Density is the measurement of compacted waste mass per volume of airspace taken up by the waste. Refuse trucks deliver waste to the landfill’s working face at a density of approximately 30 to 35 pounds per cubic foot (equivalent to 0.4 to 0.5 tons per cubic yard). The waste is then deposited on the current workface and spread into loose lifts measuring 2 to 3 feet in thickness. It is then compacted with sheepsfoot compactors specially designed to shred and compact the waste until it is reduced to half of its initial placement thickness. This doubles the in-place density of the waste to 0.9 tons per cubic yard. Higher densities than this are difficult to achieve without the use of extreme techniques like dynamic compaction or mechanical baling of waste.
Each 1-foot thickness of waste over an acre is equivalent to slightly more than 1,600 cubic yards. After compaction, this acre-foot of waste has a weight of about 1,450 tons. At a typical tipping fee of $30 per ton (a national average according to the USEPA), this weight represents about $44,000 in gross sales to the landfill.
But an actual operational workface is seldom as large as a full acre. Usually a quarter-acre (about 100 feet by 100 feet, the minimum needed to choreograph refuse trucks and compactor operations) is a good size for all but the largest landfills. The equivalent volume of an acre-foot over this smaller workface would be approximately 4 feet thick if it received the full 1,450 tons per workday.
However, only the largest landfills receive over 1,000 tons per day of waste. A typical midrange for landfill daily receipts would be closer to 500 to 750 tpd, about half the theoretical tonnage of a compacted acre-foot of waste and equal to only $22,000 in gross sales. So this typical, mid-range landfill would compact waste into a 100-foot by 100-foot workface to a thickness of around 2 feet each workday, with each foot being equal to $11,000 in gross sales.
At the end of each workday, federal and state regulations require that the exposed surface of the workface receive a layer of daily cover. Specifically, this requirement is found in Title 40 of the Code of Federal Regulations (40 CFR-“Protection of Environment”), Chapter I-Environmental Protection Agency, Subchapter I-Solid Wastes Part 258 Paragraph 21, “Cover Material Requirement.” This cover usually consist of clean soil (“earthen material”) spread over the exposed working face to a thickness of 6 inches. Daily cover is intended to perform multiple tasks:
- To prevent potential disease vectors (rodents, birds, insects, and other pests) from entering the waste
- To prevent the escape of blown dust and debris from the deposited waste
- To promote surface water runoff, and thereby minimize infiltration of precipitation (rainfall and snowmelt) into the underlying waste and reducing leachate formation
- To prevent the entrance of oxygen into the waste mass, minimizing the potential for fire
- To cover and obscure the deposited waste, making it difficult or impossible for individuals to conduct scavenging operations on the landfill
- To minimize the escape of odors from the waste, eliminating a significant nuisance
- To reduce the potential for landfill gas production.
This 6-inch layer of soil placed over the typical 2-foot-thick layer of compacted waste takes up a significant portion of a landfill’s available airspace. For the typical midrange landfill described above, daily cover takes up 20% of the landfill’s disposal volume (6 inches out of a daily total thickness of 2.5 feet). This would be equivalent to $5,500 worth of last gross revenue each day in unusable airspace, in addition to the direct material and labor costs needed to physically place the daily cover. Assuming a 6-day workweek, or 312 workdays per year, this results in an annual loss of potential gross revenue equal to $1,716,000.
Fortunately, the regulations allow for the use of alternate daily cover (ADC) materials and thicknesses, provided that the alternative meets the same performance characteristics of 6 inches of daily cover soil and provides the same level of environmental protection. By using ADC instead of cover soil, a landfill operator not only saves operating costs and regains the lost revenue described above, but he also avoids capital costs by extending the operating lifetime of his current disposal cell. All that volume previously wasted on soil cover (20% of the total in the above estimate) can then be used for waste disposal. The operational lifetime of the current disposal cell and the landfill itself can be increased by 20%, with commensurate delays in capital cost outlays.
This last cost savings may be indirect but is vitally important to the long-term financial health of the landfill. Landfills operate on a unique business model with relatively high capital costs and low per-unit (measured in tons of waste received) operating costs. After a certain break-even point, landfills operate at very high profit margins. By delaying the next round of capital outlays associated with the construction of the next disposal cell, or the installation of a final cap-and-cover system over the previous disposal cell, capital costs in real terms get reduced due to the time value of money (future outlays cost less in real terms than equivalent dollar current outlays).
So given the obvious financial advantages of using ADC, the next question is: What type of ADC is right for a particular landfill? Such inorganic waste as contaminated soil, foundry sand, coal combustion bottom ash, slag, and filter cakes can be used, but their use is severely limited by local regulations and availability. The material cannot be subject to desiccation and thereby become a dusty material that poses a windblown dust hazard. Additional safeguards must be installed to prevent the formation of contaminated runoff when precipitation comes into contact with this type of cover. Waste cover materials cannot be a potential odor source or clump together during placement, making it difficult to place a consistent cover thickness. Given these limitations, a landfill operator would be wise to consider commercially available ADC.
