We have provided this glossary to assist you with the terms frequently used within the water industry.

Air Scour:
A techniques whereby air is forced up through a filter bed to agitate the filter media. This agitation both breaks up large particles of dirt on the surface of the filter bed and causes the grains of filter media to rub together thereby removing fine dirt particles adhering to the surface of the grains.

In natural waters this is the amount of bicarbonate contained in the water.

Open channel for transporting water.

A technique used where water is pumped through a filter bed ( normally upflow ) which causes the filter media to loosen and expand. It is normally used in conjunction with Air Scour to flush released dirt particles out of the filter chamber into a collection channel for treatment After the backwash is completed the expanded filter media settles into its constituent layers.

Biochemical Oxygen Demand (BOD):
The quantity of dissolved oxygen in water (mg/l) consumed under test conditions during a given period (5 days) through the microbiological oxidation of biodegradable organic matter present in waste waters. One of the standard tests used to characterise effluent quality.

Chemical Oxygen Demand (COD):

The quantity of oxygen equivalent to the amount of oxidising agent consumed in oxidising the majority of organic matter present in waste waters. Does not distinguish between the organic matter liable to be degraded readily by biological means and the more intractable forms. Can include the oxygen demand of some inorganic substances such as sulphides. Measured by testing a sample of the wastewater with potassium dichromate and sulphuric acid.

The application of chlorine to water for the purpose of disinfection.

Settling tanks.

The precipitated matter (floc) is physically mixed to create large floc particles.

Is usually due to dissolved organic matter in the treatment water. Can be removed / treated using activated carbon within the filter bed or , for larger particles, flocculation .

Dissolved Oxygen:
A measure of the amount of oxygen dissolved in water, expressed as either: (i) mg/l which is the absolute amount of oxygen dissolved in the water mass (ii) as percentage saturation of the water with O2 (% sat).

Dissolved Solids:
The total colloidal and suspended solids in a liquid. Any particle passing a 1.2 µm filter is defined as dissolved.

The removal of harmful bacteria and pathogens using chlorine.

Effective Size:
A measure of filter media size. Defined as the size of the aperture in a sieve which retains 90% of the sample being tested. Since filter media are naturally occurring substances it is impossible to produce grains of equal size and diameter and hence a method of specifying a band of sizes ( effective size ) is used.

A process whereby suspended and colloidal matter is removed from water and wastewater by passage through a granular medium.

Are particles of filter media which are so small that they can cause an excessive pressure drop in the filter . These fines should be removed using several backwash cycles particularly after new media installations. Fines arise in the normal operation of filter plant due to abrasion between filter grains ( particularly so during backwash and air scour ).

Five-day Biochemical Oxygen Demand (BOD5):
A measure of the amount of oxygen used by bacteria to degrade organic matter in a sample of wastewater over a 5 day period at 20 ºC, expressed in mg/1.

The water treatment process in which particle collisions are induced in order to encourage the growth of larger particles.

A process by which suspended matter is lifted to the surface of a liquid to facilitate its removal. Frequently done by the bubbling of air through the liquid.

The addition of small amounts of chemicals (fluoride) to the water, to strengthen teeth.

Granular Activated Carbon ( GAC ):
Installed in rapid gravity filters to remove dissolved organic contaminants and for the control of taste and odour

A filter media which is coarser than coarse sand and is used as a support layer in filter beds. The shape of the gravel is of particular importance and is always a round type since this does not rotate during backwash cycles to allow filter sand to migrate towards the filter nozzles as can be the case with gravel which is predominantly made up of long thin pieces.

That area where the two layers of filter media physically meet

Series of pipes running across the filter chamber floor with filter nozzles installed in them. These pipes act as collectors of filtered water and channel this to the collection pipes.

Those pores in the activated carbon with a radius greater than 25nm found at the entrance to the media.


Those pores in the activated carbon with a radius between 1nm and 25nm used for transportation within the media.

