Evaporation ponds are important for a number of industries including agriculture and mining. Creating evaporation pond can be expensive and land intensive, in addition to having locality preferences (high ambient temperatures requirements with low annual precipitation). Creating scalable, economic evaporation ponds has become a hot topic for pond design. The most common method for increasing efficiency is increasing the surface area of the pond and decreasing the depth to improve evaporation rates. However, this strategy increases the material (shipping in a water impermeable material like clay) and land requirements.
A number of alternative evaporation techniques have been considered over the last decade, but none have been able to supplant the evaporation pond in simplicity and overall efficiency. Fortunately, there is an alternative to the traditional pond system that replaces clay with a hydrophobic plastic liner made of polyethylene (PE) or polypropylene (PP). Designing double-lined ponds with leak detection systems can also improve pond safety factors by decreasing the opportunity for leachate to escape the system and seep into the surrounding environment—a concern for ponds holding mining brine or agricultural wastewater.
A primer on evaporation pond efficiency
Evaporation ponds are one of the most cost-effective ways of separating waste or desirable deposits from a body of water. The driving factor in evaporation rates is exposing the pond’s water surface to as much solar radiation as possible through direct sunlight. Not all climates are suitable for this technique, which favors dry, hot weather with low annual rainfall. But even in places where the climate is suitable, evaporation ponds also require a low-permeable material like clay to line the pond’s bottom and embankments.
In many cases, the concentrated byproducts in the water being held in these evaporation ponds can be harmful if introduced to the surrounding ecology. For this reason, engineers incorporate a low permeability base layer for containment. If you are fortunate, your chosen site has suitable clay or bentonite on hand, but that is not always the case. And while you could haul in the material, it can increase the installation expense, slow down construction, and complicate logistics. An alternative to clay is a plastic liner and drainage system.
Lowering costs by replacing heavy clay with a plastic liner
Plastic liners carry many benefits. First, a PE geomembrane liner is less permeable to water than clay. Natural clay has a hydraulic conductivity (permeability) of approximately 1 × 10−9 m/s. By contrast, the equivalent diffusion permeability of a typical geomembrane will be 1 × 10−11 m/s to 1 × 10−14 cm/s—making geomembranes relatively impermeable compared to a compacted clay liner. Second, geomembrane liners are lighter and therefore less expensive to transport and install. In fact, you can cut your installation costs by more than half when you use a geomembrane liner instead of natural clay (1). Third, compacted clay liners can desiccate and crack requiring rehydration and recompacted to re-establish their integrity.
Geomembranes also carry an increased factor of safety. They have been around for more than 30 years and have a proven track record of performance. Installers know how to better work with the material and minimize leak rates to reduce impact to the local environment and improve brine retention. Some geomembrane liners are even specially manufactured to improve their performance in a pond.
- Koerner, Robert M. Designing with Geosynthetics – 6th Edition Vol. 1 (p. 65). Xlibris. Kindle Edition.