Common Impurities in Ultrapure Water Treatment

In ultrapure water treatment, we often encounter many terms related to impurities. These are not only testing indicators but also key quality attributes of pure water.

Water molecules are composed of H₂O = H⁺ + OH⁻. However, pure water, composed solely of H₂O molecules, does not exist in nature. Water typically contains the following impurities:

1. Particulate Matter

Insoluble inorganic and organic impurities larger than 10μm (those larger than 40μm are visible to the naked eye), such as sediment, metal oxides, or hydroxides commonly found in tap water.

2. Ionic Matter

Substances that dissociate into anions and cations after dissolving in water. The sum of anions and cations in water is called the salinity. If the iron (Fe²⁺, Mn²⁺) content in the water exceeds the standard (>0.3mg/L), iron ions should be removed through a sand filter or aeration device; otherwise, irremovable deposits will form on the RO membrane.

3. Colloids

Colloids are impurities aggregated and bound together by microorganisms, particulate matter, etc. Their size falls between that of ions and particulate matter. They usually carry a negative charge and readily combine with organic matter.

4. Organic Matter

Substances can be divided into two main categories: organic and inorganic. Hydrocarbons belong to organic matter, while non-hydrocarbons belong to inorganic matter. Organic matter is generally combustible and is a major component of life. Natural organic matter is mainly humic acid produced by aquatic organisms. In recent years, industrial wastewater, fertilizers, pesticides, and other synthetic organic matter have caused significant pollution to the aquatic environment. Organic matter dissolved in water is mainly large-molecule, weakly acidic organic molecules of multifunctional aromatic compounds, which are easily adsorbed onto the RO membrane filter layer, leading to a decline in RO membrane performance.

5. Gases

Some gases, such as O2, CO2, NH3, and N2, are soluble in water. After reacting in water, they generate certain acid radicals, affecting the quality of the finished water. If ultrapure water is left exposed to air, its quality deteriorates rapidly.

6. Microorganisms

Bacteria, viruses, spores, etc., with sizes on the micrometer scale.

7. Pyrogens

Pure water is a hypotrophic environment, but a small number of bacteria—Gram-negative bacteria—can still survive. If their cell membranes enter the human bloodstream, they can cause fever, high fever, or even death; hence, they are called pyrogens (also known as endotoxins). Pyrogens must be removed from water for injection and biological cell experiments.

8. Disinfectants

After treatment by the water company, raw water may contain disinfectants such as sodium hypochlorite. Residual chlorine in tap water (national standards stipulate it must be greater than 0.3 ppm) is a strong oxidizing agent and has a significant destructive effect on RO membranes. Activated carbon (KDF) and chlorine removers can effectively remove residual chlorine from water.