Groundwater, rain, snow, or runoff-any water that enters a landfill violates the goal of sequestering the waste forever in a “dry tomb.” Most landfills are dry tomb landfills. The dry tomb waste-disposal concept is based on the idea of isolating waste as completely as possible from the environment. This dryness is maintained by three landfill structures. The first is the leachate collection and extraction system, consisting of a series of perforated collection pipes directing leachate to recessed collection sumps for extraction via submersible pumps.
The second is the landfill’s liner system, which isolates the waste from groundwater and contains any leachate within the landfill. The third is a final cover system, designed to shed water and prevent it from entering the underlying waste. In addition to these permanent structures, the waste will receive daily and intermediate cover during waste disposal operations. All of these structures have to be maintained for the duration of the landfill’s operational life and its post-closure care period (30 years after closure).
Landfill Containment Structures: Liner and Final Cover
The requirements for landfill liners are mandated by Subtitle D of the Resource Conservation and Recovery Act (RCRA). The regulations required by RCRA for landfill design and construction are given in Title 40 of the Code of Federal Regulations, Part 258-Criteria for Municipal Solid Waste Landfill:
…composite liner means a system consisting of two components; the upper component must consist of a minimum 30-mil flexible membrane liner (FML), and the lower component must consist of at least a two-foot layer of compacted soil with a hydraulic conductivity of no more than 1Ã—10–7 cm/sec. FML components consisting of high density polyethylene (HDPE) shall be at least 60-mil thick. The FML component must be installed in direct and uniform contact with the compacted soil component.
Each MSW landfill has to have a composite liner system consisting of compacted clay layer overlain by a flexible membrane liner (FML). This FML typically consists of a 60-mil-thick, high-density polyethylene (HDPE) sheet. Above the composite liner, the landfill is equipped with a leachate collection layer. This usually consists of a layer of sand and a system of perforated pipes designed to extract and remove accumulated leachate. Sometimes, this leachate collection and extraction layer consists of a geocomposite drainage blanket (a sandwich consisting of a factory-bonded geotextile cushion below, a geonet drainage layer in the center, and a geotextile filter on top) either alone or in combination with the traditional sand and pipe system.
The last major containment structure installed on a landfill is its final cap system on top of the final waste disposal grades. This final cover is of similar construction to the liner system at the bottom of the landfill. As with liners, the requirements for landfill final covers can be found in Part 258:
…The final cover system must be designed and constructed to: (1) Have a permeability less than or equal to the permeability of any bottom liner system or natural subsoils present, or a permeability no greater than 1Ã—10–5[SUPER] cm/sec, whichever is less, and (2) minimize infiltration through the closed MSWLF by the use of an infiltration layer that contains a minimum 18-inches of earthen material, and (3) minimize erosion of the final cover by the use of an erosion layer that contains a minimum 6 inches of earthen material that is capable of sustaining native plant growth.
These standards effectively mandate a minimum final cap-and-cover configuration consisting of (from top to bottom): a complete cover of grassy vegetation, erosion controls and surface water runoff interception and diversion structures, a protective soil layer to prevent frost penetration and provide a rooting zone for the grassy vegetation, a drainage layer (either geocomposite or granular soil) to intercept precipitation percolating through the cover, an FML, a compacted low-permeability soil layer matching the permeability and thickness of the liner, and a gas-migration control layer (which can also be either a geocomposite or granular soil) to prevent gas buildup under the cap.
|Credit: LSC Environmental Products LLC
A Posi-Shell spraying
Daily and Intermediate Cover Requirements
In addition to the permanent containment structures described above (which represent a major capital cost to the landfill operator) the landfill also requires operational cover layers consisting of daily and intermediate cover. Daily cover is that layer of cover applied over the current workface at the end of each workday. It is intended to perform several tasks to preserve the hygienic and environmental safety of the landfill operations.
