- What is desalination?
- Why should we desalinate water?
- What is the difference between brackish water and seawater desalination?
- Is desalination affordable?
- What are the environmental impacts of desalination?
- What happens to the salt that is removed from the water?
Desalination is the process of removing dissolved salts from water. The two most common desalination technologies are membrane and thermal technologies. Membrane processes rely on permeable membranes to separate salts from water. Membrane processes can be pressure-driven (reverse osmosis or RO) or voltage-driven (electrodialysis). Reverse osmosis is currently the most common desalination treatment method. The thermal process involves heating saline water to produce water vapor, which is then condensed and collected as fresh water.
An increasing number of communities are facing water supply challenges due to increasing demand, drought, depletion and contamination of groundwater, and dependence on a single source of supply. About 97.2% of the water on earth is saline and can only be used through desalination technology. Communities with access to saline waters may need to consider desalination as an additional source of water as part of an overall plan to manage resources. Diversification and local control are essential to good planning and self-sustainability.
Brackish water refers to water supplies that are more saline than freshwater, but much less salty than seawater. Brackish water may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish aquifers. The level of salinity in water is measured in total dissolved solids (TDS). In a reverse-osmosis system, the greater the TDS concentration of the water, the higher the pressure needed for the pumps to push water through the membranes, and consequently, the higher the energy costs.
The affordability of desalination depends on the quality of the source water and a host of other factors. Brackish water desalination usually costs less than seawater desalination because brackish water is generally cleaner and contains fewer total dissolved salts. Advances in desalination technologies in recent years have reduced costs considerably. Communities decide on the affordability of desalination projects in the context of local needs when integrating a plan that combines water conservation with traditional and alternative sources of supply.
As with any major construction project, an Environmental Impact Statement (EIS) is required by regulators for a desalination plant prior to construction. EIS studies are conducted to identify and investigate possible impacts on soil, air, and water resources and to recommend steps to avoid or reduce these impacts. Because the ocean is a habitat, EIS studies look carefully at how marine life is impacted during desalination processes. The WateReuse Foundation conducts and supports research that examines both the potential environmental impacts of desalination and potential solutions.
It needs to be disposed of safely. Concentrate management can become one of the most important factors in determining the feasibility of a plant. Concentrate produced during seawater desalination can be disposed through deep well injection on land or returned to the ocean in a controlled process to avoid detrimental effect to the environment or marine life. For concentrates produced in inland facilities (brackish water), a few other options exist. These include disposal to surface water bodies, evaporation ponds, or to wastewater treatment plants. In some instances, the concentrate can also be utilized beneficially for industrial processes.