Thermal drying technology for advanced sludge treatment

With the rapid increase of urban sewage treatment rate, the amount of sludge generated will inevitably increase accordingly, and the problem of consumption is becoming more and more prominent, which needs to be solved urgently. The moisture content of sludge is the key to solve the problems encountered in sludge treatment.

To achieve deep dewatering of sludge, the more economical method is to introduce thermal drying technology commonly used in chemical operations. The use of thermal drying technology meets the requirements of sludge "three transformations":

• Reduction - Drying can greatly reduce the volume and quality of sludge (more than 90%). ease of transportation and disposal;

• Harmless - the sludge has been pasteurized to completely eliminate pathogens, and the dried sludge is safe and hygienic;

• Recycling - The dried sludge particles can be used as fertilizers, soil conditioners, fuels or building materials.

Basic principles of thermal drying

Sludge thermal drying refers to the use of thermal energy to heat and dry dewatered sludge to make it a dried product. Usually refers to the process of using heat energy to vaporize the moisture in the material and removing the generated steam. The essence is that the removed moisture 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.

The mechanism of drying can be divided into evaporation process and diffusion process: 

1. Evaporation process: the water on the surface of the material is vaporized. Since the water vapor pressure on the surface of the material is lower than the partial pressure of water vapor in the medium (gas), the water moves from the surface of the material into the medium. 

2. Diffusion process: It is a mass transfer process closely related to vaporization. When the water on the surface of the material is evaporated, the humidity on the surface of the material is lower than the humidity inside the material. At this time, the driving force of heat is required to transfer the water from the inside to the surface. The above two processes are carried out continuously and alternately, and finally achieve the purpose of drying.

Due to the different forms of moisture in the sludge, the removal rates during the drying process are also different:

Free Moisture: Moisture removed at a constant rate of evaporation.

Interstitial Moisture: Moisture removed during the first drop in evaporation rate. Usually refers to the moisture present in the capillaries between the mudcake particles.

Surface Moisture: Moisture removed during the second drop in evaporation rate. Usually refers to moisture adsorbed or adhered to a solid surface.

Bound Moisture: Moisture that cannot be removed during this drying process. This part of the water is generally chemically bound to the solid particles.

Classification and principle of sludge thermal drying equipment

There are two ways of transferring heat energy in drying equipment, namely direct heating and indirect heating, also known as convection heating and heat conduction (contact) heating.

According to the mass transfer method, it can be divided into three categories: direct heating, indirect heating, direct and indirect combined heating.

Direct heating drying equipment: The essence of direct drying is the application of convection drying technology, that is, the hot gas generated by the combustion chamber is directly contacted and mixed with the sludge, so that the sludge is heated, the water is evaporated, and a dry sludge product is finally obtained. Common equipment belt dryer, rotary sludge dryer, etc.

Indirect heating and drying equipment: Indirect drying is essentially conduction drying, that is, the hot gas generated by the combustion furnace is transferred through the steam and hot oil medium to the heater wall, so that the wet sludge on the other side of the wall is heated and the water evaporates to remove it.  Common equipment disc type sludge dryer, paddle dryer, etc.

Direct-indirect heating combined drying equipment: The direct-indirect combined drying system is the integration of convection-conduction technology. Common equipment fluidized bed sludge dryer, etc.

Bioetp disc sludge dryer is kind of double disc dryer, special mechanical design for protecting all welding seams, in-direct steam heated, continuously drying. Is perfect for industrial and sewage wastewater sludge drying, also suitable for fish meal, starch, vinasse and other feeds or foods drying.

Click this link to read more details of disc sludge dryer 

Disc Sludger Dryer