There are five basic categories of commercially available ADC: Disposable sheets of thin plastic film, reusable tarps or sheets made from high density polyethylene (HDPE), reusable tarps made from heavy geotextile, spray applications using inorganic chemicals and concrete, and spray applications that use such organic materials as pulped paper.
Whether disposable or reusable, tarps and sheets are rolled out over the workface at the end of the workday. This can be done either manually (if the workface is small enough and the cover material is light enough) or more typically by a mechanical tarp deployment mechanism mounted on a piece of heavy equipment. Mechanical deployment takes less time and manpower while reducing wear and tear on reusable tarps. There is no need to be careful with disposable sheets since they are torn up by a dozer track walking them and abandoned in place at the start of the next workday. Tearing holes in the sheets make them permeable to gas and liquids, preventing the formation of pockets of leachate or LFG in the waste mass.
Spray-on ADC of any kind is a specially formulated mixture of water, cementitious binder, adhesive additives and bulking fiber. It is sprayed over the working face, forming a complete cover as it dries out and hardens into a paper-mache-like crust. This cover is brittle and easily broken up by equipment movement prior to the start of additional waste disposal operations.
ADC is not a panacea; it does have its limitations. Extreme weather conditions (high winds, heavy rain, hail and snow, freezing temperatures, etc.) can inhibit or even prevent its use. For example, spray-on ADC often cannot be effectively placed during high winds since it will get blown away from the workface. Cold temperature will also affect the composition of the slurry. Heavy winds can also catch and tear a tarp during deployment. But other than these excessive situations, ADC is appropriate for use at any landfill. Some of the major producers of ADC are listed below.
Airspace Saver-Airspace Saver daily cover has been manufacturing long lasting, reusable alternate daily cover tarps for 22 years. These incorporate several heavy-duty fabrics, coated, non-coated, and flame resistant, to build any size tarps from small up to 150 feet by 150 feet. All tarps are sewn together with 2-inch (6,000-pound) polyester web straps across each seam and around the perimeter. D rings or hand loops are secured on all corners and edges for easy deployment. Cable or chain can be inserted in pockets around the perimeter edge for weight in high-wind situations.
Central Fiber-Central Fiber manufactures two spray-on ADC products: Waste-Cover and Topcoat. They are produced using recycled paper, wood fiber, polymers, and other proprietary ingredients. These products meet all of the requirements of ADC for a landfill and are nontoxic and biodegradable.
Environmental Products LLC-Environmental Products manufactures Posi-Shell, a spray-on ADC consisting of a mineral mortar coating, similar to stucco. Posi-Shell cover is non-flammable and durable. The Posi-Shell cover system consists of a liquid base (water or leachate) and Posi-Shell brown base mix. Depending on desired durability and color, Portland cement and various dyes may also be used in the mixture.
EPI Environmental Products Inc.-EPI’s ADC system consists of a uniquely degradable polyethylene film, Enviro Cover, and the Enviro Cover Deployer, the applicator equipment for placement of the Enviro Cover, providing ballast and seal at panel overlaps to create a continuous impermeable barrier between waste and the surrounding environment.
New Waste Concepts-New Waste developed the first alternative daily cover for the solid waste industry in 1987. NWC’s ProGuard line of environmental spray-on ADC coatings are slurry-based, made primarily of food grade polymers and clays.
Reef Industries Inc.-Reef’s Griffolyn tarps are UV-stabilized, additive, and reinforced (film with a high-strength cord grid) polyethylene products available in stock configurations up to 200 feet by 200 feet for immediate availability or custom fabricated for specific site requirements. Their durability allows their use as intermediate cover as well as daily cover.
Rusmar Inc.-Rusmar supplies a spray-foam ADC. The company’s third-generation foam, known as AC-667SE, is non-hardening and remains moist throughout its life. It is non-toxic, biodegradable, and non-combustible. The foam is applied with a trailer foam applicator, the PFU 400/25, which uses a proprietary foam generation system.
Southwestern Sales Co.-Southwestern manufactures many standard-size tarpARMOR ADC tarps, from 50 feet by 50 feet up to 150 feet by 150 feet, as well as custom-made tarps. Manufactured from USA-made polypropylene material in 6.5-, 8-, and 9.4-ounce weights, these tarps are made light enough to be spread by manpower, while the larger sizes can be spread using existing landfill equipment. The company also manufactures automatic tarp deployment machines, including the tarpARMOR TDS-30HC Tarp Deployment System, which can deploy up to 12,000 square feet of tarp in less than 15 minutes. The machine is built very rugged but light enough to work slopes and landfills with high sludge content.
Tarpomatic Inc.-Tarpomatic provides a machine for the efficient deployment of ADC tarps. The company’s Automatic Tarping Machine (ATM) is a patented, self-contained unit that attaches to heavy equipment to unroll and retrieve different types of fabric panels. Each ATM is custom fitted, allowing quick and easy attachment and removal. The ATM uses a hydraulic drive motor and engaging system to unwind and rewind the tarp spool with variable speed control. The system is designed for tarps measuring 84 by 30 feet. Each tarp will cover an area of 280 square yards, and each spool will hold three tarps. Therefore, each spool is capable of covering an area of 840 square yards, and the coverage is unlimited when multiple spools are purchased.