Leachate Resistance

All MSW leachates are different. Many PVCs have successfully contained MSW leachates for many years.

At Lycoming County, PVC that has been exposed to leachate in a leachate storage pond for 13 years is marginally more flexible than the material that was not exposed to leachate.

Artieres (paper enclosed) is testing different geomembranes in two different MSW leachates and water. Two PVC geomembranes (with DOP and ethylene copolymer/vinyl acetate plasticizers) and one HDPE geomembrane are being evaluated. After 16 months at room temperature and 3.5 months at 50 degrees, both PVC and HDPE show good resistance. Burst tests show small changes in HDPE, no changes in PVC. Infrared (IR) spectrophotometry shows PVC affected to t depth of 80 m m (loss of plasticizer) and HDPE affected to a depth of 100 m m (loss of antioxidant).

The paper by Fayoux et al. Discusses an uncovered 40 mil PVC geomembrane used to contain MSW leachate for 10 years. The liner was, therefore, exposed to UV radiation and/or leachate. The loss of plasticizer was less, (averaging only 0.35% / year) in PVC under the leachate than in PVC expose to UV radiation. The loss of plasticizer was, in fact, essentially the same as it was in plain water.

Oil-resistant PVC was subjected to an EPA Method 9090 test in a leachate, generated from a mixed municipal solid waste and industrial waste landfill, that was strongly spiked with liquids normally very aggressive towards PVC. The PVC was shown to be resistant to the leachate.

Conclusion: MSW leachates do not automatically leach out plasticizers and degrade PVC geomembranes.

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Laboratory Ageing of Geomembranes in Municipal Landfill Leachates

by O. Artieres, F. Gousse and E. Prigent

Summary: The chemical stability of geomembranes layed in municipal landfills is, even if not alone, an important choice criterion. Changes of their physical and chemical properties must be assessed in the time with an ageing simulation. Selected liner materials exposed to leachate during 16 months present no damages.

INTRODUCTION:

In many European and international legislations, geomembranes are prescribed to be used in hazardous waste landfills to prevent aquifer pollution. They are also used to watertight the bottom and the top of municipal landfills and collect leachate produced by rainfall through the domestic wastes. The duration before complete stabilization of actual domestic wastes, when not pretreated, is very long, perhaps many centuries after deposition (1). During this period, produced leachates contain a lot of chemical compounds which can react with the liner material point of the landfill security. This paper describes a long-term laboratory study undertaken to evaluate the possible modifications of the geomembranes characteristics during their contact with leachates.

METHODOLOGY OF THE LABORATORY-AGEING

The aim of this study is to simulate only the chemical compatibility of a selection of geomembranes in leachates. The laboratory-ageing must reproduce the chemical environment of a geomembrane layed at the bottom of the landfill, i.e. chemical, biological or mechanical stresses, temperatures flow,....

The general principle of the experiment is to immerse at 200C and at 50C samples of various geomembranes in two different municipal landfill leachates and in distillated water, then to test these samples periodically to follow the variation of their characteristics.

The size of the geomembrane samples is about 30 cm x 30 cm. The samples are hung up in polypropylene tanks of 60 cm x 40 cm area filled with about 35 cm of leachates or water.

Each tank holds 17 samples of one type of geomembrane in one kind of fluid. There is about 27 1 of fluid per m2 of geomembrane area.

The local environment and stresses must be simulated as far as possible:

1) Flow and dissolved oxygen rate.

An immersed pump connected to a pipe distributes the fluid between the samples. This constant flow is necessary to renew the fluid at the samples surface avoiding the saturation of the chemical reactions. The flow is high enough to move the fluid along all the samples surface, but sufficiently low to avoid turbulencies on the fluid surface preventing its re-oxygenation. The anaerobic condition must be preserved to reproduce the real environment, this condition is usually more injurious to the polymers because anaerobic micro-organisms find their energy in eating molecular chains.

2) Temperature.

