Geosynthetics’ role in pollution control
This section first discusses how to minimize the impact of contaminant transport from waste disposal facilities and mining facilities, and then discusses the use of geosynthetics to treat mud and sediment. The production of renewable energy from waste or sunlight is also discussed.
Minimize the impact of underground transport of pollutants from landfills
Landfills are an essential part of an integrated waste management strategy, not least because they are the only land sink for hazardous substances that would otherwise enter the environment. Landfill practices have evolved considerably over the past four decades, with mature facilities now subject to strict environmental regulations (Touze-Foltz et al. 2008).
Different types of liner systems can be used for waste containment with the aim of limiting the migration of contaminants to a level that has negligible impact on the environment. These liner systems are combined with drainage systems to reduce the level of hydraulic leachate on the liner (“leachate” is rainwater that has been contaminated with pollutants from waste runoff and can be harmful to the environment).
The soil barrier serves to minimize the migration of pollutants from the facility, so the environmental impact of the facility is closely linked to its design and long-term performance (Rowe et al. 2004). European legislation on waste disposal (OJ 1999) requires landfills to be constructed and designed in such a way as to avoid pollution of soil, groundwater or surface water and to ensure efficient collection of leachates. The bottom and walls of the landfill must consist of a mineral layer that meets certain requirements for hydraulic conductivity and thickness, which together affect the protection of soil, groundwater and surface water. If the geological barrier does not intrinsically meet the required conditions, it can be artificially supplemented and reinforced by other means providing equivalent protection. GCLs can be used here.
However, the mineral coating is not used in isolation. Indeed, many modern landfill barrier systems require a leachate collection system and a geomembrane (often a high-density PE or HDPE geomembrane due to contaminant transfer and durability issues) over a concrete liner. compacted clay or a GCL (Touze-Foltz et al. 2016). Composite liner systems, formed by joining a compacted clay liner or GCL and a geomembrane, provide redundancy, which in turn provides additional protection. Composite systems thus improve containment performance and ultimately environmental protection (Rowe 1998; Rowe and Brachman 2004; Touze-Foltz et al. 2006; Jones and Dixon 2011; Jones 2015).
However, the benefits of using geosynthetic liners as part of a barrier system may not be fully realized if the geomembrane is physically damaged: geomembranes only make excellent liquid barriers if there is no there are no holes in the geomembrane (Rowe 1998). Geomembranes can develop holes during installation, although quality control can prevent most holes (Rowe 1998; Touze-Foltz et al. 2008). In practice, various types of protective layers (e.g. geotextiles, sand layers, combinations of geotextiles and sand layers, sand-filled cushions, rubber mats, layered geotextile composites) are placed between the revetment and the coarse drainage gravel to avoid damaging the revetment (Touze-Foltz et al. 2008; Brachman and Sabir 2013).
Prevent water infiltration into the waste body and gas migration into the atmosphere
The goal of permanent landfill covers is to limit, but not necessarily prevent, stormwater intrusion into the waste, thereby controlling leachate production and minimizing gas emissions from the landfill. For example, to achieve these goals, European regulations require that the landfill cover comprises a gas drainage layer, a liner system, a water drainage system and a topsoil cover. The overall goal is to reduce water and gas flows. To achieve this goal, various semi-permeable or impermeable cover alternatives are possible, provided they meet performance objectives related to liquid and gas flow in and out of the landfill, reduce the risk of waste, and have geosynthetic lifetimes longer than the risk of wastage.
Compacted clay liners, GCLs and geomembranes are therefore not the only materials that can be used as landfill covers. Drainage geocomposites, consisting of either a geofoil or a continuous HDPE drainage core with overlaps to allow water flow, may meet the demand for alternative cover systems for non-hazardous waste (Meydiot and Lambert 2000, Faure and Meydiot 2002; Fourmont and Arab 2005; Fourmont et al 2009a, 2009b).
The case of radioactive waste after the 2011 Tohoku earthquake and tsunami
The 2011 Tohoku earthquake in the western Pacific off the east coast of Japan caused a widespread tsunami that combined to cause a catastrophe in Japan with massive loss of life and massive damage to buildings and infrastructure. One of the casualties was the Fukushima Daiichi nuclear power plant, which was severely damaged. The damage caused three reactors to overheat, with the resulting hydrogen gas explosions releasing large amounts of radioactive material into the atmosphere, which was then deposited on adjacent land (Inui et al. 2012). Several interim storage facilities were commissioned to store the soil and waste generated during the decontamination work. The IGS Japan Chapter Geomembrane Technical Committee has developed procedures for (1) the selection and installation of storage tanks to prevent polluted water from escaping reservoirs, (2) the design and construction of structures and the selection of appropriate barrier materials, and (3) Inspection of seams of geomembranes (Shimaoka et al. 2014). There are currently more than 16,000 temporary storage sites that include a bottom geomembrane, piles of soil or garbage bags, and a top cover to prevent rainwater from entering the garbage.