First, it is designed to prevent entrance to the waste by rodents, birds, insects, and other pests and potential disease vectors. Second, it prevents the escape of blown dust and debris from the deposited waste. Third, by promoting surface water runoff, it minimizes infiltration of precipitation (rainfall and snowmelt) into the underlying waste, thereby minimizing leachate formation. Fourth, by smothering the workface and reducing the entrance of oxygen into the waste mass, daily cover minimizes the potential for fire. Fifth, by covering and obscuring the deposited waste, it makes it difficult or impossible for individuals to conduct scavenging operations on the landfill. Lastly, it minimizes the escape of odors from the waste, eliminating a significant nuisance and reducing the potential for landfill gas production. The requirements for daily cover application are also spelled out in RCRA’s Subtitle D. These can also be found in Part 258:
Except as provided in paragraph (b) of this section, the owners or operators of all MSWLF units must cover disposed solid waste with six inches of earthen material at the end of each operating day, or at more frequent intervals if necessary, to control disease vectors, fires, odors, blowing litter, and scavenging…. Alternative materials of an alternative thickness (other than at least six inches of earthen material) may be approved by the director of an approved state if the owner or operator demonstrates that the alternative material and thickness control disease vectors, fires, odors, blowing litter, and scavenging without presenting a threat to human health and the environment.
The second part is critical to landfill operations, as it allows for the use of alternate daily covers consisting of synthetic materials. These can consist of a wide variety of materials, including spray-on foams, reusable tarps, disposable plastic sheets, and spray-applied mortar coatings. The equipment used for applying this material is equally varied.
Intermediate cover differs from daily cover, both in physical characteristics and operational purpose. Though it has to meet all of the same performance and operational requirements of daily cover, it is not just a thicker version of daily cover. The main difference is the regulatory requirement to provide cover for those waste disposal areas that will be exposed for 30 days or longer (a requirement derived from Section 258.60). Operationally, unlike daily cover, intermediate cover must be removed prior to placement of additional waste on the covered areas, and the operator must protect intermediate cover from erosion and gully formation.
Post-Closure Care and Maintenance
Post-closure care and maintenance tasks also are listed in the code. These tasks must be performed for a care period of at least 30 years after final closure of the landfill. The owner/operator is responsible for ensuring that the landfill’s structural and mechanical systems (final cover system; leachate collection, removal, and pretreatment systems; gas extraction, treatment, and monitoring systems; and groundwater monitoring systems) preserve their integrity and continue to function. This requires regular inspection and maintenance, with repairs performed as needed. Care and maintenance tasks are typically performed at least quarterly for the first five to 10 years after closure with the frequency being reduced to semiannually or even annually later on.
ADC Options: Tarps
The most innovative materials used in landfill operations are the alternate daily cover (ADC) materials that can be used in place of 6 inches of soil. ADC comes in two broad categories: tarps or sprays. Tarps include disposable sheets made from thin plastic film, reusable sheets made from more durable HDPE geomembrane, and tarps made from heavy geotextiles. Sprays include inorganic applications (various types of concrete mixtures) and organic mixes (such as pulped paper). Each has its advantages and disadvantages.
Depending on their size, tarps can be placed manually over the existing workface. But in most cases, it is easier and cheaper to use a specialized deployment mechanism to roll out the tarp. Using this equipment to roll out the tarp and (more importantly) roll it back up again at the start of the next workday, reduces wear on nondisposable tarps, greatly extending their useful lifetime and reducing operational costs. However, some degrading of the tarp material is inevitable, since waste contains many sharp and protruding objects that can catch and tear even the most carefully placed tarp. High winds can also make it difficult to deploy tarps.
On the other hand, cheap, thin, disposable plastic sheets are designed to be torn up by equipment traffic prior to the start of further waste disposal operations. In both cases (removal or destruction) the goal is to eliminate a potential barrier to the downward percolation of water/leachate through the waste mass. If left in place, these impermeable covers could cause the leachate to perch in significant quantities high in the waste mass. This could result in serious slope stability issues.
Enviro Cover System is an efficient and cost effective ADC for municipal solid waste landfills. The System consists of the Enviro Cover, a uniquely non-reusable polyethylene film developed to meet requirements for ADC. It is placed over the workface by the Enviro Cover Deployer. This is a versatile and efficient applicator for placement of Enviro Cover. Its method of application involves providing ballast and seal at panel overlaps to create a complete, conforming, continuous, impermeable barrier between waste and the surrounding environment.
