Water treatment methods include physical treatment and chemical treatment. The way of water treatment has a long history. Physical methods include using various filter materials with different pore sizes to exclude impurities in water by adsorption or barrier. Activated carbon is the most important way to adsorb impurities in water. The barrier method is to pass water through filter materials so that larger impurities can not pass through, and then obtain cleaner water. 。 In addition, physical methods also include precipitation method, that is, to let the smaller proportion of impurities float on the surface of the water, or the larger proportion of impurities precipitate on the bottom, and then obtain. Chemical method is to use a variety of chemicals to transform impurities in water into less harmful substances, or to concentrate impurities. The oldest chemical treatment method can be regarded as adding alum into water. When the impurities in water are collected, the volume becomes larger, and the impurities can be removed by filtration.
Water treatment refers to the physical and chemical measures taken to make the water quality meet certain use standards. Minimum standards for drinking water shall be formulated by the environmental protection department. Industrial water has its own requirements. The physical characteristics of water, such as temperature, color, transparency, odor and taste, are the basic criteria for judging the quality of water. The chemical characteristics of water, such as its acidity and alkalinity, the concentration of dissolved solids and oxygen content, are also important criteria for judging water quality. For example, in some Grassland Natural waters, the total soluble solids concentration is as high as 1000 mg/l, while in Canada, it is stipulated that the total soluble solids concentration in drinking water should not exceed 500 mg/l, and in many industrial waters, the concentration should not exceed 200 mg/l. This kind of water, even if its physical properties meet the requirements, can not be used casually. In addition, the content of radioactive elements from nature, nuclear accidents and nuclear power plants is also an important characteristic that must be monitored. 
Purpose of water treatment
The purpose of water treatment is to improve water quality and make it meet certain water quality standards. According to the different treatment methods, there are physical water treatment, chemical water treatment, biological water treatment and so on. According to the different treatment objects or purposes, there are two kinds of water treatment and wastewater treatment. Water treatment includes drinking water treatment and industrial water treatment, and wastewater treatment can be divided into domestic wastewater treatment and industrial wastewater treatment. Among them, boiler water treatment, recharge water treatment, main condensate water treatment of steam turbine and circulating water treatment, which are subordinate to industrial water treatment, are particularly closely related to thermal technology. Water treatment plays an important role in developing industrial production, improving product quality, protecting human environment and maintaining ecological balance. 
Editor of Water Treatment Method
Water treatment includes sewage treatment and drinking water treatment. In some places, sewage treatment is further divided into two types, namely, sewage treatment and reclaimed water reuse. The commonly used water treatment agents are: polyaluminium chloride, polyaluminium ferric chloride, basic aluminium chloride, polyacrylamide, activated carbon and various filter materials.
The effect of water treatment can be measured by water quality standards.
In order to meet the water quality requirements of finished water (domestic water, production water or dischargeable wastewater), raw water (raw water) is processed.
When raw water is used for domestic or industrial purposes, it is called water treatment.
When processing wastewater, it is called wastewater treatment. The purpose of wastewater treatment is to discharge wastewater (into water or land) or reuse (see wastewater treatment, wastewater reuse).
In the recycling water system and water regeneration treatment, raw water is wastewater, while finished water is water. The processing process has the characteristics of both water supply and wastewater treatment. Water treatment also includes the treatment and final disposal of wastewater and sludge produced in the treatment process (see sludge treatment and disposal), and sometimes the treatment and discharge of waste gases. Water treatment methods can be summarized as three ways: first, the most commonly used way is to get the required water quality by removing part or all of the impurities in raw water; second, by adding new components in raw water, the required water quality can be obtained through physical or chemical reactions; third, the processing of raw water does not involve removing impurities or adding new components.
Impurities in water and treatment methods Impurities in water include coarse substances, suspended substances, colloids and solutes. Coarse substances such as floating weeds in rivers, garbage, large aquatic organisms, gravel in wastewater and large pieces of dirt, etc. In water supply projects, coarse impurities are removed by facilities of water intake structures and are not included in the scope of water treatment.
In wastewater treatment, the removal of coarse impurities generally belongs to the pretreatment part of water. Suspensions and colloids include sediments, algae, bacteria, viruses, and insoluble substances existing in water and produced in the process of water treatment. Solubles are inorganic salts, organic compounds and gases. There are many methods to remove impurities in water. The scope of application of the main methods can be roughly divided according to the particle size of impurities (Fig. 1). Because the impurities in raw water and permissible impurities in finished water vary greatly in type and concentration, the water treatment process also varies greatly.
In terms of domestic water (or urban public water supply), raw water from high-quality water sources (well water or well-protected special reservoirs for water supply) needs only disinfection as finished water; raw water from general rivers or lakes needs first to remove turbidity impurities such as sediment, and then disinfect; raw water with more serious pollution also needs to remove pollutants such as organic matter; raw water containing iron and manganese (for example). In some wells, iron and manganese need to be removed. Domestic water can meet the water quality requirements of general industrial water, but industrial water sometimes needs further processing, such as softening, desalination and so on.
