal) compression. Following this initial compression, long-term settlement occurs under self-weight loads. If waste were a true soil, it would have a coefficient of secondary compression ranging from 0.1 to 0.4. However, it is difficult to determine exactly how much of this secondary, long-term compression is a function of self-weight and how much the result of chemical (either physical or biological) decomposition. One of the unexpected things about landfills is how much waste (as a percentage of the total) does not decompose into its organic constituents. Landfill gas well drillers routinely excavate readable newspapers and recognizable food items that date back many decades. Yet, enough decomposition occurs that significant differential settlement happens over time. This is largely the result of localized weakening of waste substrate causing regional shifting of the deposited waste. Either way, studies show that long-term settlement is a function of the deposited waste’s initial void ratio. This function can be made greater if the waste contains a high degree of decomposable organics and a warm and moist environment favorable to decomposition.

Waste Compactors
Thankfully, most landfills compact their waste in-place, either out of self-interest or because it is required by environmental regulations. The standard method of in-place waste compaction is the use of landfill compactors. These are specialized earthmovers resembling typical soil compactors that have been modified to operate in the harsher environment of the landfill. They are also modified to achieve the highest possible in-place compaction by mobile equipment operating on heterogeneous material such as municipal solid waste.

The act of compacting waste in place should be viewed as a construction effort. In effect, the landfill operator is constructing an earthwork structure called a “cell.” A cell is defined as the volume of waste and daily cover material (soil or other) contained within a previously constructed lined area. The current cell will typically (unless it is the very first cell to be constructed) overlie the previously constructed volume of an adjacent cell. The goal of this effort is to construct the highest-density cell volume in the safest possible manner. Most landfills have their cells constructed by the area method. In this method, waste is deposited at the toe of the current waste disposal slope (known as the working face) and spread over previously compacted waste. Some landfills utilize the trench method, where waste is disposed of in discrete trenches. However this is rarely done nowadays and is only suitable for landfills receiving small daily waste tonnages and whose groundwater is very deep.

For most landfills, a small fleet of vehicles is required to manage the working face and ensure that compaction is performed properly. Tractor-type vehicles are useful for spreading waste in thin layers over the working face and for providing a secondary compaction prior to direct compaction. Track loaders are occasionally used on area fills to load and deposit earth materials such as gravel or daily cover soil. Wheel loaders, though not used for waste handling, are useful of cleanup tasks and for keeping the working face tidy. These materials are usually loaded onto articulated trucks for hauling to the working face or wherever the material is needed. Wheel tractor scrapers are best at performing cover operations, pushing soil cover deposited at the toe of the working face up and over the exposed waste at the end of the working day. All this work is performed so that the waste compactors can effectively and efficiently perform their task.

Waste compactors both spread and compact deposited waste. They operate at relatively high operating speeds and torques. The minimum preferred operating weight for landfill compactors is over 45,000 lbs. They normally operate on slopes of 4 horizontal to 1 vertical (25% grade) or less, though some landfills have finished cell grades of 3 horizontal to 1 vertical (33% grades). Operating on slopes steeper than this is not recommended due to reduced compaction results and increased safety hazards.

Caterpillar’s line of landfill compactors runs from the “lightweight” 816F, with an operating weight of 52,793 lbs, through the 81,498-lb 826G (a derivative of the Caterpillar 825 soil compactor) to the heavyweight 836G (also a soil compactor derivative) operating at a hefty 113,348 lbs. Their engine powers range from 22 hp to 480 hp produced by DITA engines with displacement ranging from 10.3 L to 15.8 L. The compaction work is done by the wheeled drums, varying in width from 3 feet, 4 inches, to 4 feet, 7 inches, and in diameter from 5 feet, 10 inches to 6 feet, 9 inches. Each drum is studded with 20 to 35 steel tips measuring 6.5 inches in length and/or 20 to 28 6-inch-high chopper blades. The blades act to shred the waste while the tips concentrated the weight of the machine to perform its compaction effort.

