Domestic sewage is the wastewater discharged from residents' daily life, mainly from residential buildings and public buildings, such as residences, institutions, schools, hospitals, shops, public places and industrial enterprise toilets. In this chapter, the manufacturers of water quality online monitors will talk about what kind of harm domestic sewage will cause! The pollutants contained in domestic sewage are mainly organic matter (such as protein, carbohydrate, fat, urea, ammonia nitrogen, etc.) and a large number of pathogenic microorganisms (such as parasite eggs and intestinal infectious viruses, etc.). The organic matter present in domestic sewage is unstable and easily rots to produce stench. And pathogens multiply by using organic matter in domestic sewage as nutrition, which can lead to the spread of infectious diseases. Therefore, domestic sewage must be treated before discharge. Eutrophication of aquatic ecosystems can occur in two ways through chemical pollutants: one through the increase in the amount of inorganic nutrients that normally limit plants; the other through the increase in organic matter that acts as decomposers. Malodor is a common pollution hazard that also occurs in polluted water bodies. There are more than 4,000 kinds of stench that people can smell, and dozens of them are more harmful. The hazards of stench are as follows: 1) It hinders normal respiratory function and reduces digestive function; mental irritability, reduced work efficiency, reduced judgment and memory; working and living in a foul smelling environment for a long time will cause olfactory disturbance, damage, and damage to the cerebral cortex. Exciting and regulating functions; 2) Some aquatic products are contaminated with stench and cannot be eaten or sold; 3) Smelly water cannot be used for swimming, fish farming or drinking, which destroys the use and value of water; 4) It can also produce hydrogen sulfide, Toxic hazards such as formaldehyde. Acid and alkali pollution change the pH of the water body, destroy its buffering effect, eliminate or inhibit the growth of microorganisms, hinder the self-purification of the water body, and corrode bridges, ships, and fishing gear. Acids and alkalis often enter the same water body at the same time, and some salts can be produced after neutralization. From the perspective of pH value, acid and alkali pollution are self-purified due to neutralization, but various salts are produced, which become new pollutants in the water body. pollutants. Because the increase of inorganic salt can increase the osmotic pressure of water, which has adverse effects on the growth of freshwater organisms and plants. In salinized areas, the salt in surface water and groundwater will further harm the soil quality. The hazards of high hardness water, especially water with high hardness for a long time, are manifested in many aspects: it is unpalatable; it can cause digestive tract dysfunction, diarrhea, and abortion of pregnant animals; it is inconvenient for people to use daily; it consumes a lot of energy; it affects the life of kettles and boilers; Scaling with water can easily cause explosions; softening and purification treatments are required, and the loss of acids, alkalis, and salts into the environment will cause the hardness of groundwater to increase, forming a vicious circle. Toxic substance pollution is a particularly important category of water pollution. There are many types, but the common feature is the toxicity and harm to biological organisms. So what we have to do is try to avoid causing harm. It is very necessary to monitor the water quality online, and it can only be used after testing the health of the water quality.