The temperature of 500C is the extreme value reached by the leachate at the bottom of the landfill near the geomembrane. The value of 70 degrees C is also proposed in literature (2) (3). On the other hand, 20 degrees C can be considered as medium-temperature at the bottom of the landfill when the degradation phase of the organic matter is almost finished or when the reaction heart of the landfill is far from the geomembrane.

The knowledge of the ageing-kinetics at these two extreme temperatures also allow to extrapolate the kinetic at intermediate or, if necessary, at higher temperatures.

3) Light

The tank is covered against the light which can help to the development of not suitable bacteria or fungi, or accelerate the ageing of the samples by photo-degradation.

4) Chemical media.

Leachates:

It was decided to use real leachate instead of a synthetic one because of the wide range of compounds they contain. The aim of the study was not a work on the definition of a standard leachate. At the beginning time of these simulation tests, no artificial leachate was yet standardized. To take the variability of leachates into account, the samples are immersed at room temperature (= 20 degrees C) in two leachates of different municipal landfills :

• the first is sampled at the outlet of the leachate collector of a landfill where the domestic wastes are compacted; as a result they are in anaerobic conditions and they produce a leachate with high concentrated pollution (Table 1).
• the second one is sampled in the treatment pond of a landfill were the wastes are crushed, but not compacted. They decompose in aerobic conditions and the produced leachate is less polluted as the first one (Table 1).

The rate of dissolved oxygen in the two leachates in very low. This value is little higher in the case of the crushed wastes because of the sampling in the treatment pond. But both conditions are anaerobic and it is primordial not to introduce air in the ageing tanks during the tests.

Reference medium:

All the samples are also immersed at room temperature in distilled water used as reference medium, in the same conditions as those immersed in the leachates.

The samples at 50 degrees C are only immersed in the leachate from the compacted wastes landfill to simulate the extreme conditions.

All immersion media are usually renewed every 3 or 4 months. The physical-chemical characteristics (pH, conductivity, dissolved 02) are checked every 2 weeks to follow their evolution and to give the renewing-time.

Figure 2.) Schematic of stress cracking device after ISO 6252.

1-Beam, 2-Pivot without friction or fulcrum, 3-Anchor, 4-Inox cable, 5-Thermoregulated fluid circulation, 6-ClampS, 7-Sample, 8-Ageing fluid, 9-Masses, 10-clock circuit breaker, 11-Clock

5) Mechanical stress.

Chemical and mechanical stresses can combine to affect the polymer's service life. The combined stress on semicrystalline polymeric materials can cause stress cracks [2]. This phenomenon is now well know for polyethylene geomembranes and is one of the disadvantages of this material (5) (6).

These failures often occur in the seam areas, due to residual stresses after seaming and also due to the overlapping geometry of the seams [5]. Cracks and strain due to residual stress increase diffusion of chemicals in polymer material leading to chains break.

This acceleration of chemical damages due to synergy of stresses was also tested in this research program. A device according to ISO 6252 "Determination of environmental stress cracking Constant tensile force method" was therefore developed.

Twelve test specimens of one liner material as described in figure 4 are loaded to a portion of their break stress by means of static load. Then, 4 of them are immersed in crushed wastes leachate, in distillated water and 4 in the air. The leachate and the samples are protected from light (figure 2).

The first tests were initially leaded on continuous samples and 20 degrees C. Further tests will concern seamed samples and higher temperatures.

Figure 3.) Structure of the bituminous geomembrane 1-Non-stick film, 2-Sanding, 3-Asphalt impregnation, 4-Bidim non-woven fabric, 5-Glass fiber layer, 6-Anti-perforation film
 

Table 2 : Ageing time and media of the selected geomembranes.

(1)-since December 89, (2)-stopped after 5 months,

(3)-since December 90, (4)-since November 90



SELECTED GEOMEMBRANES

The current constituent material for the geomembranes prescribed at the bottom of hazardous, and even domestic, wastes landfills is High Density Polyethylene (HDPE). This polymer is commonly selected because of his chemical stability. But data about long-term ageing of other types of geomembranes in domestic wastes landfill leachates are so few in literature (except (12)), that it was decided to test a representative range of the present international production.

