Layer Upon Layer

Oct. 28, 2013

A landfill waste disposal operation is a two-stage process. The first stage involves the actual depositing of the waste on the workface of the current disposal cell, followed by spreading into relatively thin layers, and compaction to the maximum density possible given the waste’s physical characteristics and available compaction equipment. The second stage includes placing various types of covers over the deposited and compacted waste. These covers include daily cover (placed at the end of each work day), intermediate cover (on waste that will be exposed-not receiving either additional waste deposits or final cover-for more the 30 days), and final covers installed over completed waste slopes that have been developed to their maximum designed extent. Each type of cover has a different function, and since “form follows function,” each has a different configuration and uses different types of materials.

Daily 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.

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Daily cover placement is a specific requirement of the regulations established in accordance Subtitle D of the Resource Conservation and Recovery Act (RCRA). This section of the federal code deals with the management and disposal of nonhazardous municipal solid waste (MSW). These regulations are found under Title 40 of the Code of Federal Regulations (40 CFR-“Protection of Environment”, Chapter I-Environmental Protection Agency, Subchapter I-Solid Wastes, parts 239-282). The actual details of the regulatory standards and requirements for MSW landfill operations specifically describe the requirements for covering deposited waste at the end of the work day under Part 258 Paragraph 21 “Cover Material Requirement”:

(A) 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.

(b) 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.

(c) The Director of an approved State may grant a temporary waiver from the requirement of paragraph (a) and (b) of this section if the owner or operator demonstrates that there are extreme seasonal climatic conditions that make meeting such requirements impractical.

(d) The Director of an Approved State may establish alternative frequencies for cover requirements in paragraphs (a) and (b) of this section, after public review and comment, for any owners or operators of MSWLFs that dispose of 20 tons of municipal solid waste per day or less, based on an annual average. Any alternative requirements established under this paragraph must

  • consider the unique characteristics of small communities;
  • take into account climatic and hydrogeologic conditions; and
  • be protective of human health and the environment.
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As part of the enforcement mechanism for Subtitle D, state environmental agencies must mandate regulations that are at least as stringent as those established by the federal government. Therefore, the above regulations represent only the minimum floor of regulatory requirements. However, it should be noted that these are primarily performance specifications. While 6 inches of soil cover is specifically named, it serves only as a comparative example that allows the use of alternate daily cover material, provided that it performs as well as this baseline standard without itself directly or indirectly presenting a threat to the environment.

There are three broad categories of alternate daily cover:

  • Disposable sheets of thin plastic film
  • Reusable tarps or sheets made from high-density polyethylene (HDPE) or heavy geotextile
  • Spray applications using inorganic chemicals and such concrete or organic materials as pulped paper

Though fragile in appearance, disposal sheets laid over the current workface can meet all the requirements for alternate daily cover. Lightweight enough to be deployed manually, disposable sheets can also be efficiently deployed by a mechanical tarp deployment device attached to a standard piece of landfill equipment. When properly anchored around the edges, the deployed sheet serves as an effective barrier and seal for the workface until the start of the next workday. Being impermeable, thin plastic films are actually superior to standard soil cover in that they completely prevent infiltration of precipitation. It is for this reason that disposable sheets are deliberately shredded prior to the start of the next workday by means of track walking with a dozer or other piece of equipment. The resulting holes and tears make the sheets permeable to the downward passage of moisture, thus preventing destabilizing leachate pockets from forming above them in the waste.

Similar in application to disposable film sheets, heavier tarps made from HDPE geomembrane or geotextiles are designed for multiple uses. Being heavier, tarps tend to be placed and removed mechanically more often than film sheets. However, they are more resistance to wear and tear, resistant to being dislodged by strong winds, and can be used over and over again to provide an overall cost savings. Additional cost savings can be achieved by fashioning these tarps from scrap pieces left over from recent liner or cap construction efforts.

However, sheets and tarps have their limitations. They can be difficult to place, especially over waste with large sharp objects protruding from the workface and can be prone to slippage when placed on workfaces with relatively steep slopes. As an alternate to physical covers, other daily cover options are spray-on slurries and foams. Usually a mix of water, cementitious binder, adhesive additives, and bulking fiber, these spray-on ADC materials can be easily placed over the workface with a high-pressure hose. After spraying, it dries out to form a decomposable (if organic) or easily breakable (if inorganic) cover on the waste surface. The dried result resembles a thin crust of papier-mache. Spray-on ADC is also subject to its own set of unique operational limitations. For example, it cannot be effectively deployed during high winds or in low temperatures. But aside from these limitations, spray on ADC can be a labor-saving option to the landfill operator.

Though not normally considered as potential daily cover material, inert waste can also be used in the right circumstances and with state regulatory agency approval. Inert waste would be defined as inorganic waste that’s physical and chemical characteristics do not pose a threat to human health or environmental safety. These materials can often include energy production byproducts such as fly ash. By using fly ash instead of soil cover, a site can achieve considerable combined cost savings and additional revenues (a power plant will pay the site to take the ash as waste). However, getting approval from the applicable regulatory agency for the use of inert waste as daily cover material can be an involved process, since this material is often on a state’s exclusion list.

