Changing Demands Drive Landfill Technology

Scott F. Lockhart

After years of evolution, uniform guidelines for MSW disposal were established by Subtitle D (see map). Politics, public perceptions, technological advancements and scientific knowledge were integral to their development.

During that time, advancements have been made in landfill technology for correcting older, problem sites and outfitting new facilities.

Regulations cover a number of elements affecting the entire process, from landfill design through post-closure.

Containment, Encapsulation

A composite liner system is Sub-title D's minimum requirement for a lateral expansion or new municipal solid waste landfill (MSWLF). The system must include a synthetic flexible membrane liner (FML) at least 30 mils thick underlain by a compacted soil barrier. A minimum 60 mils thickness is required if the FML is made of HDPE.

If an alternate system is not permitted, a new site's configuration may be limited - particularly the sideslope angle - because of the difficulty in constructing compacted soil liners on slopes steeper than 2.5:1 or 3:1 (horizontal to vertical). This affects the usable airspace. However, most states allow alternative materials and liner systems to provide equivalent protection.

Some states require double composite liner systems, similar to Sub-title C facilities. Double composite systems include a secondary collection system (SCS) under the primary composite liner, typically consisting of polyethylene drainage netting or a granular drainage layer with several pipes. The SCS is placed with a geotextile filter on one or both sides.

The SCS, which is often underlain by a second FML, monitors the primary composite liner's performance and provides backup for containing and collecting leachate.

Liquids in the SCS may be: water trapped during construction in the drainage layer; groundwater which may infiltrate the system; water squeezed from the pores of the compacted soil barrier under pressure; or liquid resulting from a breach in the primary composite liner. Moni-toring is required and allowable flow rates are prescribed.

Although models can predict diffusion and advection permeation of liquids through a compacted soil barrier, SCS performance is best evaluated by testing volume and quality combined, incorporating geochemical fingerprinting of leach-ate and liquid in the SCS.

Landfill caps vary in configuration or thickness, depending upon construction constraints, regulatory criteria or performance standards. The basic components include a low-permeability barrier layer (soil, soil/ geomembrane or geomembrane), a sub-cover drainage layer and a vegetative/protective layer on top.

A landfill cap and the barrier layer must reduce infiltration, which contributes to leachate generation. A sub-cover drainage layer, either a free-draining granular material or geocomposite, drains away infiltration through the overlying vegetative layer to minimize the buildup of liquid on top of the barrier layer.

Finally, the vegetative layer can protect a compacted soil barrier or geomembrane from damage due to freeze and thaw, puncture, tearing or physical breakdown. Vegetation reduces erosion potential and peak surface water runoff rates, improves aesthetics and can help reduce infiltration through evapotranspiration.

Technological advancements have been driven by regulations, engineering re-quirements and market demands. For ex- ample, HDPE geomembranes became popular in the mid 1980s. A competitive market led to developments such as a wedge welder to produce a dual seam, which decreased in-stallation and seam testing time and re-duced construction and inspection costs.

Another advancement followed a landfill liner system failure in 1988; as a result, new polyethylene products were developed to increase the interface shear characteristics of geosynthetic liner systems. For example, it was discovered that textured HDPE used with a geotextile or geocomposite creates a Velcro effect that significantly in-creases the interface friction. It's used primarily on sideslopes of double composite liner systems or on internal berms, which may be subject to significant shear stress.

Technological advancements also have been made to increase seam quality and to minimize ultraviolet degradation and environmental stress cracking. HDPE is available with co-extruded layers of black and white polyethylene. Although many black HDPE liner materials are manufactured with carbon black and other UV chemical stabilizers, non-carbon black stabilizers can be in-corporated into the white layer, which reflects more sunlight and reduces potential degradation. This reduces subgrade desiccation, in-creases installation damage detection and adds long-term durability.

Although HDPE's advantages in-clude increased chemical resistance and decreased diffusive permeabilty, it is stiffer and less flexible than low-er density resins and is less resistant to environmental stress cracking. Consequently, co-extruded lin- ing materials were developed to combine the chemical resistance and low permeability of HDPE with the flexibility and greater environmental stress crack resistance of lower density polyethylene, like very low density polyethylene (VLDPE) and liner low density polyethylene (LLDPE).

