Desalination, the process of removing salt from water, is a practical way to provide drinking and irrigation water to communities who have lost their primary water source.  More commonly, it acts as a backup in case the inevitable should occur and is slowly being implemented into more communities, particularly along coastlines.  

A team of researchers from the University of Bath has devised a low cost and low maintenance method to desalinate water in potentially bulk quantities.  The method consists of three primary components: ionic rectifiers (positive and negatively charged filters), ionic diodes (positive and negatively charged sources of electricity), and 3-D printed water basins. 

This new technique can not only separate salt from water, but also compile the salt to be collected.

The ionic diodes work with the ionic resistors to form am electrically driven circuit.  The researchers flip a switch to shift the charge of the voltage in the saltwater from positive to negative and back again.  This blocks and opens varying chambers, resulting in a build-up of salt in outside chambers, while the desalinated water remains inside the circuit.  In short, one chamber holds the salt-free water while another collects the extracted salt.

Model and picture of the desalination machine.

The concentration of salt in the inside of the circuit, where the desalinated water is contained, is closely measured along with the intensity of electricity.  As the amount of salt in the center decreased, so did the ionic currents. If everything goes smoothly, an ideal rectification ratio of 4-5 would ensure optimization of energy/electricity use in the conversion process.

Unfortunately for the research team, this was not the case.  Instead, they recorded an abysmal ratio of two and lower! The thin film used to strain the saltwater and help conduct the positive and negative charges was determined to be the main cause of energy loss.

The film, named Fumasep FAS-130, was the thinnest material available to the team that would still (technically) work, but it rapidly degrades when attached to an activated circuit.  When all was said and done, their brilliant idea to separate salt from the water and to collect the salt with little hands-on work, resulted in a flimsy 12% efficiency rate.

water and salt

Pink salt in water. Picture courtesy of Creative Commons.

Fortunately, the team is confident their low efficiency is an easy fix.  First and foremost, they’ll need a better film, preferably one that doesn’t degrade immediately upon being placed in saltwater.  Altering the size and location of the holes in the film used to filter the water could also yield improvements. In addition, stronger electrodes (therefore stronger electrical voltage) and larger vats of saltwater could result in creating drinking water in bulk through this process.

So yes, this technique certainly has its issues, but it offers insight into a new method of desalination with incredible potential.  It doesn’t have any moving parts, only electricity from the diodes, so it needs little to no maintenance, it’s cheap compared to other methods, and (when the efficiency issue is sorted out) will be low energy as well.  This simple process could one day save countless communities struggling from lack of water after natural disasters, all by removing salt from water.

https://www.sciencedirect.com/science/article/pii/S0011916419321940?via%3Dihub

Putra, B. R.; Madrid, E.; Tshwenya, L.; Arotiba, O. A.; Marken, F.;  An AC-driven desalination/salination system based on Nafion cationic rectifier.  Science Direct 2020.  DOI: https://doi.org/10.1016/j.desal.2020.114351