Electrically Conductive Geotextiles – A Graphene Solution

Geomembranes (GMB) separate waste or nasty products from the natural environment and store valuable resources (e.g. potable water). Electrical methods of locating undesirable leaks in geomembranes need electrically-conducting media on both sides of the GMB. In the case of a single pond liner, the underside medium is usually soil/clay, but may increasingly be a man-made polymeric product (geocomposite (GC) – a geonet (GN) sandwiched between two nonwoven geotextiles (GTX) that enable the collection and removal of any primary liner leakage on top of a lower secondary GMB. While such products are non-conductive, the nonwoven textile component of the GC can be made to be electrically conductive by integrating graphene into its fibrous structure. Therefore, the conductive geotextile (CGTX) layer is placed adjacent to the GMB, thus allowing the detection and repair of very small leaks.

Background

High density polyethylene (HDPE) GMBs with 1-2 mm thicknesses are increasingly being employed to contain waste water, potable water, coal seam gas process waters (brines), municipal solid waste, and valuable mining solutions to avoid contamination, environmental pollution, and loss of valuable products (for example. copper, gold , uranium).

When installed in a construction environment, these highly efficient fluid barriers may not be well protected and hence they are subject to mechanical damage (punctures, cuts, tears) that if not repaired can lead to leakage during service. However, electrical methods are available as the final phase of construction quality assurance (CQA) to locate leaks as small as 1 mm in diameter (Figure 1). It is possible to repair such leaks.

Small pinhole. Pen tip is 0.5 mm diameter.

Figure 1. Small pinhole. Pen tip is 0.5 mm diameter.

In order to find such leaks, an electrically conductive medium that is located directly underneath the GMB to find such leaks is required. This can be a conductive bottom surface layer manufactured into the GMB, a prepared soil subgrade, a separate CGTX or a geosynthetic clay liner (GCL) (granular or powdered bentonite between two GTXs). In several cases, under the upper primary GMB, there is a secondary HDPE GMB to collect and remove leakage, but there is no conductive layer under the primary GMB. Normally, there would be a GN sandwiched between two GTXs, called a geocomposite (GC), to provide the leakage detection, collection, and removal system between the two GMBs. The upper GTX could be a CGTX.

GCLs usually contain about 12 to 20% of moisture when delivered to site, making them sufficiently conductive for carrying out surveys. However, if their moisture decreases to less than about 8% when installed they may not be sufficiently conductive to locate leaks.

An increasing number of environmental agencies and regulators are requiring that electrical integrity surveys be carried out as the last stage of on-site CQA such that all important data can be located and repaired, thus thereby minimizing subsequent in-service leakage.

“Zero” Leakage

While zero leakage is obviously the goal, experience over the past 25 years or so (Giroud 2016b) has revealed that zero can neither be ensured nor be measured. Is one drip a second out of the leakage detection system (LDS) a leak? One drip a minute? Or does it have to be a constant stream? Usually, regulators specify a Maximum Allowable (Action) Leakage Rate (ALR) above which the responsible leak(s) must be located and repaired. Essentially, the ALR enables a small amount of leakage through pinholes which is unavoidable but not through excessively large defects such as excavator bucket holes (Figure 2).

Excavator bucket damage

Figure 2. Excavator bucket damage.

ALRs vary from 10 liters per hectare per day (lphd) (Victoria EPA Australia) under 30 cm of head to 5000 lphd (10 states in the USA 2004) under a 2 m head in service. The ALR will be the de facto approach for evaluating the overall liner quality. All lining systems should require integrity surveys once the construction is completed and then whenever the ALR is exceeded. In fact, a survey should be carried out after the completion of GMB, and also after the placement of leachate drainage layer on the GMB. Statistics show that more than 70% of liner damage is done during GMB covering (Nosko et. al. 1996).

The Regulator’s Role

Just as the need to carry out independent CQA is regulator driven, so is the need to perform integrity surveys. In New York State, due to the improved performance of primary liners subjected to integrity surveys, surveying secondary liners were also discussed. As a result, the regulatory community will insist the use of integrity surveys at an increasing rate. And it is a long way to go. Estimates show that perhaps 2% of containment GMBs in the USA are surveyed (Giroud 2016a), and approximately 4% in Australia. On the other hand, about 20% of liners in Quebec, Canada are surveyed and about 18% of coal seam gas pond liners in Australia (Bennett 2017).

Taking into account the prominent failures of ash/tailings ponds that have occurred over the years it is now necessary in the USA (Betke and Ramsey 2014) to survey all GMB lining systems that need to be considered in new and updated facilities.

