A new desalination method is able to remove salts from the water with less energy than required for other methods.

According to researchers from the Pennsylvania State University, a new desalination method is able to remove salts from the water with less energy than required by other methods. “From a global point of view, there is a decreasing accessibility to fresh drinking water,” says the main researcher. “More and more water sources used by humanity are becoming damaged, both due to rising salinity and the presence of other pollutants, and the result of this is the need to rely on water sources that are not optimal.”

In order to combat the situation, the researchers developed a new desalination method known as ‘battery electrode deionization, BDI’. This method improves the usual method accepted today for desalination – CDI (capacitive deionization) – by omitting the regeneration step and while reducing the electrical voltage required to complete the process. The conventional method (CDI) desalinated water by separating the ions in the water. A typical CDI cell consists of two electrodes attached to opposite ends of a flow channel. The electrodes trap the salt ions through electrical conversions that occur when an electric current is applied to the cell. In the next step, the cell regenerates by releasing the salt ions in a second cycle by reversing the direction of the applied current. Since this method does not need filter membranes and has lower energy requirements than other methods, it becomes a competitive method for the removal of salt from water. The problem with this method lies in the fact that it is limited by the adsorption of a small amount of salt when using the typical electrical voltage of 1.2 volts.

Increasing the applied voltage does indeed improve salt adsorption, but it also increases the chance of getting unwanted side reactions that waste unnecessary energy, with the possibility of permanent consumption of the electrode. In the new system developed by the researchers, a specially manufactured flow cell is used which consists of two channels. The channels are separated by a membrane and two identical electrodes are located at each end. In order to test the efficiency of the cell, the team of researchers fed the canal with a salt solution at a defined flow rate while applying a constant electric current. The researchers measured several current densities, depending on the number of membranes in each stack. The researchers next reversed the cell voltage flow when the current reached a minimum level of minus 0.6 volts or a maximum level of plus 0.6 volts. The research team found that the new system effectively removed the salt at levels similar to those of the previous method (CDI), using a low voltage of only 0.6 volts. Moreover, the low current applied and the materials used helped prevent unwanted side reactions, while achieving better desalination performance while utilizing a smaller amount of energy.

Since the researchers simultaneously created desalinated water and concentrated water in two separate channels, the system also bypasses the two-cycle approach, so the cell regeneration phase is no longer needed. In addition, the researchers found that adding membranes between the electrodes reduced energy consumption even more. “Many researchers discuss the possibility of capturing energy from the second CDI cycle, but this possibility is very difficult to the point of impractical,” says the researcher. “Our system avoids this second regeneration step simply by replacing the trapping current by changing the direction of the applied electric current. This step is very simple, and it requires the use of little energy.”

Although the current configuration is not suitable for the desalination of particularly saline water, such as seawater, the results show that the new method could be effective as a low-energy method for low-level saline water, such as groundwater, or for the preliminary desalination of water before entering treatment facilities. The researchers now intend to improve the system and make it more suitable for large scale.