Ultrapure Water System Water Quality Basics—impurities in Water

The essence of an ultrapure water system is to filter out impurities from water to obtain extremely pure water. Therefore, pure water composed solely of H2O molecules does not exist in nature; water typically contains various impurities, including:

1. Particulate matter: Insoluble inorganic and organic impurities larger than 10 μm in water (those larger than 40 μm are visible to the naked eye), such as silt, 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 and manganese (Fe2+, Mn2+) content in the water exceeds the standard (>0.3 mg/L), iron and manganese ions should be removed through a manganese sand filter or aeration device; otherwise, they will form irremovable deposits and blockages on the RO membrane.

3. Colloids: Impurities formed by the aggregation and adhesion of microorganisms, microparticles, etc., with a size between ions and particulate matter. They usually carry a negative charge and easily combine with organic matter.

4. Organic matter: Matter can be divided into two main categories: organic matter and inorganic matter. Hydrocarbons belong to organic matter, while non-hydrocarbons belong to inorganic matter. Organic matter is usually combustible and is a major component of life. Natural organic matter is mainly humic acid produced by aquatic organisms. Dissolved organic matter in water is mainly composed of large, weakly aromatic organic acids with multiple functional groups. These 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 and react to form certain acid radicals, affecting the quality of the finished water. Ultrapure water will rapidly deteriorate in quality if left exposed to air.

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

7. Pyrogens: Pure water is a very low-nutrient 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 water used in biological cell experiments.

8. Disinfectants: After the raw water is treated by the water company, disinfectants such as sodium hypochlorite will remain in the water. The residual chlorine in tap water (the national standard stipulates that it must be greater than 0.3 ppm) is a strong oxidizing substance, which has a considerable destructive effect on the RO membrane. The activated carbon filter and filter media in the ultrapure water machine can effectively remove the residual chlorine in the water.