A Whole Lot of Nothing

What exactly does a landfill sell? What does a customer, disposing of his waste at the landfill, get in return for his $30 or so per ton? The obvious answer is that the landfill operator provides a service. According to the standard view, the service that the operator provides is an environmentally safe, politically acceptable, and economically viable means of permanently managing municipal solid waste. But the providing of services is open-ended, and is not dependent on available supplies of raw materials or finished products. In contrast, landfill operations are not forever. Instead, landfills actually sell a commodity of finite amount and limited availability. Every landfill eventually closes when it finally runs out of the commodity it is actually selling: airspace. The hard part is determining how much this commodity is worth.

Airspace and Profitability
Landfill operations are unique in many respects. They combine aspects of industrial manufacturing, mining operations, and construction sites. Furthermore, they have a relatively high break-even point resulting from very high initial fixed capital costs and relatively low variable unit operational costs (per ton of waste received) associated with the deposal of waste and subsequent environmental monitoring. These capital costs are related primarily to a landfill’s footprint. That is, landfill capital costs (aside from things like scales, access roads, fences, or maintenance buildings) are a result of how much area has to be excavated, lined with clay and geosynthetics, and eventually capped with similar materials.

To offset these fixed capital costs, a well-designed landfill should have as much disposal volume as possible within the landfill’s footprint. The design of the landfill disposal area will determine the all-important ratio of airspace (which determines profits) to area (which determines capital expenses). This is the basic metric for measuring landfill profitability. Not counting the potential disposal airspace below grade (depth of excavation is more function of hydrogeological site characteristics than anything else), the perfect shape for maximizing volume to base area is the pyramid.

A square base allows all the sides of the landfill to achieve maximum height, provided they have a consistent final grades slope. Rectangular or irregular landfill areas have their maximum heights determined by the shortest axial dimension. This limits the maximum potential height of the landfill and with it the landfills’ potential volume per acre of lined area. If a site’s property is elongated or otherwise affected by setbacks that would suggest a larger but less efficient landfill layout, the operator will have to choose between maximizing gross or net profits. The larger irregular landfill may have more volume (and thus generate more gross profits) than a smaller square landfill, but it will have fewer per-unit returns in investment.

Exchanging Tonnage for Cubic Yards
As odd as it sounds, landfill cash flow depends on tonnage, while landfill profitability depends upon airspace. The trick is to equate the two by making the conversion between tonnages at the gate and cubic yards of airspace in the landfill. The waste that gets hauled to the landfill in waste collection trucks gets measured after it enters the gates by driving over an in-ground truck scale. This measures the weight of the truck plus the weight of its waste load. After depositing its waste at the current working face, the now empty truck goes back over the truck scale and is re-weighed on its way out of the facility. This new measurement gives the weight of the truck only. The difference between these two weights is the weight of the waste delivered by the truck to the landfill.

In general, waste hauled to a landfill has a density of between 15 pounds per cubic foot to 25 pounds per cubic foot. This is the equivalent of 0.20 to 0.35 tons per cubic yard, thanks to the compaction of the collected waste inside the hauling truck itself—already approximately twice as dense as the waste had been when it was sitting on the street corner. The goal of the landfill operator is to reduce the volume of the deposited waste as much as possible by means of active compaction. Properly compacted waste can achieve another doubling of its density (and halving of its volume) as a result of compaction by heavy equipment at the working face where it is deposited. This results in an in-place waste density within the landfill of between 0.40 and 0.70 tons per cubic yard. Therefore, each ton of waste received by the landfill requires on average between 2.50 and 1.40 cubic yards of airspace.

For planning purposes, a typical rule of thumb would be an airspace utilization rate of 2 cubic yards per ton based on an assumed in-place density of 0.5 tons per cubic yard. If a landfill charges a typical tipping fee of $30 per ton, each wasted cubic yard of airspace is equal to $15 of lost gross revenue. Each lost acre-foot of airspace reduces gross revenue by $24,200. So, ironically, as improved compaction increases the value of a landfill, the same high rates of compaction make waste airspace potentially more costly.

