Surface cake layer

The additive should provide a thin layer of solids having high porosity (0.85 to 0.90) over the filter medium s external surface. Suspension particles will ideally form a layered cake over the filter aid cake layer. The high porosity of the filter aid layer will ensure a high filtration rate. Porosity is not determined by pore size alone. High porosity is still possible with small size pores. 

The presence of a liquid layer on the surface of the filter cake will cause solute to diffuse from the top layer of cake into the liquid. Also if disturbed the layer of liquid will mix with the surface layer of filter cake. This effect can be incorporated into the digital simulation by assuming a given initial depth of liquid as an additional segment of the bed which mixes at time t=0 with the top cake segment. The initial concentrations in the liquid layer and top cake segment are then found by an initial mass balance. 

Figure 6.2 depicts 3 ways in which microporous membranes can foul (a) pores can suffer closure or restriction, (b) pores or porosity can be blocked or plugged and (c) a surface cake or layer can cover the membrane. All three mechanisms could apply, probably in sequence (a) then (b) followed by (c). Nonporous membranes are fouled by cake or surface layers (c). 

The processes of interest are NFand RO where the membranes are either nanoporous or essentially nonporous. In these processes the fouling is a surface layer, the effects of which maybe exacerbated by the high retention of solutes by the membrane. Operation is with crossflow and in industry fixed flux is commonly used. This section considers particulate fouling, biofouling and scale formation and then discusses the implications of cake enhanced concentration polarization on fouling outcomes. 

Ultrafiltration and microfiltration can be backwashed occasionally to remove accumulated solids from membranes. UF and MF membranes may be used to remove micrometer-sized and upper suspended particles, namely bacteria, algae, and so on, they can also be used to remove Guardia and Cryptosporidium, as well as most viruses found in surface water. In fact, the solid layer ( cake ) adhering to the membranes in the latter two techniques acts like a dynamic membrane [8, 9], removing smaller particles even at colloidal and virus levels. 

A well-known addition to the basic machine is a separate reroll ring beyond the rim of the main disc (see Fig. 3.14). Formed product-size agglomerates are ejected over the rim into this peripheral ring where they are further smoothed and formed into more spherical shapes. A surface layer of fines, such as coke breeze in iron ore green balls or an anti-caking dust to improve storage properties, may be added in the reroll ring. 

Adjacent channels are alternatively plugged at each end in order to force the diesel exhaust gases through the porous substrate walls which act as a mechanical filter. The filtration mechanism is a combination of surface and cake filtration. As the soot load increases, a particulate layer develops at the inlet channel walls and the cake filtration becomes the prevailing mechanism. 

Thus, at each time increment, the newly formed surface layer has not been compressed and the mass in the surface layer is estimated by the measured increase in fouling layer thickness due to cake growth and the surface porosity of a... 

The cartridges consisting of a single piece of filter medium are made from a perforated cylindrical core, of metal or stiff plastic, onto which the material of the filter medium is placed or formed. There are two basic types of integral media cartridges, depending upon which primary filtration mechanism is operating thin media, which work by surface filtration, followed if necessary by cake filtration on top of the surface layer and thick media, which work by depth filtration. 

Mlcrofiltra.tlon, Various membrane filters have been used to remove viral agents from fluids. In some cases, membranes which have pores larger than the viral particle can be used if the filtration is conducted under conditions which allow for the adsorption of the viral particle to the membrane matrix. These are typically single-pass systems having pore sizes of 0.10—0.22 lm. Under situations which allow optimum adsorption, between 10—10 particles of poHovims (28—30 nm) were removed (34—36). The formation of a cake layer enhanced removal (35). The titer reduction when using 0.10—0.22 p.m membrane filters declined under conditions which minimized adsorption. By removal standards, these filters remove vimses at a rate on the low end of the desired titer reduction and the removal efficiency varies with differences in fluid chemistry and surface chemistry of viral agents (26). 

The American version of the dynamic filter, known as the Artisan continuous filter (Fig. 30), uses such nonfiltering rotors in the form of turbine-type elements. The cylindrical vessel is divided into a series of disk-type compartments, each housing one rotor, and the stationary surfaces are covered with filter cloth. The feed is pumped in at one end of the vessel, forced to pass through the compartments in series, and discharged as a thick paste at the other end. At low rotor speeds the cake thickness is controlled by the clearance between the scraper and the filter medium on the stationary plate, while at higher speeds part of the cake is swept away and only a thin layer remains and acts as the actual medium. 

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