Dewatering filter cake

Sludge is here defined as that solid or semi-solid biologically active material used in wastewater treatment plants to break down organic matter. The material to be sampled may be obtained from relatively stationary points within the plant, from a pumpable slurry return line, or from the dewatered filter cake. 

At the end of the filtration cycle, the dewatered filter cake must be removed from the fabric in preparation for the next cycle. It is important that the cake is effectively discharged at this point since any delays will lead to extended filtration cycle times and therefore reduced process efficiency. This is particularly apt in filter press operations, where manual intervention may be necessary to remove sticky cakes. As a consequence, in addition to longer cycle times, the cost of the operator must also be considered. To some extent this topic may be linked to the cake moisture content because, broadly speaking, wetter cakes will adhere more tenaciously to the cloth. This problem has been partly addressed by the equipment manufacturers with the incorporation of high pressure wash jets and brush cleaning devices, and the filter media producers also continue to pursue the development of fabrics that will facilitate the ultimate goal of perfect, unassisted cake release and hence the achievement of a fully automated operation. 

Surface tension Combined with the particle/liquid contact angle, controls the moisture content of dewatered filter cakes. 

Wakeman R.J., Rushton A. and Brewis L.N., 1976. Residual saturation of dewatered filter cakes. The Chem. Engr. (London), 314, 668-670. 

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. 

Some attempts have been made to reslurry the filter cake without having to open the filter press. However, a number of problems appear, eg, bending of the plates due to uneven cake deposition or cavitation, uneven dewatering and washing within the frames, and plugging of the inlet ports. 

In the double-neutralization process, Na2SiFg is precipitated and removed by filtration at a pH of 3—4 (9). Upon raising the pH to 7—9, insoluble phosphates of Fe, Al, Ca, and Mg form and separate. Iron can be precipitated as hydrous ferric oxide, reducing the phosphate loss at the second filter cake. Both the fluorosihcate and metal phosphate filter residues tend to be voluminous cakes that shrink when dewatered recovery of soluble phosphates trapped within the cakes is difficult. 

Cake dewatering is related to cake formation in a filter cake, the final moisture content is dependent on many variables that also control cake formation. Pretreatment processes, for example, affect both cake formation and cake dewatering. Cake dewatering is achieved by compacting the soHds ... 

Use of Surfa.cta.nts, Although the use of steam to improve dewatering is consistently beneficial, the effects of surfactants on residual moisture are highly inconsistent. Additions of anionic, nonionic, or sometimes cationic surfactants of a few hundredths weight percent of the slurry, 0.02—0.5 kg/1 of soHds (50), are as effective as viscosity reduction in removing water from a number of filter cakes, including froth-floated coal, metal sulfide concentrates, and fine iron ores (Table 2). A few studies have used both steam and a surfactant on coal and iron ore and found that the effects are additive, giving twice the moisture reduction of either treatment alone (44—46,49). 

Surfactants aid dewatering of filter cakes after the cakes have formed and have very Httle observed effect on the rate of cake formation. Equations describing the effect of a surfactant show that dewatering is enhanced by lowering the capillary pressure of water in the cake rather than by a kinetic effect. The amount of residual water in a filter cake is related to the capillary forces hoi ding the Hquids in the cake. Laplace s equation relates the capillary pressure (P ) to surface tension (cj), contact angle of air and Hquid on the soHd (9) which is a measure of wettabiHty, and capillary radius (r ), or a similar measure appHcable to filter cakes. 

Electrical Enhancement of Dewatering. Electrophoresis (qv) can be used to prevent a filter cake from forming on a filter medium while allowing water to pass through the medium from the slurry. Electrophoresis is used to move the particles upstream, opposite to the Hquid movement, in order to prevent blinding of the medium. 

When eontaminated, polluted, or valuable suspension liquors are present, it becomes necessary to wash the filter cake to effect clean separation of solids from the mother liquor or to recover the mother liquor from the solids. The operation known as dewatering involves forcing a clean fluid through the cake to recover residual liquid retained in the pores, directly after filtering or washing. Dewatering is a complex process on a microscale, because it involves the hydrodynamics of... 

Vacuum filter systems consist of a horizontal cylinder partially submerged in a tank of biosolids. A layer of porous filter media fabric or tightly wound coils covers the outer surface of the cylinder. As the cylinder surface passes through the tank, a layer of biosolids adheres to the cylinder and vacuum is applied.61 The dewatered biosolids cake is then scraped off the fabric at up to 30% solids. 

Rhenipal A sewage sludge treatment process. One of three proprietary additives is used be-fore dewatering. The resulting filter cake is smaller in volume, has less odor, and its heavy metals are insoluble. Offered by Rhenipal, UK, a joint venture between National Power and Dirk European. 

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