Geosynthetic cap materials should be flexible and resilient since landfill settlement can create depressions or voids. LLDPE, polyvinyl chloride (PVC) and polypropylene (PP) are the three main choices for capping mat-erials. PP, which can be manufactured with scrim reinforcement or a single geomembrane, is puncture resistant and can be wedge welded.

Wedge temperature, applied pressure and seaming speed all affect geomembrane seam quality. Trial seams for each welding device are usually made every four to five hours during installation, or more frequently under severe conditions.

Welding parameters are manually adjusted, although computer-controlled models are being developed.

HDPE materials, co-extruded with an electrically conductive layer un-derneath, are available. By applying an electrical current across the system, seaming defects or holes in the liner sheet are identified by a spark.

Soil is an important component of a composite liner system. Geosyn-thetic clay liners (GCL), amended soil caps or liners (i.e. soil/bentonite mix) and compacted clay barriers are typically used. GCLs provide flexibility, low permeability and maximized airspace, but may be limited on steep slopes because of low shear strength. GCL products have been developed using high-strength geotextiles stitched together with reinforcing threads to increase the shear strength for use on steeper slopes.

Amended or attenuating soil barriers can be used where low-permeability materials aren't readily available. Bentonite, zeolites or high car- bon fly ash can be mixed with a granular or fine-grained soil in a pug mill or using heavy equipment. This process is susceptible to changing weather conditions, requires strict quality control to maintain uniformity within the liner or cap and may be subject to chemical degradation.

Compacted soil barriers are commonly used for containment and encapsulation. Compacted clay works well if it meets the design criteria and is available locally. Under the right conditions, clay soil will provide a fairly uniform, low-permeability barrier, but it is susceptible to freeze/thaw, desiccation and cracking due to differential settlement and gas migration. It also may be difficult to construct a compacted soil barrier on steeper sideslopes or on a compressible waste fill.

When evaluating soils during the design stage, consider the local availability of low-permeability soils, geographic location, regulatory standards, design and performance criteria, long-term maintenance requirements and economics.

Geogrids are high strength poly-ethylene or polypropylene mesh-like materials. They can be used to help stabilize a liner system, vegetative cover or drainage layers in a cap or a sideslope.

Geogrids gained notoriety in 1988 with a piggyback expansion of an Islip, N.Y., landfill. Because regulations prohibited siting a new landfill, engineers used geogrids in the site's vertical expansion, proving the liner system could withstand the wastes' settlement and shifting.

Leachate And Groundwater Subtitle D requires that a composite liner and leachate collection system prevents more than 12 inches of leachate buildup over the liner. A network of perforated collection pipes is constructed in a highly permeable granular drainage medium, such as sand, gravel or in some cases shredded tires.

When choosing a drainage material, durability, composition, strength and permeability are considered. Computer models help predict leachate generation rates to determine proper pipe spacing. Perforated polyethylene pipe is often used be-cause of its chemical resistance, ease of installation and flexibility.

Some states require regular visual or mechanical inspection of the leachate collection system. This can limit cell configuration flexibility as well as the use of alternate technologies, such as geonet. This inspection can be conducted by video camera or by pulling or pushing a mandrel through the pipes to verify their integrity. Hydraulic flushing or jet cleaning can clean the pipes, de-pending upon whether access is upstream or downstream.

Using vertical manholes to extract leachate is difficult, especially when maintaining pumps or adding additional sections during filling.

Both customer demand and stringent regulatory requirements in-spired today's sophisticated pumps and controls. Now, the sideslope riser is a viable alternative to vertical manholes.

Items such as flow meters and level sensors gained popularity when regulatory agencies began requesting the verification of liquid levels, flow rates and leachate extraction volumes. Telemetry and remote monitoring of leachate removal and extraction systems may be a future standard, particularly for post closure monitoring.

Leachate is stored on-site in large modular tanks or discharged to a publicly owned treatment works (POTW). However, it can be difficult to secure permission.

Subtitle D allows leachate recirculation, which has several benefits. The amount of moisture reaching the waste in a closed landfill directly affects degradation. Also, managed refuse stabilization can help control leaching or immobilize pollutants, thus reducing potential long-term adverse impacts.

In addition, moisture helps form landfill gas, a potential energy source. Finally, long-term maintenance and monitoring could be minimized or reduced by accelerating a landfill's stabilization.

A groundwater monitoring program is also required and includes detection monitoring, assessment monitoring and corrective action.

