Protective Liner Uses and Landfill Application

by Joe Yack and E.J. O'Neill

INTRODUCTION

LINER TYPES AND USES

LINER APPLICATION OF SOLID WASTE LANDFILL

LINER TESTING AND WELDING METHODS

LEACHATE COLLECTION AND CONTROL

CONCLUSION

INTRODUCTION

With the increase of industrialization, companies are producing more wastes that need to be treated proir to being released into the environment. When residual or solid wastes are produced from municipal authorities or from mills as slag, a hazardous leachate usually results from the exposure to weather. A liner system is installed to prevent the contamination of the soil or groundwater by the hazardous leachate. Lining systems can be utilized in the application of leachate collection ponds, subsurface wall barriers, and to create landfill cells. The majority of the lining systems being installed today are for solid waste landfill cells. Due to the differences among industries, many chemicals must be treated from these leachates. This variation forces liner companies to produce a wide variety of liners that are chemical resistant. The installation process and the specific types of liners that can be used will be discussed in the further sections.

LINER TYPES AND USES

GEOMEMBRANE:Goemembrane liners are made of high density polyethylene to ensure maximum security of the solid waste contaminant. There are a variety of choices of geomembrane liners that the liner companies manufacture to suit the needs of the lining system. The simplest difference that is offered are geomembranes which are textured or smooth. A smooth geomembrane liner is normally used for collection ponds since they are not covered with soil or other liners. Collection ponds also do not have steep side slopes which results in minimal slipping if additional liners are placed. A textured geomembrane is typically used for mining and solid waste landfills. This creates a surface for additional layers such as geocomposite or soil to adhere to and releases pressure from the seams of the liner. Other types of geomembrane liners that are available are white geomembranes, which will cut back on thermal expansion and also create a cooler environment for the workers due to light reflection. This type of geomembrane is typically used in solid waste landfills. A flame retardent geomembrane is available from certain companies. It can be used in areas such as nuclear facilities and petrochemical applications. Finally, there is a conductive geomembrane that has been made available by the GSE Lining Company. This liner can be used for a collection pond and provides a simple procedure to test for damages. This is accomplished by using a unique type of spark testing. This was constructed for collection ponds that are exposed to the elements and have increased chances for damage. The primary use of the geomembrane liner is to contain the contaminants of solid waste.
GEOCOMPOSITE:Geocomposite is a combination of two types of liners. The first and main component is a geonet. The geonet is a high density polyethylene net that is usually placed either above or below the geomembrane liner. This net is used as a collection system for the leachate produced by the solid waste. The second component that makes up the geocomposite liner is a nonwoven geotextile. This geotextile is bonded to the geonet either on one side or both. The purpose of the geotextile is to prevent any soil from clogging the geonet. This ensures proper collection of the contaminant. The geotextile, when layed over a textured geomembrane, acts as a adherent preventing the geocomposite liner from slippping on the steep slopes of the landfill cell. The geocomposite leads to the final step of the lining system.
GEOSYNTHETIC CLAY LINER:The geosynthetic clay liner(GCL) acts as a substitute for a compacted layer of bentonite clay that is usually placed over the geocomposite to complete the lining system. The GCL, with its easy deployment, gives the contractor the ease of rolling out a clay layer that would normally have to be placed and compacted by heavy machinery, adding risk to damaging the liner system. The GCL can be used as a protective cap to cover the landfill as well. (GSE Lining Technology,1996)

LINER APPLICATION OF SOLID WASTE LANDFILL

The liner installation process for a solid waste landfill is a process that takes several weeks due to the precautions that need to be taken. These precautions help to ensure damage free installation. It is up to the design engineer and the engineer of the company having the liner installed to decide what type of liner should be used. Most primary liners used today are made of high density polyethylene (HDPE) with a minimum of 60 mil (1.5 mm) thickness; HDPE liners can be smooth or textured. With the way landfills are being constructed today, to optimize space and revenue, the sidewalls of the cell tend to be very steep. Having steep side slopes can sometimes cause the cover soil to slide and create added stress to the seams of the liner and the liner itself. This is why companies have designed the textured liners to optimize friction control and keep the liner and cover soil in place during its operation. These primary liners are used in the cell to contain hazardous leachates and protect the groundwater surrounding it. The second most important part of the liner system is the geocomposite. The geocomposite lining is an HDPE net that provides a high in-plane flow for the leachate. The geocomposite lining acts as a a leachate collection system that has relatively high resistance to chemicals. Since this geonet is placed directly on top of the primary liner there is usually a geotextile that is bonded to it to prevent clogging from the cover soil. Most systems that use the textured geomembrane will also use the double bonded geocomposite. A secondary lining system is usually installed because of requirements for leak detection systems.

