Nothing matters more to a landfill operator than effective use of his available airspace.

By Daniel P. Duffy

Landfill economics are unique compared to other industries in that there is a very high, up-front capital cost associated with landfill construction and the installation of the various structural and mechanical components (liner, leachate collection, landfill gas extraction, pumping systems, and so on). Coupled with these high fixed capital costs are relatively low operating expenses as measured per ton of waste. Often disposal operations can cost literally pennies on the ton. In short, after achieving a (relatively high) breakeven point, every ton of waste received by a landfill translates into nearly pure profit.

Fixed capital costs are directly related to landfill area as measured by acres of liner (and final cap). Operational profit is directly related to landfill volume as measured by the site's disposal capacity. Or more specifically, profitability depends on how much tonnage can be disposed of in available airspace volume. Site limitations and regulatory requirements will dictate how and where the liner, leachate management system, and final cover are built (and how much they will cost). Where the operator has a free hand is in the maximization of landfill density, squeezing as much tonnage of waste as possible into available airspace. Density can be maximized by the following methods: efficient site design, material reduction, effective compaction, and maximizing use of alternative daily cover.

Site Design

As mentioned above, local regulations will determine the extent of the landfill's liner and disposal area as defined by horizontal setbacks from the property line and other features such as airports, minimum vertical separation between groundwater and waste, and maximum height of the landfill above surrounding terrain. A well-designed landfill will take the disposal space right up to these limits, without creating odd shapes that actually detract from available airspace. The most efficient landfill design (in terms of the volume-to-area ratio) is a perfect square pyramid. So while any rectangular, oblong, or irregular landfill footprints might increase the overall disposal volume, they will result in a lower volume-to-area ratio (and be inherently less profitable). It's up to the operator to decide if he wishes to maximize landfill operating lifetime or profitability.

Belowground, a landfill with its relatively flat floor and steeper sideslopes resembles an upside down, truncated pyramid. How deep the landfill can go will depend on the site's hydrogeological conditions, especially the elevation of groundwater. Where the operator has some freedom is in the steepness of the liner sideslopes. Other things being equal (landfill depth and footprint area), a steeper liner sideslope will provide more volume. Steeper slopes are more difficult to place waste against, however, and require special operational procedures. Typically waste is placed along the entire floor of the disposal cell prior to placing the next layer of disposal cells against the sideslope. Though extremely steep (vertical) sideslopes also can be managed in this fashion, they will present unique liner construction headaches. It's usually not worth the effort to try to utilize nearly vertical sideslopes.

Final waste slopes above the surrounding terrain usually are limited to a maximum steepness of 4 horizontal to 1 vertical (4:1). This is a fixed value derived from the strength of the deposited waste in resisting slope failure. However, the operator has a choice concerning the overall configuration of the slope and the terraces utilized for surface-water runoff controls and access roadways. Terraces can be either set into the slope, causing the next highest slope segment to be set farther into the landfill, or they can be built out on the final waste grades with earthen berms. The first results in a loss of potential airspace, while the second can present construction and stability concerns. Provided that the overall height of the landfill does not result in inherent stability problems, set-back terraces should be avoided so as to maximize airspace.

Material Reduction

MSW arrives at the landfill with a density of approximately 0.20 - 0.35 ton/yd.³ (roughly 15-25 lb./ft.³, approximately one-sixth of most clays). Typical site compaction efforts reduce the volume of waste deposited on the working face by approximately 50%, resulting in a compacted, in-place density of 0.40 - 0.70 ton/yd.³ For every ton of waste diverted from the landfill by recycling or other waste reduction efforts, 1.40 - 2.50 yd.³ of airspace is preserved.

Waste-to-energy operations also produce the significant side benefit of reducing waste volume destined for ultimate disposal. Waste volume reductions as high as 90% can be achieved. The remnant of a typical mass-burn operation is ash and clinkers that cannot be compacted in place. Furthermore, the burn residue must be handled with special care and frequent applications of water from a spray truck to prevent windblown dust and particulates. However, these are minor concerns compared to the significant increase in landfill. Any cost-benefit analysis of a proposed waste-to-energy system should take into account the cost savings inherent in the subsequent reduction of waste volume going to a landfill. Deferment or complete avoidance of additional capital costs required to build a new landfill disposal cell represents a significant (though indirect) cost savings.

Compaction

The Trashmaster 3-90E, with three-position configuration of four wheels, offers full width compaction.

The primary means of effective landfill airspace management is proper compaction of the deposited waste. Four factors will determine the effectiveness of the compaction effort: layer thickness, number of passes, slope, and moisture content of the waste.

