What are raw water, softened water, demineralized water, pure water, and ultrapure water?
Natural water or municipal water supply is not always "pure"; it contains various dissolved or suspended substances. Based on different levels of treatment and the degree of impurity removal, water can be classified into several levels: raw water, softened water, demineralized water, Pure Water, and ultrapure water. They are like different stations on the road to "ultimate purity."
1. Raw Water: This is natural source water that has undergone no treatment or only preliminary physical treatment (such as sedimentation and filtration to remove large particles). It is the starting point for all subsequent Water Treatment Processes.
Main sources include surface water (rivers, lakes, reservoirs), groundwater (well water), seawater, and municipal tap water (sometimes also considered "raw water" for further treatment).
Key Characteristics:
Contains abundant dissolved substances: calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), sulfate (SO₄²⁻), bicarbonate (HCO₃⁻), and other cations and anions (collectively referred to as "salts" or "dissolved solids," TDS).
Contains colloids and suspended particles (silt, algae, organic debris).
May contain microorganisms (bacteria, viruses), dissolved gases (oxygen, carbon dioxide), and organic matter (humic acid, etc.).
Exhibits "hardness" (mainly caused by calcium and magnesium ions).
Relatively high conductivity (hundreds to thousands of microsiemens/cm, μS/cm), low resistivity.
Typical treatment processes: coagulation, sedimentation, sand filtration, activated carbon filtration (primary treatment).
Main uses: irrigation, landscaping, some industrial cooling (with less stringent water quality requirements), as raw water for softened water, demineralized water, and other higher-grade water treatments. 1. Untreated raw water is generally unsuitable for direct drinking or high-demand industrial processes.
2. Softened water: Primarily removes calcium (Ca²⁺) and magnesium (Mg²⁺) ions from water, which cause hardness. Its core objective is to prevent scaling (scale buildup).
Treatment process: Mainly uses ion exchange. When water flows through a water softener containing sodium-type cation exchange resin, Ca²⁺ and Mg²⁺ in the water are adsorbed by the resin, while the resin releases an equal amount of Na⁺ into the water. After the resin becomes saturated, it needs to be regenerated with concentrated brine (NaCl solution).
Key characteristics:
Hardness is significantly reduced, even approaching zero; this is the most crucial characteristic.
Total dissolved solids (TDS) show little change, or even a slight increase. This is because only sodium ions replace calcium and magnesium ions; the total salt content in the water is not reduced.
Other ions (such as Cl⁻, SO₄²⁻, HCO₃⁻, etc.) and impurities (colloids, organic matter, microorganisms) are usually still present unless the raw water has undergone pretreatment.
The conductivity is usually slightly higher than that of the raw water (because sodium ions are slightly more conductive than calcium and magnesium ions).
Main uses: Boiler feedwater (prevents scaling), laundry rooms (improves washing performance and prevents clothes from becoming stiff), hotel hot water systems, pretreatment for reverse osmosis systems (protects the membrane from clogging by hard scale), and certain chemical processes (scale prevention required). Softened water is not the same as potable pure water!
3. Demineralized water: Water in which most (usually >90%) of dissolved salts (cations and anions) have been removed, with the goal of significantly reducing the ion content of the water.
Treatment process:
Ion exchange method: Uses cation exchange resin (H⁺ type) and anion exchange resin (OH⁻ type) in series or mixed (mixed bed). Cation exchange resin adsorbs cations, releasing H⁺; anion exchange resin adsorbs anions, releasing OH⁻. H⁺ and OH⁻ combine to form water (H₂O). This is a traditional method for deep desalination.
Reverse osmosis (RO): Utilizes high pressure to force water through a semi-permeable membrane, retaining most dissolved salts, organic matter, colloids, microorganisms, etc. RO permeate typically has very low TDS and is the most commonly used and core technology for obtaining desalinated water.
Electrodeionization (EDI): Fills ion exchange resins in chambers under a DC electric field to achieve continuous regeneration and deep desalination. It is often used as a finishing treatment after RO.
