Natural water is usually rich in five impurities:

1. Electrolytes, including charged particles, common cations are H+, Na+, K+, NH4+, Mg2+, Ca2+, Fe3+, Cu2+, Mn2+, Al3+, etc.; The anions are F-, Cl-, NO3-, HCO3-, SO42-, PO43-, H2PO4-, HSiO3- and so on.

2. Organic substances, such as organic acids, pesticides, hydrocarbons, alcohols and esters, etc.;

3. Particulate matter;

4. Microorganisms;

5. Dissolved gases, including N2, O2, Cl2, H2S, CO, CO2, CH4, etc.

The so-called purification of water is to remove these impurities. The more completely the impurities are removed, the purer the water is.

National standard: there are drinking pure water (GB17323), analysis laboratory water (GB6682-92) and electronic water (GB/T11446.1-1997) skill indicators.

Purification of water

1. Distillation, according to the distillation vessels can be divided into glass, quartz distillation, metal materials have copper, stainless steel and platinum distillation, etc.. According to the number of distillation can be divided into primary, secondary and repeated distillation. In addition, in order to remove some of the heavy impurities, also need to adopt some special measures. For example, pre-participate in some potassium permanganate can remove easy oxides; Adding a small amount of phosphoric acid can remove iron trivalent; Adding a small amount of non - volatile acid can be made ammonia free water. Distilled water can satisfy the water needs of the usual analytical laboratory. Because it's very difficult to sweep out the carbon dioxide. So the resistivity of water is very low, not up to the M ω level. Unable to satisfy the demands of many new skills.

2. Ion exchange method, there are mainly two preparation methods:

A. Double bed type, namely connecting and producing deionized water according to the method of Yang bed-yin bed-yang bed-yin bed-mixed bed; Early selection of this method, easy to resin regeneration.

B. Mixing bed type (2-5 series), mixing bed deionization effect is good. But regeneration is inconvenient.

More than 10 M ω of deionized water can be obtained by ion exchange method. But organic matter can not be removed, TOC and COD values are often higher than raw water. This is because the resin is not good, or the pre-disposal of the resin is not complete, the oligomer, monomer, additives, etc. contained in the resin is not removed, or the resin is unstable, constantly releasing differentiated products. All of this will manifest itself in the form of TOC or COD indicators. For example, when the COD value of tap water is 2mg/L, the COD value of deionized water obtained by past ion disposal is usually between 5-10mg/L. Of course, you get good results when you use good resins, otherwise you can't make ultrapure water.

Fouling is the main factor affecting boiler life, so boiler has a high demand for water quality. Low pressure and medium pressure boiler water quality demand is slightly lower and high pressure boiler water quality demand is very high. Most or all impurities such as dissolved oxygen, soluble silicon dioxide, iron and residual chlorine that can cause corrosion and fouling of boilers, water supply systems and other thermal equipment and lead to aeration of soda water and poisoning of ion exchange resins should be removed. In boiler water treatment, the ion exchange water treatment of boiler water supplement is the most basic and important water treatment method, but after the failure of ion exchange resin it is necessary to add alkali or acid for recovery, to produce a lot of acid and alkali wastewater, direct discharge not only pollutes the environment, and the cost is high.

3. Electrodialysis, which was developed in 1950, is often used as the pretreatment process of ion exchange because of its low energy consumption. It is in addition to the direct current electric field effect, the use of anion exchange membrane separation selective anion and Yang through, so that part of the ion through the ion exchange membrane migration to another part of the water, so that part of the water purification, another part of the water concentration. That's how electrodialysis works. Electrodialysis is one of the commonly used desalination techniques. The produced water is pure enough to meet the needs of industry. For example, 1.03M ω ·cm(25°C) of produced water can be obtained from raw water with a resistivity of 1.6K ω ·cm(25°C). In other words, the total hardness of raw water is 77mg/L when the total hardness of produced water is 10mg/L.

4. Reverse osmosis, etc. The resistivity of produced water can be nearly 10 times higher than that of raw water. The removal of impurities by reverse osmosis membrane can be seen in the table: it is currently the most widely used desalination technique. Although reverse osmosis membrane was available in 1977, its large-scale production and wide use in desalination is only in recent years. Reverse osmosis membrane can remove inorganic salts, organic matter (molecular weight >500), bacteria, heat source, virus, suspended turbidity (particle size >0.1μm).

Commonly used reverse osmosis membranes are: cellulose acetate membrane, polyamide membrane and polysulfone membrane.

The pore size of the membrane is 0.0001-0.001μm. The power of reverse osmosis depends on a pressure difference (10-100 atmospheres). The ability to remove impurities is determined by the function of the membrane and the amount of water in and out. The share of water in and out is usually controlled at about 10:6 or 10:7. The removal rate of such impurities should be between 95-99.7%.

For example, when the resistivity of raw water is 1.6 K ω ·cm(25°C), the resistivity of produced water is about 14 K ω ·cm. This water is what we now call pure water, pure drinking water sold in the market. (Credit: Techno)