Pressure filtration is a process similar to vacuum filtration where sludge solids are separated from the liquid. Leaf filters probably are the most common type of unit. Refer back to Chapter 5 for a detailed discussion. Like vacuum filtration, a porous media is used in leaf filters to separate solids from the liquid. The solids are captured in the media pores; they build up on the media surface; and they reinforce the media in its solid-liquid separation action. Sludge pumps provide the energy to force the water through the media. Lime, aluminum chloride, aluminum chlorohydrate, and ferric salts have been commonly used to condition sludge prior to pressing. The successful use of ash precoating is also prevalent. Minimum chemical costs are supposed to be the major advantage of press filters over vacuum filters. Leaf filters represent an attempt to dewater sludge in a small space quickly. But, when compared to other dewatering methods, they have major disadvantages, including: (1) batch operation, and (2) high operation and maintenance costs. Some other types of pressure filters include hydraulic and screw presses, which while effective in dewatering sludges, have a major disadvantage of usually requiring a thickened sludge feed. Sludge cakes as high as 75% solids using pressure filtration have been reported.
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