Drying technologies described above have generally been developed for other applications and materials and modified for sludge drying. This transfer is complicated because sludge exhibits unusual behavior, in that there is an initial high drying rate in which the sludge is fluid, followed by a low drying rate period where the sludge becomes very sticky. The transition through the sticky zone occurs in a narrow moisture range (at approximately 60% dry matter) and presents unique technological difficulties, which are not faced in most drying processes. At lower moisture content, sludge breaks into granules, becomes powdery, and the drying rate is increased. Many sludge-drying processes currently attempt to avoid operating in the wet and sticky zones by recycling significant amounts of dry material and back mixing this into the dryer.
In drying sewage sludge, consideration must be given to the following:
• health: the initial material is offensive and possibly pathogenic; the dry product, if dusty, may pose inhalation hazards;
• safety: the dry material, if dusty, poses explosion risks; and
• environment: release of odors, toxic materials, heavy metals and pathogens.
A novel contact drying process that has recently been proposed is to immerse sludge into heated oil held at a temperature above water boiling point. This resembles a deep frying process, widely applied in food processing. Drying would typically be carried out in waste oils with an oil bath temperature ranging between about 120 and 180 °C. Compared with conventional drying techniques, sludge immersion drying has a number of major advantages.
• Heat and mass transfer rates are much higher than other drying techniques. Furthermore, the very sticky plastic phase is not observed, and mechanical stirring in the vessel is not required.
• Gases generated during frying can easily be separated and condensed.
• The latent heat of the evaporated water may be relatively easily recovered.
• Higher heating value of the fried sludge can be achieved.
• The sludge may be dried to any desired moisture content.
• The product is nonoffensive, hygenized, nondusty, and can have a high energy value.
Due to the decrease of available land-filling areas, and the restrictions on land spreading, the use of incineration for sludge disposal has been increased in European and other countries. Furthermore, the available methods for disposing recycled cooking oils has decreased considerably due to restrictions arising from bovine spongiform encephalo-pathy (BSE) and dioxin contamination events. This increases the attractiveness of using the oil for drying and co-incineration with sewage sludge.
A comparative life cycle assessment (LCA) of the method against conventional drying has been provided in the literature.
See also: Environmental Impact of Sludge Treatment and Recycling in Reed Bed Systems; Water Cycle Management.
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