Glossary of Sewage Treatment Terminology

activated sludge plant

A plant mixing aerated wastewater with a suspended biomass. A downstream settling tank is used to separate the clarified effluent form the biomass, which is recycled to the inlet of the works.


Conditions where oxygen is present.

ammonia loading rate

The average daily ammonia load present within the wastewater entering a plant expressed as kg NH4-N per m³ of filter medium per day.


Conditions where there is a lack of oxygen or sources of oxygen.

biochemical oxygen demand (BOD5)

The amount of oxygen used by microbial activity when a sample is incubated for 5 days at 20˚C
biofilm A matrix of microbial cells attached to a surface and submerged

biological aerated filter (baf)

An aerated bed containing a submerged medium. Biofilm growing on the medium purifies the settled sewage. Excess biomass is removed by washing the bed.

biological filter (bf)

A bed of medium fitted with a distributor at the top to disperse the wastewater over a biofilm growing on elements of medium. Underdrains allow both drainage of the effluent and natural ventilation of air through the bed of the medium. A downstream humus tank clarifies the effluent.


A watertight tank used for storage of wastewater that has no outlet for the discharge of waste matter. These are not permitted under the Scottish Building Regulations. Water Service (NI) will not adopt cesspools and does not provide an emptying service.

chemical oxygen demand (COD)

Amount of oxygen consumed by the chemical oxidation of matter present in a sample.

controlled waters

Receiving waters, including inland watercourses, lakes and reservoirs, estuaries, coastal waters and groundwater.


Significant reduction in infectious micro-organisms by destruction, inactivation or removal.

drum filter

Cylindrical sieve that rotates about a horizontal axis partially immersed in a horizontal flow of wastewater to remove suspended solids.

dry weather flow (DWF)

The average daily flow of wastewater to a plant (excluding that caused by wastewater)


Viruses that indicate faecal contamination.

environmental regulator

Organisation that monitors consented discharges and receiving waters to ensure compliance of the discharger with the consent.


Enrichment of waters by nutrients enabling luxuriant growth of plant life, which leads to the deterioration of water quality.

faecal coliforms

Group of bacteria of largely faecal origin and used to indicate faecal pollution.

faecal streptococci

Bacteria used as an indicator of faecal pollution.

flow to full treatment (FFT)

The maximum flow that can be treated by a plant.

hydraulic loading rate

The average rate of application of wastewater to a filter, expressed as m³ of wastewater per m³ of medium per day.

humus tank

A settling tank used to clarify effluent by the removal of humus solids that have been washed off filter media.


A filter constructed from polymer or ceramic materials to remove suspended solids and bacteria from wastewater.

membrane activated sludge plant

An aerated tank that mixes waste water with suspended biomass. It is fitted with membranes that filter the biomass from the effluent.

mixed liquor

The mixture of microbial solids and wastewater present in aeration tanks of activated sludge plants.

organic loading rate

The average daily BOD5 load present in wastewater applied to a filter expressed as kg BOD5 per m³ of medium per day.

oxidation ditch

A tank fitted with a suitable device to provide aeration and maintain a flow pattern to mix the wastewater with suspended biomass. A downstream settling tank separates the clarified effluent from biomass, which is then recycled to the inlet of the ditch.

package sewage

treatment plant A sewage treatment plant module or series of linked modules that are constructed in a factory and subsequently transported to site for installation.

per capita flows and loads

The flow rate of domestic wastewater and its organic and nitrogen load expressed as litres or grammes per person.

population equivalent (pe)

Notional population comprising the resident population, an allowance for visitors, and a conversion of trade effluent based on flow (200l/h/d) or biological load (60g BOD5 /d per pe).


Gravel bed planted with reeds to provide either secondary or tertiary treatment.

rotating biological contactor

Discs that are partially immersed in influent and rotate about a horizontal axis. The biofilm growing on the discs alternately contact the watsewater and the air. A downstream humus tank clarifies the effluent.

sand filter

Bed of filter medium that is submerged within a flow of effluent to remove suspended solids.

septic tank

A compartmentalised settling tank that provides conditions for settlement and anaerobic breakdown of solids from wastewater.

sequencing batch reactor

A tank that undergoes a cycle of filling, aeration of suspended biomass and wastewater, settlement and drainage of clarified effluent and withdrawal of the excess biomass.

sewage undertaker

The organisation responsible for providing and maintaining sewers, disposing of the contents and receiving and disposing of trade effluents. In England and Wales this role is carried out by the water utilities, in Scotland by the water authorities and in Northern Ireland by the water service.

submerged aerated filter

Plant similar to biological aerated filter but with a downstream settling tank to remove excess biomass which dislodges from the submerged medium. Forced aeration provides a source of oxygen.

surface loading rate

Instantaneous flow rate of sewage and any recycle stream applied to a filter. It is defined as the total influent flow rate per day divided by the plan area of the filter (m³/m².d).

upflow velocity

Peak flow rate of effluent divided by plan area of settling tank expressed as m³/m².h.