Desalination, or “desalting” is fast becoming a vital part of water management in many parts of the world. Although our Earth is 70% water, 97% of that water is the saltwater of our seas and oceans. In some places it is the only foreseeable resource to supply the water needed for growing populations.
What is membrane desalination, and how is it being used to provide fresh water? How will it impact the future? This guide will help answer these questions.
BWA Water Additives said, “Water quickly separates added salt into its respective sodium and chloride ions.” Humans have been trying to transform seawater into potable water since ancient times. The usual method was inefficient distillation or evaporation solutions which were inefficient and limited, but the only solutions known, at least up until modern times.
During World War II efforts were made to find a means of providing fresh water to troops in the field. Using an osmotic (diffusion of water through a membrane) process to separate salt from water did not become a familiar technology until the mid-1950s. Prior to that there were simply no membranes of filters fine enough to effectively remove sodium and chloride ions from water.
It was not until the 1970s that desalination plants using reverse osmosis (RO) and electrodialysis (ED) appeared. Today, there are thousands of desalination plants operating throughout the world, and the majority employ RO. Membrane desalination technologies today account for hundreds of thousands of gallons of fresh water each day. These processes include RO, ED-reversal (EDR), and nanofiltration (NF).
RO works by forcing water through a semi-permeable membrane at greater pressure and opposite direction than the flow of the saltwater itself, with the result that the salt is left behind. In RO, the saline feed water is first treated by removing other solids and adjusting the pH (acidity) of the water to prevent fouling. It is then pumped under pressure into a membrane system which separates out the salts and also streams off the dissolved salts and desalinated water. The desalinated water is then treated to remove any formed gases, the pH readjusted, and the final product stabilized before it is released into outgoing channels. Several generations of technology have resulted in osmotic membranes which are much more efficient that the original versions.
Electrodialysis and EDR involve separation by means of an electrically charged membrane which helps separate the sale through ionization and trapping of the ionized salt molecules.
NF is an extremely fine membrane which allows the passage only of solids less than one nanometer in size (a sheet of paper is about 100,00 nanometers thick). It’s chiefly used on water softening to remove hard mineral ions that can contaminate and discolor groundwater. As pollutants accumulate in the ground and atmosphere, even rain is more likely to spread contaminants than to remove them. Nanofiltration is also heavily used to prepare injection water for flooding reservoirs in offshore oil recovery.
One of the chief concerns in the desalinization industry has been conservation of the energy used in the process. Increasing use of renewable energy sources like wind and solar power have been adopted to help power these desalination plants, and new designs combine membranes with other energy-efficient solutions such as low-heat distillation. Another promising advance is a technique called forward osmosis, which uses a “draw” to pump the water rather than hydrostatic pressure. One of the obstacles is finding the funds to develop techniques and technologies from the research-and-development stage to the industrial-scale plants needed to cope with freshwater demand.
Desalination in the Future
The outlook for desalinization methods remain promising. While most of the desalinization plants are currently used to provide fresh water to municipalities, the same membrane process can be used to treat wastewater, or prepare raw groundwater for use in more industrial and agricultural processes. In a future of exploding populations and global warming, the need for freshwater is only likely to increase. But as membrane desalination progresses, we’ll find more plants turning seawater into potable water, and doing it more efficiently.