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Drains and Sewers Terms Definitions

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Drains and Sewers Terms Definitions


A surface at the base of an inspection chamber or manhole to confine the flow of sewage to avoid the accumulation of deposits and provide a safe working surface. The surface is sloped so that any surcharge flow runs off it.


Material that will fill interstices, irregularities and excavated soft spots in the exposed trench bottom and, when adequately compacted, will create a firm uniform formation on which to place the pipe bedding material.

Branch drain

A pipeline installed to discharge into a junction on another pipeline or at a point of access such as an access junction, inspection chamber or manhole.

Branch vent

A ventilating pipe connected to one or more branch drains.


A pit formed of concrete or brickwork in which sewage is allowed to collect. The pit will be periodically emptied using purpose-made hydraulic equipment.


Highest point of the external surface of a pipe at any cross section.

Deep manhole

A manhole of such depth that an access shaft is required in addition to the working chamber.

Drop-pipe connection

A vertical connection to or near to the invert level of a manhole from a sewer or drain at a higher level.

Flexible pipe

Pipe that deforms to a significant extent before collapse, e.g. plastic.

Flexible pipeline of rigid pipes

A line of rigid pipes with flexible joints.


The finished level of the excavation at the bottom of a shaft, trench or heading, prepared to receive the permanent work, such as the pipe bedding.

Foul Water

Water discharged after being used in households or in a process. Wastewater is now the international definition.

Inspection chamber

A covered chamber constructed on a drain or sewer to provide access from the ground surface for inspecting, testing or the clearance and removal of obstructions, and usually situated in areas subjected to light loading only.


The lowest point of the internal surface of a drain, sewer or channel at any cross section.

Inverted siphon

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


A fitting on a pipeline to receive a discharge from a branch drain.


A small shaft, constructed of pipes, for the purpose of lowering a lamp into the sewer to facilitate inspection and to indicate change of direction in a sewer between manholes.


Either a pipe or cable—a primary distribution system, normally located beneath an adaptable area such as a footway or service zone.


A working chamber with cover constructed on a drain or sewer within which a person may inspect, test or clear and remove obstructions.

Nominal size (DN)

A numerical designation of the size of a pipe, bend or branch fitting. Confusingly it may refer to either the internal or external diameter of the pipe according to the standard for the material.

Rigid pipe

Pipe that fractures before significant deformation occurs, e.g. clay-ware and concrete.

Rigid pipeline

A line of rigid pipes with rigid joints. Rarely laid now but found in drain and sewer construction prior to the 1960s.

Semi-rigid pipe

Pipe that behaves either as a rigid or flexible pipe according to diameter and thickness, e.g. fibre (asbestos)–cement, ductile iron and steel.

Septic tank

A purifier for sewage where no sewer is available. It is a tank through which sewage flows slowly enough for it to decompose and be purified. It is divided into two or more chambers separately by scum boards.


A pipe or cable connection from the main to the building, usually within the curtilage of that building.


Water-borne human, domestic and farm waste. It may include trade effluent, subsoil or surface water.


A system of sewers and ancillary works to convey sewage from its point of origin to a treatment works or the place of disposal.

Shallow manhole

A manhole of such depth that an access shaft to the chamber is unnecessary.


The highest point of the internal surface of a pipe or conduit at any cross-section.


Surface water from heavy rainfall combined with wastewater diverted from a sewer by a stormwater overflow.

Storm overflow

A device, on a combined or partially separate sewerage system, introduced for the purpose of relieving the system of flows in excess of a selected rate. The size of the sewers downstream of the overflow can thus be kept within economical limits or the flow to a sewage treatment works limited, the excess flow being discharged to a convenient watercourse.

Surface water

Water that flows over, or rests on, the surface of buildings, other structures or the ground.

Trade effluent

The fluid discharge, with or without matters in suspension, resulting wholly or in part from any manufacturing or specialist process.
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