As mentioned above, a nonconductive GC LDS is generally used for the sake of simplicity in double lining systems. The upper GTX could be replaced by a CGTX, which would enable the installation of a complete 3 layer conductive GC at one time compared to the two separate placements of CGTX and layers which is an alternative requirement. The majority of GCs are manufactured with the bottom and top GTXs bonded to the GN.

Double lining systems are increasingly being required and designed, because of their rapid response for identifying a leak, ease of leakage collection, more assured removal, and ease of monitoring. Every double lining system has the potential for incorporating CGTXs.

Application Leaders

According to the USEPA (www.zerowasteamerica.org/Landfills.htm) in the USA, there are more than 3000 active landfills and over 10,000 old “dumps” that may need replacement with lined facilities. There are tens of thousands of industrial and municipal wastewater treatment plant ponds that should be lined and surveyed.

Landfill applications of CGTX will be predominant in both the waste cells and leachate ponds due to their highly regulated structures. Nevertheless, Breitenbach (2017) writes “EPA is totally unaware of the information gained on the mining side of the fence which puts them about 30 years backward in the research and development department concerning their interest in non-leaking composite liners for landfills.” Simply put, EPA has had little interest in mining lining systems.

However, as stated above, expectations of zero leakage must be avoided and rather the concept of ALR should be accepted which can be measured and accepted by all parties. Zero leaks will not become obligatory as it is impossible to measure zero and due to the different interpretations of zero leakage by the different parties. And experience shows that when ALRs are “low” (e.g. 50 lphd), attempts to make repairs may only lead to higher leakage rates because of collateral damage caused by equipment and foot traffic on the liner while locating the leaks and making repairs.

The same applies even more when sand and drainage rock layers are being placed on top of the liner. Actually, about 74% of damage done to GMBs takes place when protective cover soil or leachate drainage rock (Figure 3) is spread over the GMB. (Nosko et. al. 1996).

Cover soil being placed on the geomembrane.

Figure 3. Cover soil being placed on the geomembrane.

The mining sector (Smith, 2017) is slowly shifting to GMB/clay or GMB/GCL composite liners for heap leach pads while true double liners are only required in more restrictive jurisdictions “which do not account for much of the industry, but this may be increasing”. A typical heap leach pad may have an area of one million square meters compared to a large landfill of about 300,000 m2.

As double liners often come with a composite secondary liner, candidates for CGTXs are common for both mining process solution ponds “and the impounding areas inside valley fill leach pads”. (Smith 2017). Shown in Figure 4 are large (~100 ha) evaporation ponds in Chile.

Large evaporation ponds.

Figure 4. Large evaporation ponds.

The mining industry also uses very large evaporation ponds (for example. ten 100 ha ponds side by side) to process lithium, boron, and potassium which owners do not like to lose but these are usually single GMBs placed on a conductive soil subgrade and thus do not require CGTXs. However, there will obviously be a trend to the use of double lining systems in evaporation ponds. In 2009, the author carried out a survey on a large (30 hectare) double lined evaporation pond in Nevada. The primary liner was a conductive HDPE geomembrane on a geocomposite LDS on an HDPE secondary liner.

Perceived Risks

In the author’s experience, the risks of liner failure (excessive leakage) are considerably reduced with the performance of an integrity survey. If they do a survey and find no leaks, then they assume that something has gone wrong. Beck (2017) has shown that if effective surveys are carried out on the GMB, then there is virtually no chance of exceeding New York State’s ALR of 200 lphd on the soil covering the GMB.

Hence, it might be expected that insurance costs will be lower with the performance of surveys and the specification of practical ALRs. However, enquiries of a few colleagues never show any tendency to premium reductions while performing an integrity survey. This may be due to a lack of knowledge or may simply reflect the fact that a liner is a liner to underwriters with an established range of performance from A to D depending on previous statistics. If performance classification E is added, then considerable effort and many clearly superior installations would be involved.

Nosko (2017) followed premium adjustments with both insurance companies and facility owners, but without positive conclusions. The insurance companies requested details of Nosko’s certificates of liner integrity, product liability insurance (to be carried by them), warranties, and detailed company information clearly expecting the liner integrity surveyor to assume a significant part of the risk. This may be the way it should be done but it will not be acceptable to the typical small companies carrying out surveys.

On the other hand, allowing integrity surveys do not need changes in the structure of a landfill that might compromise the mechanical and chemical performance of the landfill. Just like there is no reduction in premium for the performance of integrity survey, there is no associated penalty for the performance of a survey.

