Water Classification

Water varies in type and quality depending on the context.

1. Groundwater vs. Surface Water

Groundwater – Contains less organic matter and microorganisms, but more dissolved ions, resulting in higher hardness. It easily forms scale during distillation and boiling. Sometimes, iron, manganese, and fluoride ions exceed standards, failing to meet the needs of domestic and industrial water use.

Surface Water – Contains more organic matter and microorganisms than groundwater. In limestone areas, surface water often has higher hardness, such as in Deyang, Mianyang, Guangyuan, and Aba in Sichuan Province.

2. Hard Water vs. Soft Water

Hard Water – The total concentration of metal ions such as calcium and magnesium in water is called hardness. Hard water significantly impacts industrial water use, such as in boilers, and requires softening/desalination treatment. Water with a hardness greater than 200 mg/L is generally considered hard water.

Soft Water – Water with lower hardness.

3. Raw Water vs. Purified Water

Raw Water – This typically refers to the influent to water treatment equipment, such as commonly used municipal tap water, suburban groundwater, or surface water from the wild. Its quality is usually measured by TDS (Total Dissolved Solids) value. In China, the TDS value of municipal tap water is typically 100–400 ppm.

Purified Water – Water that has undergone treatment by water treatment facilities is called purified water.

4. Pure Water vs. Distilled Water

Pure Water – Raw water that has undergone treatment by reverse osmosis and sterilization equipment, removing most inorganic salt ions, microorganisms, and organic impurities. It is pure water that can be consumed directly.

Distilled Water – Pure water prepared by distillation and is generally not used for drinking.

5. Pure Water vs. Ultrapure Water

Pure Water – Deionized water prepared by reverse osmosis, distillation, ion exchange, etc. Its TDS value is typically <5 ppm, and its conductivity is typically <10 μS/cm (resistivity >0.1 MΩ·cm).

Ultrapure water – obtained by further purifying and deionizing pure water through methods such as ion exchange, distillation, and electrostatic desalination. Its TDS value is unmeasurable, and its conductivity is typically <0.1 μS/cm (resistivity >10 MΩ·cm). Ions are almost completely removed. Theoretically, the resistivity of the purest water is 18.25 MΩ·cm.

6. Purified water and water for injection

Purified water – pure water used in the pharmaceutical industry, with a conductivity requirement of <2 μS/cm.

Water for injection – purified water that has been further purified by multi-effect distillation/ultrafiltration to remove pyrogens, making it suitable for preparing injections.