Land Application
In certain circumstances, low concentrations of brine can be used in agriculture for irrigation. The plants need to be either naturally or hybridized to tolerate a certain level of salt in the soil and care must be taken that those limits are not exceeded. There is also a potential for contamination of surface and groundwaters, either through soil seepage or unmanaged runoff. The choice and management of irrigation techniques, plus careful consideration of soil characteristics, are critical to ensure that the added salt won’t damage the productivity of agricultural land.
Other surface application options include the reuse of brine for dust control and as a de-icing agent for roads in winter months. These are limited-demand opportunities and clearly won’t solve the issue of increasing volumes of waste brine.
Conversion
A large percentage of brine produced in industrial operations and during desalination processes contains valuable trace elements. Mining for elements like lithium is frequently managed through evaporative technology, for example, but there are other valuable materials to be gathered even from highly concentrated brines. For example, most RO plants use fairly large amounts of caustic soda (sodium hydroxide) to adjust acidity of incoming seawater and maintain efficiency of their filtering membranes. Caustic soda and other valuable chemicals like hydrochloric acid can be produced from waste brine using common, straightforward, chemical processes. If RO plants incorporate steps to recover valuable chemicals like these that are in demand across a wide span of industries, operating and disposal costs could be reduced, and they could even see income from sale of surplus chemicals.
This is an exciting concept because disposal of waste brine is such a concern, with more than 27 billion gallons produced per day worldwide. Any reasonable strategy that allows extraction of additional value from waste brine along with a reduction in the volume to be disposed of is a win. It’s true that these extra processes would increase equipment and power use, but since desalinators tend to be located in climates where freshwater is at a premium and solar energy is abundant, it makes sense to design plants that can take advantage of inherent synchronicities to create a system that has substantially more benefits than the sum of its parts.
Mechanical Wastewater Evaporators
After brine has been minimized, treated, reused and converted, it is still considerably more efficient to dispose of the remaining waste produced by desalination plants, industrial processes, food manufacturing plants and agriculture as solid blocks rather than as liquid. Some plants, in fact, have already set a goal of zero liquid discharge, which is a key marker for ultimate sustainability.
There are already multiple proven options for reducing even heavily concentrated brines to a solid state through evaporation. Mechanical evaporators are widely used, but they have historically tended to be energy-intensive and expensive to operate. Fortunately, development of ever more efficient systems continues as demand for this kind of solution increases. Today’s evaporators can take large volumes of brine maintained in evaporation ponds and render it completely down to a solid product - lighter, smaller, and easier to handle than the original liquid form.
These days, mechanical evaporators can be found in land-based and floating configurations, often featuring highly controllable spray nozzles to increase plume control and reduce risks of environmental contamination through wind dispersal. Recent improvements in design and materials allow mechanical evaporators to operate at much lower energy costs, while some companies are working to incorporate solar energy technology with their own thermal evaporators, with an eye to improving evaporative efficiency even more. In some cases, these new solar arrays may even be able to produce enough additional power to reduce carbon-based energy requirements for other plant operations. This could be another win-win situation for the desalination industry.
Whether powered by renewable fuels or otherwise, some combination of enhanced desalination processes and evaporative treatments that result in an easily disposable solid product should be the benchmark. In that process, any valuable minerals or compounds should be harvested in order to adhere to the highest standard.