When the water quality of wastewater discharge or reuse is low, only coarse impurities and suspended solids can be removed by sieving and sedimentation (often referred to as primary treatment); when organic matter is removed, biological treatment (commonly referred to as secondary treatment) and disinfection are generally adopted after primary treatment; for wastewater after biological treatment, the treatment process is generally referred to as tertiary treatment or advanced treatment. For example, the removal of nitrogen and phosphorus by wastewater discharged into the water body which needs to prevent eutrophication belongs to the third-level treatment (see the physical and chemical treatment of water). When wastewater is used as a source of water, the quality requirements of finished water and the corresponding processing flow depend on its use. In theory, modern water treatment technology can produce any high quality finished water from any inferior water.
With reasonable water treatment process and advanced treatment of water, the treated water can reach the standards of GB5084-1992 and CECS61-94 for reclaimed water recycling. It can be reused for a long time and save a lot of water resources.
Water treatment is a process of improving water quality by physical, chemical and biological methods for source water or water that does not meet the water quality requirements.
The commonly used sewage treatment technologies are biochemical methods, such as Activated Sludge Process, Fixed Biofilm Processes, Combined Biological Processes, etc. Physicochemical methods, such as Granular Media Filtration, Activated Carbon Adsorption and Chemical Precipitation. Precipitation, Membrane Processes, Natural Processing, such as Stabilization Ponds, Aerated or Facultative Lagoons, Constructed Wetlands, Chemical Coloured Cosette Resin. Separation Principle of Nanofiltration Membrane
Nanofiltration membrane is also called ultra-low pressure reverse osmosis membrane. Japanese scholar Tagu Minlang once defined the separation principle of nanofiltration membrane in detail: the membrane with operating pressure less than 1.50 mPa, molecular weight 200-1000 and NaCl rejection rate less than 90% can be considered as nanofiltration membrane. Nanofiltration membrane separation technology has been separated from reverse osmosis technology, and has become an independent separation technology between ultrafiltration and reverse osmosis technology. It has been widely used in seawater desalination, ultra-pure water manufacturing, food industry, environmental protection and many other fields, and has become an important branch of water treatment technology.
Principle of nanofiltration technology
Principle of dissolution and diffusion: Permeate dissolves in the membrane and diffuses along its driving force gradient, forming chemical equilibrium between the phases on the surface of the nanofiltration membrane in the form of energy = concentration o, mobility o, driving force, so that a substance must overcome the osmotic pressure when it passes through the membrane.
Electrical effect: The electrostatic interaction between nanofiltration membrane and electrolyte ions results in different ion rejection rates due to different charge intensities of electrolyte salt ions. In multicomponent systems containing different valence ions, the selectivity of membrane to different ions is different due to DONNAN effect, and the proportion of different ions passing through the membrane is also different.
The ionic selectivity of the nanofiltration process is due to the negative charged groups on the nanofiltration membrane or in the membrane, which interact by electrostatic force to hinder the penetration of multivalent ions. The possible charge density of nanofiltration membrane is 0.5-2 meq/g.
Separation Principle of Nanofiltration Membrane
Nanofiltration membrane is between RO and UF membrane, and the removal rate of NaCL is below 90%. Reverse osmosis membrane has a high removal rate for almost all solutes, but nanofiltration membrane only has a high removal rate for specific solutes.
The nanofiltration membrane mainly removes solute particles with a diameter of about 1 nanometer (nm) and a molecular weight of 100-1000. In the field of drinking water, it is mainly used to remove trihalomethane intermediates, odor, chroma, pesticides, synthetic detergents, soluble organic substances, hardness components such as Ca, Mg and evaporative residues.
Processing process editing
Generally speaking, sewage treatment consists of three stages: the first stage is mechanical treatment, such as grille, precipitation or air flotation, to remove stones, sand and fat, iron ions, manganese ions, grease and so on. Secondary treatment is biological treatment. Pollutants in sewage are degraded and converted into sludge by microorganisms. Three-stage treatment is the advanced treatment of sewage, which includes the removal of nutrients and disinfection of sewage by chlorination, ultraviolet radiation or ozone technology. Some wastewater treatment processes may not include all of the above processes, depending on the treatment objectives and water quality.
Pure water treatment process depends on the quality of raw water.
If raw water is municipal tap water, the general process is
Sand filter - activated carbon filter - softening (optional) - security filter - reverse osmosis - ultraviolet disinfection - water production
If the surface water is normal, sterilize and add flocculant before entering the above process.
If it is well water, iron and manganese removal filters should be added after sand filtration.
Water is recycled and purified.
Quartz sand filtration is one of the most effective means to remove suspended solids in water. It is an important unit in advanced sewage treatment, sewage reuse and water supply treatment. Its function is to further remove the flocculated pollutants in water. It achieves the purpose of water purification through the interception, sedimentation and adsorption of filter materials.
II. Scope of application
1. For industrial water, domestic water and municipal water supply systems requiring effluent turbidity less than 5 mg/L to meet drinking water quality standards;
2. Removal of suspended and solid substances from industrial sewage;
3. It can be used as pretreatment equipment in ion exchange softening and desalination system and as crude filtration equipment for industrial feed water with low water quality requirements.
It is also used in swimming pool circulating treatment system and cooling circulating water purification system.
Method Principle Editing
Commonly used water treatment methods are:
(1) Precipitate filtration method
(2) Hard water softening method
(3) Activated Carbon Adsorption Method
(5) Reverse osmosis method
(8) Ultraviolet disinfection
(9) Biochemical method.
(10) Mixed ion exchange method