It’s simply not enough to excavate a well-designed impoundment basin and expect it to suffice for holding coal ash slurries and “dry” mixtures. Impoundments, even in the most clay-heavy soils, will slowly lose water to the surrounding environment from seepage. All soils lose water through the tiny gaps between individual particles, and even adding most liner materials won’t entirely stop this water loss. Water seeping from a pond full of freshwater may not be a problem because it’s clean and free from contaminants. When you’re dealing with a hazardous material like coal ash, the resulting wastewater isn’t safe to release unexpectedly into the soil or groundwater. Find out why flexible geomembrane liners from a source like BTL Liners is the best choice for all lined coal ash pits and impoundments.
Impermeable Materials
Many of the liner materials used for reinforcing loose soils are mistaken for impermeable materials. Yet, the concrete or clay-coated fabrics used to keep the embankment around a pit or impoundment from collapsing isn’t necessarily designed to hold back water over time. Concrete is particularly is more permeable than many people realize due to its capillary action. Microscopic openings in the surface actively pull water through the material over time and leave it outside of the containment zone. Whether you’re aiming for primary or secondary containment of coal ash residues and by-products, it’s essential to stick with impermeable, flexible liner materials. Only polymers can offer the amount of seepage and leak control you need for fly and bottom ash storage, especially if you need to meet strict state or EPA requirements for new pits and ponds.
Ring Embankments
Most impoundments designed specifically for holding coal ash residues and slurries feature ring embankments around the edges to prevent overflowing as rainwater fills in the pit or pond. These embankments must be lined, along with the rest of the impoundment, or they’ll be susceptible to erosion and other forms of damage. Yet, the steep slope and curved shape of these embankments make them challenging to line with rigid or inflexible materials. Flexible liners conform to the exact shape and slope of the embankment, ensuring better erosion control both from surface runoff and wave actions from the slurry in the pit.
Chemical Compatibility
Coal ash residues do contain small amounts of many reactive chemicals, but it’s considered more inert than many other waste products of the gas and oil industry. Nonetheless, it can still contain a wide enough spectrum of reactive chemical components that can shorten the lifespan of the liner material. Using a chemical resistant liner material like ArmorPro, is the best way to ensure that variations in the specific composition of the waste stream won’t threaten the integrity of the impoundment. In general, UV resistance is a greater concern for these installations than chemical resistance since most designs call for exposed liner installation.
Which Flexible Liner is Best?
Of course, it’s not enough to simply install just any flexible liner product and expect good results. Some liners may work well enough for fishponds or retention ponds, but they aren’t durable enough for the challenging environment of the constantly dredged coal ash pit. This round-up of the most common flexible liner materials will help guide your choices by helping you compare them based on the features most important for coal ash storage.
Best: Reinforced Polyethylene
Reinforced polyethylene, or RPE, is the single best option for lining coal ash storage pits. It’s widely used in the gas and oil industry for its durability and chemical resistance, but it works even better for coal ash impoundments in need of primary and secondary containment. It’s highly durable due to both the polyethylene materials used and the reinforced design. Reinforced liners are far more tear resistant than liners made without the feature, making them easier to install and less likely to leak over time. It’s durable enough to withstand routine dredging to remove solid waste from the bottom of the impoundment, a routine challenge that can ruin other liner materials. BTL Liners recommends RPE products for coal ash use for the advanced chemical and UV resistance as well.
Good: High- or Low-Density Polyethylene
When RPE liners aren’t available for a coal fired, power plant project, the next best choice is either high density polyethylene (HDPE) or low-density polyethylene (LDPE). On their own, neither material is quite as good as RPE. HDPE is strong, but it’s stiff and thicker than RPE. This makes it more difficult to conform to the curved impoundments and ring embankments commonly used for coal ash storage and treatment. LDPE is more flexible, but it lacks the durability a liner needs to withstand the abrasive materials and dredging equipment used in this industry. Using either HDPE or LDPE alone for a liner is likely to result in a shorter lifespan than is desired; especially for a direct burial pit that will be used for permanent coal ash storage. Stick to RPE liners instead, since they can be shipped almost anywhere in the world.
Acceptable: Polyvinyl Chloride
Polyvinyl chloride, or PVC, is one of the most widely used polymer materials for lining small and decorative ponds. Yet it’s simply not tough enough for most commercial purposes, including for coal ash impoundments and pits. PVC is flexible and soft enough to conform to curves and it sacrifices a lot of strength and durability to achieve that. It’s also known for leaching out chemicals that could possibly interact with the contents of the coal ash residue pit and react to the waste by-products. If you’re planning to reuse or recycle the ash, that could be a serious problem.
Avoid: Bentonite and Clay-Composite Liners
The latest loosening of restrictions on the federal level for coal ash storage now allows for the use of clay-composite liner products. These are woven or other open materials that are coated with bentonite clay that swells and expands when it comes in contact with water. The idea is that the slurry will wet the clay enough that it seals tightly together and creates a barrier to stop seepage and leaks. Unfortunately, extensive testing has shown that the coal ash residues contain chemicals that interfere with this effect. This leaves the porous fabric exposed and leaking rather than forming a water-tight barrier. Avoid clay-composite and bentonite-based lining products for coal ash impoundments and ponds; even if they’re allowed by state and federal law. You’re likely to end up dealing with clean-up and remediation costs later if you try to rely on a clay-based product of any kind.
Careful liner selection is as important to coal ash containment success as any other detail of impoundment design. Without the right liner, even the best pit or basin will begin leaking shortly after installation. Don’t let remediation costs and environmental damage interfere with your operation of a successful coal burning power plant. Turn to BTL Liners for all of the RPE materials you need to reliably and safely build coal ash residue pits and ponds. Our containment solutions work for all kinds of storage areas, including for secondary containment basins under tanks and transfer stations.