A CGTX will be deployed and treated in exactly the same manner as a standard nonwoven GTX, except it requires connection to wires to integrate it into the electrical circuit (Figure 5). The connection could be made using a small clamp or a flat conductive plate.

There is no increase in perceived risks with the use of CGTXs.

Survey electrical circuit.

Figure 5. Survey electrical circuit. Sensor electrodes are about 1 m apart.

First Installation

Imagine Intelligent Materials has developed a novel approach by using graphene to coat a nonwoven geotextile so that the appropriate level of conductivity is created in the material.

After a series of preliminary laboratory and field tests to determine the best method and amount of graphene to add to the basic Bidim geotextile, the first installation was constructed in March 2017. A segmental concrete evaporation tank with horizontal wall segments laced together with circumferential cables was used as the containment facility. It was 59 m diameter, 2.5 m deep, with a volume of 6.5 Ml. The double lining system was made as follows from the top down:

  • Primary geomembrane. 1 mm Enviro Liner EL6040 HD
  • Conductive A19 Bidim geotextile
  • Secondary geomembrane. 1 mm Enviro Liner EL6040 HD
  • Copper wire grid
  • Standard A34 Bidim geotextile cushion
  • Subgrade

The copper wire grid is a standard layer for the Owner with water being poured into/onto the standard geotextile to make sure it had sufficient uniform conductivity. This allowed the survey of the secondary geomembrane with the ARC spark discharge tester. In fact, a single conductive geotextile could replace the entire copper wire grid, standard Bidim geotextile, and the water. Inspection of the large pre-welded GMB parts found holes because of transport and handling. 90% of holes were on creases and 10% on patch welds and “T” joints.

Following installation, two additional holes were found in the primary and no holes were found in the secondary geomembrane. When the holes in the secondary geomembrane were repaired, the primary geomembrane and the conductive geotextile were installed. Once the primary geomembrane was in place, the secondary geomembrane could no longer be tested. With the ARC tester set at a voltage of 20 to 30 kV, the primary geomembrane was surveyed. A number of crease holes were indicated and one additional hole. The smallest hole was 0.7 mm in diameter.

A second installation was made in a 79 m diameter tank (Figure 6) with essentially the same results.

The conductive coating had a specified mass per unit area of 50 g/m2. It was coated to one side of the 190 g/m2 Bidim GTX, giving the CGTX a total mass of 240 g/m2. Its presence made the CGTX a little stiffer which enabled easier handling and adjustment. In addition, it prevented electrostatic attraction to the GMB.

While the CGTX rolls in both of these installations were thermally bonded (coated side to uncoated side) with a 200 mm overlap, previous preliminary field trials have shown that the joints are adequately conductive with a simple 100 mm overlap. Figure 7 depicts a much wrinkled geomembrane being successfully tested with the ARC tester.

General view inside tank

Figure 6. General view inside tank.

ARC tester on wrinkled geomembrane.

Figure 7. ARC tester on wrinkled geomembrane.

Both the general contractor and owner were very satisfied with the performance of the CGTX and committed to use it in future tanks.

Obviously, the conductive geotextile performed as planned.

Further Down the Road

The pilot installation has confirmed that the conductive geotextile will satisfy the current requirements for carrying out leak location surveys, where no other conductive medium is present under the primary geomembrane liner. If there is a GCL under the geomembrane there would typically be 12 to 20% moisture in the bentonite which would make it sufficiently conductive. However, if the moisture content falls below about 8%, due to exposure to hot dry conditions, for example, the conductivity may not be adequate. Yet, a conductive geotextile as the surface upper layer of the GCL would offer adequate conductivity regardless of the moisture content of the bentonite. A conductive geotextile would also offer uniform conductivity that overcomes locations where the GCL’s moisture content varies and might affect the uniformity and sensitivity of a survey.

Looking further ahead, if GCLs and geotextiles could be made as “smart” as trending clothing textiles and wound dressings, such that, as the lower layer of an HDPE/GCL composite liner, the treated smart geotextile could collect data for liner performance analysis. One might know when a liner develops a leak, how large that leak is, where it is, and what its flow rate is. Such parameters could be calculated and monitored remotely.

To some extent, this can be done presently using a 5 m grid of electrodes under the geomembrane but this is quite costly and will only approximately locate any leak. A more detailed survey is then needed to precisely locate the leak.

Overview

Although electrical methods can accurately locate some very small leaks in GMB-based containment systems, it is mainly the requirements of the environmental regulatory agencies that have led to the growth and application of survey technologies. While there is no question that the survey industry is growing, it will not be an explosive growth and instead would be a slow accelerating growth, mainly in regions that are sensitive to groundwater pollution. In general terms, landfill-related work in Europe is slowing down due to legislation concerning waste separation, waste management, waste to energy, recycling, and incineration etc.

