Deionized water (DI water) is high-purity water obtained by removing ions (such as sodium ions, calcium ions, chloride ions, etc.) from water. Its conductivity is usually ≤ 10 μ S/cm (resistivity ≥ 0.1M Ω· cm), and it is widely used in fields such as electronics, chemical engineering, and laboratories. The following provides a detailed explanation from two aspects: working principle and core structure:
1、 Working principle of deionized water equipment
The core of deionized water equipment is to remove anions and cations from water through ion exchange technology or electrodialysis/EDI technology, and the specific principle varies depending on the type of technology:
（1） Ion exchange method (principle of traditional deionized water equipment)
Ion exchange method is the use of ion exchange resin to selectively adsorb and exchange anions and cations in water, achieving desalination.
The function of cation exchange resin
Resin is of the “sodium type” or “hydrogen type” (commonly used in industry), and its active groups (such as – SO3H) can release H ⁺, which can exchange with cations in water (such as Na ⁺, Ca ² ⁺, Mg ² ⁺):
▶ Reaction equation: R-SO ∝ H+Na ⁺ → R-SO ∝ Na+H ⁺
After exchange, cations in the water are adsorbed by the resin, which releases H ⁺ into the water.
The function of anion exchange resin
Resin is of the “chlorine type” or “hydrogen oxygen type” (commonly known as hydrogen oxygen type), and its active groups (such as – N (CH3) ∝ OH) can release OH ⁻, which exchanges with anions in water (such as Cl ⁻, SO ₄² ⁻, HCO ∝⁻):
▶ Reaction equation: R-N (CH ∝) ∝ OH+Cl ⁻ → R-N (CH ∝) ∝ Cl+OH ⁻
After exchange, anions in the water are adsorbed by the resin, which releases OH ⁻ into the water.
The synergistic effect of mixed bed resin
Cationic resin and anionic resin are mixed and packed in a 1:2 ratio (mixed bed), where H ⁺ and OH ⁻ combine to form water (H ⁺+OH ⁻ → H ₂ O), ultimately removing all ions from the water and resulting in higher water purity (conductivity ≤ 0.1 μ S/cm).
Principle of resin regeneration
After the resin is saturated with adsorption, it needs to be regenerated with chemical agents:
The cationic resin is regenerated with 5-8% hydrochloric acid (HCl) and the adsorbed cations are replaced with H ⁺;
The anion resin is regenerated with 4-6% sodium hydroxide (NaOH) and the adsorbed anions are replaced with OH ⁻;
The mixed bed resin needs to be separated first (by backwashing and layering), and then regenerated separately.
（2） Electrodialysis/EDI method (principle of continuous deionization technology)
Electrodialysis (ED) and EDI (Continuous Electro Desalination) are physical desalination technologies that drive ion migration through an electric field, without the need for chemical regeneration.
Principle of Electrodialysis (ED)
Alternating cation exchange membranes (allowing cations to pass through) and anion exchange membranes (allowing anions to pass through) between positive and negative electrodes, forming a “fresh water chamber” and a “concentrated water chamber”;
After being powered on, cations in the water migrate towards the cathode (only through the cation membrane), while anions migrate towards the anode (only through the anion membrane);
The ions in the freshwater chamber migrate to the concentrated water chamber, and ultimately the freshwater chamber produces ionized water, while the concentrated water chamber discharges high salt wastewater.
Principle of EDI (Continuous Electro Desalination)
Integrating electrodialysis and ion exchange technology: filling a small amount of ion exchange resin (assisting ion conduction) in the freshwater chamber, and driving ion migration through an electric field;
Resin continuously self regenerates under the action of an electric field (without the need for acidity or alkalinity): H ⁺ and OH ⁻ are generated by the electrolysis of water, continuously regenerating the resin and achieving continuous desalination;
The purity of the produced water is higher (conductivity ≤ 0.06 μ S/cm, resistivity ≥ 16M Ω· cm), and there is no chemical pollution.
2、 Core structure of deionized water equipment
According to the type of technology, the construction of deionized water equipment can be divided into two categories: ion exchange equipment and EDI equipment. The specific components are as follows:
（1） Construction of ion exchange deionized water equipment
1. Pre treatment system (protecting ion exchange resin)
Multi medium filter: stainless steel or fiberglass tank body filled with quartz sand and anthracite, removing suspended solids and colloids (turbidity ≤ 1NTU) in water to avoid clogging resin pores.
