Glossary of Wastewater Terms

All the sewage terms and words explained in simple English

Activated Sludge
Sludge which is produced by the growth of bacteria and other organisms in sewage, usually flocculated (in small clusters) when it is continuously aerated.
Activated Sludge Process
A sewage treatment process in which a mixture of raw sewage and activated sludge is aerated, usually via an air blower. The activated sludge is subsequently separated from the treated sewage by settlement in a clarifying chamber
Aerobic Digestion
Digestion of sewage promoted by the action of bacteria in the presence of oxygen.   This occurs in the biozone of sewage treatment units

Anaerobic Digestion
A biological process promoted by the action of bacteria in the absence of oxygen.  This occurs in septic tanks and in the primary settlement tanks of sewage treatment units.

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 ).  This is usually expressed in mg/L
This is a term used for a sewage treatment system using a Rotating Biological Contactor (R.B.C.) It usually refers to a Klargester.

Biological Filter (Media)
Material used to provide surface area for bacterial growth to assist natural aerobic degradation of sewage. It can be discs, plastic shapes, etc.

Biological Treatment
The natural aerobic digestion of the effluent by bacteria and other organisms.
The microbial colony that builds up in a sewage treatment plant
The section of the sewage treatment plant responsible for the digestion of the sewage via micro-organisms.

Cesspool, Cesspit
An underground watertight tank which has no outlet, used for receiving and storing raw sewage   It is emptied by tanker and is illegal in Scotland.

Chemical Oxygen Demand
The amount of oxygen used in the chemical oxidation of a substance by a specified oxidising agent under standard conditions.

Crude Sewage
Raw sewage which has received no treatment.

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

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

These are the drainage pipes that serve a property. There may be foul water, and surface water drains, which are usually in separate systems. Many problems are caused by smells and odours from drains which are badly vented.
Dry Weather Flow ( DWF )
This is the average daily flow to the treatment plant during seven consecutive days without rain following 7 days during which the rainfall did not exceed 0.25mm on any one day.  As rainwater is not now allowed into the sewage treatment unit anyway, this figure is the same as the wet weather flow, usually taken as 180 litres per head per day on domestic properties
The liquid sewage in all its stages of treatment.

Effluent Polishing or Tertiary Treatment
An additional stage of sewage treatment which removes more suspended solids and pollutants. This can be a reed bed, sand filter or Zabel Filter. 

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.

Environment Protection Agency - (E. P.A.)
This body exists in Ireland to provide protection of the environment.

The enrichment of water in watercourses and lakes by chemical substances, especially compounds of nitrogen and phosphorous.
Extended Aeration
A sewage treatment process which aerates the sewage for an extended period of time, usually without any filter media, for more complete reduction of the solids and pollutants.

Filter Medium or Media
The material of which the biological filter is formed and on which a biological film (or biomass) containing bacteria and fungi develops. It is usually inert, eg plastic.

Final Effluent
The effluent discharged from a sewage treatment plant.

Foul Sewage
This is the sewage that is derived from all sinks, baths, and toilets in a property.

Humus Tank (or final settlement tank )
A secondary settlement tank which follows the biozone for the purpose of separating more settleable solids.

The distance from ground level to the bottom of the sewer pipe.
All systems will necessitate some form of maintenance, varying from just emptying the sludge from septic tanks  to considerably more involved maintenance of the system and process in more complicated sewage treatment plants. The more complicated the sewage plant, the more maintenance is required.
Material used to provide surface area for bacterial growth to assist natural aerobic degradation of sewage. It can be discs, plastic shapes, etc.

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 geo-textile membranes and natural stone-fibre in the treatment of sewage effluents.

Sewage is not odourless, but some sewage treatment plants are.

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

Population Equivalent
The equivalent, in terms of a fixed population, of a varying or part-time population (eg. of an office or pub ) based upon a figure of 60 g. BOD per head per day and/or 180 L per head per day and 8mg of ammoniacal nitrogen/day.

Primary Settlement Tank
The first tank in a 3 stage sewage treatment plant, in which the majority of settleable solids are removed from the crude sewage that will flow into it.  They are usually not without some odour problems unless vented, like the Biorock.

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 which pump the sewage.
Reed Beds
These are usually specially constructed reed beds which contain reeds (frequently the Phragmites Australis reed) to biologically treat sewage treatment unit effluents to a higher standard.   They don't work well for septic tank effluent.  Many natural reed beds exist in wetland areas.

Rotating Biological Contactor (RBC)
This is a system of sewage treatment, that uses closely spaced parallel discs mounted on a horizontal shaft, which rotate about a horizontal axis, and the discs are alternatively exposed to sewage effluent and air as the discs turn to biologically treat the sewage.  The most well known one is the Klargester biodisc. 

Secondary Settlement tank
A tank in which settleable solids or humus is separated from the effluent flowing through it.

Separate Drainage System
A drainage system in which foul and surface water are discharge into seperate pipe systems.

Septic Tanks
A type of settlement tank in which the sludge is retained for sufficient time for the organic matter to undergo some anaerobic decomposition.   The old brick-built ones tend to achieve a better quality of effluent than the modern 'bottle' shaped units.

The water-borne wastes including toilet, sink, bath, washing machine, etc. of a house or community.

Sewage Treatment Works
The plant which treats the sewage.

Sewage Treatment Plants
The modern version of the septic tank which cleans the effluent to a higher standard.

The network of pipes which conveys the sewage

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

Submersible Pumps
These are usually electrically driven pumps, housed in a water proof casing, which can be submerged in a liquid and pump the liquid away to a drain.
Surface Water
This is rain water which falls on Roof's, Drives, Roads, Car Parks 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 settled solids at the bottom of the tank and any floating scum or crust.

Suspended Solids (SS )
Solids in suspension in sewage liquors as measured by filtration through a filter paper followed by washing and drying at 105 deg. 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 telephone land lines.  Often causes problems. 
Tertiary Treatment
An additional stage of sewage treatment which removes more suspended solids and pollutants. This can be a reed bed, sand filter or Zabel Filter, etc. 

This usually refers to the toxic element of waterborne wastes, eg. metals, pesticides, or other chemicals which cause pollution of streams, watercourses, rivers or ground water.

See suspended solids

Water Quality
The quality of water in rivers, lakes, streams or watercourses, as regards pollutants, BOD, etc. as well as the quality of Drinking Water

Water Table
The level below ground at which the ground is saturated with water.  This must NEVER rise to more than 1 metre below the soakaway pipe.