Conventional filters

No internal piping and no conventional filter valve are needed with single-cell dmm filters where the entire dmm also operates under vacuum. The cake discharge is effected by air blowback from an internal stationary shoe mounted inside the dmm at the point of discharge. There are very close tolerances between the inside surface of the dmm and the shoe in order to minimize the leakage. The inside of the dmm acts as a receiver for the separation of air and filtrate conventional multicompartment dmm filters require a separate external receiver. This type of filter permits operation of the filter with thin cakes so that high dmm speeds, up to 26 rpm, can be used and high capacities can be achieved. Sizes up to 14 m are available. 

The vertical recessed plate automatic press is shown schematically in Figure 15. Unlike the conventional filter press with plates hanging down and linked in a horizontal direction, this filter press has the plates in a horizontal plane placed one upon another. This design offers semicontinuous operation, saving in floor space, and easy cleaning of the cloth, but it allows only the lower face of each chamber to be used for filtration. 

The Flat-bed pressure filter (Hydromation Engineering Co. Ltd.) (19) is based on the above principle. The pressure compartment consists of two halves, top and bottom. The bottom half is stationary while the top half can be raised to allow the belt and the cake to pass out of the compartment, and can be lowered onto the belt during the filtration and dewatering stage. The filter can be considered as a horizontal filter press with an indexing cloth in comparison with a conventional filter press, however, this filter allows only the lower face of the chamber to be used for filtration. 

Results of test work with this filter, producing cakes of 1 mm thickness using a 3 mm clearance, have been pubUshed (33,34). The cake formed on the medium was generally stable, giving high filtration rates over long periods of time, and the precoat type cake did not blind with time. There was no evidence of any size selectivity of the process the only exception was conventional filter aids which were preferentially picked up by the rotating fluid. This... 

Dynamic membranes are concentration—polarization layers formed in situ from the ultrafiltration of coUoidal material analogous to a precoat in conventional filter operations. Hydrous zirconia has been thoroughly investigated other materials include bentonite, poly(acryhc acid), and films deposited from the materials to be separated (18). 

This deposit is composed of suspended particles similar to conventional filter cakes, and more importantly, a slime that forms as retained solutes exceed their solubility. The gel concentration 6 is a function of the feed composition and the membrane-pore size. The gel usually has a much lower hydrauHc permeabihty and smaller apparent pore size than the underlying membrane (27). The gel layer and the concentration gradient between the gel layer and the bulk concentration are called the gel-polarization layer. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

Flat-sheet modules are similar in some ways to conventional filter presses. An example is shown in Figure 16.17. This consists of a series of annular membrane discs of outer diameter 0.3 m placed on either side of polysulphone support plates which also provide... 

Doubles the volume of sample filtered compart with other conventional filters... 

Filtration can remove fine suspended solids and microorganisms, and microfiltration membranes of cellulose acetate or polyamides are available that have pores 0.1-20 /xm in diameter. Clogging of such fine filters is an ever-present problem, and it is usual to pass the water through a coarser conventional filter first. Ultrafiltration with membranes having pores smaller than 0.1 fim requires application of pressures of a few bars to keep the membrane surface free of deposits, water flows parallel to the membrane surfaces, with only a small fraction passing through the membrane. The membranes typically consist of bundles of hollow cellulose acetate or polyamide fibers set in a plastic matrix. Ultrafiltration bears some resemblance to reverse osmosis technology, described in Section 14.4, with the major difference that reverse osmosis can remove dissolved matter, whereas ultrafiltration cannot. 

The two main types of conventional filter used for cell separation are platefilters (filter press) and rotary drum filters-. 

Bowles et al.67 have studied the photolysis of 1,1,3-trichlorotri-fluoroacetone using a medium pressure mercury vapor arc, isolating the 3130 A. line with a conventional filter assembly. 

Metals precipitated with chemicals are removed by settlement in a basin. Because industrial metals removal processes frequently include filtration, basin effluent passes through a rudimentary filter of straw, conventional filters being considered impractical for unattended operation at a site lacking electricity. Also, provision is made to spray neutralized effluent upstream of the basin to strip carbon dioxide that may inhibit flocculation by depressing the pH,11 as ample hydraulic head is available at the site. Dewatered solids would be placed in a mine cavity. 

Another problem encountered was the impurity content of the filter paper used in the high volume samplers to collect the particulate samples. The conventional filter material used by EPA was glass fiber filter media. However, this was not compatible with INAA because of its high and varied impurity content. Discussions with K. Rahn of the Ford Reactor at the University of Michigan revealed that Whatman-41 filter paper was the most desirable medium for use with INAA (see Ref. 2). Our analyses showed Whatman-41 to be very low in impurities with consistent impurity levels from batch to batch. Average impurity levels, based on 12 batch analyses, are shown in Table III. Although the levels for calcium, chlorine, sodium, aluminum, and iron appear large, they rarely affected elemental levels found in filtered particulates. Impurity levels did not vary more than 25% from the mean. 

Ideolly the coalescer should be ploced upstream of the separator to provide o uniform woter droplet size in the condensate stream to the cross flow interceptor, however the presence of solids in the product streom would quickly plug conventional filter coalescers. 

High rate deep bed. This filter is much like a conventional filter except a coarse grade of media serves as the top layer. The second layer is finer. 

Filtration rates as high as 15-20 gpm/sq ft are quoted for these filters. Before backwashing, the bed is given an air purge to expand the media. Backwash with filtered water is from boltom-to-top as with conventional filters. 

In the sludge-thickening and dewatering section, polymer is added to thicken the sludge from 1 percent total suspended solids content to about 3 percent. Ferric chloride and diato-maceous earth are then added, prior to dewatering in conventional filter presses. The filter cake will have a total solids content of about 20 percent, which is high enough that no free liquids will be present. 

Conventional filters, such as a coffee filter, termed depth filters , consist of a network of fibers and retain solute molecules through a stochastic adsorption mechanism. In contrast, most membranes for the retention of biocatalysts feature holes or pores with a comparatively narrow pore size distribution and separate exclusively on the basis of size or shape of the solute such membranes are termed membrane filters . Only membrane filters are approved by the FDA for sterilization in connection with processes applied to pharmaceuticals. Table 5.3 lists advantages and disadvantages of depth and membrane filters. 

Conventional filter papers retain only particles with diameters in excess of at least 1 pm and are, therefore, permeable to colloidal particles. 

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