Gas release may begin as soon as the first layer of waste is added to the cell, but these early discharges of carbon dioxide and methane are usually permitted as long as they’re limited. In general, capping is the first stage of preparing to use the installed gas collection system. Gas collection pipes and wells are installed in the trash layers as the lifts are added over time, stabilized and held in place by the trash compacted around them. Yet, all this equipment can’t work if there’s not an impermeable cap at the top to stop the loss of the gases to the environment. A thick layer of clay or dirt may seem sufficient, but it’s far from enough for gas control. Find the right cover material to build a cap that will control both gas and liquid for the lifespan of the closed landfill cell throughout all the challenges of long-term gas collection.
Impermeability and Soil
Soil lacks the impermeability needed to serve as an only capping material, especially for gas control and collection. With just a layer of compacted soil on top, gas will continue to escape over the lifespan of the landfill. Levels of gas loss may stay within what’s allowed by the state or federal guidelines, but this can change unexpectedly as the cap ages and vegetation suddenly increases or is lost. Adding a more reliably impermeable layer of liner, in the form of a geomembrane, is a much better idea. A truly impermeable barrier is essential to gas collection, especially if volume is a concern. Without an impermeable barrier at the top, it’s impossible to pressurize the system to extract as much gas as possible for use elsewhere.
Applying a Vacuum
Pressurizing the gas collection system allows for the most complete capture of landfill gas possible. Without a vacuum applied by the wellheads above the cap, it’s impossible to retrieve more than just what rises to the surface naturally. Natural gas release by ambient pressure alone may be sufficient for wells just designed to vent to the atmosphere, but collection systems can’t operate solely on that kind of production. Applying a vacuum to the entire gas collection system increases wear and tear on the entire cap and cover system, so the impermeable barrier material must be able to withstand that force without tearing, rupturing, or moving out of place.
Penetration for the Wellheads
Gas collection systems are rarely passive. So, if there’s a demand to maximize production, wellheads for actively pressurizing the gas are generally installed on top of the capped landfill cell. The piping to connect these wellheads to the wells or pipes that collect gas must penetrate multiple layers of the landfill cap without risking damage to it. Fragile materials won’t last through decades of constant maintenance and inspections for the wellheads. A reinforced geomembrane is generally a good idea for any cap or cover material that is expected to have penetrations running through it, even if they’re only temporary.
Cover Material Durability
The covers that go in place as part of a landfill cell cap must be more durable than other types of geomembranes. First, the material stays in place for decades while foot and vehicle traffic occurs above it. The weight of the dirt and other cover materials added to complete the cap presses down on the geomembrane for all that time, stretching it and adding wear and tear. Second, the cover also has to withstand pressure from underneath it as gas presses up from below. This is unusual for most in-ground cover situations, which are usually only used on stable water supplies that don’t produce gas or pressure. Reinforced materials, such as reinforced polyethylene (RPE), are a good choice for the impermeable hydraulic barrier in any landfill cap; especially those designed for gas collection.
Choosing a Polymer
Aside from selecting a geomembrane that can handle wear and tear, it’s important to consider how the polymer used for the cover affects its lifespan. Some materials react badly to the compounds and chemicals in the gas condensate, increasing the likelihood of a breakdown. Other materials can’t take the pressure of weight pressing down on them while sharp gravel collection layers are below.
The best material for a geomembrane cover for a landfill is RPE. The reinforced material is easy to install because the material spreads easily and lays flat quickly to reduce wrinkles and folds. Wrinkled material tends to crack and leak after weight is applied over the top, especially when a foot or more of dirt is added to the top. RPE is also chemical resistant, ensuring that contact with condensate or leachate won’t shorten the lifespan of the material. It’s also possible to find highly UV resistant RPE covers that can handle extended periods of exposed installation. While most landfill caps require covered installation instead, gas collection caps may include at least temporary exposure to direct sunlight that could damage other types of material.
Of course, RPE is far from the only material on the market used for geomembrane covers. Other materials you’ll see marketed as being available for landfill covers, that aren’t ideal for gas collection, include:
- Polyvinyl chloride (PVC), which isn’t durable or chemical resistant enough to be a good choice for landfill capping. It’s flexible and easy to spread, but that flexibility also reduces its durability and makes it prone to UV damage. The pressure of gas collecting under the cover is likely to tear the material as well, especially over time as it becomes more brittle and less flexible.
- High- and low-density polyethylene (HDPE and LDPE), which offer good UV and chemical resistance but neither of which compare to the durability of reinforced polyethylene. RPE combines both of these polymers to get the strength and flexibility of each in a single package. Trying to settle for either one will either compromise the installation process or lead to a cover that fails more quickly than expected due to low durability.
- EPDM rubber, a thick material used for commercial roofing due to its ability to hold water even when it stands for long periods over the surface. However, it’s far too thick and inflexible to serve as a good cover material for a landfill. Piling dirt on top of the rubber sheet will only cause it to rise up in other areas, leaving gaps for gas to escape or at least accumulate and stress the material. Avoid rubber geomembranes for this application since the material is rarely thin and flexible enough for capping.
Still need more guidance on selecting the perfect cover material for your landfill gas collection system? Reach out to us here at BTL Liners. Our team is happy to put our 40 years of industry experience to good use for recommending just the right cover materials for landfills of all kinds. Even if gas collection isn’t a goal for your landfill, our cover materials are ideal for controlling leachate levels and preventing odor issues.