The single most important factor in maximizing landfill densities and airspace is the thickness of the waste layer being compacted. Ideally this should be no more than 2-3 ft. of spread, loose waste prior to compaction. Given the irregular nature of trash, however, it isn't always possible to achieve this thickness consistently. Pressure exerted on the waste by a compactor dissipates with depth. So whenever possible, the operator of a large landfill operation should try to spread waste as thinly as possible with a track-type tractor specifically outfitted for waste disposal operations prior to actual compaction. Operators at smaller landfills with lower disposal rates probably can get by with using a compactor with a dozer attachment to spread the waste.

Once the waste has been spread out over the current working face, the compactor(s) makes several passes over the loose layer. It usually takes three to five passes (a pass is defined as movement back and forth across the working face) to achieve optimum density. A greater number of passes won't result in increased density; so additional passes will be wasted effort.

Whenever possible, the operator should compact waste up against a relatively steep sideslope, preferably 3:1. The compaction effort should be in the direction upgrade to the slope. This way the braking force of the compactor delivers its force more directly into the hillside. If the configuration of the current working face does not allow compaction up a slope, then reasonably effective compaction can be achieved along a flat surface. Whenever possible, the operator should avoid attempting to compact waste in a downslope direction.

Unless the waste being compacted has inherently high moisture content, then high moisture would be the result of excessive rainfall during operations. Waste has a field capacity (the amount of water in a "soil" which remains after extensive gravity drainage) as high as 30% by volume. Even after compaction, waste has a typical total porosity (volume of voids/total volume) of approximately 67% and can hold much more liquid for short durations. For short durations after a major rainfall, moisture content can reach as high as 80%. Given its highly irregular and heterogeneous nature, however, deposited waste can form impermeable layers that hold pockets of water or fissures, which allow for a more rapid drainage of water. Either way, high moisture content will tend to result in higher compacted in-place densities since moisture weakens the bridging strength of the waste components. Though optimum moisture content for waste compaction is approximately 50% (based on field studies), it rarely pays for the operator to purposely add water to the working face and artificially raise the waste's moisture content since all that extra water eventually shows up at the bottom of the landfill as leachate. For this reason, most state agencies forbid the application of water for anything other than dust control.

Compaction Equipment

The type and number of waste compaction equipment should be based on the estimated daily tonnage received: low (less than 500 tpd), medium (500-750 tpd), and high (more than 750 tpd).

Caterpillar Corporation has a range of steel-wheeled landfill compactor equipment for each level of tonnage. The Cat 816F Landfill Compactor (usually working with a pair of medium-size dozers, such as the D4, D5, and D6) is suited for low-tonnage sites. It has an operating weight of 50,115 lb. and a Cat 3306 TA engine producing 234 hp. With a wheel width of 3.33 ft., it covers a width of 14.75 ft. with two passes. Typical operating speed is 8 mph. The 826G Series II Landfill Compactor is designed for sites receiving a medium level of waste tonnage and usually works with one or two heavier dozers (either D8 or D9). It has an operating weight of 81,498 lb. and a Cat 3406E ATAAC diesel engine producing 380 hp. With a wheel width of 3.92 ft., it covers a width of 15.67 ft. with two passes. Typical operating speed is 12 mph. The 836 G-Series landfill compactor is designed for sites receiving a high level of waste tonnage and also works with a pair of heavy dozers. It has an operating weight of 118,348 lb. and a Cat 3456 DITA engine producing 525 hp. With a wheel width of 4.58 ft., it covers a width of 18.58 ft. with two passes. Typical operating speed is 18 mph.

The CMI Environmental Machinery Corporation (a Terex Company) has a similar range of waste compaction equipment in its Trashmaster product line: the Trashmaster 3-35C with an operational weight of 44,000 lb. for low tonnage rates; the Trashmaster 3-70C, which can apply a compaction force of 648 lb./lin. in. for medium tonnage rates; and the Trashmaster 3-90C with a fully hydrostatic drive system and 525 hp for heavy tonnage rates.

Al-Jon Inc. also supplies three types of landfill compactors, suitable for each range of a waste tonnage. The Impact 81K landfill compactor has an operating rate of 86,000 lb. and uses a CAZT C15 water-cooled diesel engine that produces 425 gross hp. The Impact 91K is suitable for midrange tonnages, has an operating weight of 101,000 lb., and uses a Cat C15 electronic engine that generates 525 gross hp. Both models come with an oversized radiator and full hydrostatic drive and have a ground clearance of 30 in. Its Advantage 600 is advertised as the world's heaviest landfill compactor at an operating weight of 126,000 lb. A Cat C16 electronic engine provides 600 gross hp. It has hydrostatic drive, full-time all-wheel drive, and 32 in. of ground clearance.

