Grantee Research Project Results

Final Report: Waste-Liner Compatibility Studies Using the Comprehensive Testing System for Geomembrane Liners

EPA Grant Number: R822485
Title: Waste-Liner Compatibility Studies Using the Comprehensive Testing System for Geomembrane Liners
Investigators: Stessel, Richard Ian , Garcia-Rubio, Luis Humberto
Institution: University of South Florida
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1995 through September 1, 1997
Project Amount: $182,621
RFA: Exploratory Research - Engineering (1995) RFA Text |  Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management

Objective:

This research project made use of the geomembrane Comprehensive Test System (CTS), initially developed at the University of South Florida, and involved use of the CTS to study waste-liner compatibility, yielding both physical test data and data from laboratory analyses of the resultant samples. Comparisons of these data provided information concerning how liners are affected by waste materials during in-service conditions. Due to its clear popularity, HDPE was the liner material tested.

There were five specific goals. The first goal was to determine cyclic loading rates. The standard displacement rate used in current geomembrane testing was too high. The second goal was to construct two new CTS cells. The original CTS cells were subject to substantial modification since their original construction in 1987. Most of the older modifications were retrofits that caused the system to be difficult to seal and use. Although several important components of the old cells were incorporated into one of the new cells, it was important for the stability of the testing program to incorporate design changes in a proper manner. The third goal was to use the new CTS testing protocols determined in goal 1 to expand the test database. A considerable range of chemicals required testing. Of particular concern were organic compounds. Test chemicals were as follows: (1) benzene, toluene, ethylbenzene, and xylene (BTEX) because they were representative of leaking underground storage tank problems and automobile fuel wastes; (2) trichloroethene and tetrachloroethene because they represent degreasing solvents; and (3) the Toxicity Characteristic Leaching Potential (TCLP) synthetic landfill leachate, spiked with lead and (separately) cadmium, because these are the two heavy metals of most current concern in solid waste. The specimens from each test sample were analyzed using differential scanning calorimetry and scanning electron microscopy. These were employed to provide information as to the cause of failure, particularly with regard to retention or replacement of plasticizers and remaining bond integrity. Test data were compiled in tabular form. Visual inspection and statistical analysis were undertaken to reach understandings and draw conclusions concerning the effect of wastes on liner integrity. Studies of correlations between analytical measures and physical testing results were undertaken to draw conclusions concerning mechanisms of attack.

Summary/Accomplishments (Outputs/Outcomes):

Differences were observed in the results of testing of geomembranes using the CTS and EPA Method 9090. The differences in results were explained in terms of the mechanisms of the chemical interactions between the attacking chemical and the geomembrane. The Method 9090 testing provided long-term interaction between the surface of the geomembrane and the attacking chemical providing time for penetration into the geomembrane matrix, and could be thought of as a thermodynamic equilibrium test. The CTS test provided simultaneous mechanical loading and chemical exposure, and could be considered a kinetic test. Results of Method 9090 testing could not be used to determine the timeframe required for mechanical effects of chemical exposure to be revealed. The time required for a CTS evaluation was significantly less than that needed to achieve changes in properties, had the only effect on the geomembrane been based on diffusion of the chemical into the unstressed geomembrane under zero head.

Method 9090 mechanical results only revealed that the aromatic compounds had an effect. The CTS testing clearly showed that: (1) chlorinated solvents had the most serious effect; (2) gasoline and smaller molecule organic compounds taken as representative of fuel wastes had an intermediate effect; (3) independent of heavy metal content, the TCLP fluid, representing sanitary-landfill leachate had an effect significantly above that of water; (4) xylenes, a relatively larger organic molecule also taken as emblematic of fuel wastes, had a relatively small effect; and (5) motor oil, often a great public concern, had no discernable effect. The above distinctions were not available from concurrent Method 9090 testing.

The CTS and test method developed for this project seemed to produce more theoretically reasonable data, making distinctions of which Method 9090 was not capable. The CTS' mode of operation was designed to be obviously more relevant to application than Method 9090. Enhanced test speed is critical to enabling evaluations of chemical compatibility in a timeframe of use to the practicing community, and to permit the development of a usefully comprehensive database of chemical compatibility. This study, itself, produced the first such database, giving significant information for leachate components of greatest concern.