Geosynthetics in mining for metal extraction
Mining is the backbone of many economies around the world, including developed countries like Australia. In many developing countries, mining provides more than 50% of export earnings (Bouazza 2013). The incorporation of geosynthetic barriers in modern mining is expected to reduce environmental risks associated with the migration of contaminants abroad and improve mine water recovery for either reuse or recovery of products from the solution (Bouazza 2013). The liner systems are designed to meet similar requirements as landfill bottom liner systems, with particular emphasis on chemical and mechanical environments specific to mining applications (Bouazza 2013). Another special feature is that the geomembrane has no punching protection due to the slope stability (Touze-Foltz et al. 2008). Geosynthetic materials are now commonly used and coated in lining systems for heap leach pads (HLPs), tailings deposits (TSFs), waste rock or tailings deposits, and ponds and channels.
Heap leach pads
HLPs are lined facilities where ore is placed. A leach solution is applied to the ore to dissolve the minerals it contains. The leach liquor depends on the type of ore being processed and may consist of a strong acid (e.g., as the leach liquor seeps into the ore it dissolves the precious metals creating what is known as a ‘mother liquor’. In the case of the mother liquor losses should be avoided are minimized and, where possible, eliminated by the underlayer system in order to maximize recovery (Bouazza 2013). embedded in a drainage layer These include geotextiles, corrugated and perforated pipes (single and double wall), perforated or non-perforated HDPE pipes and geodrains (Touze-Foltz et al. 2008).
Tailings storage facilities
When water is used in the mining process to concentrate valuable substances from the ore, the waste produced is sludge, commonly referred to as “tailings”, which are basically solid particles dispersed in water. TSFs are man-made structures built to contain waste. Depending on the geochemistry and treatment solution of the tailings, the tailings may be stored in a geosynthetic lined TSF.
A continuous or intermittent drainage layer may also be placed over the liner to enhance consolidation of the tailings or to provide internal drainage for the TSF. Geosynthetic materials such as geotubes, geotextiles and geodrains are often incorporated into the design of the drainage system.
Fluid fine tailings
Oil sands, phosphate mining and alumina refining produce very fine sludge (d80 < 20 μm) with a high clay content. The oil sands industry produces waste sludge called Fluid Fine Tailings (FFT). FFTs have a high clay content and contain 70–80 wt% water and 1–3 wt% residual bitumen, making them particularly difficult to dewater (Mikula et al. 1996; Allen 2008; Jeeravipoolvarn et al. 2009). The two main problems caused by FFTs are the large volume of material and its very poor geotechnical properties, both caused by its high-water content (Mikula et al. 1996; Snars and Gilkes 2009; Farkish and Fall 2013). FFT dewatering is a preferred way to increase tailings shear strength and reduce the volume of material to be contained, thereby reducing the risk of failure, reducing footprint and minimizing water consumption (Farkish and Fall 2013). One solution is to place an electrokinetic geocomposite in the FFT deposition zone during filling to dewater the mud (Gastaud et al. 2015, 2017), induce hydraulic head and drive off the water (Faure et al. 1993) . The application of an electric field across the mud layers creates electro-osmosis in the mud, leading to water migration (Jones et al. 2008; Fourie and Jones 2010).
Mining waste storage
During the mining process, waste rock or overburden is removed to allow access to the ore. Tailings material is often placed in a special facility designed for stability and drainage. Lining systems for tailings storage facilities are used where there is a possibility that the tailings could develop low-grade leachate by percolation of rainwater through the material and subsequent reactions with minerals in the tailings, creating leachate.
Tailings storage facilities often include solution collection systems to collect and direct leachate to ponds or sumps for further analysis, treatment if necessary, and discharge. These installations usually consist of collecting drains dug into the ground. The drains are surrounded with filter geotextiles to prevent migration of fines from the overburden and foundation into the header (Touze-Foltz et al. 2008).
