Filtration

Air Filtration

Smokestacks, garbage incinerators, and other producers of potentially noxious emission are commonly subjected to various types of chemical and water spray scrubbers and baghouse filters to trap undesirable gaseous and particulate pollutants. Homeowners are familiar with replaceable fiberglass air filters that are installed in their home air conditioning or forced-air heating systems.

Water Filtration

Two main types of water filters are in use: (1) granular-medium filters and (2) surface membrane filters. Both types are designed to remove undesirable materials or contaminants, but the membranes can achieve very fine filtration down to 3 Å (angstroms; 1 Å = 10–10 meter), which is at the atomic level.

Granular-medium filters are designed with various configurations of materials such as sand, gravel, anthracite coal, activated charcoal, and other compounds. In the filtration process, the water or wastewater is applied to the top of the filter bed so that the suspended matter in the water is removed in various ways, including mechanical and chance-contact straining; inertial impaction and sedimentation; interception; adhesion and absorption, or the sticking of particles to the filtrate; and flocculation, wherein the particles clump together and are too large to pass through the filter's openings. At some point, the granular-medium filters may become clogged with particulates and have to be backwashed or back-flushed with water or air so that undesired particulates can be disposed of.

Semipermeable-membrane processes of filtration involve separation effected by water passing through the membrane as osmosis or hyperfiltration. The opposite process, wherein solute molecules and ions pass through the membrane, is called dialysis, which is familiar to many as the medical procedure of blood dialysis for those suffering from kidney failure. Membrane processes may be used to desalinize water, treat industrial wastes, or recover economically valuable materials such as precious metals from waste streams. Reverse osmosis is a case in which pressure is applied to a material such as saltwater to drive water without salts through the membrane, leaving the saltier water behind.

Solids and sludge left over after filtration processes need to be further processed and disposed of because they are among the most difficult to deal with in wastewater management. This is because the sludge contains much of the original offensive material of the incoming liquid, only a small part of it may be solid matter, and the waste biological material is organic and will decay. Further filtration may remove more water to thicken the remaining waste. This can be accomplished by vacuum filtration, heating, freezing, pressing, centrifuging, or simple gravity thickening. Drying beds and lagoons may also be employed.

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