What is EDI in a Water Treatment Plant? – Electro deionization

In this article, we will discuss the EDI Electro deionization technology used in a water treatment plant. Water treatment plants are present in all industries as well as in major cities also to treat the water. The treated water is used for drinking as well as other purposes like washing and cleaning in industries as well as for domestic use.

The main function of a water treatment plant in an industry is to convert bad or contaminated water or raw water into a suitable form that can be used by humans. Here, the suitable form means, water with proper pH, conductivity, turbidity or TDS, and hardness. Various sensors are used for measuring these chemical parameters.

The operation of the plant is decided on the basis of these chemical parameters measured by the mentioned sensors. Many processes are used to convert bad or raw water to a suitable form. EDI is one of the processes used to treat bad or raw water. EDI stands for Electro Deionization. Using the EDI process, the conductivity of the water is lowered.

Why is EDI required?

Water has many chemical properties. One of the important chemical properties of water is conductivity. The conductivity of the water is simply the ability of the water to pass electricity due to the presence of the ions present in the water. Low conductivity of water means there are a smaller number of ions present in the water. This means that water will pass less electricity which also means that the water is pure.

On the other hand, when the conductivity of the water is high, then the water contains more ions. This also means that the water can pass more electricity and the water is impure. This type of water can contain dissolved salts, chlorides, sulfides, etc which increases the conductivity of the water. To reduce the conductivity of the water, EDI is used as a process. EDI is used after the RO process for treating the water to obtain the desired levels of conductivity.

What is EDI (Electro Deionization Technology)?

From the name itself, it is clear that Electro Deionization means the process of removing the ions. The ions are removed from the water in this process. The Electro Deionization process uses electricity, ion exchange membranes, and resins to remove the ions or any other charged species from water which can be either salts or organic acids.

The EDI process is the best alternative to a conventional mixed bed polishing unit. The advantage of the EDI is that this is a continuous process with no downtime required, due to the electrochemical regeneration.

Working of EDI (Electro Deionization)

EDI is simply a chamber of membranes.

EDI in a Water Treatment Plant

One special feature of the membranes used for EDI is that only the ions present in the water can pass through it. The water flows from the feed line to either the reject line or the product line through the resins between membranes. A DC electrical field is through an external power source using electrodes present at the end of membranes.

Now let us assume that the external power is applied using the electrodes. Now, as the water passes through this membrane. The water contains both positive ions and negative ions. The positive ions will start getting attracted to Cathode and negative ions will start getting attracted toward Anode.

The membranes used are cationic exchange and anionic exchange types. Cationic exchange membranes are the orange ones shown in the figure which will allow only positive ions to flow through them. Anionic exchange membranes are the light blue ones shown in the figure which will allow only negative ions to flow through them. So, when we look into the figure, it becomes clear that from line number 2 positive ions will go towards line 1 and negative ions will go towards line 3.

This flow of ions is due to the supplied external power as discussed before. Hence, line 2 will give pure water product without any ions. Similarly, from line 4 we will get pure water without any ions in it. This is because the negative ions will go to line 5 and positive ions will go to line 3. Line numbers 1, 3, and 5 will have water with ions that are rejected.

How is EDI Technology used in Industrial Automation Control?

To control the process through Electro Deionization Technology (EDI), PLCs are used in the water treatment plant. The flow of the inlet water is controlled using the ON-OFF valves or the control valves. Pumps are used to inject the water into the water treatment plant. So, the functioning of the pumps is also controlled using the PLC logic.

The external power to the membranes is also controlled using the PLC logic. The analog output from the PLC will apply a variable voltage to the EDI rectifier unit installed in the control panel. The interlock to turn ON the EDI voltage with the pump in running condition or a minimum flow is always present to avoid dry EDI.

At the outlet of the water treatment plant, conductivity analyzers are installed to measure the quality of the water. If the conductivity measurement is high, then the product is again diverted for treatment.

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