Drainage of waste and contaminated sediments
Many industries use water for processing as well as for transporting and storing by-products and wastes, which produce large volumes of liquid or slurry material. For example, the treatment and disposal of sewage sludge is one of the most problematic issues in wastewater treatment in developed countries (Glendinning et al. 2006). Pre-treatment significantly reduces the volume of manure waste, making it manageable for handling, transportation and disposal (Lawson 2006, 2008). Such treatment usually consists of dehydration. Geotextile tubes for sludge drainage offer significant cost savings and are environmentally friendly: they have a relatively small footprint, allowing for a greater number of drainage sites and optimizing land use when the surface area available for containment and drainage is limited (Lawson 2006, 2008; Hsieh 2016).
Geotextile tubes are an ideal medium for the drainage of sludge-like waste streams and contaminated sediments such as municipal solid waste, agricultural waste, food and food processing waste, industrial and mining waste and sediment. contaminated (Lawson 2006, 2008, 2014). Effective drainage requires a geotextile tube drainage platform installed at the foot of a solid barrier to support the geotextile tube drainage units and prevent the loss of wastewater from the tubes into the foundation. A drainage mat is installed over the barrier, which may be a geomembrane, to facilitate complete drainage of the geotextile tube (Lawson 2006, 2008).
Renewable energy
An increasingly popular practice is the non-penetrating mounting of photovoltaic modules on contaminated surfaces (eg landfills, mines); more specifically on the geomembranes already used on these sites to contain the pollution (Brooks et al. 2011). The solar energy blanket is a composite system that integrates flexible photovoltaic laminates (i.e., flexible solar panels) with an enhanced exposed geomembrane cap.
Other rigid solar panel technologies represent an alternative approach to converting landfills into solar energy sources (Alexander 2010). Based on the various examples presented, work is being carried out to show that these systems contribute to energy independence and reduce unsustainable dependence on fossil fuels (Alexander 2010). Moreover, although currently not quantifiable, the benefits in terms of public perception and transition to a more sustainable source of energy are significant (Alexander 2010).
As in agriculture, methane recovery is also an issue in environmental applications, whether biodegradable waste landfills, leach ponds (Ng et al. 2009) or treatment plants where anaerobic ponds cover sedimentation to promote wastewater solids and their anaerobic decomposition into methane (Craggs et al. 2015). Covering the landfill enhances the production of biogas for energy production and at the same time prevents the emission of greenhouse gases and associated odor nuisance (Craggs et al. 2015). For urban lagoons, DeGarie et al. (2000) report that the production and use of biogas should pay for cover systems.
Climate protection through the use of geosynthetics
Dixo et al. (2017) indicate that two categories of actions are necessary to combat climate change and its impacts: (i) mitigation to reduce greenhouse gas emissions and (ii) adaptation. Geosynthetics can contribute to mitigation by reducing greenhouse gas emissions from the construction and operation of infrastructure. In fact, the environmental benefits of building with geosynthetics are well known using geosynthetics instead of soil can significantly reduce emissions compared to excavation, transport and landfill.
The sustainability of materials and processes is usually assessed by calculating the CO2 emissions generated. Although this is a simplification, the simplicity of the calculation encourages comparisons between solutions and makes these assessments accessible, transparent and reproducible, making it easier to inventory the CO2 emitted for comparison to industry, national and international comparisons (Dixon et al. 2016).
Such calculations have been performed for various applications of geosynthetics, including filtration (Ehrenberg et al. 2012; Laidié et al. 2012), foundation stabilization (Ehrenberg et al. 2012; Elsing et al. 2012), landfill drains (Ehrenberg et al. 2012; Werth et al. 2012), construction of retaining walls (WRAP 2010; Bouazza and Heerten 2012; Ehrenberg et al. 2012; Fraser et al. 2012; Damians et al. 2017, 2018; Russell et al. 2017) , implementation of Slope stabilization (Bouazza and Heerten 2012; Heerten 2012), road construction (Bouazza and Heerten 2012; Heerten 2012), implementation of landfill cover systems (Bouazza and Heerten 2012), slope development using electrokinetic geosynthetic treatment (Jones et al. 2014) and the Use of geosynthetics to strengthen bridge piers (Beauregard et al. 2016). The results of these studies indicate that structures containing geosynthetic layers tend to have lower embodied carbon (EC) levels than structures using traditional granular solutions. The use of geosynthetics leads to massive improvements in CO2 savings compared to almost all alternative civil engineering materials used.
In recent years, the field of geosynthetics has shown greater mastery of life cycle analysis techniques. The latest calculations by Dixon et al. (2016) and Damians et al. (2017, 2018) indicate advances in this area, where an increasingly engineered standard approach is evolving, using EC values representative of geosynthetics (Raja et al. 2015) and comparing EC values for whole engineering solutions. The results are recognized and reliable when they claim that the use of geosynthetics significantly reduces the environmental impact.
