What are the aspects of pollution in sewage treatment?

Sewage refers to the entry of various wastes and other pollutants into the water, which exceeds the self-purification and pollution-holding capacity of the water body, resulting in adverse changes in the physical and chemical properties of the water body and its sediment and the composition of biological communities, destroying the inherent ecosystem in the water and the function of the water body, thereby reducing the use value of the water body. According to the cause of pollution, it can be divided into natural pollution and man-made pollution. Natural pollution refers to the enrichment of some chemical elements due to special geological or natural conditions, or some toxic substances or biological pathogens produced in the decay of natural plants enter the water body, thereby polluting the water quality. Man-made pollution refers to the pollution of surface water bodies caused by human activities (including productive and living). From the division of pollution sources, it can be divided into point pollution sources and surface pollution sources. Point pollution refers to the discharge of pollutants into water bodies from concentrated places (such as industrial wastewater and domestic sewage discharge outlets); it is characterized by frequent sewage discharge, and its changing law obeys the discharge law of industrial production wastewater and urban domestic sewage. Surface pollution refers to the pollutants originating from the ground (or underground) of the catchment area, such as the use of chemical fertilizers and pesticides in farmland, which often contain pesticides and chemical fertilizers after irrigation and drainage. During the rainy season in cities and mines, rainwater washes away the ground to form pollutants. surface runoff, etc. According to the nature of pollution, it can be divided into physical pollution, chemical pollution and biological pollution. Physical pollution refers to the change of turbidity, temperature and color of water, the increase of floating oil film, foam and radioactive substances contained in water, etc.; chemical pollution includes the pollution of organic and inorganic compounds, such as the decrease of dissolved oxygen in water , increased dissolved salts, changes in pH, or some toxic chemical substances in the water; biological pollution refers to the entry of bacteria and sewage microorganisms into the water body, mainly from animal feces and some industrial wastewater, especially hospital sewage. Sewage In fact, the water body is not only polluted by one type, but by multiple types of pollution at the same time, and various pollutions interact with each other, constantly decomposing, compounding or biological precipitation.