Seven kinds of material constituting the geomembranes were selected (Table 2) :

- Bituminous geomembrane-

The geomembrane is made up of a polyester non-woven geotextile and a glass voile arming the sheet, both impregnated with oxidized biumen (Figure 3).

Bitumen is particularly sensitive to hydrocarbons, ether, xylene, benzene,...

Polyester is sensitive to hydrolysis, especially over 50-70 degrees C, but it has a good chemical stability to petrol and solvents (7).

- Modified bituminous geomembrane.

The structure is the same as the previous one, but the impregnation is made with bitumen modified with a copolymer of styrene-butadiene-styrene (SBS). SBS is a styrenic elastomer which gives a part of its elastic properties to the mixture.

SBS is sensitive to oxidation, hydrocarbons and solvents (7).

- Plasticized PVC geomembranes,

Polyvinylchloride (PVC) polymer has a good stability to chemical compounds (oil, water, oxydizing agents) (7). But it is a rigid material. One must include plasticizers (between 20 and 50 %) to produce soft geomembranes. These platicizers are very sensitive to oxidation (alcohols, hydrocarbons... ) because of their polarity and they migrate out of the material. others additives are then added to protect the plasticizers: anti-oxidizing and blocking agents (7).

The kind of plasticizers and additives is therefore very important for the long-term evolution of the geomembrane two PVC geomembranes were tested to evaluate the influence of the plasticizers

- PVC plasticized with dioctylphtalate (DOP)

- PVC plasticized with a copolymer of ethylene and vinylacetate (EVA).

EVA which belong to the polyolefinic family (like PE or PP) is more resistant to oxydation than the phtalates

- Polvolefinic geomembranes.

Polyolefines comes from polymerisation of ethylene and/or propylene. The resulting polymers have a good resistance to chemical compounds because of the low chain ramifications and the crystalline structure, but they are sensitive to oxidation (especially UV) [7]. They contain very few additives (< 5%): mainly anti-oxidizing agents.

Two kinds of polyolefinic geomembranes were tested

High Density Polyethylene (HDPE)

Copolymer of Ethylene and Propylene (co-PP)

- EPDM geomembrane.

EPDM is an elastomer terpolymer of ethylene-propylene-diene monomer. Like the polyolefines, EPDM is sensitive to oxidation, but also to aromatic solvents and chlorinated hydrocarbons Some additives are used to protect it against oxidation.

TESTS

The selected tests intend to quantify and understand the ageing of the geomembranes in the time. These tests are mechanical, hydraulical or analytical.

Figure 4.) Sample of ISO type for uniaxial tensile test.

 

 

L	A	B	F	C	D	R
50+/-0.5	60±0.5	10±0.5	150 min	20+/-0.5	115+/-3	60 min
D-initial distance between clamps, L-Distance between the marks, A-Constant width part. All the sizes in mm-

5
.1 Mechanical tests

5.1.1 Uniaxial tensile test

The classic uniaxial tensile test is used to assess the mechanical values of the geomembranes, i.e. tensile strength and strain at break and/or yield points, secant tensile modulus at 10 % strain. All these values are very common and easy to use, even if the test doesn't describe exactly the real behaviour of the geomembrane in place.

The sample size shown in figure 4 is of ISO type defined in the ISO/RS27 standard. The thickness of the sample is the thickness of the geomembrane.

The strain rate of the machine is 50% per minute. Strain is measured with an optical extensometer following 2 lines previously drawn on the sample.

5.1.2 Biaxial tensile test

The principle consists in blowing up the sample of geomembrane facing a circular opening by air pressure while clamps prevent from shortening. The air pressure is raised by steps of 100 kPa per 2 minutes and the geomembrane forms a spherical dome which grows in relation with the pressure up to the bursting failure. The result of the test is a relationship between air pressure and increase of the dome.

