IEEE Future Directions
Related Articles
Some 300 million people drink thanks to desalination plants. Many of these plants, as you would expect, treat sea water, removing salt to make it drinkable, but quite a few operates inland purifying brackish water.
Desalination is not new, it goes back several decades. Originally it was based on thermal processes (you heat the water and condense the vapour into pure H2O leaving all salt behind), now close to 70% of desalinated water is produced through a process based on reverse osmoses.
Getting water salty is easy and cheap. Removing the salt is complex and expensive. More than that. For every litre of purified water you produce from 1 litre to 1.5 litre of brine, a waste water dense with salt and other minerals that pollute the environment.
If you use a thermal process you need to heat the water (plenty of it!) and that requires a lot of energy. Middle East Countries have plenty of energy available -oil- and were the ones that needed most desalination plants so they started long time ago, using thermal processes (Saudi Arabia, UAE, Kuwait, and Qatar are by far the larger user of thermal desalination, resulting in 55% of brine production).
Reverse osmoses is better, uses fewer energy and produces less brine. Osmoses is the process through which a fluid separated by a membrane (like the cell membranes in our body) tends towards evening out the “density” (physicists, excuse my simplification). If you want to increase the difference in density, have salty water on one side and sweet water on the other, you need to provide energy, in this case increase the pressure on one side of the membrane. This process is less power hungry than the one used in thermal desalination but still requires quite a bit of energy.
The amount of energy can be reduced by using better filter, membrane. This is what has been done by researchers at the University of Connecticut who have invented a new process based on additive nano-manufacturing resulting in a membrane that is just 15 nm think (or, rather, thin!) and very vey smooth (thickness is controlled within 2nm). The thickness can be controlled in increments of 4nm (depending on the type of filtering needed). Today’s membrane on the contrary have a thickness between 100-200nm and “bumpiness” (roughness) in the order of 80nm. The smoothness of the surface and the thickness of the membrane reduce the energy required for the reverse osmoses.
Other researchers are exploring the feasibility of using graphene to create the membrane (see the clip). This material is extremely strong and thin (just one atom thick), hence it promises greater efficiency and lower cost.
An extensive report on emerging desalination technologies can be found here.
As desalination will increase (in 2018 every day 300 million cubic metre of sweet water were produced through desalination by the 16,000+ desalination facilities all around the world) so will the production of brine and the problem of its management will become a major issue. We are already seeing in the Gulf substantial damage to the sea ecosystem and actions is needed. New technologies for managing the desalination waste are a must and more research is needed.
