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 basicity, 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.

　　Water treatment methods

　　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 use, 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 treatment 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: firstly, some or all of the impurities in raw water are removed to obtain the required water quality; secondly, the required water quality is obtained by adding new components to raw water through physical or chemical reactions; thirdly, 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 weeds floating 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 sediment, 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 the allowable 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 needs to remove pollutants such as organic matter; raw water containing iron and manganese (such as some well water), 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 called primary treatment); when organic matter is removed, biological treatment (commonly called secondary treatment) and disinfection are generally used after primary treatment; for wastewater after biological treatment, the treatment process is generally referred to as tertiary treatment or advanced treatment, such as when waste is treated. The process of nitrogen and phosphorus removal by water discharged into the water body to prevent eutrophication belongs to the third stage 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.

　　Treatment process

　　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