Lining a landfill is quite possible, but it is a bigger challenge than some operators first assume. It’s not simply a matter of finding a compatible liner material and applying it to every cell and holding area. Using the wrong impermeable liner material or failing to install a collection layer with permeable materials will result in long-term issues. Leaking is common even in the most well-built landfill cells when the wrong materials are chosen. Since landfill cells almost always require more than a single layer of liner material, to achieve leachate management goals, the materials used must also work well together without clashing or reacting to each other. Here are some basic guidelines on designing multi-layered landfill liner systems to guide product selection from BTL Liners.
Risks of Leachate Loss
Leachate is unavoidable in every large-scale landfill facility because it’s simply impractical to completely cover all the open cells. Even in covered cells, water can still enter horizontally to create more leachate before or after capping. Due to high levels of chemical compounds, heavy metals, dissolved solids, organic nutrients, and other materials, landfill leachate can create environmental and health issues like:
- Degradation of soil quality surrounding the landfill, even at a long distance if there is extensive runoff
- Groundwater and surface water quality problems, which may affect human health if residents nearby are using wells
- Long-term habitat loss if the leachate escapes into wetlands or other sensitive areas commonly found around remote landfills
- Air pollution and odor issues if leachate evaporates and forms various gases like methane and ammonia.
What’s Too Permeable?
Many materials recommended for lining other kinds of waste storage pits or ponds are too permeable for this kind of system. While permeable materials are needed for the upper layers of the leachate accumulation system, they’re not part of the actual impermeable liner designed to control the loss of these liquids. Permeable materials can slow down the loss of liquids, but they’ll eventually let hazardous waste escape into the surrounding environment. Even materials that might seem impermeable are actually quite permeable, such as asphalt, clay textiles liners, and concrete. If these materials are required for structural support in the project, make sure they’re backed up with truly impermeable liners like RPE from BTL Liners.
Liner Systems
Each liner system is different because it must be designed around the geography of the site, soil conditions, average rainfall, and the type of waste intended for storage above it. A compacted cell of construction waste will need a tougher liner than one that will only hold sorted municipal plastic or composting materials. However, most landfill cell liner systems share common features like multiple layers of material. Some projects call for just three to four layers of material to trap leachate, while others can use seven or more distinct layers. Impermeable and flexible sheets of polymers known as geomembranes are recommended as the main layer of protection against leaks. However, some designs also call for burying an expansive clay liner below it to act as a reactive backup layer in case of a rare leak. Other materials used in complete liner systems for landfills may include geotextiles that are open enough to allow water to flow through, various pipes to collect and redirect leachate, and various types of gravel and sand to keep the other layers apart.
Issues with Asphalt and Clay
Asphalt and clay were some of the first materials proposed for lining landfill cells and related types of waste management construction. Asphalt is still sometimes used as a capping material, but it’s no longer recommended for most projects as a type of liner. First, asphalt is only moderately impermeable and still allows leachate to seep out over time. Much like concrete, it also inevitably shifts and cracks as the material ages and loses elasticity. Flexible geomembranes, made from the right polymers, are much more resilient than asphalt as both liners and caps. Clay is often used to seal the prepared base of the raw soil to smooth it out and prevent penetrations in the liners above, but it’s not dense enough to control seepage in the long run either. Even the best clay liners are still partially permeable, allowing for the slow escape of leachate over the years. Both clay and asphalt layers in landfill cells should be paired with flexible and impermeable materials to ensure they’re truly holding back the liquid waste as desired.
Choosing a Material for the Impermeable Liner
Aside from choosing various types of geotextiles to control the leachate accumulate layer, the most important decision in designing a landfill cell is picking the impermeable liner. Choosing the wrong material for this layer may result in hidden damage since it’s buried so deep under layers of compacted trash and other liner materials. There are few opportunities to check or repair these liners after installation, unlike a pond that is drained regularly for cleaning. This means that choosing a less durable material to save money up front could result in expensive remediation costs later after a liner failure. Make sure you’re pairing the right geomembrane liner with your landfill cell project by working with us here at BTL Liners.
