Filtration filter layers function

Porosity constitutes a important criterion in a description based on straining. Porosity is determined by the formula V /Vc, in which V c is the total or apparent volume limitated by the filter wall and is the free volume between the particles. The porosity of a filter layer changes as a function of the operation time of the filters. The grains become thicker because of the adherence of material removed from the water, whether by straining or by some other fixative mechanism of particles on the filtering sand. Simultaneously the interstices between the grains diminish in size. This effect assists the filtration process, in particular for slow sand filters, where a deposit is formed as a skin or layer of slime that has settled on the... 

The choice of the filter medium is often the most important consideration to ensure efficient operation of a filter. Its function is generally to act as a support for the filter cake, while the initial layers of cake provide the actual filter. The filter medium should be selected primarily on the basis of its ability to retain solids without binding. It should be mechanically strong and corrosion resistant, and should offer as little resistance as possible to the flow of the filtrate. The media are made from widely different materials such as cotton, wool, linen, nylon, jute, silk, glass fiber, porous carbon, metals, rayon and other synthetics, and miscellaneous materials like porous rubber. Cotton fabrics are most commonly used because they are available in a wide variety of weaves, and are cheap. 

Cake filtration is based on passing a suspension through a permeable, relatively thin medium (chapter 8). The sohds are deposited in the form of a cake on the upstream side of the medium. As soon as the first layer of cake is formed, the subsequent filtration takes place on top of this cake and the medium provides only a supporting function. These so-called surface filters are best used for filtration of suspensions of solids concentrations in excess of 1% by volume in order to minimize medium blinding which occurs in the filtration of dilute suspensions. If cake filtration is to be applied to clarification of liquids, which imphes low feed concentrations of sohds, addition of filter aids is usually necessary—see section 2.2. 

It has to be stressed that geotextiles cannot always replace the granular filter completely (see Sec. 19.2.3). A granular layer can often be needed to reduce (damp) the hydraulic loadings (internal gradients) to an acceptable level at the soil interface. After that, a geotextile can be applied to fulfill the filtration function. 

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