Depth type media

A vast variety of materials in diverse forms are used as filter media for which Purchas (1981) produced a guideline classification, which is reproduced in Table 2.1. Alternative methods of classification are available, but there always exist media that cannot be fitted neatly into the classification scheme. For example, Flood et al (1966) classified filter media into surface and depth media types. Surface type media are distinguished by the fact that the particles in suspension are mostly retained on the surface of the medium, with little penetration into the pores. Examples are filter paper, filter cloths and wire mesh. Depth type media, used mainly for liquid clarification, are characterised by the fact that the particles penetrate into the pores, where they are retained. The pores of such media are considerably larger than the sizes of the particles in... 

Typical beta ratios P3, and for depth-type media... 

On the other hand, in dead end filtration the retention is achieved by particle or gel layer buildup on the membrane and in the pores of the medium such as when a depth type filter is used. This condition is analogous to that encountered in packed-bed geometries. 

By far the largest number of ventilation filters is of the flat panel type, square or rectangular in shape, and of a standard size to be accepted as a push fit into appropriately sized spaces in the dividing wall separating the ventilated area from the outside atmosphere. The simplest form has a flat panel of thick, depth filtration medium (Figure 6.3) held in a cardboard or thin metal frame. This pad filter has a relatively low surface area available for filtration. 

The filter selected must be able to retain particles removed from the subject fluid. Depth-type filters of the type whose pores can increase in size as pressure is increased are subject to unloading. With surface filters or fixed structure filters, one selects a medium that will not change its structure imder system-produced stress. 

For the case of electro-magnets, inclusions detection in welds situated at 1-2 mm of depth is very important, because the reluctance variation between the two mediums is not important, and thus the detection of this type of defect is very difficult. It will be sufficient to be in optimal conditions to eliminate this problem. 

All filters require a filter medium to retain solids, whether the filter is for cake filtration or for filter-medium or depth filtration. Specification of a medium is based on retention of some minimum parficle size at good removal efficiency and on acceptable hfe of the medium in the environment of the filter. The selection of the type of filter medium is often the most important decision in success of the operation. For cake filtration, medium selection involves an optimization of the following factors ... 

Chemically inactive These are made from a variety of materials that include wound yarn, felt, flannel, cotton waste, wood pump, mineral wool, asbestos and diatomaceous earth. The solid particles are trapped and retained within the medium. Certain types will remove water, as well as large and small particles of solids in a range down to 10 microns. Ceramics are sometimes employed for depth filtration, as also are special sintered metals. 

Fig. 7.ia-c. Medium depth chemical peel for widespread lentigines in type 11 skin, (a) Pre-operative, (b) 10 days status post medium depth chemical peel, (c) One month status post medium depth chemical peel... 

Reactor type Electrochemical sheet micro flow reactor Reaction medium layer depth 600 pm... 

Type of Filter BOD5 Loading (kg/m3 d) Hydraulic Loading (m3/m2 d) Depth (m) BOD Removal (%) Medium... 

Grendahl and Sung [96] tested a simple, reliable, and inexpensive assay method for the quantitation of serum levels of miconazole and other imidazole drugs. This assay, which is similar to Kirby-Bauer test, was sensitive to 0.2 pg of drug/mL and linear from <0.2 to 10 pg/mL. Concentration of inoculum and agar depth in test plates was not as critical as the type of medium, amount of inoculum or type of drug used. 

In the second type of filtration, depth or deep-bed filtration, the particles penetrate into the pores of the filter medium, where impacts between the particles and the surface of the medium are largely responsible for their removal and retention. This configuration is commonly used for the removal of fine particles from very dilute suspensions, where the recovery of the particles is not of primary importance. Typical examples here include air and water filtration. The filter bed gradually becomes clogged with particles, and its resistance to flow eventually reaches an unacceptably high level. For continued operation, it is therefore necessary to remove the accumulated solids, and it is important that this can be readily achieved. For this reason, the filter commonly consists of a bed of particulate solids, such as sand, which can be cleaned by back-flushing, often accompanied by... 

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