Water treatment is an important part of water supply engineering. Its task is to pass the raw water through the process of dosing, mixing reaction, precipitation (clarification), filtration, disinfection, etc., to remove various harmful impurities contained in the raw water, and to meet the water quality standards of people's living and production water.

Most domestic water plants use surface water sources, which are taken from rivers, lakes and reservoirs. According to the different raw water quality conditions and water treatment requirements, surface water sources generally have the following 5 categories:

1. Unpolluted or lightly polluted surface water.
The water body conforms to the water quality indicators of Class I and II of the "Ground Water Environmental Quality Standards" prescribed by the state, and the turbidity and water temperature are within the normal range. The purpose of treatment is to remove turbidity and achieve microbiological hygiene indicators.

2. Micro-polluted surface water.
Water bodies are polluted by the environment, and some indicators have exceeded the requirements of Class III water bodies in the Environmental Quality Standards for Surface Water Body. At present, China's seven major river systems and inland rivers have been polluted to varying degrees in recent years, especially in industrially developed and urbanized areas. The main pollutant indicators are ammonia nitrogen, potassium permanganate oxygen consumption, volatile phenol and biochemical needs. Oxygen, etc.

3. High turbidity ground water.
In the Yellow River and the upper reaches of the Yangtze River, a large amount of mud and sand flowed into the water body during the flood period, forming raw water with high sand content. The maximum sand content of the middle reaches of the Yellow River and its tributaries can reach more than 1000kg/m3. The highest sand content of Jialing River in the upper reaches of the Yangtze River also reached 252kg/m. The high turbidity water of the Yellow River and the Yangtze River are quite different. The high turbidity water of the Yellow River generally refers to the river water with muddy liquid surface during the sedimentation process, and the high turbidity water of the Yangtze River refers to the frequent turbidity greater than (20-30d) during the flood period 1000NTU, and muddy water above 5000NTU appears several times.

4. Low temperature and low turbidity ground water.
In the vast areas of northern my country, the low temperature lasts for a long time and the raw water turbidity is low, which makes water treatment difficult. It is necessary to choose an appropriate treatment method to meet the effluent quality requirements. Low temperature and low turbidity water generally refers to the surface water whose winter temperature is 0～4℃ and the turbidity is lower than 30NTU.

5. High algae-containing surface water.
High-algae surface water mainly appears in lakes and reservoirs. Due to the impact of polluted water discharge and agricultural fertilization, many lakes in my country are becoming more and more eutrophic, with high levels of nitrogen and phosphorus, resulting in the proliferation of algae. In eutrophic lake water, the amount of algae is generally hundreds of thousands per liter to tens of millions per liter, which brings difficulties to the conventional treatment process. In response to the characteristics of the above-mentioned different water sources, the majority of water supply workers have conducted scientific research on different source water treatment processes and have continuously practiced, enriched and perfected them.

Usually the water treatment methods are conventional treatment methods: coagulation, precipitation (clarification), filtration, disinfection and special treatment methods: deodorization, deodorization, iron reduction, softening and desalination, etc.

1. Conventional treatment method: The treatment objects are mainly suspended solids and colloidal impurities that cause water turbidity. Treatment methods mainly include coagulation, precipitation (clarification) and filtration. After the agent is added in the raw water, the suspended matter and colloidal impurities in the water are formed into large-sized flocculants that are easy to settle through mixing and reaction (belonging to the coagulation process), and then separated by gravity sedimentation through the sedimentation tank. The clarifier is a structure that integrates reaction and precipitation. The filter tank is a structure that uses granular filter materials with pores (such as quartz sand and anthracite) to trap small impurities in water. It is usually installed after the precipitation (clarification) process to further reduce the turbidity of the water. When the turbidity of the raw water is low, it can also be filtered directly without precipitation after adding the agent.
While removing turbidity, coagulation, precipitation, and filtration also have a considerable effect on the removal of organic chromatic substances, bacteria, and even viruses, especially filtration.
For high-turbidity water sources, sedimentation tanks or pre-sedimentation tanks are usually used to remove sediment particles with larger diameters. The pre-sinking tank may or may not be dosed with chemicals.

2. Disinfection: The treatment object is pathogenic microorganisms in water. Usually after filtering. The main disinfection method is to add chlorine gas, bleaching powder and other disinfectants, such as ozone, chlorine dioxide or ultraviolet radiation. Among various disinfection methods, the use of chlorine gas is the most common.

3. Deodorization and deodorization: The method for removing odors and odors depends on the source of the odors and odors in the water. For example, the smell and smell generated by organic matter can be removed by oxidizing with activated carbon adsorption, adding oxidant or aeration and oxygenation; the smell and smell generated by the propagation of algae can be added to remove algae by adding copper sulfate in water; The odor and smell produced can be removed by desalting.

4. Iron removal: The treatment object is soluble second-order iron (Fe2+) in water. The main methods of iron removal are: natural manganese sand contact oxidation and natural oxidation. The former is equipped with an aeration device for oxygenation and oxidation, and then filtered through a manganese sand filter. The latter is usually equipped with aeration device, reaction, precipitation and sand filter. After being oxidized, the ferric iron is converted into a ferric iron precipitate and retained.

5. Softening: The treatment object is calcium and magnesium ions in water. The main softening methods are: ion exchange method and agent softening method. The former is to exchange the calcium and magnesium ions in the water with the ions of the exchange agent to achieve the purpose of removal; the latter is to add chemicals such as lime and soda to the water to convert the calcium and magnesium ions into precipitates and separate them from the water.

6. Desalination and desalination: The removal objects are various soluble salts in water, including anions and cations. The process of treating high-salinity water such as seawater and "brackish water" to meet the standards of domestic drinking water and desalination of water. The process of making high-purity water (pure water) is called water desalination. The methods of desalination and desalination are: distillation, ion exchange, electrodialysis, and reverse osmosis.
According to the water quality of different water sources and the water quality requirements after treatment, the above-mentioned various treatment methods can be used alone or in combination of several methods to form different water treatment systems. In water treatment, several methods are usually used in combination.