An Overview of Current Sludge Drying Practices

The trend of drying dewatered sludge in sewage treatment plants is motivated by ascending disposal cost and more stringent regulations. The product of the drying process generally contains water content lower than 10% and with one-fourth of the original volume. This significantly reduces the transportation cost to distant landfill and disposal fee. Besides, the dried biosolids can be utilized as marketable fertilizer for soil conditioning or as alternative fuel for co-generation. Even though incineration of sludge is a viable option of reducing volume, the emission of dust poses a risk of explosions and fires.There are three major types of sludge drying technology, thermal drying, solar drying, and non-thermal drying. Thermal dryer dries wet sludge by direct, indirect, or combined process. Direct thermal dryer employs hot gas as the drying media. Hot gas is injected to the drying chamber and removes moisture from sludge by convection. Indirect dryer dries sludge by conduction. The drying media (e.g. hot gas or oil) is separated from sludge by heat conducting surfaces which dries sludge through the agitation process. The combined type of thermal dryer utilized both convection and conduction method. Thermal dryer is usually compact in size and it provides consistent dried products. The high energy and maintenance cost are the major disadvantage of thermal dryer. Solar dryer utilized solar energy as the exclusive energy provider in the drying process. A greenhouse-like enclosure is used to contain the drying bed for odor reduction. An electric mole is functioned as a tilling device to agitate the sludge layer to promote drying. The environmental condition including humidity and ventilation within the drying bed and the motion of electric mole are controlled by microprocessor. The solar drying bed has the advantages of low capital and operating cost. It has limited amount of machinery and the energy cost is insignificant compare to thermal dryer. However, the drying process is heavily relied on the humidity difference between the environment and the sludge. The dried product quality may vary even with the optimization of microprocessor. This technology may not be practical in the area with lots of precipitation. Non-thermal dryer is a relatively new technology which removes moisture by accelerating the wet sludge and injected to a series of cyclones and conditioning chambers. This design has the advantages of space saving and the product is independent environmental conditions. Nevertheless, this technology lacks proven examples to justify the performance.