Depth filtration

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... 

There are three (3) types of pharmaceutical filtrations depth, cake, and membrane. Cake and depth are coarse filtrations, and membrane is a fine, final filtration. Membrane filtration and cross-flow filtration are discussed in Ch. 7. 

There are two main types of barriers depth and screen. A depth filter retains particles both on its surface and within its matrix. Figure 37-6, p. 433, shows a depth filter made of fiberglass bonded with acrylic resin to prevent pieces from getting into the filtrate. Depth filters have a random matrix of fibers. The pore structure is irregular so they retain a variety of particle sizes. These filters are given a nominal rating, and they will retain 98% of all particles larger than that size. 

Filtration processes may be classified as either depth filtration or surface filtration. Depth filtration relies on a layer of porous media in which suspended particles in the beer are trapped within the media. Examples in brewing include filter aid filtration, sheet filtration and some forms of filter cartridge. Surface filtration normally refers to membrane technology. A thin layer of membrane has pores throughout the structure. This means that it is possible to achieve very exact filtration, perhaps enabling sterilisation, but typically the quantity of suspended beer particles that may be removed is less than for depth filtration. 

Filtration. In filtration, suspended solid particles in a liquid or gas are removed by passing the mixture through a porous medium that retains the particles and passes the fluid. The solid can be retained on the surface of the filter medium, which is cake, filtration, or captured within the filter medium, which is depth filtration. The filter medium can be arranged in many ways. 

Mud filtrate invasion is normally restricted to within a few inches into the formation, after which the build up of mudcake prevents further filtrate loss. If resistivity tools with different depths of investigation (in the invaded and non-invaded zones) are used to... 

Several types of aggregate-bed filters are available which provide in-depth filtration. Both gravel and particle-bed filters have been developed for removal of dry particulates but have not been used extensively. Filters have also been developed using a porous ceramic or porous metal filter surface. 

Fiber Bed Alist Filtration. In-depth fiber bed filters are used for the collection of Hquid droplets, fogs, and mists. Horizontal pads of knitted metal wire (or plastic fibers), 100—150 mm thick, and gas updow are used for Hquid entrainment removal. Pressure drop is 250—500 Pa (1.9—3.8 mm Hg). 

Depth filters are usually preferred for the most common type of microfiltration system, illustrated schematically in Figure 28. In this process design, called "dead-end" or "in-line" filtration, the entire fluid flow is forced through the membrane under pressure. As particulates accumulate on the membrane surface or in its interior, the pressure required to maintain the required flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particulate loading of the feed solution. In-line microfiltration of solutions as a final polishing step prior to use is a typical apphcation (66,67). 

By filtration mechanism. Although the mechanism for separation and accumulation of solids is not clearly understood, hvo models are generally considered and are the basis for the apphcation of theoiy to the filh ation process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within tne pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration. 

By objective. The process goal of filtration may be diy solids (the cake is the product or value), clarified liquid (the filtrate is the product of value), or both. Good sohds recovery is best obtained by cake filtration, while clarification of the liquid is accomplished by either depth or cake filtration. 

The depth of cut involved in precoat filtration is a veiy important economic factor. There is some disagreement as to the method required to accurately predic t the minimum permissible depth of cut. Some investigators maintain that the depth of cut can be evaluated only in a quah-tative manner during bench-scale tests by judging whether the process solids remain on the surface of the precoat beck This being so, they indicate that it is necessaiy to run a continuous pilot-plant test to determine the minimum permissible depth of cut. The use of a continuous pilot-plant filter is a veiy desirable approach and will provide accurate information under a variety of operating conditions. 

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 ... 

Use of filter aids is a technique frequently applied for filtrations in which problems of slow filtration rate, rapid medium blinding, or un-satisfactoiy filtrate clarity arise. Filter aids are granular or fibrous solids capable of forming a highly permeable filter cake in which veiy fine solids or slimy, deformable floes may be trapped. Application of filter aids may allow the use of a much more permeable filter medium than the clarification would require to produce filtrate of the same quahty by depth filtration. 

Fig. 5.9. AFM image of a Lavsan filtration membrane. Image size 5 )xm x 5 )xm, depth scale 500 nm from black to white.

Generally, sampling of waterways should be at the fastest flowing part of the stream/river, usually mid-depth unless the eontaminant is less dense than water and eould float, or is more dense and eould aeeumulate near the river bed. For lakes representative samples should be taken near to the inflow, outflow and other loeations. If two phases are present both may require sampling. Sample preservation by refrigeration, pH adjustment, elimination of light, filtration, and extraetion may be important. 

It should be noted that the total loss of head of a filter bed is in inverse ratio to the depth of penetration of the matter in suspension. In a normal wastewater treatment plant, the water is brought onto a series of rapid sand filters and the impurities are removed by coagulation-flocculation-filtration. Backwashing is typically performed in the counterfiow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters. 

Quebracho-treated freshwater muds were used in drilling at shallow depths. The name of red mud comes from the deep red color imparted to the mud by quebracho. Muds treated with a mixture of lignite and quebracho, or a mixture of alkaline organic polyphosphate chemicals (alkaline-tannate treated muds), are also included in the quebracho treated muds. The quebracho thinners are very effective at low concentrations, and offer good viscosity and filtration control. The pH of red" muds should be 8.5 to 10 mud temperature should be lower than 230°F. 

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