Reading guidance：In the early part of the 20th century, Chinas urbanization process further speed up, which puts forward higher requirements of the city sewage processing. More urban sewage source, the sludge is in sew

   In the early part of the 20th century, China's urbanization process further speed up, which puts forward higher requirements of the city sewage processing. More urban sewage source, the sludge is in sewage treatment process. Therefore, the sludge contained kinds of pollutants is also more diverse, containing heavy metal sludge will directly to the surrounding urban environment brings serious threat. Improper disposal of sludge, the secondary pollution can also cause environmental and aggravated the burden on sewage treatment, forming a vicious cycle of pollution control.
   From perspective of content of heavy metals in sewage, content of heavy metal in the town is much higher than the suburban and rural areas, which increases the impact of human activities is the main cause. From the overall situation, the highly centralized city population and industrialization has caused serious heavy metal pollution aggravated. Heavy metal waste water contained a large part is transferred to the soil. The heavy metal pollution in the main city shows the characteristics of point source pollution, as a source of pollution as the center, with the increase of the distance of heavy metal content in the environment also decreased gradually. According to a city of heavy metal pollution situation analysis can be seen in the city of pollution of heavy metals in different functional areas are different. The most serious heavy metal pollution in industrial areas, followed by the old residential area, tailing area, green city and the new development zone.
   Heavy metals in urban sewage mainly came from the surrounding nonferrous metal factory, chemical factory, people to use in daily life of pharmaceutical, tobacco, paint and pigment contain heavy metals. The existence of heavy metals in sewage, the first cause pollution of groundwater. In to groundwater as the main water source of the area, the pollution will cause poisoning, and even induce cancer.
   Different with other heavy metal pollutants, which cannot be decomposed into harmless microbial substances, once into the water, the water will be a small part of the fish biological absorption, most of the rest are adsorbed on colloidal particles and water, after sedimentation in the bottom of the water, but the settlement process is very long. The concentration of heavy metals in water going along with the change of water temperature and the change in the summer when the water temperature is high concentration is the highest. These heavy metal substances on the human body are different, such as mercury, in the form of organic matter by means of the respiratory and digestive tract into the body, will cause harm to the human body kidney, spleen and liver, but also by the the blood accumulated in the brain tissue, can cause inflammation and restlessness, irritability and other emotional changes, what will appear mental disorders than the phenomenon. Such as copper, will seriously affect the human hematopoietic function, blocking the cell's normal development, will lead to gastrointestinal disorders, such as diarrhea and vomiting phenomenon. From these aspects can see the harm caused by heavy metal pollution is very serious.
  Stability of kinds of sludge heavy metal classification and content of the polluted water, sewage treatment process, at the same time, sludge heavy metal speciation in sludge in different forms is different, the speciation of heavy metals in sludge of biological toxicity is not the same. Therefore, heavy metal sludge must get some control, keep the sludge harmless. 

：Suggestions on sludge disposal in china ：Current application situation of using ultrasonic to disposing sludge