Filtration is the process by which solid materials are removed from a fluid mixture, either a gas or liquid mixture. Anyone who has ever prepared foods in a kitchen has probably seen one of the simplest forms of filtration. After cooking pasta, for example, the contents of the pot may be poured through a colander or sieve. (A colander looks like a big pot with holes in it.) The pasta is captured in the colander, and the wastewater runs through holes and is usually thrown away.

Beginning science students often use filtration, too. The solid material (precipitate) formed in a chemical reaction can be separated from the liquid part of a mixture by passing the mixture through a filter paper. The filter paper traps solid particles in the mixture, while a clear solution passes through the filter, down the funnel, and into a receiving container.

Filtration is carried out for one of two general purposes: in order to capture the solid material suspended in the fluid or in order to clarify the fluid in which the solid is suspended. The general principle is the same in either case, although the specific filtration system employed may differ depending on which of these objectives is intended.

In the world outside of chemistry laboratories, a very great variety of filtration systems are available. These systems can be categorized according to the fluids on which they operate (gaseous or liquid) and the driving force that moves fluids through them (gravity, vacuum, or pressure). They also can be subdivided depending on the type of material used as a filter.

Liquid 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 pressure applied to the mixture. In some filtration systems, a vacuum is maintained in the receiving container. Then normal atmospheric pressure has a greater effect in forcing the mixture through the filter.

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 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 behind fluid 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 also have 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; atmospheric pressure then 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.

Clarifying filters

The filters described thus far are used most commonly to collect a solid material suspended in a liquid. Clarifying filters, on the other hand, are designed to collect a liquid that is as free from solid impurities as possible. The most important feature of a clarifying filter, then, is the filter itself. It must be constructed in such a way as to remove the very smallest particles suspended in the liquid. A number of systems have been developed to achieve this objective. Some rely on the use of wires or fibers spaced very closely together. Others make use of finely powdered materials, such as diatomaceous earth.

Gas filtration

Examples of gas filtration are common in everyday life. For example, every time a vacuum cleaner runs, it passes a stream of dust-filled air through a filtering bag inside the machine. Solid particles are trapped within the bag, while clean air passes out through the machine.

The removal of solid particles from air and other gases is a common problem in society. Today, air conditioning and heating systems not only change the temperature of a room, but also remove dust, pollen, and other particles that may cause respiratory problems for humans.

The cleansing of waste gases is also a significant problem for many industrial operations. Gases discharged from coal- and oil-burning power plants, for example, usually contain solid particles that cause air pollution and acid rain. One way to remove these particles is to pass them through a filtering system that physically collects the particles leaving a clean (or cleaner) effluent gas.

 

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