Precoat filter

Filtration. Any type of clarification is foUowed by filtration through leaf-type vertical or horizontal pressure filters. Carbonatated Hquors, containing calcium carbonate, may require addition of diatomaceous earth as a filter precoat. Phosphatated Hquors are generally filtered with the addition of diatomaceous earth as precoat and body feed. 

The majority of the minimum design criteria in the Ten State Standards for diatomaceous earth (DE) systems meet the SWTR (2). However, two design criteria in addition to the Ten State Standards are necessary to meet current regulations (a) The minimum amount of filter precoat should he 1 kg/m (0.2 Ih/ft ) to enhance Giardia cyst removal and (b) the minimum thickness of the precoat should be increased from 0.3 to 0.5 cm (1/8 to 1/5 in.) (found to be more important than the size graduation of the diatomaceous earth), also to enhance Giardia cyst removal. 

The Boyden chamber is a simple apparatus used to test for chemotaxis, especially of leukocytes. It can also be used to assess tumor cell transmigration across an endo-thehal monolayer in vitro. It consists of two compartments separated by a MiUi-pore filter (3-8 pm pore size). A chemotac-tic factor is placed in one compartment, and a gradient develops across the thickness of the filter (ca. 150 pm). Cell movement into the filter is measured after an incubation period less than the time taken for the gradient to decay. Cell motility can be measured in Boyden chambers containing filters precoated with different materials, for example fibronectin or fibronectin fragments. The method, when apphed to malignant and non-mahgnant cell hnes, shows that the variable invasive potentials of these cells correlate with their abihty to disrupt the endothelial cell monolayer. 

It is recommended that both precoats should be prepared with water or filtered wine. Filtration of the wine may start as soon as the filter precoat has been prepared. The outer surface of the filter layer is constantly renewed by continuous accretion, generally with the same adjuvant or mixture of adjuvants. This prevents rapid clogging of the precoat and increases the length of the filtration cycle. 

These filters are used on slurries with small amounts of solids, usually less than 0.1%, and generally do not form any visible cake. Solids to be removed are usually very small panicles that may be trapped on the surface of the filter medium or within the medium. This type of filter is usually used in a polishing application where excellent quality liquids are needed as in food or beverage, pharmaceutical, and electronic processing operations. The most common clarifying filters are disk and plate presses, cartridge filters, precoat pressure filters, deep bed filters, and membrane filters. 

A more specialized application is the filtration of mercury-cell liquor to remove mercury from the product. We consider two types of filter here, the candle filter using carbon tubes and the pressure-leaf filter precoated with activated carbon. The operation of candle filters also has been described in Section 7.S.4.2. 

The Precoat Discharge This is a special type of discharge of the filter cake used on the so-called precoat filters. Precoat filters are drum filters in which the filter medium is composed of a layer of diatomite, perlite, wood-flour, synthetic granulate, and so on. This layer is applied in thicknesses of approximately 100 mm before the actual filtration of slurries mostly having a very low solids content and with very fine particulate composition (Figure 18.11) commences. 

Centrifugation or filtration in candle filters or in disk filters precoated with charcoal gives sodium hydroxide solutions containing mercury concentrations of < 0.05 ppm (mgAg of 50 % caustic soda). 

Figure 8 Dose-response of SCM-chitin III on haptotactic migration of tumor cells. B16-BL6 cells (2 X 10" ) were seeded on the filter precoated on the lower surface with 5 xg laminin. Various concentrations of SCM-chitin III were added into the lower compartment of a Transwell chamber. After a 6-h incubation, the migrant cells on the lower surface were visually counted. P > 0.001 compared with the control by Student s two-tailed r-test.

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