Tarps can be quickly deployed and removed.

Intermediate Cover
Wrongly considered as just a thicker layer of daily cover, intermediate cover has a different function. In addition to meeting the same operational requirements as daily cover, intermediate cover has to provide resistance to longer-term weathering and erosion by providing protection to underlying landfill components against both. Those areas that will be exposed for 30 days or more will usually be required to receive intermediate cover consisting of at least a 12-inch-thick layer of soil. A site operator can prove to the regulators that an alternate (longer) time period is acceptable if it can be demonstrated that infiltration into the underlying waste will not be increased.

Intermediate cover will also usually have to meet more stringent material specifications. These include being nonputrescible, low permeability to water, good compactability, cohesiveness, relatively uniform texture, and not containing a significant number of large objects in such quantities as may interfere with its construction. Typically, this means a soil layer consisting of well-compacted loam, silt loam, clay loam, silty clay loam, silty clay, or some combination of these soil types.

Alternate intermediate cover can also be used. Different materials and layer thicknesses can be substituted if it can be shown that the proposed alternate configuration provides equal or better protection. Furthermore, any intermediate cover should be removed prior to subsequent placement of additional waste so as to not impede the downward flow of leachate to the leachate collection system. And while the intermediate cover is exposed, the operator should protect the cover from erosion (for example, by providing erosion control structures such as silt fences and straw bale barriers), and repair any rills or gullies forming in the intermediate cover while ensuring that the minimum required thickness is maintained.

The maximum 30-day limit for waste exposure necessitating the use of intermediate cover derives from 40 CFR Part 256.60 Closure Criteria. This regulation requires the start of final closure operations within 30 days of final receipt of wastes. Therefore, the requirement is in conformance with the need to minimize exposure time for waste. Covering up waste for long periods during operations has another important benefit. It generally improves the landfill’s appearance and general aesthetics (including the minimization of odor and blown litter). In the landfill business, perceptions are reality to a concerned public. So, shaping public and political attitudes towards a landfill may be the most important function of intermediate cover.

Final Cover
This cover system is installed on those waste slopes that have achieved final development grades at the completion of disposal operations. Final cover is usually placed in partial as individual sections of the waste slopes achieve final development. A typical operational phase or disposal cell, once it has achieved full capacity, tends to be at final grades along its outboard slopes and at intermediate grade on the interior slopes. So a completed disposal cell will be roughly half final cover and half intermediate cover.

Like daily cover, final cover placement is a specific requirement of the regulations established in accordance Subtitle D of the RCRA. The detailed regulatory standards and requirements for final covering of deposited waste are listed under Part 258 Paragraph 60 “Closure Criteria”:

(a) Owners or operators of all MSWLF units must install a final cover system that is designed to minimize infiltration and erosion. 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 1×10–5 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.

(b) The Director of an approved State may approve an alternative final cover design that includes:
(1) An infiltration layer that achieves an equivalent reduction in infiltration as the infiltration layer specified in paragraphs (a)(1) and (a)(2) of this section, and
(2) An erosion layer that provides equivalent protection from wind and water erosion as the erosion layer specified in paragraph (a)(3) of this section.
(3) The Director of an approved State may establish alternative requirements for the infiltration barrier in a paragraph (b)(1) of this section, after public review and comment, for any owners or operators of MSWLFs that dispose of 20 tons of municipal solid waste per day or less, based on an annual average. Any alternative requirements established under this paragraph must:
(i) Consider the unique characteristics of small communities;
(ii) Take into account climatic and hydrogeologic conditions; and
(iii) Be protective of human health and the environment.

The federal regulations provide, in effect, a minimum cap configuration consisting of at least 18 inches of compacted, low permeability soil acting as an infiltration layer and an overlaying 6-inch layer of vegetative soil for growing grass and establishing rot systems, 24 inches total. This is the basic design of an all-soil (no geosynthetics utilized) final cover system. For older landfills, this is the type of final cover that is most likely to have been constructed.

The design of a soil cap can be modified to include a geomembrane barrier layer to form a composite (low permeability soil and geomembrane) cap. Since a geomembrane is by definition impermeable to percolation of water and migration of gas, a composite cap should also include a drainage layer to wick away precipitation from building up on top of the geomembrane and a permeable gas layer to prevent landfill gas from building up under the geomembrane. A complete composite cap final cover system includes the following (from top to bottom):

  • Grassy vegetation and other erosion controls and surface water runoff management structures.
  • A soil protective layer sufficient to guard against frost penetration and provide sufficient depth for vegetative cover roots.
  • A drainage layer to manage percolation. This layer can either be high-permeability sandy soil (if slope analyses show this to be stable) or a geocomposite (factory-bonded geotextile filter, geonet drainage layer, and geotextile cushion).
  • A flexible geomembrane made from applicable materials such as high density polyethylene (HDPE), very low density polyethylene (VLDPE), and poly vinyl chloride (PVC)-where allowed. The geomembrane’s thickness can vary from 40 mil to 80 mil, depending on design considerations and regulatory requirements.
  • A compacted, low-permeability soil barrier layer. The combination of thickness and permeability should match that of the compacted soil component of the underlying liner system.
  • A gas-permeable layer to allow free migration of landfill gas. Again, this can be either high-permeability sand or a geocomposite.
  • A soil-leveling layer directly on the waste (and its daily or intermediate cover soil) to ensure proper final grades are achieved by cover construction and provide a firm foundation for this construction.