Bomag’s mainstay waste compactor is its BC772RB. At an operating weight of 81,000 lbs, it is powered by a 442-hp diesel engine. The engine has been refitted to comply with the most recent Tier 2 emissions regulations. Bomag’s heavyweight compactor is its BC1172RB, weighing in at 120,000 lbs operating weight. It is powered by a 511 HP diesel engine that allows it to operate at speeds of 7 mph. All Bomag compactors have been modified for optimal operation in a landfill environment, including dust protection provided by a fully sealed and enclosed frame, oscillating joints that deliver maximum pressure on waste slopes as steep as 30 degrees, and a hydrostatic power delivery system that utilizes four different circuits maximizing drive torque on the compaction wheels.

Several factors besides equipment weight affect the results of compaction. First, waste should be spread out in thin layers by the tracked dozers prior to direct compaction by the waste compactors. These layers should be less than 2-feet thick, the thinner the better, though it is probably impractical given the heterogeneous nature of waste to achieve consistent layers of less than 1-foot thickness. A certain minimum number of passes with the compactor is required to achieve maximum density. This number is usually 3–4 passes, with a full pass being defined as rolling over and backing down from the working face. After 4–5 passes, no more significant compaction can usually be obtained, and further compactor operations are not economical. Moisture content of the waste also affects its compactability, with wetter waste tending to be more easily compacted. However, most states forbid the direct application of water for anything other than dust control, and any large quantities of water introduced into the working face will have to be paid for later as treated leachate.

Aljon’s Advantage model series of waste compactors (500, 525 and 600) fills the needs of the heavy duty end of the waste operations spectrum.  The Advantage 600 weighs in at 126,000 pounds, making it the heaviest compactor in the North American market.  Its 600 HP John Deere or CAT engine operates an all wheel hydrostatic drive that does not need a torque converter, clutch, differentials or axle shaft.  Its 55 inch wheels are studded 10 inch cleats arranged so as to not need supplemental cleaner bars.  What it lacks in sophistication it more than makes up for with raw power and heft, durable reliability, simplicity of design and ease of maintenance.

Sakai America provides a wide range of soil and waste compactors utilizing smooth, pad foot and combination drums.  Sakai’s compactors come with an option strike off blade for backfilling and light dozer applications.  Their series of vibratory soil compactors (SV201, SV400, SV505, and the SV510) are complemented by the CV550D (smooth drum) and VV550T (padded foot) suitable for landfill applications, complicated soils, operations on steep grades and embankments, and work in rough terrain.  At an operating weight of almost 31,000 lbs., the VV550 punches above its weight class by delivering a vibratory force of about 50,000 pounds provided by a 169 HP engine.
The Terex model 3-75E and 3-90E Trashmaster compactors are designed with a unique triangular wheel configuration.  The front wheel is a single sold drum that covers the gap between the two back wheels.  The resultant overlap of the wheels coverage allows for completed compaction over the width of each pass.  Powered by a 350 HP engine, the 3-75E operates at 45,000 lbs and delivers a compaction pressure of 708 pounds per linear inch over a compaction width of 14 feet.  The 32-90E is in the heavy duty category with an operating weight of 108,000 pounds providing 2 more feet of compaction width and 8% more compaction force.

Geometry also plays a role. The slope of the working face should not exceed 3 horizontal to 1 vertical (33%). The flatter the slope, the more of the equipment’s weight is translated into direct compaction as a result of the geometry of the applied forces. For example, the 33% slope has an angle to the horizontal of 18.43 degrees. The cosine of this angle is 0.948 so only about 95% of the equipment’s weight is actually compacting the waste underneath. Since the compaction tends to create a small wave of waste in front of it and to its side as a result of the pressure distribution through the waste, it is often useful to have a pair of compactors working in tandem. If a landfill has skilled and experienced waste compactor operators (as well as a relatively flat working face) these compactors can operate in parallel and opposite directions.