Compaction is performed with specialized earthmoving equipment called landfill compactors. These landfill compactors are basically soil compactors modified for operating in the harsh environment of a landfill. They have been equipped with special extensions to their front dozer blade to contain low-density waste objects, protective shields to the undercarriage and the coolant system to prevent damage from sharp or blowing waste, and steel wheels with feet attachments designed not just to compact the waste but to shred it as well. Operative standards have been established that, when combined with field experience, determine the optimum number of passes with the compactor required to achieve maximum in-place density.

The act of compacting waste is still more art than science, despite a dozen studies by equipment manufacturers to determine those factors that affect compaction performance. Unlike soil, waste is very heterogeneous. So, unlike the compaction of earthen berms, the same compaction effort won’t always achieve the same results with waste. Otherwise, waste compaction should be considered as a construction activity. The landfill isn’t disposing of waste so much as it is using waste as a raw material to construct waste cells. Waste is placed in loose lifts typically 2 feet thick and compacted to a thickness of about 1 foot.

Specially designed compactor wheels are used to achieve this combination of shredding and compaction. The Caron Compactor Co. utilizes Pin-On Maximizer wheels that are interchangeable with standard Caterpillar 826G and 836H series compactors. Additionally, Caron provides BMAX pin-on teeth. They can be fitted to a wide range of compactors up to operational weights in excess of 120,000 pounds. Its dual-purpose tip is engineered to work with factory cleaner arrangements and axle protection guarding.

Terra Compactor Wheel supplies a wide variety of teeth and cleat patterns. The Terra Twist Torque is a six-sided rhomboid coercion cleat with opposing mirror-image surfaces. The right-hand and left-hand sides twist during compaction to achieve extreme reduction while being self-cleaning. Their modified steeple cleats come with an extreme-service rolling wire guard to prevent entanglement during operations.

Daily waste cells are constructed of multiple waste lifts, one on top of the other until they reach a height between 8 and 12 feet. The areal extent of each waste cell is a function of the daily waste receipt, which determines the size of the landfill’s equipment fleet, which in turn determines the minimum-required area of the working face needed to safely and efficiently choreograph the equipment and the arriving waste collection trucks. Daily waste cells are covered with and delineated by daily and (when necessary) intermediate cover.

Subsequent waste cells are typically constructed adjacent to and overlapping the previously constructed cells. They are usually developed to the same elevation, until they completely cover the available floor space within the current landfill operational phase. This forms a layer of waste cells whose top surface creates a new floor for the next layer of waste cells. Layers consisting of individual daily waste cells are then constructed until all the available airspace defined by the current phase’s configuration is used up.

The Impacts of Daily Cover
State and federal regulations require that some sort of cover material be applied to the current working face at the end of each day and to those interior waste slopes that may be exposed for extended periods of time. This is done to minimize the potential of landfill nuisances affecting the public and the environment. These nuisances include odors, dust, blown litter, and disease vectors (vermin, insects, and birds).

The traditional method of daily cover (the one most mentioned directly by state landfill regulations) is the spreading of a minimum 6-inch layer of cover soil over the current working face at the end of each workday. How much volume is taken up by this cover soil varies from landfill to landfill, depending on the amount of waste received during the workday.

For example, take a landfill that receives an average of 600 tons per day of waste. Good compaction practices yield an in-place density of 0.60 tons per cubic yard. As a result, the amount of space used up on average each day by waste placement operations would be about 1,000 cubic yards (27,000 cubic feet, or 0.62 acre-feet). Assuming a typical lift thickness of about 6 feet, the daily waste cell would cover an area of about 4,500 square feet (a square roughly 65–70 feet in dimension). This would also be the area that would have to receive daily cover at the end of each workday. Assuming a 6-inch layer of soil is utilized (realistically, this daily cover soil layer would be closer to 12 inches in thickness given the irregularity of the waste surface) it would consume approximately 83 cubic yards of airspace each day, increasing the daily airspace utilization rate to 1,083 cubic feet. Daily cover in this example would represent almost 8% of the total landfill airspace.

