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 wastewaters. One of the standard tests used to characterise effluent quality.

biological filtration

A process in which settled sewage uniformly trickles downward through a bed of inert material such as slag, moulded plastics or clinker, thus permitting contact with the biological film with which the surfaces of the medium are coated so that oxidation and clarification take place.

discharge consent/registered standards

An authorisation issued by an environmental regulator to allow discharge of treated water. Consents are set to minimise pollution of receiving waters. Consents specify the discharge location, total volume of treated water permitted and the quality conditions of the discharged water, such as the biochemical oxygen demand, limits for suspended solids, ammonia, metals and toxic substances.

descriptive consent

A consent under which discharges are controlled by imposing general rather than numerical standards.

coliform bacteria

A group of bacteria found in the intestine and faeces of most animals. Coliforms can sometimes be found in untreated water. The treatment process removes them and disinfection prevents their reappearance in the distribution system. In water receiving discharges, faecal coliform bacteria are used to indicate the presence of sewage.


The destruction of pathogens by physical or chemical means.


The enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus, causing an accelerated growth of algae and higher forms of plant life to produce disturbance to the balance of organisms present in the water and to the quality of the water concerned.

final effluent

Typically, the effluent discharged from a treatment plant after completion of treatment of a domestic or industrial wastewater.

e. coli (escherichia coli)

A bacterium taken as an indicator of faecal contamination.


Water occurring in permeable underground strata, eg chalk and sandstone.

industrial wastewater

Any wastewater which is discharged from trade or industrial premises, other than domestic wastewater and run-off rainwater.

numeric consent

A consent in which numerical limits are set for the concentration or load of substances discharged and also for the effluent flow.


The site of discharge of a liquid from a pipe. Applied particularly to the point at which a sewer discharges to a treatment works or receiving water, or the point at which a conduit discharges the effluent from a treatment works into a receiving water.


A organism which is capable of producing disease.

polluting load

The quantity of polluting matter entering a treatment plant or in the effluent discharged into a receiving water during a given period.

population equivalent

The unit of measure used to describe the size of a waste water discharge. Population equivalent is the biodegradable load (matter) in waste water having a 5-day biochemical oxygen demand (BOD) of 60g of oxygen per day. Population equivalent doesn't necessarily reflect the actual population of a community.

private sewer

A sewer which is not a public sewer as defined in Section 20 of the Public Health Act 1936.

public sewer

A sewer vested in a water authority by virtue of the provisions of Section 20 of the Public Health Act 1936, as amended by Section 40 and Paragraph 33 of the 8th schedule to the Water Act 1973.


The gross solids, including rags and plastics, in sewage are intercepted by screens and removed manually or by raking mechanisms.

septic tank

A type of sedimentation tank in which the sludge is retained sufficiently long for the organic content to undergo anaerobic digestion. When sludge is eventually removed to a sewage treatment works, some is left in the tank to act as a 'seed' to initiate further digestion. Used for receiving the sewage from houses and other premises which are too isolated for connection to a foul sewer.


The water-borne wastes of a community

sewage sludge (also known as 'biosolids' or 'sludge')

A by-product arising from the treatment of sewage or from septic tanks or similar installations.

sewage (treatment) works (STW), waste water treatment plant (wwtw)

A term for the structures, plant and equipment used for collecting and treating sewage, normally with some sludge drying.


A pipe conveying wastewater or sewage discharged into it from two or more house drains.


A system of pipes and mechanical appliances for the collection and transportation of domestic and industrial wastewaters.

storm sewage tank

A tank into which, in wet weather, is diverted all the sewage and rainwater reaching a treatment works in excess of that which is to receive biological treatment. Its purpose is to store as much of the storm sewage as possible, for return to the works inlet after the flow has returned to normal, and to remove settleable solids from the remainder which overflows from the tank to a receiving water.


Bacterial indicator of faecal contamination of water.

surface water

The run-off from paved and unpaved roads, buildings and land.

suspended solids

In sewage analysis, those solids retained after filtration.

total dissolved solids (TDS)

The concentration of dissolved solids in a wastewater or effluent, ie the residue after evaporation and drying, expressed in milligrams per litre of sample.

trade effluent

Wastewater produced by trade and industry; not domestic sewage.

ultraviolet (UV) treatment

Treatment of drinking water or effluent with UV rays to neutralise bacteria.