A relatively new type of final cover system dispenses with cover soil entirely and relies exclusively on exposed geomembrane. The types of material most used for these applications are thermoplastic polyolefin (TPO) membranes. These membranes have been traditionally used as durable, easy to install roofing materials. But recently, they have found new life in landfill geomembrane applications, specifically as alternate covers that integrate photovoltaic cell (PVC) arrays that convert a closed out landfill into a solar energy facility.

By combining landfill final protective cover with energy production, the landfill operator simultaneously solves many problems inherent with the development of renewable energy sources (finding available land for large solar cell arrays) and what to do with otherwise unproductive closed landfills.

However, the potential remains for the utilization of exposed TPO geomembrane as an acceptable alternate final cover system for closed landfills regardless of other post-closure uses. In order to properly examine the potential of exposed TPO geomembrane final covers, a landfill operator and his design engineer must consider the following: physical characteristics and composition, ease of construction and CQA requirements, overall cost, aesthetics and public relations, integration with the landfill’s gas management system, impacts on surface water run-off and erosion control structures, structural stability and resistance to sliding failure, long term resistance to weathering and UV exposure, the effects on percolation of precipitation through the final cover and subsequent leachate formation, and the post-closure care and maintenance requirements.

TPO in Action
Several landfills currently use exposed TPO geomembranes as final covers.

The Hickory Ridge landfill in Conley, GA, is owned by Republic Services. Hickory Ridge combines a TPO geomembrane final cover with integrated solar cells, a system referred to as an exposed geomembrane solar cover (EGSC) and branded the Spectro PowerCap by its developer, Carlisle Energy Services, a division of Carlisle Construction Materials. It is both the largest TPO geomembrane landfill cover and was the largest solar photovoltaic cell array in Georgia.

The Tessman Road Landfill outside of San Antonio, TX, also owned by Republic Services, uses flexible solar panels welded directly to the exposed TPO geomembrane that generate electrical power that is tied directly in with the site’s landfill gas-to-energy system. Its 1,050 flexible laminate solar panels are installed over 5.6 acres of the landfill.

A recipient of the Environmental Project of the Year Award from the Central New York Branch of the American Public Works Association, the Madison County Landfill near Lincoln, NY (another Carlisle Energy Services project), utilizes a combined solar array and TPO geomembrane cover. The system provides 50% of the energy needed to run the site’s collocated recycling facility.

The physical characteristics and chemical composition of TPO differ significantly from standard high-density polyethylene (HDPE) and polyvinyl chloride (PVC) geomembranes. Unlike these two standard geomembranes TPO is the composite material. It consists of polypropylene (PP) and ethylene-propylene (EP) rubber, with the EP added to provide flexibility to the membrane. Unlike typical thermoset rubber, TPO can be processed and recycled like thermoplastic materials. But unlike other thermoplastic membranes, TPO does not contain chlorine or chlorine-based additives. Material specifications for TPO manufacture are governed by the recently published ASTM specification D6878.

Installation procedures differ for TPO covers. Though lacking typical vegetation and soil cover, a TPO geomembrane can provide better environmental protection. Typically 60 mil in thickness, a TPO geomembrane cover is anchored directly into the landfill at regular intervals instead of relying on overlying soil cover for stability. The intervals between the anchors depend largely on local weather conditions, especially anticipated wind speeds. The TPO is installed on a compacted and graded bedding layer consisting of a minimum 12-inch-thick intermediate cover layer and additional grading soil to provide a smooth surface for geomembrane installation as needed. Engineering design analyses are performed to ensure, slope stability on final waste grades, resistance to wind load tears, and to calculate anticipated surface water runoff flow quantities.

Performance history is an important consideration, given the exposed nature of the TPO geomembrane final cover configuration. Being exposed to thermal, ultraviolet, and physical impact degradation, care must be taken to ensure proper maintenance throughout the site’s post-closure care period. Therefore, major suppliers of TPO geomembranes, like Carlisle, provide a scrim-reinforced, three-ply TPO geomembrane specially formulated for exposed landfill cover applications. Even with these improved materials, inspection and repair of damage due to weathering remains a critical post-closure activity.
About the Author

Daniel P. Duffy

Daniel P. Duffy, P.E., writes frequently on the topics of landfills and the environment.

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