Detection monitoring requires installing and monitoring upgradient and downgradient groundwater monitor wells. Chemical parameters are used to determine if leachate has been released into the groundwater. If a release has occurred, existing wells must be sampled and as-sessed, using an expanded list of pa-rameters. Assessment defines the extent of contamination. Periodic monitoring will determine if the site can return to detection monitoring or proceed to corrective action. Cor-rective action ranges from remediation to closing a facility.

Some of the technological ad-vancements in sampling and testing include stainless steel submersible and pneumatic pumps, Teflon bailers or gas driven piston pumps. Be-fore selecting the technology for sample collection, consider construction details, hydrogeologic conditions, cost, sample integrity cost and the monitoring parameter list.

Stormwater Management

Surface water controls are re-quired to manage the run-off from a 25-year, 24-hour storm. Run-on controls, designed to prevent flow of water from surrounding areas into the active disposal area, must ac-commodate the peak flow from a 25-year storm. These controls must comply with the Clean Water Act, including NPDES criteria.

Sites with a general stormwater permit also must develop a Storm-water Pollution Prevention Plan (SWPPP), which describes efforts to minimize stormwater contact with potential pollutants and sedimentation off-site. These demands also have yielded products aimed at sedimentation and erosion control.

Methods for stormwater control include earthen berms, perimeter and diversion ditches, terraces or benches, rock or gabion letdowns, strong vegetation and sedimentation basins (see Figure I). Structural aids include silt fences, rock filters, straw bales, natural or synthetic erosion control blankets, revegetation mats and sometimes brush barriers.

Since clay or silty soils are susceptible to erosion and are the slowest to settle once suspended in storm-water, the percentage of bare earth exposed should be minimized during site development and operation.

Minimizing leachate generation from stormwater run-on can be accomplished in various ways. Pre-cipitation is considered leachate as soon as it has contact with refuse or a drainage layer through which leachate may be flowing. Large open cells are particularly susceptible. Techniques for minimizing leachate generation include installing temporary stormwater flaps within the drainage layer; building diversion berms; and covering unfilled portions of a landfill cell with a thin geo-membrane or plastic sheeting.

Remediation Techniques

Remedial actions at MSWLFs in-clude containment and encapsulation, leachate removal, groundwater pump-and-treat or gas migration control.

Containment and encapsulation isolate the contamination from the groundwater, soil or air. Methods to contain lateral migration of contaminants include capping a site with a low-permeability barrier to reduce infiltration and constructing a barrier, such as a slurry wall or sheet piling, around the site's perimeter.

An innovative technology is an in-situ treatment of contaminated groundwater within a permeable wall placed across a groundwater flow path.

The permeable wall section(s), or gate, is placed within a low-permeability barrier wall. Groundwater flow paths carrying the dissolved contaminants are then directed into the gate for treatment.

Steel sheet pilings or HDPE vertical barriers are viable options for barrier wall applications. The latest technology is the HDPE barrier, which minimizes or prevents contaminant migration. Individual panels are vibrated into place and then locked together to form a watertight seal. Also, HDPE panels can be in-stalled in conjunction with a traditional slurry wall.

The type of barrier or encapsulation system depends on regulations, site configuration, cost, long-term O&M requirements and performance criteria.

Pump-and-treat technologies have been combined with barrier walls, groundwater injection, bioremediation, and air sparging. Pumping systems, using single or arrayed vertical wells, capture contaminated groundwater and remove contaminant mass and/or achieve hydraulic containment of plumes.

Treatment technologies are selected based on the types and concentrations of contaminants extracted by the pumping system, natural groundwater chemistry, treated discharge restrictions (both air and water), flow rates and O&M considerations. Treatment technologies include: air stripping, UV/oxidation, granular activated carbon adsorption, biological treatment, chemical precipitation, ion exchange and single-/multi-media filtration.

Vertical extraction wells or perimeter collection systems will remove leachate from an older site. This method is affected by site configuration, installation limitations, O&M requirements and disposal options. Bioremediation or natural attenuation should be considered when limited amounts of released contaminants are not a threat to human health and the environment. Using gravel trenches, sloping from old to new lined areas, preferential pathways through the refuse can be created to move leachate from unlined areas into lined areas.

Landfill gas control involves active extraction wells or passive vents and gravel filled trenches.

A gas extraction system requires attention to the gas field balance, maintaining the flare, header pipes, wells and blowers, and managing the residual condensate.