Basic knowledge of water treatment

1. What is the hardness of water?

Some metal cations in water are combined with some anions. During the heating process of water, due to evaporation and concentration, scale is easy to form, which is attached to the heating surface and affects the heat conduction. We call the total concentration of these metal ions in water as the hardness of water. For example, the common metal ions in natural water are calcium ion (Ca +) and magnesium ion (Mg +), which are combined with anions in water such as carbonate ion (CO3 -), bicarbonate ion (HCO3 -), sulfate ion (SO4 -), chloride ion (Cl -) and nitrate ion (NO3 -) to form the hardness of carbonate, bicarbonate, sulfate, chloride and nitrate of calcium and magnesium. Iron, manganese, zinc and other metal ions in water will also form hardness, but they can be omitted due to their small content in natural water. Therefore, the total concentration of Ca + and Mg + is usually regarded as the hardness of water. The hardness of water has a great impact on boiler water. Therefore, the water should be softened or demineralized according to the requirements of boiler water quality with different parameters.

2. What are the hardness of water ?

The hardness of water is divided into carbonate hardness and non carbonate hardness.

3. Carbonate hardness

It is mainly the hardness formed by bicarbonate of calcium and magnesium, and a small amount of carbonate hardness. Bicarbonate hardness is decomposed into sediment after heating and removed from water, so it is also called temporary hardness.

4. Non carbonate hardness

It is mainly the hardness formed by sulfate, chloride and nitrate of calcium and magnesium. This kind of hardness cannot be removed by heating decomposition, so it is also called permanent hardness, such as CaSO4, MgSO4, CaCl2, MgCl2, Ca (NO3) 2, Mg (NO3) 2, etc.

The sum of carbonate hardness and non carbonate hardness is called total hardness. The content of Ca + in water is called calcium hardness, and the content of Mg + in water is called magnesium hardness. When the total hardness of water is less than the total alkalinity, the difference between them is called negative hardness.

5. What is colloidal substance in water?

Colloidal substances in water refer to particles with a diameter of 0.1-0.001 microns. Colloids are collections of many molecules and ions. The inorganic mineral colloid in natural water is mainly iron A compound of aluminum and silicon. The organic colloidal substances in natural water are mainly humic substances produced by the decay and decomposition of the limbs of plants and animals. Among them, the humus content in lake water is the most, so it often makes the water yellow green and brown. Due to the small particles, light weight and large surface area per unit volume of colloidal material, its surface has great adsorption capacity, often adsorbing a large amount of ions and electricity. Similar colloids repel each other because they have the same charge. They can’t adhere to each other in water and are in a stable state. Therefore, colloidal particles cannot be removed by gravity. Generally, agents are added to water to destroy its stability, so that colloidal particles increase and settle for removal.

6. What is the total solids of water Dissolved and suspended solids?

All impurities in water except dissolved gases are called solids, and the solids in water are dissolved solids and suspended solids. The sum of the two is called the total solid of water.

Dissolved solids refer to various inorganic salts that are still dissolved in water after filtration organic compound. Suspended substances such as microorganisms.

The determination of total solids is obtained by steaming dry water and then weighing. Generally, the steaming dry temperature is controlled at 105-110 degrees.

7. What is the salt content of water?

The salt content of water (also known as salinity) is the amount of salt in water. Since various salts in water generally exist in the form of ions, the salt content can also be expressed as the sum of various cations and anions in water.

The salt content of water is different from that of dissolved solids, because dissolved solids include not only dissolved salts in water, but also organic substances. At the same time, the meaning of salt content of water is different from that of total solids, because total solids include not only dissolved solids, but also suspended solids that do not dissolve water, so the quantity of dissolved solids and total solids is higher than that of salt content.

8. What is the turbidity of water?

Because the water contains suspended and colloidal particles, the original colorless and transparent water produces turbidity. The degree of turbidity is called turbidity. Turbidity is the expression of different sizes of water Different relative densities Suspended solids of different shapes Colloidal substance The effects of plankton and microorganisms on light. It does not directly indicate the content of impurities, but is related to the amount of impurities.