Compaction America (a Bomag Company) provides a range of equipment for moderate (the BC672RB) and heavy (the BC772RB) refuse rates. The BC672RB/BC772RB combines a hydrostatic drive system with independent four-wheel-drive motors to provide greater tractive effort regardless of operating conditions. The BC672RB has a water-cooled diesel engine with 421-hp output at 2,100 rpm. The BC772RB has an additional 21 hp, pushing the rating to 442-hp output at 2,100 rpm. Engine power on the BC672RB/BC772RB drives a hydrostatic system with independent four-wheel-drive motors. Both models utilize a Deutz BF6M1015 series engine with a 726 C.I.D. and turbocharger that will meet emission regulations and gives high torque at low revolutions. BC672RB/BC772RB wheels have polygonal disk segments and one-piece cast, high-wear-life teeth as standard equipment.

Specialty Wheels

Terra Compactor Wheel manufactures a wide variety of cleated steel wheels specifically designed for landfill compaction. Patterns include collateral, inline, inverted, dual helix, and standard chevrons. The cleats range from standard 7-in. to megasized 8-in. specially designed steeple cleats, soil-tamping cleats, and cleats for use at transfer stations. Terra also provides rolling wire guards that prevent refuse wire and other entanglements from wrapping around the compactor's axle. Upper kick and rear bumper guards also are provided as standard safety equipment.

The Caron Compactor Company manufactures a pin-on teeth system for steel compactor wheels utilized by many compactor suppliers. The teeth are designed to be self-cleaning and come in two types: wedge-shaped for compaction and contoured for demolition and stability. The teeth are held in place by high-strength retainer pins that allow for changing with minimum downtime.

Compaction Performance

Dual Helix wheel with Twist Torque Cleats

The production of all types of compactors is measured in terms of compacted cubic yards per hour. The production rate is determined by multiplying the width per pass (twice the wheel width) by the average speed in miles per hour, by the compacted lift thickness in inches, by a conversion factor of 16.3.

The number of passes needed to achieve the desired compaction then divides this parameter. The number of passes is empirical and depends on the characteristics of the "soil" being compacted. In this case, waste is very heterogeneous and its characteristics will vary across the working face and even from day to day. For planning purposes, however, we will assume an average number of passes per lift.

Assume a Cat 816F with a wheel width of 3.33 ft. and an average speed of 8 mph. The waste is spread out in a loose lift of 2 ft. with the goal to compact the lift thickness down to 1 ft. The compaction parameter in this case is 648 yd.³/hr. divided by the number of passes. The site receives 400 tpd of loose waste, an average of 50 tph. The 50 tons (100,000 lb.) of loose waste has a density of 20 lb./ft.³ (resulting in a loose volume of 5,000 ft.³) and will be spread out over 2,500 ft.² (50 ft. x 50 ft.) in a loose lift thickness of 2 ft. The postcompaction waste volume will be 2,500 ft.³ or 92.6 yd.³ In order for the compactor to compact the waste that arrives each hour, compaction must be achieved with no more than seven passes (648 yd.³/hr. ¸ 7 passes = 92.6 compacted yd.³/hr.). If compaction can be achieved in three to four passes, the compactor can complete its work in only 30 minutes each hour. This is preferable as it allows for easier choreography of the other disposal equipment (dozers, trucks, and so on).

GPS-Guided Dozer Operations

While global positioning system (GPS) survey guidance has been utilized extensively in standard construction of roadways, embankments, and landfills, it has not been used extensively for proper placement of landfill waste and daily cover. Where it has been used, it has shown itself to be a very valuable airspace management tool that more than pays for itself. A differential GPS-based system offers extremely precise real-time machine position monitoring, enabling precision control of landfill operations. With GPS systems, waste can finally be considered as finesse material, which can be accurately placed and controlled.

GPS tracking can be used to monitor compaction efforts and slope placement in real time. The landfill benefits from increased in-place density and reduced cover-soil usage, all of which result in airspace savings and extend the life of a fill. GPS surveying also provides the operator with faster (and less expensive) surveying, improving his site record by providing continuous logs of waste disposal operations.