Balers and Balefills
Balers are machines that take in waste, compact it to a high density and bundle it with wire so it holds its shape, though some waste balers rely on the post compaction adhesion of the waste to hold the bale’s shape. This shape is usually a rectangular block whose dimensions typically vary from 3 feet to 8 feet. The bales resemble larger versions of standard hay bales or very large blocks. As such, the waste bales can be stacked like blocks and laid like bricks.

The standard method of in-place waste compaction is that of using specialist earthmovers.

So why bale waste instead of compacting it with standard waste compaction equipment? First, baling achieves a much higher density with an additional one-third reduction in waste volume. While compaction equipment increases in-place waste density to 40 or 50 lbs per cubic foot, a baler can achieve densities of 60 to 75 lbs per cubic foot. The bales have inherently better internal strength characteristics and therefore more resistant to slope failure than compacted waste. Stable bale slopes can be steeper than compacted waste slopes. This method requires less landfill volume and footprint. Bales are resistant to wind and disease vectors and therefore require little or no daily cover to prevent windblown debris and infestations. The bales also tend to shed precipitation, reducing the level of contaminants in the leachate, since most percolation flows around instead of through the bales. In short, baling results in a cleaner, more efficient, and safer landfill.

So why isn’t everyone baling waste? First off, a baler represents a significant up-front capital cost. It can be very expensive to own and operate a baling machine. All that compaction requires a lot of energy, so monthly electrical costs may be prohibitive. By comparison, operating and maintaining a fleet of standard compaction equipment is relatively cheap. Baler operations are difficult in cold temperatures. So landfills in northern tier states will have to install their balers in heated enclosures. Operating a baler requires specialized training and safety standards. Lastly, given the nature of baling operations (the confined compaction of large hard-and-sharp objects) wear and tear on a baler can be extreme.

Balemaster USA offers a line of over 100 different baler models whose capacity ranges from a single half-ton bale per workday to 20 minibales per hour. The company’s E-series is used in a variety of landfill and recycling operations. The system’s density controls automatically adjust the force applied to the compacted material. Sizes range from the 200-series, which handles low-volume applications and paper, film and foil, through the 400-series, which manages hard-to-compact materials like folding cartons, chipboard, boxboard, and a variety of coated stocks, to the 1800-series, which compacts PET bottles, aluminum cans, and other slippery materials.

More and more municipal solid waste landfills have adopted balers as part of their disposal operations. The city of Rawlings, WY, has successfully used a baler at its landfill for the past 25 years. Its Mosley-Badger baler is housed in an enclosed structure. They have found that not only has the baler improved operational efficiencies and reduced airspace needs by over two thirds (the landfills remaining operating life has been extended from 8 to 25 years), it has had a positive aesthetic effect with the elimination of blown debris and litter. The baler produces easily handled bales measuring 3 feet by 3 feet by 4 feet, which are taken out to the current disposal area and stacked. Each stack is covered with daily cover soil except for the next day’s working face.

The Montgomery County and Stewart County (“Bi-County”) Solid Waste Authority in Tennessee has been using a pair of Harris Waste Management Group refuse balers to compact the waste, extending the life of the landfill by reducing airspace needs. Here, the operators have found that the balers control pests (virtually eliminating insect and rodent vectors), improve the landfill’s appearance with its neatly stacked bales and lack of blown litter, reduce operating costs (since waste disposal operations with bales require few operators and equipment at the working face), greatly reduce the threat of fire hazard, and minimize odor problems.

Dynamic Compaction
Dynamic consolidation is an extreme method of compacting refuse and waste fill. This technique involves dropping heavy weights (15–20 tons) onto the surface of the fill from a height of 30 to 60 feet on a grid pattern whose spacing is determined by the weight being dropped and the nature of the waste. The impacts produce shock waves that extend deep into the landfill; therefore there is no need for the impact points to overlap. Dynamic compaction isn’t just used to minimize airspace. The high densities achieved by this method create very stable foundations for construction. Despite its organic content, municipal solid waste compacted with this method has superior bearing capacities and limited long-term differential settlement. Instead of using the landfill in post closure as a green space, a dynamically impacted landfill is suitable for commercial development and even the construction of roadways.

