At present, many countries have adopted thermal drying technology in sludge treatment. According to whether the thermal medium is in contact with the sludge, the current sludge thermal drying technology can be divided into three categories: direct thermal drying technology, indirect thermal drying technology and direct-indirect combined drying technology. Drying usually refers to the process of using heat energy to vaporize the wet part of the material and remove the resulting steam. The essence of drying is that the moisture removed is transferred from the solid phase to the gas phase, the solid phase is the material to be dried, and the gas phase is the drying medium.
Direct heat drying technology is also known as convection heat drying technology. Convection heat drying is the removal of water from the surface of the sludge by hot air. The efficiency of drying depends on two factors: the operating conditions of the air (stable point, relative humidity, speed) and the structure and characteristics of the sludge itself. In the operation process, the hot medium (hot air, gas or steam, etc.) is in direct contact with the sludge, the hot medium flows through the sludge layer at a low speed, and absorbs the water in the sludge during this process. The dry sludge after treatment needs to be separated from the hot medium. Part of the exhaust gas is returned to the original system through the heat recovery system for reuse, and the remaining part is discharged after harmlessly. This technology has high heat transfer efficiency and evaporation rate, which can increase the solid content of sludge from 25% to 85%~95%. However, due to the direct contact with the sludge, the thermal medium will be polluted, and the discharged wastewater and water vapor must be discharged after harmless treatment; At the same time, the thermal medium and the dry sludge need to be separated, which brings some trouble to the operation and management.

1. According to the treatment method and separation process: activated sludge, humus sludge, chemical sludge;
2. According to the different production stages of sludge: domestic sludge, digested sludge, concentrated sludge, dehydration and drying sludge;
3. According to the composition and nature of sludge: organic sludge, inorganic sludge, hydrophilic sludge, hydrophobic sludge, dry sludge, etc.