Turbidity is an optical effect. It is the degree to which light is blocked when passing through the water layer. It indicates the ability of the water layer to scatter and absorb light.

Controlling turbidity is not only an important content of industrial water treatment, but also an important water quality index. According to different uses of water, the requirements for turbidity are different. The turbidity of domestic drinking water shall not exceed 1 NTU; The turbidity of circulating cooling water make-up water is less than 2-5 NTU, and the reverse osmosis demineralized water inlet is required to be less than 0.5 NTU

9. What is the chromaticity of water?

The chromaticity of water is an index for quantitative color test of natural water or treated water. Natural water often shows light yellow Different colors such as light brown or yellow green. The reason for the color is due to the humus dissolved in water Caused by organic or inorganic substances. In addition, when the water body is polluted by industrial wastewater, it will also show different colors. These colors are divided into true colors and surface colors. True color is caused by dissolved substances in water, that is, the color after removing suspended substances in water. The surface color is the color when the suspended solids in the water are not removed. The quantitative degree of these colors is chromaticity.

Industrial wastewater may make the water body produce various colors, but the humus in the water The presence of suspended sediment and insoluble minerals will also make the water color. For example, clay can make the water yellow, iron oxide can make the water brown, sulfide can make the water yellow, algae can make the water green, rotten organic matter can make the water black brown and so on.

10. What is the resistivity of water?

When measuring the conductivity of water, it is related to the resistance value of water. If the resistance value is large, the conductivity is poor, and if the resistance value is small, the conductivity is good.

The resistivity of water is related to the amount of salt in water Ion content in water The charge number of ions is related to the movement speed of ions. Therefore, the pure hydraulic resistivity is large, and the ultra pure hydraulic resistivity is even greater. The purer the water, the greater the resistivity.

16. What is the conductivity of water?

The conductivity of water is commonly called conductivity; Conductivity reflects the amount of salt in water and is an important indicator of the purity of water. The purer the water, the less salt content, the greater the resistance and the smaller the conductivity. Ultrapure water can hardly conduct electricity. The conductance is equal to the reciprocal of the resistance.

Since the dissolved salts in aqueous solution exist in ionic state, they have conductivity, so the conductivity can also indirectly represent the content of dissolved salts (salt content)

11. What is the pH value of water?

Negative logarithm of hydrogen ion concentration in water.

12. What is electrolyte?

Electrolytes are substances that can conduct electricity in aqueous solution, while non electrolytes do not have this ability. All acids Alkali Salt is an electrolyte. The reason why electrolyte has conductivity in water is that it dissociates or ionizes in water to produce positively charged cations and negatively charged anions.

13. What is chemical oxygen demand (COD)?

The amount of oxidation consumed when a certain strong oxidant is used to treat water samples under certain conditions. It is an indicator of the amount of reducing substances in water. The reducing substances in water are mainly organic substances. Therefore, cod is often used as an indicator to measure the content of organic matter in water. The higher the COD, the more serious the water is polluted by organic matter.

14. What is biochemical oxygen demand (BOD)?

Under aerobic conditions, due to the action of microorganisms, the amount of oxygen consumed when the organic substances that can be decomposed in water are completely oxidized and decomposed is called biochemical oxygen demand, which is referred to as biochemical oxygen demand for short. The amount of BOD indicates the degree of organic pollution of water body and reflects the quality of water quality.

15. What is total oxygen demand (TOD)?

The measurement of total oxygen demand is the amount of oxygen consumed by burning and oxidizing organic matter at 900 ℃ in a special burner with platinum as catalyst. The measurement result is closer to the theoretical oxygen demand than cod.

16. What is total organic carbon (TOC)?

The content of organic matter in water is expressed by the amount of carbon, the main element in organic matter, which is called total organic carbon.

TOC is determined by gasification and combustion of organic matter in water samples at high temperature to generate CO2. The total organic carbon can be known by measuring the amount of CO2 generated.

If the water sample is filtered through 0.2um microporous membrane, the measured carbon is dissolved organic carbon (DOC).

TOC and DOC are commonly used water quality indicators.

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