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Researchers at the Faculty of Mechanical Engineering present an improved system for water desalination and water recovery for agricultural purposes

Description of the dissolved boron removal process using a CDI cell. First, the cell creates a basic environment, which causes boron to appear in its charged form. The boron ions are then stored in the electrodes. (Credit: Paul Gerlach, Houten, The Netherlands)
Description of the dissolved boron removal process using a CDI cell. First, the cell creates a basic environment, which causes boron to appear in its charged form. The boron ions are then stored in the electrodes. (Credit: Paul Gerlach, Houten, The Netherlands)

Researchers at the Faculty of Mechanical Engineering at the Technion and their colleagues in the Netherlands have developed an innovative method for purifying water from pollutants. They showed that reversing the order of electrodes in the process leads to a significant improvement in water purification. the research which was published in PNAS, the journal of the American Academy of Sciences, was led by Technion researchers Prof. Matti Sass and doctoral students Amit Shukron and Eric Geiss with Dr. Jauka Dykstra from Wageningen University in the Netherlands.

Water purification for agricultural purposes is a complex technological challenge that is particularly important in Israel, where most wastewater is used for agricultural purposes and most agricultural crops in the Negev are based on such water. Irrigation of fields and orchards with high-salt water causes various agricultural damages, including damage to leaves, the formation of clearings in the field, and in severe cases, the destruction of the land and its deactivation as agricultural land.

Due to the growing water shortage, desalination plants have been established in Israel in recent decades, which currently provide us with a significant portion of our drinking water. However, desalination of seawater and purification of water for drinking and agriculture are complex technological challenges, partly because in these processes we seek not only to remove the toxins from the water but to preserve in it the essential substances for our health and the health of the agricultural crops.

One of the main components of the standard desalination process is the membrane - a sophisticated filter that removes some of those toxins. In this process, the water is transported under pressure through the membrane and some particles are blocked according to their size or electrical charges. However, the membrane is an expensive part to purchase, install and maintain, and must be frequently replaced or treated - which necessitates stopping the desalination process. Furthermore, the membrane "misses" small particles that it fails to block.

The research published in PNAS presents a breakthrough in this field. The study focused in Boron - A toxic substance that is important to remove from drinking water. Boron is found in washing powders and other cleaning materials, and when it penetrates into agricultural land it harms it and later also us, the consumers of water and crops. The uniqueness of boron is that it is not eliminated in a sufficient amount in the standard desalination step, which is conducted in an environment of neutral acidity (pH=7). To remove it, an additional desalination round is required, which is performed at high acidity (around pH=10).

The system built by the Technion researchers is based on simple and cheap electrodes and the concept of CDI - treatment without a membrane. This concept is based on a cyclical process of charging and discharging electrodes (anode and cathode); During the charging phase, the ions stick to the electrode and are thus removed from the water, and during the discharging phase they are released into the wastewater stream - the salty stream that is not intended for irrigation and drinking. 

Traditionally, in CDI systems for boron removal, the water is flowed through the cathode to the anode; The system developed by the researchers reverses this order: first the water passes through the anode and only then through the cathode, thus creating a more efficient dynamic of acidity and a more efficient removal of boron. Furthermore, the researchers showed that increasing the electrical voltage does not necessarily improve the efficiency of the system, in other words - there is an optimal voltage, which can be calculated. 

The researchers showed that although the new model was developed as a theoretical concept, it indeed achieves the aforementioned purification in an experimental setup, and they estimate that this concept will also be found effective in commercial facilities that will be built based on it. They emphasize that although the research was done on boron, it is relevant to a wide range of pollutants and similar valuable substances.

Prof. Matthew Sass
 He is a faculty member in the Wolfson Faculty of Mechanical Engineering and Chemical Engineering and a member of the Grand Energy Program.

for the article in the journal PNAS click here

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