Activated carbon filter: filled with coconut shell activated carbon to adsorb organic matter and residual chlorine (which oxidizes the resin), reducing COD (chemical oxygen demand).
Security filter: 5 μ m PP filter element, intercepting residual fine particles and protecting subsequent resin.
2. Ion exchange unit (core desalination component)
Cation exchange column
Construction: 304 stainless steel tank body (diameter 300-1000mm), filled with 001 × 7 strong acidic cation exchange resin (hydrogen type).
Function: Remove all cations from water, and the effluent is acidic (pH ≈ 3-4).
Anion exchange column
Construction: Same material tank body, filled with 201 × 7 strong alkaline anion exchange resin (hydrogen oxygen type).
Function: Remove all anions from water, combine with H ⁺ in the effluent of the cation column to generate water, and the effluent pH is approximately 7-8.
Mixed ion exchange column (mixed bed)
Structure: stainless steel tank, containing 1:2 mixed cation and anion resin (usually gel resin).
Function: Deeply remove residual ions, with a water production resistivity of ≥ 10M Ω· cm (conductivity ≤ 0.1 μ S/cm), it is a terminal precision processing unit.
3. Regeneration system (essential for ion exchange columns)
Regenerative agent storage tank: made of PE material, storing hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions (concentration 5-8%) separately.
Regeneration pump: a corrosion-resistant metering pump that pumps chemicals into the cation and anion exchange columns in proportion.
Self control valve group: solenoid valve or pneumatic valve, controls the regeneration process (backwash → feed → displacement → forward wash), and achieves automatic regeneration.
4. Auxiliary system
Pure water tank: made of PE or stainless steel material, storing deionized water, equipped with a liquid level sensor (linkage device start stop).
Pipes and valves: UPVC or 304 stainless steel pipes (to avoid ion leaching), manual/automatic valves control the direction of water flow.
（2） Construction of EDI deionized water equipment
1. Preprocessing system (stricter than ion exchange method)
Reverse osmosis (RO) system: As a pre-treatment for EDI, the conductivity of the produced water is ≤ 50 μ S/cm (removing more than 99% of ions), providing qualified inlet water for EDI (otherwise it will cause scaling in the EDI concentrated water chamber).
Precision filter: 1 μ m PP filter element, intercepts trace particles in RO produced water, and protects the EDI membrane stack.
2. EDI membrane stack (core component)
Construction: Composed of alternating cation exchange membranes, anion exchange membranes, fresh water chamber (filled with ion exchange resin), concentrated water chamber, polar water chamber, and positive and negative electrodes.
Cation exchange membrane (CEM): allows cations such as H ⁺ and Na ⁺ to pass through, while blocking anions;
Anion exchange membrane (AEM): allows anions such as OH ⁻ and Cl ⁻ to pass through, while blocking cations;
Resin: A small amount of 001 × 7 cation resin and 201 × 7 anion resin to enhance ion conductivity efficiency;
Electrode: Made of titanium coated ruthenium material (corrosion-resistant), providing a direct current electric field (voltage 50-300V).
3. Auxiliary system
High pressure pump: Provides 0.2-0.4MPa pressure for EDI inlet to ensure uniform water flow through the membrane stack.
Concentrated water circulation system: Partial concentrated water reflux (increasing concentrated water concentration and enhancing conductivity), reducing energy consumption.
PLC control system: Real time monitoring of water conductivity, pressure, and flow rate, automatic adjustment of voltage and concentrated water discharge, and fault alarm (such as membrane stack blockage and electrode failure).
3、 Comparison and Applicable Scenarios of Two Technologies
Technical advantages, disadvantages, and applicable scenarios
The initial cost of ion exchange method is low, the purity of the produced water is high (mixed bed), acid-base regeneration is required (polluting the environment), resin needs to be replaced regularly, and the operating cost is high for small-scale water use (such as laboratory)
EDI method does not require chemical regeneration (environmentally friendly), continuous water production, low operating costs, high initial investment, strict requirements for inlet water quality (requiring RO water production), and large-scale water use (such as electronic factories and pharmaceuticals)
summarize
Deionized water equipment removes ions from water through ion exchange or EDI technology, with the core being the use of resin exchange or electric field driving to achieve desalination. The ion exchange method is suitable for small-scale and low-cost needs, while the EDI method has become mainstream due to its advantages of environmental protection and continuous operation. Its structure includes pre-treatment (protecting core components), desalination unit (core deionization), and auxiliary system (ensuring operation). The specific selection needs to be determined according to the water production, purity requirements, and environmental standards.
Post time: Aug-04-2025