Analysis of the Structure and Working Principle of Ultrapure Water Equipment

Foreword:

Ultrapure water equipment is specialized water treatment technology that processes municipal tap water and other source water through multi-stage purification processes to produce ultrapure water with resistivity ≥15MΩ·cm (at 25°C) and TOC ≤50ppb. It is widely used in scenarios with extremely high water quality requirements, such as pharmaceutical cleanrooms, electronics and semiconductors, precision instruments, and laboratories. Its core function is to meet the demand for ultrapure water free of ions, particulates, microorganisms, and organic compounds.

Below, we analyze the ultrapure water system from three dimensions: equipment structure, working principle, and operational considerations.

I. Equipment Structure

The ultrapure water system features an integrated modular design comprising four major modules: the pretreatment unit, purification unit, deep refinement unit, and auxiliary control unit. The composition and functions of each module are as follows:

1. Pre-treatment Unit:

Serves as front-end protection, incorporating quartz sand filters, activated carbon filters, softeners (ion exchange resin), and precision filters (5/10μm). For highly turbid feedwater, ultrafiltration units are added. Primarily removes sediment, suspended solids, residual chlorine, organic matter, calcium/magnesium ions, reduces water hardness and turbidity, and prevents contamination or scaling of downstream core components (membrane modules and resin).

2. Purification Unit:

The purification unit is the core water production component of the equipment, predominantly employing RO reverse osmosis systems (single-stage/two-stage) with high-pressure pumps, reverse osmosis membrane modules, membrane housings, and concentrate flow control valves. It removes over 99% of ions, organic matter, microorganisms, and colloids from the water, purifying the raw water into pure water (resistivity 0.1~10 MΩ·cm) and laying the foundation for subsequent advanced refinement.

3. Advanced Purification Unit:

Transforms purified water into ultrapure water through:

- EDI (Electrodeionization) modules

- Polishing mixed-bed resin columns

- TOC (Total Organic Carbon) removers

- UV (Ultraviolet) sterilizers

- Micro/ultrafiltration terminal filters (0.22μm) EDI employs an electric field to deeply remove trace ions, the polishing mixed bed precisely adsorbs ions, the TOC remover decomposes trace organic compounds, the UV sterilizer inactivates microorganisms, and the terminal filter removes minute particulates, ultimately producing compliant ultrapure water.

4. Auxiliary Control Unit:

Includes raw water tank, pure water tank, ultrapure water tank (sealed to prevent secondary contamination), variable frequency booster pump, flow meter, online conductivity/resistivity monitor, TOC analyzer, PLC control cabinet, touchscreen, and piping valves (UPVC/PPR/316L stainless steel); Responsible for water storage, pressure regulation, flow control, and real-time water quality monitoring. Enables automated start/stop functions, fault alarms, and parameter adjustments to ensure stable system operation.

II. Working Principle

The ultrapure water system employs a multi-stage purification process: “pretreatment → reverse osmosis → deep refinement → terminal filtration.” It operates without chemical dosing (except for minimal cleaning, descaling, and reducing agents). The core process progressively removes impurities from the feedwater through physical filtration, membrane separation, electrochemical ion removal, and sterilizing adsorption, ultimately producing ultrapure water. The overall workflow is as follows:

1. Pre-treatment Stage:

Raw water first passes through a quartz sand filter to remove large particles like silt and suspended solids. It then traverses an activated carbon filter to adsorb residual chlorine, odors, and some organic compounds, preventing residual chlorine from oxidizing subsequent reverse osmosis membranes. A softener employs ion exchange resin to adsorb calcium and magnesium ions, reducing water hardness and preventing membrane scaling. Finally, a precision filter removes minute colloidal particles, ensuring the feedwater entering the reverse osmosis system meets turbidity and Silt Density Index (SDI) standards (SDI ≤ 5).

2. Reverse Osmosis Purification Stage:

The pretreated feedwater is pressurized by a high-pressure pump to the reverse osmosis membrane operating pressure (0.8–1.5 MPa). Through the selective permeability of the reverse osmosis membrane, water molecules pass through the membrane layer while impurities such as ions, organic matter, microorganisms, and colloids are retained by the membrane and discharged with the concentrate. Single-stage reverse osmosis purifies raw water into pure water. For poor-quality raw water, a second-stage reverse osmosis can further enhance purity by reducing ion and organic content.

3. Advanced Purification Stage:

The reverse osmosis permeate (pure water) enters the EDI (Electrodeionization) module. EDI combines ion exchange resin with electrochemical technology. Under an electric field, trace cations and anions in the water are adsorbed by the ion exchange resin and then driven by the electric field into the concentrate chamber for discharge. This enables continuous ion removal without regeneration, producing high-resistivity pure water. The water then passes through a polishing mixed-bed resin column for precise adsorption of residual trace ions, further enhancing resistivity. Subsequently, a TOC remover (UV oxidation + adsorption) decomposes trace total organic carbon, while a UV sterilizer (254nm/185nm) inactivates microorganisms and decomposes some organic matter, ensuring microbial-free water with low TOC. 4. Final Output Stage:

The deeply refined water passes through a 0.22μm microfiltration/ultrafiltration terminal filter to remove residual microscopic particles and microbial bodies. It is then delivered via sealed piping to the ultrapure water storage tank. Throughout the process, parameters such as resistivity and TOC are monitored online. Only compliant water is supplied for use, while non-compliant water is returned to the upstream tank for reprocessing.