That may not sound like much, but it is equivalent to airspace that could have received up 50 tons of waste in a single workday. With 312 workdays per year, this is equivalent to almost 15,600 tons of potential waste disposal lost to daily cover each year. Further assuming the landfill is of moderate size with a 20-year operational lifetime, the total disposal capacity utilized by daily cover would be equal to almost 312,000 tons of waste. Assuming a tipping fee of $30 per ton, this represents over $9 million in lost gross revenues over the lifetime of the landfill. In fact, landfills often have much higher percentages of their airspace volume given over to daily and intermediate cover soil, with some landfills being as high as 20%.

This doesn’t even factor in the equipment and operator costs required to place daily cover soil or the cost of the soil itself (assuming there is no convenient borrow source onsite). The time required for a dozer to spread and place daily cover soil can be up to two hours, depending on the size of the current working face. If the total cost per hour for equipment and operator to place daily cover is $70 per hour, the total daily operational costs associated with daily cover placement is $140 per day, equivalent to over $43,500 per year. These numbers alone should be justification enough for the use of alternate daily covers.

So what is needed is an easy-to-apply alternate daily cover (ADC) that does not take up a significant amount of potential disposal airspace. At first glance, such organic materials as yardwaste would make good candidates for ADC. Though easily placed and often already provided in the arriving wastestream, organic waste is usually forbidden by state regulations from being used as cover. Far from containing odors, the decomposition of organic waste gives off its own odors. Instead of serving as a barrier to disease vectors, organic waste can serve as a breeding ground. For these and other reasons, most states ban the use of organic materials (“putrescible waste”) as daily cover.

There are five general kinds of man-made ADC. These include: thin sheets of disposable film, reusable high-density polyethylene tarps, reusable heavy geotextiles, spray applications that utilize such nonorganic materials as chemicals and concrete, and spray foams that utilize pulped paper or other bulking agents.

Tarps and sheets can be manually or mechanically placed and can either be abandoned in place or rolled back again for reuse at the end of the next day. Disposable sheets are track-walked with a dozer at the start of the next work day to tear them up and prevent them from blocking the downward migration of leachate. Reusable tarps are used over and over again until they wear out.

However, tarps can be difficult to place, catching on sharp protrusions of the underlying waste as they are dragged into place. Even if a serious tear is avoided, general day-to-day abrasion can limit the lifetime of reusable tarps.

One example of a disposable tarp ADC system is Environmental Technology’s Enviro Cover System, a two-part system consisting of polyethylene film and application equipment for covering waste in landfills. It is a degradable polyethylene film. When it is used as an ADC, it is nonretrievable and does not require removal when the next lift of waste is placed. Enviro Cover occupies minimal volume and offers a wide range of benefits, including those of saving valuable waste disposal space and reducing soil-operating costs.

Another example is Tarpomatic’s Automatic Tarping Machine (ATM), a patented, self-contained unit that attaches to heavy equipment to unroll and retrieve different types of fabric panels. Each ATM can be custom fitted to be transported and lifted by the blade of a dozer (or waste compactor) or the bucket of a front-end loader. The ATM can be readily attached and removed from the equipment as needed. The ATM uses a hydraulic-drive motor and engaging system to unwind and rewind the tarp spool at variable speeds, allowing easy placement over irregular surfaces.

The potential difficulties associated with the placing of tarps have given impetus to the use of spray-on foams and other sealants. One of the first spray-on ADCs was Landfill Service Corp.’s Posi-Shell, a mixture of shredded paper, polyester fiber, and cement kiln dust (later formulations would eliminate the paper component).

In general, any spray-on ADC is a slurry mixture of water, cementitious binder, adhesion enhancing admixture, and fiber. However, spray-on ADC also has operational limitations. Often, they cannot be properly applied in high winds and low temperatures. Special storage units may also be required.

