Glossary

A

Absorption

Uptake of a substance into the bulk of another material, as a sponge takes up water. Often confused with adsorption.

Activated alumina

A highly porous form of aluminium oxide, usually supplied as balls, used as a desiccant and for removing fluoride from water.

Activated carbon

Carbon processed to develop an extensive internal pore structure, giving it a very large surface on which impurities are held by adsorption.

Activation

The step in making activated carbon that opens up its pore structure, using steam or gases at high temperature, or chemicals.

Adsorption

The attachment of molecules from a liquid or gas to the surface of a solid. It is how activated carbon, activated alumina and silica gel work.

Apparent density

The mass of a packed volume of carbon, including the spaces between particles. Used to work out how much carbon fills a vessel.

Ash content

The mineral, non-carbon fraction of activated carbon that remains after burning. Lower ash matters in sensitive processes.

B

Body feed

Filter aid dosed into a liquid during filtration so the growing filter cake stays porous.

Breakthrough

The point at which impurities start to appear in the outlet of an adsorbent bed, showing the bed is nearing exhaustion.

D

Decolourisation

Removal of colour bodies from a solution, commonly with powdered activated carbon followed by filtration.

Defluoridation

Reduction of fluoride in drinking water, commonly using beds of activated alumina.

Desiccant

A material that adsorbs water vapour, such as silica gel or activated alumina, used to dry air, gases and packaged goods.

Dew point

The temperature at which water vapour in a gas begins to condense. Dryers are specified by the dew point they achieve.

E

Empty bed contact time (EBCT)

The volume of an adsorbent bed divided by the flow rate through it; a measure of how long the liquid is in contact with the bed.

F

Filter aid

A fine, porous powder that forms a permeable layer on a filter medium, trapping fine solids and keeping filtration steady.

G

Granular activated carbon (GAC)

Activated carbon sized into granules and used in fixed beds or columns for continuous treatment.

H

Hardness (carbon)

The resistance of carbon granules to breaking down in handling and use, important for beds that are backwashed.

Hydrated lime

Calcium hydroxide, Ca(OH)₂, an alkali used to raise pH, soften water and neutralise acidic effluent.

I

Iodine value

The milligrams of iodine adsorbed by one gram of carbon under standard test conditions; an indicator of capacity for small molecules.

M

Mesh size

The particle size range of granular carbon expressed as sieve numbers; for example, 8x30 means particles pass an 8 mesh sieve and are retained on a 30 mesh sieve.

Methylene blue value

The amount of methylene blue dye adsorbed by carbon under standard conditions; an indicator of capacity for larger molecules, relevant to decolourisation.

P

Powdered activated carbon (PAC)

Activated carbon ground to a fine powder, dosed into liquids and then removed by filtration or settling.

Precoat

A thin layer of filter aid laid on the filter medium before filtration starts.

R

Reactivation

Thermal treatment of spent granular carbon to remove adsorbed material and restore much of its capacity for reuse.

S

Silica gel

A porous, amorphous form of silicon dioxide that adsorbs water vapour; used as a desiccant.

W

Washed activated carbon

Activated carbon treated with acid and water after activation to reduce ash and soluble matter.