This secondary system is to monitor the primary liner on a regular basis. The secondary system is the same as the primary system consisting of a layer of geomembrane and either geocomposite or geonet. The use of any of these layers is dependent on whether a clay layer is used under the primary liner. This two liner system containing the primary and secondary systems is becoming the most widely used in order to ensure complete protection. Once the underlying system is complete a protective cover goes over the primary liner. This protective cover is usually a 18-36 inch compacted clay cover. The clay cover is placed at a specified moisture content to achieve ultimate compaction and protection of the primary liner. (GSE Lining Technology,1996)

The final stage in completing a landfill liner system is the landfill cover itself. Once the cell is filled with waste it must be covered to eliminate problems such as odor, litter, and most of all leachate production; leachate in a solid waste landfill is a result from precipitation. In order to contain the waste a flexable geomembrane cover is installed. It is crucial that the protective cover be able to contain hazardous methane gas production from the decomposition of waste. The decomposition and shifting of waste is the primary reason for a flexible cover. The liner system does not involve a complicated set up; however the precautions that have to be taken are never ending. Successful installation of a lining system can ensure the proper protection from hazardous leachates entering the groundwater or soil. (GSE Lining Technology, 1996)

Schematic of Liner System

LINER TESTING AND WELDING METHODS

Laboratory Testing : There are several tests performed on the geomembrane liners in the laboratory and they all follow the guidelines of the American Society for Testing Materials (ASTM). Some of these tests include Tensile, Stess Cracking, Multi-Axial, Density, and Melt Flow Index testing.

Tensile Testing (ASTM D638): Is the measure of the greatest tension the geomembrane can handle without tearing. In this test a liner specimen is placed in a machine and stretched until it breaks. This is recorded on a tensometer and sent to the engineer in charge.
Stress Cracking Test (ASTM D1693): This test is to find out how susceptable the geomembrane is to cracking when exposed to the environment. These stress cracks are less than tensile strengths and are caused by exposure to the environment. For this test a bend speciman is placed in a test vessel and submerged in a surface-active agent. After a given time interval the speciman is analyzed for cracks. Ideally the liner needs to be impervious to stress cracking.
Multi-Axial Testing (GM 4): Is a test that gives a more accurate representation of field performance of the liner. The liner is stretched 360 degrees all at once in a pressure vessel. This test lets there be no exposed edges as would be in the field. The peak deflection point is measured and recorded.
Density Testing (ASTM D1505): The determination of the density of liner through this test identifies certain properties that can predict chemical resistance, and some other physical properties. This density is determined by how far a sample will sink in a water/alcohol liquid column.
Melt Flow Index Test (ASTM D1238): This test gives the lab technician an idea of the polymer's molecular weight, viscosity, and processability. This test will help find uniformities in other polymer properties.

Welding Techniques:

Hot Wedge Weld : This welding technique allows the worker to guide the welder along the length of the liner fusing two liners together to create a leak proof seal. The hot wedge welder is self propelled and is capable of welding up to 15 feet/ minute. The hot wedge welder leaves a small gap between the two sealed liner sides. this gap enables the workers to use an air test to test the seam quality. The air test is a field test in which a needle and gauge are placed at one end of the seam and then pressurized. The seam, which can be up to 500 feet long, has to remain under pressure for 5 minutes. After the 5 minutes has passed and the seam passed the test, the needle is then removed and an extrusion weld is placed over the needle hole.
Extrusion Weld : This welding technique is used for placing patches that may be in the middle of a liner section, for welding boots around piping, and just fixing penetrations of the liner. The weld is different from the hot wedge weld because it leaves an extrudate on the top of the liner being welded. In this case an air test will not work so another field test known as the vacuum box test is used. The vacuum box test consist of placing a soapy solution all over the extrusion weld, then a box with a plexiglass top is then put on the weld. The weld then receives a 5 psi vacuum from the box and a visual inspection of the weld is performed. If the seam fails there will be noticable air bubbles forming from the soap.

