Optimizing Evaporation Efficiency
The rate of evaporation is the key factor affecting efficiency in a brine pond. Aside from selecting a site with natural advantages such as copious amounts of sun and low humidity, a range of enhancements can be introduced that will speed evaporation. Shorter processing time means more volume can be managed in a smaller footprint, which leads to savings in construction, materials, maintenance and repairs.
Methods (some proven, some under study) for increasing evaporation rates include:
- Using spray systems to continuously wet sails made of permeable membranes. This exposes an increased surface area of brine to the air and enhances evaporation. In these operations, membranes need to be cleaned frequently to prevent clogging and additional care must be taken to prevent spray from being carried outside the containment area by errant winds.
- Raising the water temperature leads to an increase in the brine’s stored energy which supports more rapid conversion to water vapor. This could be accomplished by adding dye to the brine, which enhances the absorption of solar energy. In these cases, the cost of the dye must be compared to the improved rate of evaporation, but if the destination of fully dewatered brine is permanent underground disposal, small amounts of an approved dye may not present a significant problem.
- Oddly enough, the rate of evaporation does not increase as air temperature increases, but rather as vapor pressure difference between the water and air increases. Higher air temperatures actually tend to equalize the vapor pressure with the water and can significantly reduce the speed of evaporation. This may not be a concern in areas with very low humidity but could be a deciding factor in more humid climates.
- Employing fans or other mechanisms to increase wind velocity and air turbulence at the pond’s surface layer may itself create enough of a vapor pressure difference to contribute significantly to increased evaporation, especially in cases where the humidity is already high. In this case, artificial air movement continuously carries away saturated air from the surface of the pond, pulling in dryer air (lower humidity). One factor to note is that very low wind speeds may allow a diffusion barrier or film effect to build up, limiting the vapor differences between air and the surface of the water. Higher wind speeds can reduce or even eliminate the formation of such a barrier.
- In short, regardless of the air temperature, a decrease in humidity equates to lower vapor pressure and the increased differential increases speed of evaporation. The beneficial effects of wind speed, then, are greatly enhanced in areas where the humidity is relatively high but decrease as the overall humidity falls.
- Finally, in setups that involve significant air movement, it’s important to ensure that the increased wind does not cause problems with wave action or splashing and spraying that would carry salts beyond the containment area.
More specific investigation would be helpful to understand the correspondences between evaporative rates and wind speed when working with different saline concentrations. If a set of ratios can be defined based on climatic conditions and variables such as salinity, humidity, air temperature and wind speed, it may prove that evaporative ponds are viable solutions for handling brine waste across a much broader range of environments.
A Stitch (or patch) in Time
Evaporation ponds are fundamentally simple tools. Ponds that are designed and constructed for safety and efficiency can be operated at a relatively basic level without requiring equipment on an ongoing basis. This alone makes ponds an attractive solution.
Ongoing activities should include
- Regular inspections of the primary and secondary containment systems, checking for obvious damage or degradation of the liner materials.
- Leak detection systems must be continuously monitored and periodically inspected to ensure they’re operating properly.
- It’s important to record the volume of brine entering the pond and take regular measurements of water levels in the pond. If the numbers fail to correspond properly, and unusual weather patterns can’t explain the discrepancy, it’s time to start investigating - if too much water is being lost, you may have a potential disaster on your hands, with brine entering the local environment. If not enough water is evaporating, you may be able to take some measurements, pinpoint the problem, and take remediation measures before the annual evaporation cycle is completely muddled.
- On the same principle, salinity of surrounding soil and local groundwater should be monitored for any changes.
Typically, on an annual basis:
- Crystallized salts and minerals should be removed from the pond to maintain usable volume and avoid spillovers.
- It is sometimes recommended, however, that no salt be removed for the first year or so of operation. When a pond dries out completely during summer months, salts remaining at the bottom form a hardpan. Essentially, that’s a rock-hard layer which can’t be removed with regular scraping activities and therefore provides extra protection to the liner.
- Salts should be removed annually during dry months, but before they have had a chance to form additional hardpan, which will permanently reduce the usable volume of your pond.
- Removal of salt may be a relatively simple process of shoveling it out, one pond at a time, or for large ponds it may require heavy equipment.
- In any case, take care with both shovels and heavy equipment to avoid damaging your liner along the sides and any other exposed places.