III. Precautions

The core of ultra-pure water system operation lies in maintaining full control over water quality, pressure, and flow parameters, ensuring equipment protection and regular maintenance. This prevents membrane module or resin failure due to improper operation, or secondary contamination of ultra-pure water.

I. Daily Operation Precautions

1. Pre-Startup Checks:

Verify all module piping and valves are in proper open/closed positions. Confirm adequate water levels in raw water and purified water tanks. Ensure online monitoring instruments display correctly. Never operate pumps dry to prevent damage to high-pressure and booster pumps.

2. System Startup/Shutdown Procedures:

Follow the sequence “water flow first, then pressurize, then power on” when starting. Before starting the reverse osmosis system, flush the membrane modules for 5–10 minutes to expel air and impurities from the membrane housings. When shutting down, follow the sequence “depressurize first, then stop water flow, then power off.” Flush the membrane modules before shutdown to prevent scale formation from residual concentrate on the membrane surface.

3. Real-time Monitoring of Operating Parameters:

Monitor reverse osmosis feed pressure, concentrate pressure, and product water flow rate. Ensure EDI feed resistivity ≥ 0.5 MΩ·cm and feed pressure ≤ 0.3 MPa. Verify ultrapure water resistivity ≥ 15 MΩ·cm and TOC ≤ 50 ppb. Immediately shut down and investigate any parameter deviations. Never operate beyond specified parameters.

4. Storage Precautions for Finished Water:

Ultrapure water tanks must employ sealed designs with nitrogen purging or air filtration to prevent secondary contamination from atmospheric CO₂, dust, or microorganisms. Avoid prolonged stagnation of tank water; circulate and refresh periodically.

5. Piping System Precautions:

Prioritize 316L stainless steel or PVDF piping for ultra-pure water transport. Prohibit ordinary carbon steel or PVC piping to prevent corrosion and ion leaching. Use seamless welding for connections to eliminate dead zones where contaminants accumulate, ensuring unobstructed flow.

II. Regular Maintenance Considerations

1. Pre-treatment Unit:

Backwash quartz sand filters and activated carbon filters every 3–6 months. Replace filter media promptly when filtration efficiency declines. Regenerate softener resin every 1–2 years and replace immediately upon failure. Replace precision filter cartridges every 1–3 months; never exceed the replacement interval.

2. Reverse Osmosis System:

Chemically clean reverse osmosis membrane modules every 3–6 months (using acid/alkali cleaning agents based on contamination levels) to remove scale and organic fouling from membrane surfaces. Replace membrane modules after 3–5 years of normal service life, or when water production drops by 30% or salt rejection falls below 95%. Regularly inspect concentrate flow control valves to ensure proper concentrate discharge and prevent backflow.

3. Advanced Purification Unit:

Regularly clean EDI modules to remove membrane and resin contamination, preventing reduced ion removal efficiency. Replace polishing mixed-bed resin promptly when resistivity continuously declines, as it is a disposable consumable. Replace UV sterilizer lamps every 1–2 years to maintain disinfection and TOC decomposition effectiveness. Replace terminal filter cartridges monthly to ensure final filtration precision.

4. Monitoring Instruments:

Calibrate conductivity/resistivity meters and TOC analyzers every 6–12 months to ensure accurate monitoring data. Regularly inspect flow meters and pressure gauges to prevent parameter display errors caused by blockages or damage.

5. System Cleaning:

Periodically flush all equipment piping. Especially after prolonged shutdowns, flush the entire system for over 30 minutes to expel stagnant water. Resume water supply only after monitoring indicators meet standards.

III. Special Protection Precautions

When raw water quality fluctuates (e.g., sudden increases in turbidity or residual chlorine), immediately halt water intake. Resume operation only after raw water quality stabilizes and undergoes pretreatment before entering downstream systems to prevent impurities from directly entering core units.

During winter operation, implement freeze protection measures for pipelines and equipment. Maintain ambient temperatures above 5°C to prevent pipe freezing/rupture or membrane/resin failure due to low temperatures.

Avoid frequent equipment startups/shutdowns. For short-term shutdowns (hours), maintain low-flow circulation without powering down. For extended shutdowns (over 7 days), protect reverse osmosis membranes and EDI modules by filling with dedicated protective solution to prevent membrane drying and resin contamination.

During equipment maintenance, thoroughly clean and disinfect the work area to prevent introducing contaminants or microorganisms. When replacing consumables, use original manufacturer parts or certified alternatives; never use substandard consumables that could compromise water quality.

Maintain a detailed equipment operation log, recording daily operating parameters, maintenance schedules, and consumable replacements. This facilitates tracing water quality issues and equipment failures while providing data for future maintenance planning.