Nosko (2017) believes that Africa, China, and Australia offer waste disposal opportunities. Mining applications are “at the starting line”. Waste water is “a sleeping market” and somewhat behind potable water. However, the driver will be a champion in the regulatory sector regardless of the field, as clearly showed by Kapila Bogoda at Victoria EPA in Australia.

Conductive geomembranes (CGMB) and GCLs are the alternatives and direct competition to conductive GTX to facilitate the performance of electrical surveys. To the author’s knowledge, there is only one other CGTX which is made conductive by the incorporation of a grid of fine metal wires. The main advantage of a CGMB is that whether flat or wrinkled, the conductive layer is always in contact with the GMB so that water in the holes that occur due to the stress in the wrinkle will always be in contact with the conductive layer. With a GCL and CGTX, the GTX may remain flat on the subgrade and not lift when in contact with the underside of the GMB at a wrinkle. It is the same for the GCLs as well.

However, welds in the CGMB require special equipment and more care to make welds so that false positive leak indications along welds are prevented, and also special attention must be paid to make sure there is a conductive pathway between adjacent rolls. In comparison, the conductive GTX works as a continuous conductive sheet with any degree of overlap. A minimum of 100 mm overlap is recommended to ensure that reliable contact is maintained. Overlaps do not require any special bonding. It is sufficient to simply overlay the non-conductive back of one roll on top of the adjacent conductive top surface. A spark exceeding 5 kV will bridge across any number of overlaps. If the conductive GTX is laid upside down (conductive face down), sensitivity to small holes is decreased, but holes as small as 1 mm are still readily detected at 10 kV (Dehghan, 2016).

As the trend to installing liners with no major wrinkles continues, there will be a decrease in this integral-layer advantage of the CGMB. Finally, but very practically, the CGMB must be installed with the correct surface down whereas the CGTX can be deployed with either surface down or up. Another concern with wrinkles is that they usually provide an interconnected network of wide pathways for leaking water to move along the GMB/substrate interface to wherever there might be a leak in/through the substrate. In this regard, eradicating wrinkles, as proposed by Koerner (2017) in his Geosynthetic Institute webinar will reduce the advantages of CGMB.

It appears that a graphene-modified GTX will possess a higher friction coefficient than a standard nonwoven GTX which may enable it to be stable on steeper GMB slopes, thus increasing air space for waste.

To summarize, conductive GTXs play a distinctive role in the minimization of leachate leakage from landfill containment systems and, by extension, from storage and containment facilities in the mining, municipal/industrial wastewater treatment industries, and potable water.

Their most important application is in double lining systems with GC and GN leak detection systems.

CGTXs also have the ability to develop as sensors, providing data that will allow the nondestructive determination of when a leak starts, how large it is, where it is located and its flow rate.

References

Ten States, Great Lakes – Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers (2004) report on “Recommended Standards for Wastewater Facilities”.

Beck, A., (2017), TRI Environmental CQA Training Course: Liner Integrity Surveys/Assessments (LISA)

Bennett, P., (2017), private communications

Betke, B., Ramsey, B, (2014), “The Clean Up Act”, World Coal Magazine, March 2014

Dehghan, M., (2016), “Conductivity Testing of a Graphene Coated Geotextile (Factory Trial)”, ExcelPlas Report 5967A, prepared for Imagine Intelligent Materials, p. 30

Giroud, J.P., (2016a), private communications

Giroud, J.P., (2016b), “Leakage Control using Geomembrane Liners”, Soils and Rocks, São Paulo, 39(3): 213-235

www.zerowasteamerica.org/Landfills.htm, “Landfills: Hazardous to the Environment”

Koerner, R., (2017), “Wave (or Wrinkle) Management [For Proper Deployment of Geomembranes]”, GSI Webinar

Nosko, V., Andrezal, T., Gregor, T., and Ganier, P., (1996), “SENSOR Damage Detection System (DDS) – The

Unique Geomembrane Testing Method”, Proceedings of the First European Geosynthetics Conference

EuroGeo1, A.A. Balkema, Rotterdam, 1996, pp 743-748

Nosko, V., (2017), Private communications

Smith, M., (2017), private communications

Victoria EPA Australia, (2015), Publication 788.3: Siting, Design, Operation and Rehabilitation of Landfills, published by Environment Protection Authority Victoria

Imagine Intelligent Materials.

This information has been sourced, reviewed and adapted from materials provided by Imagine Intelligent Materials.

For more information on this source, please visit Imagine Intelligent Materials.

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