The bursting test has the following advantages regarding the uniaxial tensile test :

• it generates a 2D-tension which is very close to field conditions
• it tests a larger area of material,
• it displays leaks in the geomembrane under strain (see further)

The two mechanical tests complement one another.

5.2 Hydraulical test

The ageing processes has also an effect on the tightness Of the geomembrane to leachates.

Mass transfer in geomembranes is due to diffusion process. This mass transport is due to 2 driving forces : concentration and pressure gradients between the 2 sides of the sheet [8]. In landfill applications, the leachate level over the geomembrane is low (< 1 M) and the pressure gradient is negligible. The diffusion due to concentration gradient is therefore the main cause of permeation of leachate through the geomembrane.

The sorption test is an easy method to quantify the diffusion by concentration gradient. It may be performed with any kind of compounds. Water is chosen as permeant to characterize the tightness of the aged geomembranes, because it is the main compound in the case of municipal landfill.

The test consist in following in the time the mass of absorbed water (Mt) in samples of geomembrane which are immersed in water till they reach mass stabilization (M). A mathematical interpretation of diffusion process with the mass increase leads to the diffusivity (diffusion coefficient) in measuring the half-sorption time t1/2 (Mt1/2 =M/2). This model is described in [8]. The relationship between diffusivity D (in m2/s) and the half - sorption time t1/2 (in s) is:

D = 0.0492 . Tg2 / t1/2

Tg : Thickness of the geomembrane in m.

Analytical test

All the above macroscopic tests are benefit to assess the general behaviour of the geomembrane after ageing, to evaluate the possibly loss of characteristics and to compare then with the design values. But these tests doesn't allow the interpretation of the changes which happen at the molecular level.

An analytical test is useful to know the kind of degradation and its extent. Many analytical tests are employed to characterize the polymers (see, for instance, (9) and (10). Each of them describes one aspect of the polymer structure. But among these methods, the micro (Fourier Transformation Infra Red) spectrophotometric technique applies to almost all the polymer materials tested, gives the chemical and morphological changes in the polymer matrix and quantifies them. The basis of the method described in (11) consists in sampling with a microtome small slices of geomembranes (thickness between 80 and l00 mm for PVC and HDPE; only 7 mm for EPDM which is very opaque because of its high black carbon content) normal to its surface and in analysing these films by IR light (Figures 6 and 7) . The micro (FTi.r.) spectrophotometric measurement gives distribution profiles of the compounds which could appear during the ageing of the material in the thickness of the liner.

 

Figure 6.) Cutting of a geomembrane slice to make an IR spectrum after CNEP report.
 

1-Microtome blade, 2-Thickness of 80 or 7 m m, 3-geomembrane, 4-PE plates maintaining the sample,

5-Slice cutting with the microtome blade,

The opacity of the bituminous geomembranes is to high to allow micro(FT i.r.) spectrophotometric measurement. The photoacoustics (FT i.r.) spectroscopy is a sustitution method which analyses surfaces of sheets with a high black carbon load.

These tests are made at the Centre National d’evaluation de Photoprotection (CNEP) in Aubi6res (France) .

FIRST RESULTS

These following results describe the changes of characteristics in the geomembranes after an ageing of about 16 months at room temperature and 3.5 months at 50 degrees C.

6.1 Mechanical chances

The uniaxial tensile tests shows no sensible changes of the mechanical properties between unexposed and exposed geomembrane samples.

The bursting tests agree this point. But they are a little more sensitive comparing to the uniaxial tensile tests for the reasons explained in 5.1. The curves drawn in figures 8 indicate small variations between unexposed and aged samples for HDPE geomembranes. These latest are softer (lower modulus) and have greater pressure and elongation at break. For the Bituminous/SBS geomembrane, the tendency is quite opposite. The differencies are very small for PVC and EPDM materials.

The bituminous geomembrane was took away from the initial liner selection, because the bursting test shows that this material was porous under little deformation (<6%) and little pressure (<200 kPa). It was assumed that such a behaviour is inadequate for an secure use in landfills.