The mechanisms of chemical attack of the geomembrane sample need to be investigated further. The differences between the Method 9090 results and CTS results suggest that both long-time-scale and short-time-scale changes are occurring in the polymer. Early time CTS data revealed that short-time-scale phenomena differ from the relatively steady-state situation after the first few cycles. To gain an understanding of the effect of chemicals on the geomembrane, testing should be conducted to determine the effects of more classes of chemicals, such as pure alkanes, chlorinated aromatics, naphthalene, and polynuclear aromatic hydrocarbon. All of these chemicals can be found in the waste stream and may cause chemical attack on the geomembrane liner. In addition, testing of the interaction of two pure chemicals of different classes, such as aromatics and alkanes, should be conducted to determine if the effects are additive.

Further study of the effects of visco-elasticity of polymers used in landfills, and the effects of visco-elasticity on the performance on the liner should be conducted. Additional development of the mathematical description of visco-elastic materials, combined with diffusion, is required. Analytical testing of geomembranes should be further studied. The analytical tests conducted as part of this project revealed the differences in the results of differential scanning calorimetry and thermogravimetric analysis were not significant under CTS testing, even though mechanical properties of the polymer had changed significantly. The clear implicaton is that these tests are not sensitive to the changes that have occurred in the geomembrane polymer. Candidate tests for further evaluation include small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS), which are able to provide insight into crystallinity of the polymer. Transmission electron microscopy and associated electron scattering techniques also can be used to determine polymer crystallinity.

This testing program utilized polyethylene geomembrane material, which is only one class of polymers available for use as a geomembrane material. Further testing of the effects of exposures on other geomembranes, such as polypropylene and polyvinyl chloride, should be conducted. Polypropylene is again being supplied by major geomembrane manufacturers. Also, fiber-reinforced geomembranes have not been tested using the CTS, nor are there data available in the open literature for Method 9090 testing. Tests of these materials should be conducted.

The mechanical forces that a geomembrane is subjected to while in operation are not well documented. Literature indicates that strain in geomembrane liners under active sanitary landfills may be large. The applied stresses that result from this strain are not known, nor are the effects of this strain on the impermeability of the liner. Further investigations need to be conducted to understand the operating conditions imposed on geomembranes and whether the conditions of CTS testing are similar to these operational conditions. A limited study of larger scale test installations, perhaps a quarter-hectare, sufficient to mimic actual design and operation using actual construction equipment is needed to connect CTS results with the field.

Geomembranes are typically designed to not be load-bearing. However, loads applied to geomembranes may have large effects on the properties of the liner. The effect of cyclic loads such as placement of wastes, or compaction of the wastes can greatly affect the performance of the liner material. Further investigation of the geotechnical aspects of liners under use need to be conducted. Such testing is common to pavement engineering.

CTS testing may have application in geotechnical engineering beyond testing the effect of chemical exposure from sanitary landfills on geomembranes. It could potentially be a tool for geotechnical engineers to use in selection of geosynthetics for use as a road subgrade or in enhancing slope stability. The CTS is unique in its capability of applying live loads to a geomembrane. Further theoretical development of the loads applied to the geomembrane by the CTS should be undertaken. In addition, better understanding of the loads applied to geomembranes while in service also should be developed.

Journal Articles:

No journal articles submitted with this report: View all 3 publications for this project

Supplemental Keywords:

waste management, treatment, storage, disposal, hazardous chemicals, emissions, geomembrane liners, compatibility studies, test system reflective of field conditions, comprehensive approach, high density polyethylene, liner, interaction of liner with fuel, solvent, differential scanning calorimetry, ultraviolet spectrophotometry., RFA, Scientific Discipline, Waste, Environmental Chemistry, Hazardous Waste, Engineering, Hazardous, hazardous waste disposal, hazardous waste management, hazardous waste treatment, subsurface, waste liner compatability, hazardous waste storage, geomembrane liners, environmental engineering, liners, ultraviolet spectrophotometry, waste management, hazardous chemicals, differential scanning calorimetry, mobility of contaminants, high density polyethylene, leachate

Progress and Final Reports:

Original Abstract

1996