Types Of Filter Media To Choose From

There are many filter media from which to choose from; however, the optimum type depends on the properties of the suspension and specific process conditions. Filter media may be classified into several groups, however the two most common classes are the surface-type and depth-media-type.

Surface-type filter media are distinguished by the fact that the solid particles of suspension on separation are mostly retained on the medium's surface. That is, particles do not penetrate into the pores. Common examples of this type of media are filter paper, filter cloths, and wire mesh.

Figure 1. Pressure filtration

Depth-type filter media are largely used for wastewater clarification purposes. They are characterized by the fact that the solid particles penetrate into the pores where they are retained. The pores of such media are considerably larger than the particles of suspension. The suspension's concentration is generally not high enough to promote particle bridging inside the pores. Particles are retained on the walls of the pores by adsorption, settling and sticking. In general, depth-type filter media cannot retain all suspended particles, and their retention capacity is typically between 90 and 99 percent. Sand and filter aids, as examples, fall into this category. Some filter media may act as either surface-type or depth-type, depending on the pore size and suspension properties (e.g., particle size, solids concentration and suspension viscosity).

It is also common practice to classify filter media by their materials of construction. Examples are cotton, wool, linen, glass fiber, porosmooth surface caused by carrying the warp (or the weft) on the fabric surface over many weft (or warp) yarns. Intersections between warp and weft are kept to a minimum, just sufficient to hold the fabric firmly together and still provide a smooth fabric surface. The percentage of open area in a textile filter indicates the proportion of total fabric area that is open, and can be determined by the following relationship:

(mesh opening + thread diameter)2

The other type of common weave is called a Dutch weave. Table 1 provides a conversion chart between the two types of weave and micron ratings. Discussions that follow focus on different types of common flexible filter media.

Table 1. Weave Conversion Chart.

Dutch Weave

Square Weave

Micron Rating

12x64

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