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SPECIAL PUBLICATION SJ2008-SP11 - page 21 / 48





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Literature Review & Summary Report Design Basis Memorandum St. Johns River Water Management District Contract # SK940AA

BCI Project No. 19-15089 September 2007 Page 13

2.3.5 Equipment Type- Thermal Dryer

Thermal drying (Figure 8) removes water from sludge to a significantly higher degree than all other dewatering processes. Solids reaching 90% are attainable, but the sludge is typically dewatered to a minimum of 18-20% solids before it is directed to the drying facility.

Sludge drying occurs in a three-step process. The first is preliminary drying, during which the temperature of the sludge is increasing to a prescribed, constant value. The second phase is called essential drying and is period during which moisture evaporates from the surface of the sludge particles with a constant speed. The sludge particle surface is covered with water, which constantly evaporates and is replaced by water from inside the particle. The temperature of sludge during drying the phase is constant and is the same as the temperature of the surrounding water, typically 50-85oC. The last phase of sludge drying is final drying. Water from the surface of sludge evaporates quicker than it is replaced from the inside of the particle. The speed of drying in the last phase is decreasing until balanced hydration is achieved.

Since thermal drying, in this application, is intended to increase the solids content of pre- thickened sludge to a level beyond that necessary for transport and storage, this alternative is not seen as a practical option. Figure 8 Drying Plant

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