On different occasions, there are different types of water, and there are obvious differences in water quality.
1. Groundwater and surface water
Groundwater-less organic matter and microbial pollution, while calcium and magnesium ions dissolve more, with higher hardness and easy scale formation; sometimes iron/manganese/fluoride ions exceed the standard and cannot meet the needs of production and domestic water.
Surface water-more organic matter and microbial pollution than groundwater. If the area is a limestone area, the surface water often has greater hardness, such as Deyang, Mianyang, Guangyuan, Aba and other areas in Sichuan.
2. Hard water and soft water
Hard water-the total concentration of metal ions such as calcium and magnesium in the water is called hardness. Hard water has a great influence on production water such as boilers and should be softened/desalted. Hard water with a hardness greater than 200mg/L is usually called hard water.
Soft water-water with less hardness.
3. Raw water and purified water
Raw water-usually refers to the inflow of water treatment equipment, such as commonly used urban tap water (internationally referred to as drinking water) / suburban groundwater / field surface water, etc. The water quality is often tested by TDS value (total dissolved solids content in water) , The TDS value of Chinese urban tap water is usually 100～400ppm.
Clean water-raw water is called clean water after being treated by water treatment facilities.
4. Purified water and distilled water
Purified water-after the raw water passes through complete sets of water treatment facilities such as reverse osmosis and sterilization devices, most of the inorganic salt ions, microorganisms and organic impurities in the raw water are removed, and the raw water can be directly drunk.
Distilled water-pure water prepared by distillation, usually not used for drinking.
5. Pure water and ultra-pure water
Pure water-deionized water prepared by reverse osmosis, distillation, ion exchange, etc., its TDS value is usually <5PPm, and its conductivity is usually <10μs/cm (resistance value>0.1MΩ.cm).
Ultra-pure water-pure water is further purified and deionized by methods such as ion exchange, distillation, and electric desalination. Its TDS value is unmeasurable, and its conductivity is usually <0.1μs/cm (resistance value>10MΩ.cm). Ions are almost completely removed. Theoretically the purest water resistance value is 18.25 MΩ·cm.
6. Purified water and water for injection
Purified water—Pure water used in the pharmaceutical industry is called purified water, and its conductivity is usually <2μs/cm.
Water for injection-Purified water is re-purified by multi-effect distillation/ultrafiltration to remove pyrogen and used to prepare injections.
Raw water type and water quality analysis. The raw water composition is an important basic data for determining the appropriate water treatment process, selecting the appropriate water quality treatment process, and calculating the water treatment equipment.
1. PH value
The pH value of the raw water reflects the acidity and alkalinity of the raw water. PH=7 is neutral; PH=0~7 is acidic; PH=7~14 is alkaline. The change of PH affects the removal rate of ions, that is, the change of the rejection rate of the system.
2. Electrical conductivity, total dissolved solid content (TDS)
Electrical conductivity is an indicator of the conductivity of dissolved ions in water. Conductivity is a convenient way to measure ion concentration in water, but it cannot accurately reflect the composition of ions. The conductivity increases as the ion concentration increases.
TDS (Total Dissolved Solids) is the inorganic substance remaining after filtering out suspended solids and colloids and evaporating all water. The TDS value can be measured directly with a measuring instrument, or by measuring the conductivity of water and then converting it into a TDS value.
The hardness of water refers to the concentration of calcium and magnesium ions in the water, and the unit of hardness is mg/L in Caco3. For water sources with high hardness and alkalinity, special attention should be paid to prevent Caco3 scaling during the pretreatment process.
Turbidity refers to the degree of obstruction of light transmission by suspended matter in water. Suspended substances in water are generally soil, sand, fine organic and inorganic substances, plankton, microorganisms and colloidal substances.
5. Ionic components
Inorganic salt dissolved in water, the insoluble salt or slightly soluble salt formed after the combination of anion and cation has a certain solubility at a certain temperature. In the RO system, it is continuously concentrated with the incoming water. When the solubility limit is exceeded, they will Scaling on the RO membrane surface. Common insoluble salts are CaCO3, CaSO4, and other compounds that may be produced are CaF2, BaSO4, etc. If the anions and cations in the water can form the above insoluble or slightly soluble salts, the pretreatment must consider scaling control measures to prevent the insoluble or slightly soluble salt from exceeding its solubility and cause precipitation and scaling.
Alkalinity refers to the content of alkaline substances in water that can react with strong acids. Alkalinity mainly characterizes the content of bicarbonate, carbonate and hydroxide ions in water, and is divided into phenolphthalein alkalinity and total alkalinity.
Different types of water sources correspond to different process pretreatments and different types of membrane elements. For small engineering projects that do not have water quality analysis, the design can be made by referring to the pretreatment of the same type of water sources that have been put into operation, but for large-scale engineering projects A comprehensive analysis of water quality must be carried out.