The uniaxial tests under constant stress show for PVC/DOP and HDPE neither failure nor differences between the 3 media after 3 months. The future tests must be conducted at 50 degrees C to increase stress cracking, on samples with and without seams.

Finally, the most important result of all these tests is that there is no noticeable difference between the exposed samples, i.e. there are no mechanical degradation due to leachate.

This first result agrees with those presented in [12]. In this study, Low Density polyethylene, PVC, EPDM geomembranes were exposed during 56 months to landfill leachate at 10-20 degrees C. They changed only modestly in physical properties.

6.2 Hydraulical changes:

Sorption tests were carried out on after 16 months ageing. Because of the long duration of the test (many months at 20 degrees C), it is not possible to get the diffusivity. But the comparison of the beginning of the curves for the PVC shows an acceleration of diffusion, which is higher for samples exposed to distillated water (Figure 9). The differences are however very small. Because of the very low difusivity of HDPE, no changes are noticeable yet.

Figure 9: Absorption of water in PVC/DOP Samples 1-Unexposed, 2-Compacted wastes leachate, 3-Crusched wastes leachate, 4-Distillated water

6.3 Chances at molecular level

These tests are more sensitive than the macroscopic tests.

The HDPE matrix shows no oxidation trace, even in the superficial layer (0-24 mm) , as well at 20 degrees C or at 50 degrees C. The HDPE geomembrane contains an anti-oxidizing agent owning an ester function (extrenum at 1736 cm-1) . This ester is hydrolised producing OH groups in the hydroxile zone (3100 to 3500 cm-1).

The hydrolisis is very small at 20 degrees C in distillated water, and still smaller with the compacted wastes leachate at 20 degrees C (<5%) (Figure 10) . It is located in the superficial zones (0-25 m m) This reaction is higher with leachate at 50 degrees C, where the ester is quite consumed in the first 30 m m but remains intact after 100 m m (Figure 11, available on hard copy) . This last result is conform to the fact that hydrolisis increases with temperature.

Like HDPE, the PVC matrix is oxidized in any environment. On the other hand, the DOP plasticizer (extremum at 1580 and 1600 cm-1 On figure 12) is hydrolised producing an acid of phtalic type and alcohols (range from 3100 to 3500 =-1) (Figure 13, available on hard copy). At 20 degrees C, this reaction is more important in leachate than in distillated water, because hydrolisis is higher when pH is different from 7, and faster at 50 degrees C. In all the cases, the loss of plasticizer is limited to the first 80 m m.

After 16 months exposure to leachate at 20 degrees C, EPDM matrix presents also no oxydation.

On the other hand, there is a small oxidation in bituminous/SBS samples which is faster at 500C, due to the thermical evolution of SBS elastomer (Figure 14, available on hard copy).

The water absorbtion in these 2 materials is important.

As conclusion of these analytical tests, it was found there is no strong oxidation of the geomembranes, whatever the environmental conditions. There is only an hydrolisis of the additives, but this one is very limitated in quantity and in thickness.

All the observed facts are of minor importance.

CONCLUSION

With the increasing use of geomembranes in domestic wastes landfills, the question of their chemical compatibility to leachate and their long-term durability becomes crucial.

The study here described aims to give some information on this point. The first results after an exposure of 16 months at room temperature and 3..5 months at 5O degrees C shows that all the tested geomembranes keep their initial characteristics.

But this too short ageing time cannot lead to any conclusion yet. The ageing kinetic must be calculated on longer period to assess the durability of the liner when the degradations, if they occur, are of greater importance. It is therefore provided to make a complete tests programme every one or two years during at least 5 years.

It was shown that an ageing programme is based on one hand on a good definition and simulation of the environmental stresses (temperature, strains, seams,..) taking into account their synergy, and on the other hand, on judicious selected tests to assess the changes. The macroscopic tests are used to compare the geomembrane characteristics to reference design criteria. The microscopic tests aim to describe the ageing processes to be interpreted, quantified, and to assess their durability.