Materials for Lining Landfills
- Reinforced polyethylene: RPE is the best material for the challenging environment of the landfill cell. The reinforced scrim design helps it resist tearing and puncturing from the sharpest types of waste while allowing the material to be manufactured thinner for lower shipping and installation costs. A high chemical resistance ensures that compounds mixed into the leachate won’t react with the polymers and break them down prematurely. RPE is also more flexible than stiffer and thicker materials, making it easy to conform to cell shapes and fit into tight curves. The reinforced design also gives it extra surface friction for good adhesion and grip against the berms and sides of the cell pit.
- High density polyethylene: HDPE is a little too stiff and thick to be the best choice for use in landfill cells. It lacks the friction and grip of the RPE liners, making it harder to get it to stay in place without anchors or extensive cover material. HDPE is also harder to find in reinforced styles than RPE, making it less puncture resistant. This can be a problem in a landfill used for construction waste or mixed municipal waste that might have scrap metal and other sharp objects in it. Chemical resistance is good with this material, but not as good as RPE.
- Low density polyethylene: LDPE is more flexible than HDPE, but it lacks the durability and longevity of RPE. This really matters in the landfill cell since it’s usually built to stay in place for 50 to 100 years. There’s usually no desire to excavate the covered mound every few decades just to keep it properly lined. While LDPE might work for low-risk or short-term facilities, like composting sites, it’s not a great choice for permanent landfill cells. It’s a little too prone to cracking and puncturing for use in most landfill situations, so it’s only a good option if RPE or HDPE aren’t available for the project.
- Polyvinyl chloride: PVC used to be one of the only materials widely available in large flat sheets like geomembranes. This led to its use in some of the earliest landfill cells and sanitary systems that included liners. Unfortunately, those early experiments largely proved that this material is unsuitable for this purpose. It’s too prone to tears and ripping under the heavy and fluctuating weight of the compacted waste; especially as leachate accumulates while the cell is still in use. It’s also not as good as any polyethylene product at resisting chemical reactivity, which can be an issue due to the complex nature of leachate. Finally, PVC is particularly prone to damage while it’s exposed, which can happen for months at a time during the construction or capping process.
- Ethylene propylene diene monomer rubber: EPDM rubber is very thick, heavy, and stiffer than any of the materials listed above. While it’s sometimes used for other applications, it’s inappropriate for most landfill uses. It’s too heavy and stiff to use for lining or capping cells and doesn’t offer a long enough lifespan for a permanent installation.
Protecting Liners with Underlayments and Grids
It’s not enough to layer various liners and permeable barriers with gravel to create the complex liner system required for trapping leachate and removing it. Landfill cells often call for the use of multiple underlayments to keep the layers of the liner from being torn, punctured, or worn down over time. Landfills are particularly full of friction and puncture risks due to the combination of heavy solid waste and a constant slow flow of water from the top to the bottom. Underlayments are thick, felt-like fabrics that aren’t impermeable but act as cushions to protect thinner impermeable layers. They are often used as the permeable layers in leachate systems since they can wick waste liquids through themselves to appropriately placed drains and pipes. If underlayment layers are too thick to serve this purpose, open geo-grids are usually required instead to encourage a steady flow towards the drains.
Considering Friction and Grip
As mentioned briefly above, the friction and grip of a particular liner material are important to consider, especially if no underlayments will be used beneath the final liner. Slick materials, with no reinforcement grid, often slip off of the embankments and sloped sides of these landfill cell pits. The addition of a smooth, clay layer to protect the liner from punctures doesn’t help with the grip or adhesion. Keeping the various layers together is essential to prevent them from slipping down and ripping or releasing leachate into the environment. Using anchors to pin the material to the soil often backfires and creates leak points instead. Choosing a reinforced material, with high friction and grip, is recommended to ensure that steep and shallow slopes alike stay covered in the landfill cell.
Of course, these are just some of the considerations that go into choosing the right liner material. If you still need more guidance, reach out to us here at BTL Liners. We’re happy to make recommendations and guide you to both the right liner and cap materials.