GPS systems, such as the Caterpillar Corporation's Computer Aided Earthmoving System (CAES), include the following hardware: tractor-mounted GPS receivers and operator displays, radio network and associated software that enables wireless transmission of grading plans and as-built data between the operator and the landfill office. The plans are done in AutoCAD drafting software and consist of three-dimensional digital terrain models, which are derived from the site's contour grading plans.

Equipment rendered superfluous includes all of the old, traditional ground markers (e.g., stakes, cones) normally found at a landfill's working face set on a 50- or 100-ft. grid. Instead the GPS unit provides the equipment operators with a graphical display terminal inside their cabs. These color-coded displays show plan views of either areas of cut and fill (for dozers) or numbers of passes and resultant elevations (for compactors). As the equipment is moving and operating, the system provides the operator with a continuous real-time stream of easy-to-read graphical information showing profile views, site cross-sections, and position/slope information, relative to the position of his equipment.

The color-coded plan view provides a bird's-eye view tied to a series of virtual elevation markers. So there is no need for inaccurate physical markers that are prone to misplacement, accidental burial, or leaning off center. The operator no longer has to make a best guess as to his position, elevation, slope, or effectiveness of his operations. As a further guide to proper grading, important changes in slope (benches, slope break lines, and so on) can be superimposed on the plan view.

The profile and cross-sectional views provide a comparison between the current surface and the design surface. The current surface is recorded as the equipment moves across the existing terrain with the machine's GPS antenna adjusted to take into account the high differential to the bottom of the dozer treads or the compactor's wheel. Profiles can be provided along the direction of the equipment's movement or at a 90° angle to this alignment.

A CAES system was installed at the County of Orange (CA) Olinda-Alpha Landfill as a test bed for this system. Olinda-Alpha is a canyon-type fill located approximately 40 mi. south of downtown Los Angeles. It disposes of an average of 7,000 tpd of waste, six days a week. The landfill found that soil placement efficiencies were greatly improved, with less airspace wasted to excessive cover. This increased the effective gate income by more than $1.3 million and reduced soil expenses by $3 million - plus annually. Side benefits included improved operational safety, clear location of potential hazards, reduced survey work hours, better teamwork between operator and engineer, and improved surface-water runoff and landfill gas control.

Use of Soil as Daily Cover

The self-contained Automatic Tarping Machine

At the end of each working day, the site operator will be required to place daily cover on the current working face. This is done to give the landfill a better appearance, prevent the breeding of disease vectors (insects and rodents), minimize odors, and prevent windblown debris. The standard material used for daily cover is locally available soil. Usually this soil is a stockpile leftover from landfill cell construction operations. Cover soil is spread loosely over the working face and is not compacted in place. Its in-place density is typically 60-70 lb./ft.³ (0.80 - 0.95 ton/yd.³). With typical daily cover-soil applications, the overall in-place density of compacted waste and its soil cover is 0.50 - 0.90 ton/yd.³ Daily cover soil is typically spread in 6-in.-thick lifts.

Take, for example, a landfill that receives an average of 700 tpd of waste. Once this waste has been properly compacted in place, it will take up approximately 1,000-1,750 yd.³ (at 0.4 - 0.7 ton/yd.³). Assume a typical compacted in-place daily waste volume of 1,000 yd.³ per day, or 27,000 ft.³ If a typical daily operation involves the compaction of four layers of waste (two in the morning and two in the afternoon) that start at 2-ft. loose thickness and end up as 1-ft. compacted thickness, then the overall compacted waste thickness each workday would be 4 ft. The current working face then would be approximately 7,000 ft.² (about one-sixth of an acre). If 6 in. of daily cover soil is placed on this working face, then the amount of volume used increases by 12.5% to a thickness of 4.5 ft. Only 89% of the potential waste disposal airspace is being utilized. For every 10 years of potential landfill operational lifetime, this site would be missing out on an additional 1.25 years of operations (and the resulting profits).

The above example represents almost ideal conditions where only the minimum 6 in. of soil is placed as cover. In the real world, it is almost impossible to avoid placing between 6 and 12 in. of cover soil to prevent flagging and to completely cover an irregular waste surface. At 12 in. of cover, the amount of airspace utilized increases by 25% to a thickness of 5 ft. Only 80% of the potential waste disposal airspace is being utilized. Though waste decomposition and eventually settlement of daily cover into underlying waste voids will reduce the overall in-place volume over time, these effects usually don't become apparent until after a disposal cell has been closed and capped. In this case, for every 10 years of operating lifetime, the landfill would be missing out on 2.5 additional years of life.