Landfill Service Corp. manufactures Posi-Shell spray-on ADC and supplies its application equipment. Posi-Shell is a spray-applied, cement-mortar coating similar to stucco, and is both nonflammable and durable (making it useful in erosion control applications as well as for daily cover operations). Its mixture consists of a liquid base (water or leachate), Posi-Pak P-100 Fibers, PSM-200 Setting Agent, and (optionally) Portland cement. It even comes in different colors since various dyes may also be used in the mixture.

Tracking Airspace Utilization
The necessity of maximizing airspace has increased the importance of tracking airspace utilization rates. Not so long ago, an annual topographic survey of the landfill considered sufficient to track disposal volumes. While still performed at most sites, annual surveys have been augmented by quarterly or even monthly ground surveys of the current work face. Furthermore, modern technologies derived from the Global Positioning System (GPS) can track surface elevations on a daily basis in real time.

Automated positioning systems, operating earthmoving equipment with a GPS-based guidance system, can effectively minimize the need for manual onsite surveying, staking, measurement, and certification. By reducing labor costs, such systems can greatly improve overall cost efficiencies while ensuring greater accuracy. Time is saved as project durations are reduced. All this is achieved without sacrificing the quality of the fieldwork.

So what does this do for the landfill operator? Near-real-time tracking of airspace utilization allows for a running tabulation of current compaction operations. The GPS-derived data can be used to create three-dimensional surfaces in AutoCAD. These surfaces (which can be generated as frequently as each workday) can be used to create volumes of in-place waste. AutoCAD software can use an upper and a lower surface to calculate a volume and then compare this volume with the recorded tonnages of waste received during the same period.

The resultant in-place density can be evaluated for consistency, the effectiveness of the operators, and the effect of weather conditions on waste compaction options. Armed with this data, an operator can accurately project airspace utilization needs and plan accordingly.

Topcon Positioning provides a whole series of GPS instrumentation suitable for tracking and controlling rough grading, utility grading, finish, and fine grading. Making the system work is the company’s HiPer Series of integrated receivers and standalone receivers, all of which feature GPS+ technology. Topcon’s Millimeter GPS allows an equipment operator to follow a highly productive and accurate 3D-GPS+ stakeless grading system.

Recycling and Airspace
Every ton that gets diverted from the landfill by recycling results in an average of 2 cubic yards of airspace saved. This airspace can be used for the disposal of additional nonrecycled waste.

As a result, airspace-saving recycling can be seen as the mirror image of airspace-consuming daily cover operations. Typically, any waste recycling program is evaluated in terms of its direct costs and actual revenues achieved on the scrap resale markets. However, a potentially greater financial benefit of recycling is its effect on landfill operational longevity.

For example, take a moderately sized landfill that receives an average of 500 tons of waste per day. Suppose that the community being served by this landfill achieves a recycling rate of approximately 20%, primarily by removing ferrous and nonferrous metal scrap for resale and by diverting organic yardwaste to composting operations. This reduces the amount of waste entering the landfill to about 400 tons per day. Assuming an overall in-place density of 0.6 tons per cubic yard (though this will vary greatly for the different types of recycled materials) the 100 tons per day not going into the landfill reduce the daily waste volume rate from 833 cubic yards to 667 cubic yards, a savings of 166 cubic yards per day (equivalent to almost 52,000 cubic yards or 32 acre-feet per year).

By reducing the overall airspace utilization rate to 80% of what it would be before recycling, the operational lifetime of a landfill can be increased by 25%. In other words, if the landfill had a designed operational lifetime of 20 years it would actually operate an additional five years.

This five-year delay pushes back the point at which a new landfill or an extension to the existing landfill must be developed, reducing the net present value of the capital costs required for this new construction.

No matter what the current market price for recycled scrap materials, the delay in having to pay for additional lined phases by itself represent a significant budget savings.