J&M Industries manufactures its Airspace Saver Daily Cover system as part of a broader line of tarps used in sandblasting and paint screens, grain storage tarps, athletic field covers, nursery tarps, frost protection, and compost tarps, as well as ADC. Since a workface can vary in size, J&M provides custom manufactured daily cover tarps that can range from 25 feet by 25 feet up to 150 feet by 150 feet. The company’s ADC tarps are sewn with a heavy-duty UV-resistant thread and 2-inch (6,000-pound breaking strength) UV-resistant seatbelt webbing on a double-needle lockstitch sewing machine. The lockstitch sewing pattern is a must in the rugged landfill industry. It makes the seams stronger and more resistant to wear and tear when being deployed and removed. While pulling a tarp through a landfill, it is going to encounter many objects on a workface. A lock-stitch thread can be cut and not keep pulling out. The company’s ADC tarps are manufactured out of high-density woven polyethylene tapes with a low-density polyethylene coating on each side. J&M’s main selling fabric is its 9.4-ounce, 24-mil-thick, flame-resistant Fabrene material. This has a 2-mil coating on each side. J&M also uses other polyethylene fabrics constructed the same way, but a little more lighter weight and economical: 7.5-ounce, 16-mil thick fabric with 2-mil coating on each side; 6.8-ounce, 14-mil-thick fabric with 2-mil coating on each side; and 6.0-ounce, 12-mil-thick fabric with 2-mil coating on each side. These fabrics can be heat-sealed in lengths up to 300 feet, with any required width. The fabrics are perfect for rain caps, intermediate cover, or compost covers, as well as any temporary cover. All of the fabrics have UV inhibitors in them, so they can last several years in direct sunlight.
Layfield Geosynthetics manufactures, fabricates, and installs a wide variety of geomembranes. With manufacturing plants at three locations in North America, Layfield makes geomembranes to requested size for landfill containment and cover systems. Leading its product line is the Enviro Liner 6000HD. This geomembrane is made with a durable lining material, NSF 61, having superior resistance to UV, temperature effects (when covered with a white surface), and weathering. Therefore, it can also be used in the roll of an ADC tarp. This material’s formulation consists of a proprietary polyolefin material and UV inhibitor/antioxidant additive package. It is available in 12-foot or 22-foot-wide roll stock with thicknesses ranging from 20 mil to 60 mil.
LiteEarth is unique in that it resembles artificial turf. It is an advanced, engineered, and fully tested capping system. Consisting of a lightweight impermeable material, it exceeds the regulatory requirements for both infiltration and erosion control. It can be used for long-term closure of all types of landfills (MSW, coal-combustion residuals, monofils, mine spoils, etc.). Runoff from the LiteEarth surface is nontoxic. The use of LiteEarth eliminates maintenance costs associated with post-closure mowing and landscaping while eliminating the need for fertilizers, pesticides, and fungicides.
Reef Industries Inc. is a maker of polyethylene laminates designed for a wide range of applications and an even wider range of weights, thicknesses, and special composites. These materials are highly resistant to tears and punctures while resisting weathering, UV, and chemical contaminants. The company’s Griffolyn products are used as ADC. Available in standard 200-foot-by-200-foot sizes, they are UV stabilized with reinforced fabric. Engineered as a high-strength, durable, lightweight material, the Griffolyn 20-Mil Reinforced is manufactured from a multiple laminate combining UV-stabilized film with a high-strength cord grid. Similarly, the company’s Griffolyn TX-1200 is a three-ply laminate combining two layers of linear, low-density polyethylene and a high-strength cord grid.
Southwestern Sales Co. carries a full line of ADCs for landfills under its tarpARMOR brand. The durable, easy-to-install tarps meet all of the regulatory requirements for ADC. They come in several standard finished sizes, including 48 feet by 50 feet and 96 feet by 100 feet, as well as custom-engineered designs and sizes. Also available for the company’s automated tarp deployment system (TDS) with patented tarpLOX structural support. The tarps are made from durable polypropylene material weighing 6.5 ounces, 8.0 ounces, or 9.4 ounces per square yard. Each tarp is reinforced with heavy-duty seatbelt webbing or cargo strapping reinforcement with heavy pull-straps and D-rings for easy connection.
