Choosing a Liner (part 2)
All brine containment ponds must be constructed with an impermeable, chemically resistant liner that can withstand long-term exposure to the caustic nature of concentrated brine. The expected chemical composition of brine should be compared to the resistance characteristics of any liners or containment materials under consideration.
- Metals are particularly susceptible to corrosion when exposed to dissolved salts. Alloys may be added to help resist corrosion, but those alloys tend to be expensive. Additional chemicals may still be required to balance pH during processing. Even so, these measures can only delay the effects of corrosion. Inevitably, regular repairs and replacement of metal components will be required.
- Plastics are generally inert to salts, but some types, such as PVC, are more susceptible to damage from the heat inherent in some desalination processes. Certain plastics are also prone to permeation or leaching of organic compounds which are found in seawater.
- For extremely high concentrations of salts and other highly corrosive fluids, even high-density polyethylene (HDPE) and low-density polyethylene (LDPE) can break down rapidly. Reinforced liners that are constructed with a combination of resistant materials offer the best corrosion resistance in flexible geomembranes, making it the best material for containing concentrated brine. BTL Liners’ AquaArmor line offers an array of durable liners with high chemical and corrosion resistance.
Primary Containment
Evaporation ponds are generally constructed with walls raised above ground level, while deep ponds intended for longer-term storage may involve some amount of in-ground excavation as well. The raised walls permit easier inspections for leaks and help prevent incursion of surface runoff and any debris carried along during storm events. The walls should be constructed with enough freeboard to accommodate precipitation accumulated during strong or prolonged rainy periods and to contain wave action in windy conditions. The ponds themselves should be constructed with a three-layer system to detect and prevent leaks.
- The deepest layer should be a durable, chemically resistant and impervious liner. This is the final barrier between the brine and the surrounding environment should a leak occur, so this is not the place to save a few pennies. The barrier should cover the entire bottom of the pond and reach up the sides, with several inches of skirt laying over the top of the wall. There should be no area exposed that would allow brine to move underneath this bottom layer.
- The middle layer is typically composed of about 12” a loamy, absorbent soil with a web of moisture sensors arrayed under and along the sides of the pond. The concept is that the absorbent soil will soak up any brine that might leach past the primary liner and allow the sensors to register increased moisture levels before it even soaks down to the final barrier. These moisture sensors should be constantly monitored so that action can be taken immediately before a minor leak turns into an environmental disaster.
- The topmost layer is another durable, chemically resistant and impervious liner. It makes sense to use the same liner used for the bottom, since they can be securely welded together wherever necessary. This topmost layer is most likely to be exposed to UV rays, so be sure the liner you’re using has high UV resistance. BTL’s AquaArmor line is an excellent choice here.
Secondary Containment
When designing a brine pond, secondary containment systems are critical. Their job is to prevent contamination of the local environment in the case of uncontrolled leaks and spills, heavy rainfall, or other accidents. This secondary system should be at least large enough to contain all the brine from your largest pond, in case of a berm collapse, or 10% of the combined volume of all ponds in the containment area, in case a heavy rain event causes the ponds to overtop. The secondary containment system should also surround the entire pond area with a wide enough margin to contain any splashing or wave action kicked up during storms.
The secondary containment system should also be completely covered with a chemically resistant, highly durable liner. Durability is exceptionally important since it’s expected that foot and equipment traffic will be common in these areas and the liner should be resistant to abrasion, tears and punctures. A RPE liner, with a reinforcing scrim, is a good option here. This secondary containment area may be covered with a layer of soil or fine sand to help reduce wear - discuss this option with your liner dealer first, to make sure you’re looking at the best option for the conditions.
Finally, you’ll need to design a drainage system to remove any spills or precipitation that do end up in your secondary containment area. Even if it’s “just” precipitation, it’s potentially contaminated and should never be released into the local environment.
Covers
Covers are often overlooked when designing an evaporation pond - after all, the whole point of the process is to maximize exposure to the sun. However, retractable covers may be the ideal solution in certain circumstances and they’re always worth at least a momentary consideration. In areas where rain comes only occasionally, but in large amounts, covers will prevent potentially several inches of precipitation from setting back your processing schedule by weeks or even months.
Covers may be useful, as well, if wildlife access is a problem, especially overnight. A retracting system can make deploying and removing covers a quick, simple process that can save trouble and heartache, especially during migratory bird season.
