We define sludge dewatering as water removal operations without evaporation. In contrast, sludge drying involves evaporation, which requires significant thermal energy. Sludge drying is also sometimes called thermal drying or thermal dewatering. Since thermal drying is an energy intensive process, dewatering is usually carried out as a prelude to thermal drying. Conventional dewatering operations are listed in Table 15.
Thermal drying of dewatered sludge is necessary for the following cases:
• requirement of low water content of the final product, such as in fertilizer production;
• improvement of heating value for efficient incinera tion; and
• reduction of transportation costs in disposal.
The main sludge drying processes are summarized in Table 16. Two relatively new drying techniques, namely reed bed drying and sludge immersion drying, are explained in the following.
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