Domestic sewage is the wastewater discharged by residents in their daily lives, which mainly comes from residential buildings and public buildings, such as houses, institutions, schools, hospitals, shops, public places and toilets of industrial enterprises. The pollutants contained in domestic sewage are mainly organic substances (such as protein, carbohydrate, fat, urea, ammonia nitrogen, etc. ) and a large number of pathogenic microorganisms (such as parasitic eggs and intestinal infectious viruses). ). The organic matter existing in domestic sewage is extremely unstable, and it is easy to rot and produce stench. Bacteria and pathogens feed on organic matter in domestic sewage and multiply in large quantities, which can lead to the spread of infectious diseases. Therefore, domestic sewage must be treated before it can be discharged.