Term used to define pores ( usually in activated carbon filter media ) with a radius less than 1nm used in adsorption .

A condition associated with dead spots within a filter chamber where the accumulation of dirt particles builds up into a clay like mass which , over time, becomes heavier than the filter bed itself and, during wash cycles, falls to the bottom of the filter bed. Mudballing often occurs near the edges of the filter chamber . Proper attention to the backwash and air scour processes can prevent the occurrence of mudballing.

A measure of the acidity or basicity of a solution i.e. the negative of the logarithm of the hydrogen ion concentration.

Rapid Gravity Filter (RGF):
A filter used in water treatment which removes suspended solids from water by passing it through a sand bed, where the solids collect as a surface mat and in the sand interstices. The water should previously have been treated by coagulation, flocculation and sedimentation.

Retention Time:
The length of time a wastewater remains in a clarification tank, an important design parameter in the optimisation of settling of suspended solids.

Rotating Biological Contactor:
A form of biological treatment in which fixed media is grown on circular discs mounted on a horizontal axle. These discs are partially submerged in wastewater while the axle rotates, allowing bio-oxidation of the wastewater, using oxygen from the air.

Roughing Filter:
A high-rate trickling filter of depth 1 - 2m through which wastewater may be passed prior to an activated sludge treatment.

Service Reservoir:
A reservoir where treated water is stored.

Settling Tank:
A rectangular or circular tank in which particle velocities within the liquid are sufficiently reduced to allow the suspended material to be removed from the liquid by gravity settling.

Settling Velocity:
This is the velocity at which a particle will fall to the bottom of a settling tank and is equal to the surface overflow rate for a rectangular tank.

Slow Sand Filter:
A filter which removes suspended solids from raw water by passing it through a large shallow sand bed, where the solids collect as a surface mat and in the sand interstices. Filtration rates are in the order of 2 - 51m3/hr.

The accumulation of solids resulting from chemical coagulation, flocculation and sedimentation after water or wastewater treatment.

Sludge Conditioning:
Addition of chemicals, polyelectrolytes or heat treatment to improve the rate of dewatering.

Sludge Dewatering:
The mechanical unit operation used to reduce the moisture content of sludge to 70 - 75 percent and thus ensure that the remaining sludge residue effectively behaves as a solid for handling purposes.

Sludge Stabilisation:
The process of destroying or inactivating pathogens.

Sludge Volume Index (SVI):

A measure of the ability of sludge to settle, coalesce and compact on settlement.

Sludge Cake:
Sludge that has been dewatered to the extent that it can be handled as a solid, usually containing more than 15 per cent dry solids, depending on the type of sludge and method of dewatering.

Sludge Disposal:
Removal of the by-product (large amounts of floc, termed sludge) from the site.

Suspended Solids:
Solids in suspension in a water or wastewater which can be removed by filtration.

Total Dissolved Solids (TDS):
The concentration of dissolved solids in a wastewater or effluent, i.e. the residue after evaporation and drying, expressed in milligrams per litre of sample.

Trickling Filter:
A biological reactor in which micro-organisms, growing as a slime on the surface of fixed media, oxidise the colloidal and dissolved organic matter in wastewater using atmospheric oxygen which diffuses into the thin film of liquid as the wastewater is trickled over the slimed surfaces at regular intervals.

The clarity of water, i.e. a measure of the accumulation of colloidal particles, determined by light transmission through the water.

Uniformity Coefficient:
A measure of the uniformity of filter media grain size obtained by dividing the sieve opening ( in mm ) which retains 40% of the sample by that which retains 90% of the sample.

Void Volume:
Is defined as the volume existing between filter media particles which is between 30% and 50% of the gross bed volume.

Water Treatment:
The process of converting raw untreated water to a public water supply safe for human consumption. Can involve, variously, screening, initial disinfection, clarification, filtration, pH correction, final disinfection.

Water Treatment Works (WTW):
Works which treat raw water to produce potable water for public supply.