Drainage, dewatering

For a cake drainage dewatering, scale up would be by one of the cake conveyance formulae, equations (4.65) or (4.68), and pond depth and conveyor differential would he judiciously adjusted to ensure the residence time in the critic.nl areas was kept the same. 

Cake Dewatering. Dewatering (qv), identified as a separate entity in filtration, is used to reduce the moisture content of filter cakes either by mechanical compression or by air displacement under vacuum pressure or drainage in a gravitational or centrifugal system. Dewatering of cakes is enhanced by addition of dewatering aids to the suspensions in the form of surfactants that reduce surface tension. 

Remove the leaf from the slurry at the end of the cake-formation period and note the time. If the slurry is particularly thick and viscous, the leaf may be gently shaken to remove excess slurry and prevent the dam from scooping up extra material. Maintain the leaf in an upright position (cake surface on top) and elevated so that hquid within the drainage passages may pass to the receiver. Tilt and rotate the leaf to help the filtrate reach the drain outlet. Continue this dewatering period until ... 

Simple, but gives relatively inefficient separation. Used for primary dewatering where the separations involve large feed volumes, and underflow drainage is not critical. 

Method Sludge dewatered by means of gravity drainage and natural evaporation. 

Combinations of cationic starch and anionic microparticles are useful commercial systems. Shear-sensitive flocculation occurs, allowing microscale reflocculation in the formed paper sheet, which improves dewatering and retention.63,75,76 The microparticles can be colloidal silica, aluminum silicate, poly(silicic acid) or bentonite of specific size and surface area.77 79 Cationic, anionic or polymeric aluminum-containing compounds can be additional components. A three-part coacervate system uses a high molecular weight anionic polyacrylamide, cationic starch and silica.80 Cooking cationic starch in the presence of an anionic silica hydrosol was reported to improve drainage and retention.81... 

As the sludge is forced through the unit at the bottom part of the assembly (at a pressure of 270 to 1,000 kPa), the filtrate passes through the filter cloth into the drainage paths, leaving the solids on the cloth to accumulate in the cavities of the frames. As determined by the cycle, the press is opened to remove the accumulated and dewatered sludge. Figure 7.4b shows an installation of aplate-and-frame press unit. 

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