One option is to strip and reuse daily cover at the start of each workday. This procedure will serve to reduce the amount of disposal airspace wasted on cover soils. The process is time and equipment intensive, however, imposing on the operator unneeded additional operating expenses. Furthermore, it is far from 100% effective. At most, a dozer blade can strip only the top half of the soil cover. An attempt to strip more soil will result in the mixing of waste with the stripped soil, making it useless for further cover applications.

Alternative Daily Cover

Pneumatic Foam Unit 2500/60 self-propelled foam-generating system

The ProGuard slurry of recycled fibers, polymers, and water contains blowing litter, vectors, dust, and fires on the work face.

Clearly, most landfills could benefit from the use of alternative daily covers (ADCs) that do not utilize airspace. By using ADC instead of soil, the landfill operator saves a considerable amount on earthwork equipment operating expenses and maximizes the landfill's utilization of disposal airspace. ADC comes in two basic types: degradable/disposable plastic tarps or spray-on foam applications. ADC of any type must meet Subtitle D requirements: provide a barrier to vectors, be noncombustible, form a barrier to odors, and prevent blowing litter.

Tarpomatic's Automatic Tarping Machine (ATM) is a self-contained unit that attaches to heavy equipment to unroll and retrieve different types of fabric panels. Each ATM is custom fitted to be lifted and transported by dozer blade or related equipment. The ATM uses a hydraulic drive motor and engaging system to unwind and rewind the tarp spool with variable speed control. Spools can be disconnected and reconnected, using a single ATM to link a series of tarps together to cover or uncover a landfill's working face. The operator can control the ATM's engine, height of the spool, and forward or reverse rolling through a controller unit placed in the cab. The system is designed for 40-ft.-wide panels of various lengths and can be adapted to a wide range of your heavy equipment.

ATM operation procedures are relatively simple: Drive the ATM to the top of the working face with a full tarp spool in place. Deploy the tarp by activating the hydraulic drive and simultaneously backing your equipment until the working face is covered. Removing a tarp is also simple: Hook the tarp to the ATM spool. Activate the hydraulic drive and move forward to roll the tarp onto the spool. If the tarp is not rolling onto the spool straight, tilt the blade to straighten the tracking. Avoid getting a lot of slack in the tarp so that you can wrap it uniformly on its tube.

EPI Environmental Technologies manufactures the Enviro Cover System, a degradable geosynthetic-alternatives cover system comprising degradable plastic film and cover. The covers are designed to be left in place, so no time is required for retrieval. Despite its light weight, it can be used year-round. Rain, snow, or wind does not affect its use. The Enviro Cover System also is used in intermediate and long-term cover applications for infiltration and erosion control (in landfills and other engineering projects), all without the need for removal and disposal.

New Waste Concepts Inc. manufactures ProGuard SB and ProGuard II spray coating that combines recycled fibers and polymers with water. The mixture forms slurry that sprays on the current work face, forming an effective barrier against blowing litter, vectors, dust, and fires. Despite its use of recycled fibers, it is nonflammable and nontoxic, adding no contaminants to the site. Preapplication mixing is easy as only one of the three components is dry.

Rusmar Inc. manufactures a spray-on foam that is used instead of daily soil cover. Rusmar Soil Equivalent Foam does not consume airspace, yet it meets or exceeds all the performance criteria for ADC material as required by Subtitle D of the Resource Conservation & Recovery Act. The Rusmar foam application is a one-person operation. The foam itself is biodegradable and can be used year-round regardless of temperature or precipitation. Equipment rental and maintenance agreements are available.

Reef Industries Inc. produces Griffolyn, a thin plastic sheet used as ADC. The sheets are 20 mil thick and are reinforced against wear and tear. The material is three-ply laminate combining two layers of an ultraviolet-stabilized, co-extruded polyethylene and a high-strength core grid. Though not specifically designed to be biodegradable, the operator may choose to tear up the previous day's cover by track-walking with his dozer prior to spreading new waste over the working face. Conversely, the operator may decide to leave these impermeable sheets in place so as to minimize leachate formation by blocking percolation into the landfill. Its strength and resistance to weathering makes Griffolyn a good choice for alternative cover over areas that will be exposed for extended periods of time.