Tarpomatic manufactures a patented Automatic Tarping Machine (ATM). This is a self-contained unit that easily attaches to a piece of heavy equipment (such as a dozer, frontend loader, or tool carrier). It is engaged by a hydraulic-drive motor to wind up and place or rewind and remove. The tarp’s spool can be operated with a variable-speed, 20-horsepower, electric-start Kubota diesel engine, and can be easily disconnected or reconnected. The deployment arm can be adjusted for height, tilt, and direction via a control mechanism installed in the operator’s cab. This allows for easy deployment and removal even over uneven terrain. It is designed for 40-foot wide panels of various lengths, weighs 6,000 pounds, and supports a load of 2,500 pounds.
Watershed GEO is a developer of synthetic liners and covers for multiple applications. Its patented ClosureTurf is a final-cap system for landfills that meets all EPA requirements for landfill closure. ClosureTurf is somewhat of a hybrid in that it offers the protective benefits of a traditional soil cover by using a specialized turf and infill covering the membrane, but without the drawbacks of erosion and high maintenance of the traditional method of closing a landfill. It’s most simply described as a “non-exposed” geosynthetic cover. At its heart is a structured geomembrane layer with studs on top to provide quick drainage even in high-intensity storms, spikes on the bottom to provide high friction that locks it into place on a prepared subgrade, and an overall thickness 20% greater than standard regulatory requirements. Its engineered cover “turf” provides structural stability and a high-friction surface while being natural in appearance and aesthetically attractive. It is a low-maintenance material, resistant to extreme weather (winds, rains, and temperatures) and long-term UV exposure. This cover is installed directly on a prepared subgrade having no special requirements beyond what the state regulatory agencies currently require.
For shorter term intermediate cover applications, Watershed Geo has developed VersaCap, another engineered turf cover with a reinforced backing made from a high-strength polymer membrane. This cover was developed to give operators the ability to provide ongoing control of gas, odors, and leachate. With a green-colored polyethylene (PE) “grass” finish, it is aesthetically pleasing as well as functional. Its impermeable surface sheds precipitation from rainfall and snowmelt, preventing it from percolating into the waste mass and producing leachate. High-strength woven geotextile under the PE grass cover provides structural stability. Underneath the geotextile is the impermeable PE membrane barrier. Given its self weight and aerodynamic properties, it does not require anchoring with sandbags or old tires. It can also provide horizontal in-plane transmissivity to convey landfill gas, while its impermeable nature contains these gases, allowing for greater gas extraction efficiency. The cover was designed so operators can install it themselves as well as remove and relocate it to other areas if needed. It can do all of this for up to 15 years in-place, compared with standard temporary membranes that may only last few years due to wind and exposure damage. This operational lifetime can be further extended to 50 years with the addition of a proprietary pozzolan infill, HydroBinder.
ADC Options: Sprays
A radically different approach is the use of spray on cover materials. This is usually a free-flowing slurry mix of water, bulking fiber (paper pulp being common), adhesive additives, and binding agents such as cement. The material is applied by means of what is essentially a large water cannon. Once sprayed over the workface, it dries to form a hard crust. If the spray is mostly organic in makeup, it can be left in place, as it will decompose in time and remove any barrier to leachate migration. Inorganic sprays need to be broken up by sheepsfoot or tractor tread vehicles to make them permeable before the placement of additional waste. High winds also alter the efficacy of spray-on covers, as do freezing temperatures, heavy rains, hail, or any other adverse weather conditions.
Central Fiber Corp. is a manufacturer of Topcoat, a nontoxic, biodegradable, environmentally friendly cellulose fiber product that can be applied as spray-on cover. This product is made from recycled materials, primarily obtained from recovered post-consumer newspapers, magazines, and wood materials. Not only is it used as ADC on landfills, Topcoat can be used for erosion control, insulation, industrial products, and other applications. Sprayed as a slurry, Topcoat provides an alternate material that meets all of the regulatory requirements for daily cover. Its application uses a convenient one-bag system containing all of the dry chemicals and materials necessary. One simply adds water; there is no mixing time or waiting prior to application (no more than 20 minutes from mix to cover). It is applied to the waste surface with standard hydro-seeding equipment.
