Filtration

Liquid filtration occurs when a suspension of a solid in a liquid passes through a filter. That process takes place when the liquid is pulled through the filter by gravitational force (as in the laboratory example mentioned above) or is forced through the filter by some applied pressure or by a pressure differential supplied by the existence of a vacuum.

One of the most familiar gravity filters in the industrial world is that used for the purification of water. A water filtration system generally makes use of a thick layer of granular materials, such as sand, gravel, and charcoal. Such a filter may be many feet thick and is known, therefore, as a deep-bed filter. When impure water passes through such a filter, suspended solids are removed, allowing relatively pure water to be collected at the bottom of the filter. In commercial water purification plants, the deep-bed filter may be modified so as to remove other impurities. For example, dissolved gases that add unpleasant odors and taste to the water may be removed if activated carbon (finely divided charcoal) is included in the filter. The gases responsible for offensive odor and taste are absorbed on particles of charcoal, leaving an effluent that is nearly odorless and tasteless.

The filtration of smaller volumes of solution than those normally encountered in a water filtration plant is often accomplished by means of positive pressure systems. A positive pressure system is one in which the fluid to be filtered is forced through a filtering medium by an external pressure. A number of variations on this concept are commercially available. For example, in one type of apparatus, the fluid to be filtered is introduced under pressure at one end of a horizontal tank and then forced through a series of vertical plates covered with thin filtering cloths. As the fluid passes through these filters, solids are removed and collect on the surface of the cloths. The material that builds up on the filters is known as a cake, and the filters themselves are sometimes called cake filters.

In another type of pressure filter a series of filter plates is arranged one above the other in a cylindrical tank. Liquid is pumped into the tank under pressure, which forces it downward through the filters. Again, solids suspended in the liquid collect on the filters while the clear liquid passes out of the tank through a drain pipe in the center of the unit.

A variety of vacuum filters have also been designed. In a vacuum filter, the liquid to be separated is poured onto a filtering medium and a vacuum is created below the medium. Atmospheric pressure above the filter then forces the liquid through the medium with suspended solids collecting on the filter and the clear liquid passing through.

Probably the most common variation of the vacuum filter is the continuous rotary vacuum filter. In this device, a drum with a perforated surface rotates on a horizontal axis. A cloth covering the drum acts as the filter. The lower part of the drum is submerged in the liquid to be separated and a vacuum is maintained within the drum. As the drum rotates, it passes through the liquid and atmospheric pressure forces liquid into its interior. Solids suspended in the liquid are removed by the filter and collect as a cake on the outside of the drum. Because the cake can constantly be removed by a stream of water, the drum can continue to rotate and filter the suspension in the pan below it.