Enviro-Cover supplies a range of automatic tarp deployers that are hitched to and towed behind standard landfill equipment, such as a large track-type tractor. The RK680 is designed for megasize landfills and can deploy 25,000 ft.² of rolled material 18 ft. in width. It can apply tarps at a rate of 40,000 ft.²/hr. The RK 650 is slightly smaller, deploying rolls of tarp 16 ft. wide at a rate of 20,000 ft.²/hr. The IJ630 is suitable for midrange landfills and can be towed by a smaller track-type tractor or other equipment, such as a track loader or a compactor. Its production rate is similar to that of the RK650. Smaller landfills can be serviced by the RK610/RK616 model that deploys 10- or 16-ft.-wide tarps at a rate of 12,000 ft.²/hr. All models deploy a degradable polyethylene tarp.

Cost-Benefit Analyses

The site operator should consider two cost-benefit analyses. The first involves the cost-effectiveness of the in-place waste compaction effort. The second concerns the cost-effectiveness of utilizing ADC. Each landfill is unique in its tonnage rates, equipment fleet, tipping fees, and a dozen other factors that will affect its profitability. Therefore, the following analyses will make some basic assumptions concerning these factors:

The landfill receives on average 800 tons per eight-hour workday, equivalent to 200,000 lb./hr. This waste, on average, arrives and is deposited on the working face at a density of 0.27 ton/yd.³, equivalent to 20 lb./ft.³ Freshly deposited waste requires 10,000 ft.³/hr. of airspace. The waste is spread out in a loose lift thickness of 2 ft. over an area of 5,000 ft.² (0.11 ac.) each hour. The compaction effort is made to reduce this lift thickness to only 1 ft. thick, halving its volume and doubling its density to 0.54 ton/yd.³ or 40 lb./ft.³ Each hour's compaction operation frees up enough airspace to accommodate another 100 tons of waste. Further assuming that the landfill has a tipping fee of $40/ton of waste, the compaction effort should cost the site less than $4,000 per hour in order to break even.

The above looks at operational expenses only and does not include fixed capital costs involving siting, permitting, and construction of the landfill. These factors vary considerably from landfill to landfill and most often will not allow for apples-to-apples comparisons. These capital costs are considerable, however, and cannot be ignored in a site-specific analysis as they determine the site's break-even point. For the purpose of simplicity, this analysis assumes that the site receives enough waste tonnage to meet its break-even point, allowing for an expenses-only analysis.

The costs of the compaction effort (equipment and operators) depend on the size and make-up of the equipment fleet. For a site receiving more than 750 tpd, a typical compaction fleet will consist of a Cat D9 dozer and a waste compactor. Each is assumed to work no more than 30 minutes each hour to allow for disposal, spreading, and compaction. Ownership period for track-type tractors and wheeled compactors should be considered as equivalent to "severe" operations, no more than 10,000 and 8,000 hours, respectively. Costs are divided into owning and operating costs.

Ownership costs depend on six factors. The "total delivered price" less the "residual value at replacement" (derived from applicable depreciation methods) results in the "value to be recovered through work." This value is then divided by the anticipated work hours per year to produce an hourly equivalent ownership cost. To this value is added the hourly equivalent costs for interest, insurance, and property taxes to derive the total hourly ownership cost.

Hourly operating costs include operator wages and benefits, fuel, lube (including oils, filters, and grease), tires or undercarriage repair, a repair reserve fund, and special wear items. Equipment operating in waste needs special racks and screens to prevent fouling of engine and transmission parts by wires and other debris. The total compaction operation costs are the sum of the hourly ownership and operating costs. As the factors affecting this value vary greatly from site to site, there is no standard compaction operation cost. Each operator must evaluate his site-specific equipment costs with his capital cost and break-even point with regard to required site profitability to determine what compaction effort makes financial sense.

Soil-cover costs include both the direct cost of placing the cover soil and the opportunity costs associated with unusable disposal airspace. Given the example above, 6 in. of cover soil each day would be used to cover up to 40,000 ft.² (0.9 ac.). Each day, cover soil would use up approximately 740 yd.³ of airspace. The cost of placing cover soil (assuming a locally available borrow source) will cost $2-$3 per in-place yard. The cover soil direct cost in this case could be as high as $2,000 per day. Furthermore, this soil is displacing airspace that could be utilized for waste disposal. At a compacted, in-place density of 0.54 ton/yd.³ or 40 lb./ft.³, the soil displaces approximately 15 tons of waste. This is equivalent, at a tipping fee of $40/ton, to $600. The total soil-cover cost in this case would be $2,600 per day or more than $800,000 per year. It would make financial sense for this hypothetical landfill to utilize an ADC system with annualized costs that are less than this figure.

Daniel P. Duffy, P.E., is a professional environmental engineer in Cincinnati, OH.

MSW - May/June 2003