Finn Corp. manufactures ADC from recycled paper and wood mixed with polymers, an enzyme complex, and proprietary chemicals. Applied as a slurry by a specialized Finn landfill spray application machine, it quickly hardens to form a cement-like crust over the workface. This application machine, the LF-120 HydroSeeder, an exclusive Finn Landfill Solution, is specifically designed for use in landfill operations. It can also double as a mobile watering source to wash equipment, put out fires, water vegetation, and hydroseeding.
LSC Environmental Products LLC is the maker of the Posi-Shell Cover System, a spray-applied mortar coating for workfaces. Posi-Shell makes it possible to significantly reduce and eliminate soils in landfills through both daily workface cover and intermediate cover, freeing up cell space for revenue-generating trash. Easy to use, Posi-Shell Brown Base Mix comes in 50-pound bags that can be mixed with water or leachate. When more durability is needed, Portland cement may be added to the mixture, making Posi-Shell ideal for ditch lining and slope retention when seeding isn’t possible. The unique and versatile nature of Posi-Shell has taken it beyond the landfill to provide cover for cement clinker piles, mining applications, compost cover, contaminated soil cover, and erosion control in highway and industrial settings. Along with Posi-Shell, LSC has a family of cover products that include Posi-Cube Seed and Soil Guard, a unique blend of mineral binders and wood fibers for a seeding coat that works on the challenging slopes of landfills. Posi-Clear is a polymer-based dry powder product that mixes in a water truck or hydroseeding unit to create a liquid that bonds with the surface of roads to reduce the need for watering. And LSC’s newest coating, Odor-Shell, enables managers to eliminate at the source odors emitted by sewage sludge or other odorous intake items, allowing the landfill to maintain good relations with neighbors while taking in some more challenging wastestreams.
New Waste Concepts provide a wide range of active systems for the suppression of dust, VOCs, odors, and gasses. The newest addition to its line of products includes the Typhoon Evaporation Head, which offers low-energy-use evaporation and misting technology. These systems include the indoor Cocoon Evaporation chamber, which is currently evaporating over a million gallons in three months in Tennessee. The company’s odor control and leachate evaporation systems are being used by composting, oil-and-gas, utility-and-energy, construction-and-demolition, manufacturing, and livestock facilities as well as by solid waste landfills. The ProGuard spray-on ADC is a low-cost, non-cement material consisting of a blend of polymers and recycled fibers. In its SB2 version, ProGuard has an applied cost of less than a penny per square foot and can be applied using a centrifugal pump. The company’s ConCover spray-on covers have higher density and higher viscosity, making them suitable for steep workfaces and interim and long-term covers. These long-lasting cover materials carry the well known name, ConCover, with the SW being used as an intermediate cover and the 180 version lasting up to 18 months.
Since 1986, Rusmar Inc. has been a full-service, specialty chemical manufacturer and a leading provider of non-hardening aqueous foam products and application equipment for the solid waste and environmental remediation industries. Its spray-on ADC products are nontoxic, biodegradable, nonhazardous, and inflammable, and they consume no valuable airspace. The company’s patented AC667 Soil Equivalent Foam was designed to withstand the elements and provide cover for periods up to 72 hours. It meets all the regulatory requirements for daily cover performance. Being unaffected by ambient temperatures and moderate precipitation, it can be used year round. Rusmar’s AC667-SE Soil Equivalent Foam is a liquid concentrate composed of a starch-modified, hydrolyzed protein surfactant, even providing a cinnamon scent for odor control. The company’s spray foams can be used in other landfill applications besides ADC, such as landfill excavation for cell liner tie-ins, landfill-gas trenching, landfill mining/reclamation projects, and nonhazardous and hazardous waste remediation. In addition to the chemical foams themselves, Rusmar manufactures and services its application equipment.