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Definitions of Words or Terms - Glossary

Activated Sludge

Flocculent sludge produced by the growth of bacteria and other organisms in raw or settled sewage, when it is continuously aerated.

Activated Sludge Process

A biological treatment process in which a mixture of sewage and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated sewage by settlement and may be re-used.

Aerobic action

A biological process promoted by action of bacteria in the presence of dissolved oxygen.

Anaerobic action

A biological process promoted by the action of bacteria in the absence of dissolved oxygen.

Biochemical Oxygen Demand (BOD)

The amount of dissolved oxygen consumed by micro-biological action when a sample is incubated, usually for 5 days at 20 deg. C. (in the UK expressed as BOD5). In some countries the BOD test is carried out over differing periods such as 7 days (BOD7), and 10 days (BOD10).


This is a generic term used for a packaged type of complete sewage treatment system using a Rotating Biological Contactor (R.B.C.)

Biological Filter

A bed of relatively inert material to promote or assist natural aerobic degradation of sewage.

Biological Treatment

This is a stage in the treatment of sewage or other effluents that biologically treats the pollution frequently by the natural aerobic degradation of the pollutant.

Centrifugal Pumps

A lot of sewage pumps are of the centrifugal type, where an electrically driven impellor rotates at a high speed and discharges the sewage through a volute into the pumping main. (See also submersible pumps.)


A covered watertight tank used for receiving and storing sewage from premises that cannot be connected to the public sewer and where conditions prevent the use of a small sewage treatment works, including a septic tank.

Chemical Oxygen Demand

The amount of oxygen used in the chemical oxidation of the matter present in a sample, by a specified oxidising agent under standard conditions.


Colloidal Material

The finely divided solids which will not settle but which may be removed by coagulation.

Crude Sewage

Sewage that has received no treatment.


The biochemical decomposition of organic matter using anaerobic bacteria, which results in the formation of simpler and less offensive compounds.


A device for spreading settled sewage over the surface of a biological filter.

Dosing Chamber

A small tank that receives settled sewage until the desired quantity has accumulated, when it is discharged automatically to the distributor of a biological filter.


These are usually the smaller pipes that serve a single property, and they join up to form a Drainage System. There may be foul water and surface water drains, which are usually in separate systems. Many queries are caused by drainage problems and odours or smells from these systems.

Dry Weather Flow (DWF)

When the sewage flow is mainly domestic in character, this is the average daily flow to the treatment works during seven consecutive days without rain, following seven days during which the rainfall did not exceed 0.25mm on any one day. Usually taken as 200 litres per head per day on domestic properties. (Note - this figure will vary according to your location and specialist help must be sought.)  Effluent Polishing or Tertiary Treatment.  A further stage of treating sewage or effluents, by removing suspended solids and or pollutants. Consequential removal of suspended solids may also remove residual BOD or other pollutants.

Environment Agency

In England & Wales the main body for protection of the environment is the Environment Agency.  In Scotland there is the Scottish Environment Protection Agency (S.E.P.A.) In other parts of the world similar bodies exist and you can usually contact them locally.

Environmental Laws

In many parts of the world there are now Environmental laws that investigate, protect and legislate to ensure our environment is protected.

Environment Protection Agency  (E.P.A.)

This body exists in the USA to provide protection of the environment.


The enrichment of water in watercourses and lakes by chemical substances, especially compounds of nitrogen and phosphorous.

Filter Medium

The material of which the biological filter is formed and on which a biological film (or biomass) containing bacteria and fungi, develops.

Final Effluent

The effluent discharged from a sewage treatment plant.

Foul Sewage

This is the sewage that is derived from all sinks, baths, and water closets in a property.

Humus Tank (or final settlement tank)

The secondary settlement tank through which biologically treated effluent flows, for the purpose of separating settle-able solids.

Hydraulic Gradient

The surface slope of a liquid in a pipeline. This is generally taken as parallel to the invert in a smaller gravity sewer, but it must be checked. In a treatment works this will be the overall head losses.


The lowest point of the internal cross section of a channel or sewer.

Inverted Syphon

A portion of pipe or conduit in which the sewage flows under pressure, due to the sewer dropping below the hydraulic gradient and then rising again.


All systems will necessitate some form of maintenance, varying between just emptying from tanks the residual sludge in simple systems, to considerably more involved maintenance of the system and process in more complicated plants.


New technology has been developed whereby the pollutants in liquids can be removed by the use of ultra fine membranes, such as the Reverse Osmosis method in the treatment of fresh potable water, or membranes in the treatment of polluted water or effluents.


Sewage by its very nature will produce odours, and these can sometimes be a problem at sewage pumping stations or sewage treatment works. At a good treatment works there will be very little odour, but a lot will depend on the sludge produced at the works. It is therefore vital when deciding the site for a pumping station or treatment works that possible odours from the plant be considered, regarding the proposed site’s proximity to the property etc.


The chemical change that a substance undergoes when it takes up oxygen.

Percolating Filter

See Biological Filter.

Population Equivalent

The equivalent, in terms of a fixed population, of a varying or transient population (eg. of a hospital or restaurant) based upon a figure of 60g BOD per head per day and/or 200 litres per head per day.