In practical applications, they are often used in combination, such as "RO + mixed bed" or "RO + EDI".
Main characteristics:
Extremely low dissolved salt (ion) content. TDS is typically in the range of 1-50 mg/L.
Very low conductivity (typically in the range of 1-20 μS/cm), high resistivity (0.05-1 MΩ·cm).
Most of the ions that cause hardness have been removed.
It may still contain small amounts of weakly ionized substances (such as silicic acid and carbon dioxide), trace organic matter, colloids, and microorganisms (depending on the integrity of the pretreatment and RO/DI system).
Main uses: High-pressure boiler feedwater, water for chemical raw material and product preparation, pharmaceutical industrial mixing water, general laboratory glassware cleaning, preliminary cleaning in the electronics industry, battery electrolyte preparation, etc.
4. Pure water: This is a broader concept than "demineralized water" but often includes desalination requirements. It refers to water that has undergone multiple advanced treatment processes to remove most impurities (not just ions) and achieve a specific purity standard. Pure water usually means lower levels of microorganisms and organic matter than demineralized water.
Treatment processes: Based on demineralized water processes (such as RO, EDI), the following are usually added:
Ultraviolet (UV) sterilization: Kills microorganisms in the water.
Ultrafiltration (UF): Removes smaller particles, colloids, and microorganisms.
Activated carbon filtration: Deeply removes organic matter, residual chlorine, and odors. Sometimes it also includes a degassing step to remove dissolved gases such as carbon dioxide.
Key Features:
Inherits the low ion content (low conductivity) of demineralized water.
Microbial limits and total organic matter (TOC) are more strictly controlled.
Lower particulate matter content.
More stable water quality, meeting stricter industry standards (such as pharmacopoeia purified water standards).
Main Uses: Pharmaceutical industry (excluding injectables), cosmetics production, food and beverage industry, high-end laboratory analysis (HPLC mobile phase, etc.), medical dialysis water, general cleaning in the microelectronics industry, etc. Pure water can generally be considered a higher standard of demineralized water, emphasizing the control of microorganisms and organic matter.
5. Ultrapure Water: This is the pinnacle of water purity. It removes almost all impurities from water, including ions, organic matter, particulate matter, microorganisms, dissolved gases (even silicon), etc., reaching the limit of theoretically pure water. It is the top-grade water for electronic or analytical purposes.
Treatment Process: Based on pure water (or deeply demineralized water), extremely stringent final purification is performed:
Multi-stage mixed bed/polishing mixed bed: Removes residual trace ions.
Ultrafiltration (UF)/membrane filtration: Removes nanoscale particles, microorganisms, and pyrogens.
UV oxidation (UV 185nm+254nm): Thoroughly kills microorganisms and oxidizes and degrades trace organic matter (reducing TOC).
Degassing membrane/vacuum degassing: Removes dissolved oxygen, carbon dioxide, and other gases.
Final precision filter: 0.04 or 0.1 micron filtration ensures particulate matter meets standards.
The entire system uses high-purity inert materials (PVDF, PVP, etc.) to avoid secondary contamination.
Key Features:
Extremely low conductivity: Approaching the theoretical limit of pure water (18.2 MΩ·cm @25°C), resistivity >18 MΩ·cm.
TDS approaches zero.
Particulate matter count (>0.1 μm) is strictly controlled to single digits/mL.
TOC content is extremely low: typically at the ppb (μg/L) level or even lower.
Microbial/bacterial endotoxin content is extremely low or undetectable.
Dissolved gas (e.g., O₂, CO₂) content is extremely low.
The water quality is extremely "fragile," rapidly absorbing CO₂ and particulate matter upon exposure to air, leading to a decrease in purity; therefore, it must be used immediately after production.
Main applications: High-tech fields with extremely stringent water quality requirements, such as integrated circuit (chip) manufacturing, semiconductor silicon wafer cleaning, photolithography processes, liquid crystal display panel manufacturing, ultrapure material preparation, trace element analysis (ICP-MS), and cutting-edge life science research (cell culture, molecular biology).