(1) Organic matter pollution: Organic pollutants in sludge mainly include benzene, chlorophenol, PCBs, PCDFs and PCDDs. The organic pollutants contained in the sludge are not easily degraded and have toxic residues for a long time. These toxic and harmful substances will cause environmental pollution when they enter the water and soil.
 
(2) Pathogenic microbial pollution: pathogenic microorganisms and parasite eggs in the sewage will enter the sludge. The pollution ways of pathogens in the sludge to humans or animals include direct contact with the sludge, direct contact with the sludge through the food chain, and water source contaminated by pathogens, and the soil and then the water body polluted by the pathogens.


 
(3) Heavy metal pollution: During the sewage treatment process, 70%~90% of heavy metal elements will transfer to the sludge by adsorption or precipitation. Some heavy metal elements are mainly derived from industrial wastewater, such as cadmium and chromium. In addition, there are some heavy metals derived from household plumbing, such as copper and zinc.
 
(4) Other hazards: The secondary pollution of the sludge to the environment also includes the pollution of sludge salt and the pollution of nutrients such as nitrogen and phosphorus. Higher sludge salinity will significantly increase soil conductivity, destroy plant nutrient balance, inhibit plant nutrient uptake, and even cause direct damage to plant roots. After a large amount of sludge rich in nitrogen and phosphorus is applied to the loose soil in the area with large rainfall, when the decomposition rate of the organic matter is faster than the absorption rate of nitrogen and phosphorus by the plant, the nutrients such as nitrogen and phosphorus may run off and enter the surface water body along with water, which will cause eutrophication of the water body and underground water pollution.