Schematic of weld types

LEACHATE COLLECTION AND CONTROL

With the increase in solid waste that municipalities are generating (almost 200 million tons/ year) the solid waste collection and disposal constituants are growing fast. In the United States the most popular method for solid waste disposal is sanitary landfills. Many of the solid waste landfills that were developoed before 1988 had to either be shut down of reconstructed due to Subtitle D of the Resource Conservation and Recovery Act (RCRA) going into effect in the later part of 1993. Under Subtitle D leachate management must be properly implemented. The previously discussed landfill lining system prevents the leachate from seeping into the groundwater. The Subtitle D allows for the treatment of the leachate collected to be mandated by the states. One rule that must be followed by all states is that the leachate collected must be treated as industrial waste. Publicly owned treatment works have to be sure the leachate does not interfere with other treatment processes or sludge quality.

Production of leachate from sanitary landfills is an environmental hazard. There are several factors that influence the generation of leachate. Some of these are precipitation, runoff, evaporation, waste density, and depth of the landfill. It is said that not much leachate is produced until the landfill is fully saturated. The leachate that is generated comes from the decomposition of the landfill waste. This occurs in three possible stages: First aerobic decomposition dominates , in this phase the temperatures raise well above ambient temperatures and produce leachates mainly of soluble salts. The second stage is believed to be anaerobic decomposition. The first part of this decomposition produces large amounts of volatile fatty acids and carbon dioxide. The facultative bacteria then get taken over by the methane producing bacteria. These mathane producing bacteria, which require neutral pH, convert the facultative anaerobes into methane and carbon dioxide. The decomposition process eventually decreases with landfill age due to substrate depletion. Through all the decomposition processes and the water that percolates through from precipitation carries these contaminants to the bottom of the landfill producing the leachate. The liquid state leachate is not the only contaminant that must be regulated. Over time the decomposition process produces gases that may be emitted to the atmosphere. These gases also have to be regulated. As for the liquid leachate once it has been collected into the sump from the bottom of the landfill it must then undergo treatment before it can be distributed into receiving waters.

Treatment of landfill leachate is a difficult task due to the nature of the leachate. A typical landfill leachate usually starts out as a high-strength wastewater, having low pH, high biochemical oxygen demand (BOD) and chemical oxygen demand (COD), and the presence of toxic chemicals. This wastewater profile can change from landfill to landfill as well as within the same landfill as it ages. Due to the changes of the wastewater composition over time, sometimes 30 years, the conventional biological waste treatment and chemical treatment processes seperately do not achieve high removal efficiency in the effluent. When it comes to the design of treatment facilities factors that influence the design are leachate characteristics, effluent discharge regulations, costs, and permit requirements. Some of the common waste treatment processes that have been applied to the landfill leachates are Activated Sludge, Waste Stabilization Ponds, Aerated Lagoons, Trickling Filters, Rotating Biological Contactor, and Anaerobic Digestion.(Qasim and Chiang, 1994)

Leachate Collection Process

CONCLUSION

For solid waste the future brings about a large emphasis on resource recovery and solid waste reduction. There will be less landfills and more innovative ways of disposing solid waste. One way will be through incineration with some sort of energy recovery system. Even though there will be less landfills in the future they will still play a major role in solid waste and residual disposal. Each year the design of the landfills and leachate control strategies will become more and more strict in order to protect the groundwater. So when the permit application is submitted by the municipality to the state, these applications will be looked at very closely to be sure the design engineer has properly designed the landfill. Desinging a landfill is not just the application of the liner system, there are issues of proper slopes for runoff, there has to be a sophisticated monitoring well system around the landfill, and most important the leachate must receive proper treatment before discharge. Under Subtitle D of RCRA all of these regulations are mandated and inforced through each state to ensure the safety of the soil and groundwater to be free from any solid waste contaminants

Acknowledgments

Tom Eld, Construction Quality Engineer at Almes & Associates
All photographs were provided and used with permission from Almes & Associates

References

Bedient, P.B., Rifai, H.S., and Newell, C.J. Groundwater Contamination Transport and Remediation. Prentice Hall, New Jersey. 1994.
Gundle Lining Systems. 1996. GSE Lining Technology.[Brochure]. Gundle Lining Technology Inc.
Sincero, A.P. and Sincero, G.A. Environmental Engineering A Design Approach. Prentice Hall, New Jersey. 1996.
Qasim, S.R. and Chiang, W. Sanitary Landfill Leachate Generation, Control and Treatment. Technomic Publishing Company, Inc. Lancaster, PA. 1994.

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Send comments or suggestions to:
Student Authors: Joe Yack, jyack@vt.edu and E.J. O'Neill, edoneill@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998