Yet this technique is not quite as simple as using a crane to raise a weight and then letting it drop. The equipment performing this task is subject to often-severe operational strains. Optimizing the impact of the dropped weights involves detailed analysis of the counterweights, bending moments in the crane’s framework, power conversion applied to the torque used to raise the weight, strain elongation on the lifting cable, the size and diameter of the dropped weight, clutch and brake operations, and the potential for crane overturning on an initially unstable foundation. This operation should be left to professional contractors who specialize in dynamic compaction.

Waste compactors spread as well as compact deposited waste.

Different energy levels are used to compact waste at different depths. Initially, the heaviest impact (with the weight dropped from the highest point) is used to achieve compaction in waste at significant depths. Waste nearer the surface is often displaced by the craters created by these initial deep impacts. Moderate drops with less impact energy are then used to compact waste at middle depths. Finally, shallow drops with the least impact energy are used to compact waste at or just beneath the surface. Moderate and shallow compacting also requires proportionally fewer numbers of blows per unit area than deep compaction. All the impacts dislodge waste near the surface instead of effectively compacting it. Therefore, waste near the surface to a depth of 3 feet may have to be compacted with a traditional roller compactor. For area compaction in landfills, a wide-diameter weight is best suited. Narrow diameter weights are usually used to create compacted columns of soil in otherwise soft conditions.

Shredders
Shredders have been traditionally used to render large objects such as white goods (major appliances), brown goods (large pieces of furniture), tires, and tree trunks into smaller pieces prior to disposal in the landfill. Shredders are also useful for creating fluff at transfer stations that can be more easily transported to the landfill. Shredders have also been traditionally used to prepare waste for use as refuse derived fuel in waste to energy plants.
The material produced by shredders is more easily handled, spread, and compacted than the whole goods fed into the shredders. Most of the objects in question consist mostly of air. Just imagine how much space is taken up by a tire’s bulk but how little is actually made up of the tire body itself. A cabinet or other piece of furniture has large voids for shelves or drawers. However, none of these large goods can be easily compacted in place even by the heaviest compactor.

Landfill Mining
Landfill mining removes and reclaims primarily inorganic materials that have a market value that justifies the expense of the mining operation. As a side benefit, landfill mining removes a certain percentage of the landfill’s volume, freeing up more air space. Its effects are therefore similar to that of an extreme compaction effort. The primary goal of landfill mining is the extraction of valuable metals, though other recyclables, combustible materials, and soils can also be excavated for reuse. Landfill mining equipment usually consists of excavators and backhoes, vibratory screeners or rotating trommels for material separation, conveyor belts or trucks for mass hauling, and storage bins for the extracted materials.

Metals typically make up 8% of the wastestream with plastics accounting for 11%. The availability of combustible materials such as paper (35% of a typical waste mass) depends on the age, degradation, and moisture content of the landfill. Daily and intermediate cover soils, provided they exist as discrete layers within the landfill, can occupy from 5% to 25% of the landfill’s volume. However, most soils have come into contact (or are even saturated) with landfill leachate and need to be remediated prior to reuse.

Landfill mining is neither cheap nor easy. It can be very messy and, if not done right, can damage the integrity of the landfill’s structural elements (containment berms, geomembrane liners, etc.). Only very high costs of scrap metal and real estate justify the expense and trouble inherent in landfill mining. The scrap metal prices represent direct gain, while real estate prices represent cost avoidance. It is rare that scrap metal prices alone will justify the expense of landfill mining.

However, in areas where real estate development costs are prohibitive, the additional airspace created by excavating recyclable materials and the resultant extension of the landfill’s operating life will push back the costs of new landfill construction or expansion of an existing landfill.