But the environmental durability must not be the only choice criterion for a geomembrane. Its long-term mechanical behaviour, its aptitude for laying and seaming, its adequation with the other elements of the tightness system, are also of great importance and must be taken into account.

It is therefore necessary to put the chemical behaviour of the materials at the right level of stability referring to the real environmental stresses.

For instance, the nature of the domestic wastes leachate, even if it is of more complex composition, is certainly less agressive for geosynthetics as hazardous wastes leachate. Then, the materials chosen after simulation tests for hazardous wastes (for instance EPA 9090 method) are not necessary the best ones for some domestic landfills applications.

In these conditions, the procedure for the choice of the products will give an enough range of acceptable materials for the design of the best liner in each situation.

 

REFERENCES

[1] STIEF, K. (1989) "Deponietechnik im Umbruch. Nachbesserung bestehender Deponien". ZeitgemiBe Deponietechnik III. Stuttgarter Berichte zur Abfallwirtschaft. Erich Schmidt Verlag, Bielefeld. pp-7-31.

[2]LANDRETH, R.E. (1988) "Durability of geosynthetics in waste management facilities : needed research". Proceedings of a Seminar on Durability and Ageing of Geosynthetics. GRI, Drexel University, USA.

[3] HAXO, H.E. and MAXO, P.D. (1988) "Environmental conditions encountered by geosynthetics in waste containment applications". proceedings of a Seminar on Durability and Ageing of Geosynthetics. GRI, Drexel University, USA.

[4] PRIGENT, E. (1990) "Contribution A l’etude de la compatibilite chimique des geomembranes aux percolate de de d6charges dlordures m6nag&res'. CEKAGREF. Memoire de 36me ann6e de IIENITRTS. 133 P.

[5] HALSE, Y.H.; KO@ER, R.M. and LORD, A.E. (1982) "Laboratory evaluation of stress - cracking in MDPE geomembrane seams". Proceedings of a Seminar on Durability and Ageing of Geosynthetics. GRI, Drexel University, USA.

[6] PEGGS, M.D. and CARLSON, D.S. (1988) "Stress cracking of polyethylene geomembranes : field experiences". Proceedings of a Seminar on Durability and Ageing of Geosynthetics. GRI, Drexel University, USA.

[7] REYNE, M. (1990) "Les plastiques. Applications et transformations." Traitte des Nouvelles Technoloqies. Serie ,materiaux. Ed. Hermes, Paris. 268 P.

[8] FAME, Y.H.; PIERSON, P; ARTIERES, 0. and GOUSSE, F. (1990). "Tests of geomembranes water permeability". Proceedings of the 4th International Conference on Geotextiles, Geomembranes and Related Products. Ed. G. den Hoedt, A.A. Balkema, Rotterdam. pp. 543-553.

[9] VERSCHOOR, K.L.; WHITE, D.F. and THOMAS, R.W. (1990) "An cverview of practicies used in the United States to determine the compatibility of geosynthetics with chemical wastes". Proceedings of the 4th International Conference on Geotextiles, Geomembranes and Related Products. Ed. G. den Hoedt, A.A. Balkema, Rotterdam. pp. 715-718.,

[10] VAN LANGENHOVE, L. (1990) "Conclusions of an extensive BRITE-research programme on ageing". Proceedings of the 4th International Conference on Geotextiles Geomembranes and Related Products. Ed. G. den Hoedt, A.A. Balkema, Rotterdam. pp. 703-707.

[11] JOUAN, X. and - GARDETTE, J.L. (1987) "Development of micro (Fti.r) spectrophotometric method for characterization of heterogeneities in polymer films". Polymer commnications, Vol. 28, december, pp 329-331.

[12] HAXO, E.E. and NELSON N.A. (1984) "Factors in the durability of polymeric membrane liners". Proceedings of the International Conference on Geomembranes, Denver, IFAI, pp. 287-292.

 

 

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