Positive Displacement (or PD) Pumps

These types of pumps invariably sit at ground level, and pump the liquid through an archimedian screw system into a pumping main.

Primary Settlement Tank

A tank in which the majority of settle-able solids are removed from the crude sewage that will flow into it.

Public Health

There have been many public health acts produced throughout the world to reduce public health problems and stop pollution of the environment. In the UK the first Act published was the Public Health Act 1848, and subsequent acts have been enacted in, Public Health Act 1875, Public Health Act 1936 and Public Health Act 1961.

Pumping Main (or Rising Main)

This is a continuous main through which sewage or effluent is pumped and running full and at a pressure greater than atmospheric, to a final destination.

Pumping Station

This is usually an underground structure that the foul (or surface water) sewage is discharged into. The types vary but in smaller systems these comprise of a wet well, into which the sewage is discharged, and the wet well also houses submersible pumps that pump the sewage to its destination. In a larger station there may be a separate dry well, adjacent to the wet well, which houses the pumps. On some pumping stations the pumps may be housed above ground near the wet well.

Reed Beds

These are usually specially constructed beds which contain reeds (frequently the Phragmites Australis reed) to biologically treat sewage and other effluents. Many natural reed beds exist in wetland areas.

Rotating Biological Contactor (RBC)

This is a system of sewage or effluent treatment that uses closely spaced parallel discs mounted on a horizontal shaft, which rotate about a horizontal axis.  The discs are alternatively exposed to polluted liquors and air as the shaft rotates, to biologically treat the sewage.

Secondary Settlement Tank

A tank in which settle-able solids or humus is separated from the effluent flowing through it, from biological filters or other biological treatment units.

Separate System

A drainage system in which foul and surface waters are conveyed by separate pipes.

Septic Tank

A type of settlement tank in which the sludge is retained for sufficient time for the organic matter to undergo anaerobic decomposition.

Settlement (or Sediment) Tank

This is a tank that is used in the treatment of sewage or effluents to settle out the suspended solids contained in the liquids. The tank may be sited near the works inlet as a 'primary settlement tank' or alternatively, after biological treatment, as a final (humus) settlement tank.


The water-borne wastes of a house or community.

Sewage Treatment Works

The site that contains all the necessary plant for the treatment of sewage.

Sewage Treatment Plant

The items of equipment or structures that treat the sewage.


This term is used to describe such items as a 'sewerage system' or 'sewerage network'. (For example - sewage runs in a sewerage system.)


A mixture of solids and water produced during the treatment of wastewater or sewage. This will frequently have to be removed from the treatment system by de-sludging.

Sludge Removal or De-Sludging

This is the process of removing sludge from treatment systems or tanks and can be carried out manually or automatically. All sludge removed from tanks or systems should be transported with care and in accordance with current legislation.


See Odours.

Storm Sewage (or Surface Water Sewage)

Sewage flowing to a treatment works in wet weather or discharged from storm overflows when the sewage is diluted with rainwater.

Submersible Pumps

These are usually electrically driven centrifugal pumps, housed in a waterproof casing, which can be submerged in a liquid, to pump the liquid through a pumping main.

Surface Water

This is rainwater which falls on Roofs, Drives, Roads, Car Parks etc which should be discharged into a surface water sewerage system. It should not be discharged into a foul sewerage system, (although in many urban areas it is already connected to the foul system) and should be discharged into nearby watercourses, streams, rivers, lakes or to sea.

Supernatant Liquor

The liquor in a settlement tank, lying between the deposited solids and any floating scum.

Suspended Solids (SS)

Solids in suspension in sewage liquors as measured by filtration through a filter paper, followed by washing and drying at 105 degrees C.


This is a system of remote monitoring, and in some cases controlling, outstations comprising pumping stations or sewage treatment systems from a central control point. The transmitting of information between locations may be carried out via a radio system or alternatively landlines (PSTN system). Sometimes referred to as a SCADA system.

Tertiary Treatment (See also Effluent Polishing)

There are many differing types of tertiary treatment of effluents, the most common being: Grass Plots, Reed Beds, Upward flow Clarifier, Rapid Gravity Sand Filter, Microstrainer, Sand Filter, Drum Filter, Lagoons, Nitrifying Filter.

Top Water Level (TWL)

The maximum water level in a settlement tank, an aeration tank or a sludge storage tank.


This usually refers to the toxic element of waterborne wastes, and the toxic elements may comprise of metals, pesticides, or other chemicals that cause pollution of streams, watercourses, rivers, beaches, or ground water.


See Suspended Solids.


There are many types of valves that are used to control sewerage or effluent systems. These can vary: non return valves, gate valves, sluice valves, penstock valves etc, each of which has a specific purpose.

Water Quality

This term is used to describe the quality of water in rivers, lakes, streams or watercourses, as well as the quality of Potable or Drinking Water

Water Table

The level below which the ground is saturated with water.


Many natural wetland areas exist throughout the world and these are frequently found where there is some form of pollution that they use as a nutrient feed. In some areas, natural wetlands have been constructed by man in order to treat sewage or other forms of effluent pollution.

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Redditch, Worcestershire, B98 8LG

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Glossary - Sewage Pumps and Sewage Treatment Plants Glossary - Sewage Pumps and Sewage Treatment Plants