Sewer Appurtenances – Manhole, Drop manhole, Lamp holes

Sewer Appurtenances

Sewer Appurtenances are those structures that are constructed at suitable intervals along with a sewerage system. These structures help in the effective operation and maintenance of the sewerage system. These devices include the following.

  1. Manhole
  2. Drop Manhole
  3. Lamp Holes
  4. Street Inlets
  5. Catch Basins 
  6. Ventilating Shaft
  7. Inverted Siphons
  8. Flushing Tank
  9. Grease and Oil Traps
  10. Storm Regulators
  11. Clean-outs

1) Manhole

Manholes are masonry or R.C.C. chambers constructed at suitable intervals along the sewer lines to provide access to them.

Manhole - Location, Classification, Function & Construction

Location of Manhole

The manholes are provided at a specified interval, change of direction, change of grade, junction points, change of sewer dia, etc.

Function of Manhole

The manholes help in joining sewer length. They also help in inspection, cleaning, and maintenance of sewers.

Construction of Manhole

A manhole essentially consists of a working chamber, an access shaft and a strong cover over top.

The lower portion of the manhole is known as the working chamber. It provides a working space for inspecting and cleaning operation of the sewer. Its minimum size for rectangular manhole is 1.2 m x 0.9 m and for a circular manhole, the minimum diameter is about 1.2 m. The height of this chamber should generally be not less than 1.8m.

The upper portion of a manhole is called access shaft. It provides access to the working chamber. This shaft is formed by corbelling or arching the working chamber. It’s minimum size about 0.75 m x 0.6 m for a rectangular manhole and for a circular manhole the minimum diameter is about 0.6 m to 0.75 m. Its depth depends upon the depth of manhole and the height required for the working chamber. 

The manhole is provided with a Cast Iron cover and frame at its top. The manhole cover may be rectangular or circular. The size of the rectangular cover is about 0.6 m x 0.45 m. The dia of a circular manhole cover is about 0.5 to 0.6 m. 

The bottom portion of the manhole is constructed in cement concrete. A semi-circular or U shaped channel is provided at the bottom. Cast Iron steps are provided in the manhole for the entry and exit of workers into them. 

Classification of Manholes

Depending upon their depth, the manholes may be classified as:

  • Deep manholes: A deep manhole is having depth of more than 1.5 m.
  • Shallow manholes: A shallow manhole is about 0.7 m to 0.9 m in depth.
  • Normal manholes: A normal manhole is about 1.5m in depth.  

2) Drop Manhole

The manhole which is constructed to connect high-level branch sewer to the low-level main sewer with minimum disturbance is called drop manhole.  

Drop manhole - Location, Function & Construction
Sewer Appurtenances – Drop Manhole

Location of Drop Manhole

A drop manhole is provided at the junction point of a high-level branch sewer and a low-level main sewer.

Function of Drop Manhole

The drop manhole serves the following function of drop manhole: i)It avoids unnecessary steep gradient of branch sewer and thus reduces the quantity of earthwork. ii)It avoids the possibilities of sewage being thrown on persons entering the working chamber of the manhole.

Construction of Drop Manhole

The details of a typical drop manhole are shown in the figure. The branch sewer is joined to the manhole through a vertical pipe. A plug is provided at the point where the branch sewers intersect the manhole wall straightly.
The length of the branch sewer between the vertical pipe and the plug is called the inspection arm. The inspecting arm is used for inspecting and cleaning the branch sewer.   

3) Lamp Holes

A lamp hole is a small opening on sewer for the purpose of lowering a lamp inside it.

Lamp holes - Location, Function & Construction
Sewer Appurtenances – Lamp Holes

Location of Lamp Holes

The lamp holes are provided at places where. i)A bend in the sewer is necessary. ii)Construction of manhole is difficult. iii)The spacing of manholes is more than the usual.

Function of Lamp Holes

The lamp holes serve the following functions:

  • It helps in examining the sewer length between adjacent manholes. 
  • Under some circumstances, it may be used as flushing devices.
  • If its top cover is kept perforated, it can be used for ventilation of sewer. Such a lamp hole is also known as a fresh air inlet.

Construction of Lamp Holes

The lamp hole consists of vertical cast iron or stoneware pipe (20 to 30 cm in dia) Which is connected to the sewer line by a Tee joint. The vertical pipe is surrounded by concrete to make them stable. A manhole cover is provided at the top to make up a load of traffic.  

4) Street Inlets (Gullies)

An inlet is an opening on the road surface through which stormwater is admitted and conveyed to the underground stormwater sewer or combined sewer.

Street inlets - Location, Function & Construction
Sewer Appurtenances – Street Inlets (Gullies)

Location of Street Inlets

On the straight portion of a road, the inlets are located or placed along the roadside at an interval of 30 m to 60m. They are also placed at road intersection points. The inlets are placed in such a way that stormwater is collected in a short period and the crosswalks are not flooded.

Function of Street Inlets

Street inlet collects the stormwater flowing along the streets and convey it to the underground stormwater sewer or combined sewer. Thus it prevents the accumulation of stormwater on the road pavement.

Construction of Street Inlets

A street inlet is a simple concrete box. It may have grating or openings in a vertical direction or in a horizontal direction. The former is known as vertical inlet or curb inlet and the later is known as a horizontal inlet. The inlets are connected to the nearby manholes by pipelines.  

5) Catch Basins

Catch basins are rectangular chamber provided along the sewer line to admit clear rainwater free from silt, grit, debris, etc into the sewer.

Catch basins - Location, Function & Construction
Sewer Appurtenances – Catch Basins

Location of the Catch Basin

The catch basin is placed along roadsides below the street inlets.

Function of the Catch Basin

A catch basin prevents the entry of silt, grit, debris, etc. into the sewer. It also prevents the escape of foul sewer gases.

Construction of Catch Basin

A catch basin consists of a masonry chamber or basin with a perforated cast iron cover at the top. At the bottom of the basin, space is provided for the accumulation of impurities. The basin is connected to the sewer by a pipe having a hood. The hood prevents the escape of foul sewer gases.  

6) Ventilating Shaft

The ventilating shaft or column is a device provided along the sewer line for the ventilation of sewer.

Ventilating shaft - Location, Function & Construction
Sewer Appurtenances – Ventilating Shaft

Location of Ventilating Shaft

The ventilating shaft is provided along the sewer line at an interval of 150 m to 300 m. They are also provided at the upper end of every branch sewer and at every point where sewer dia changes. 

Function of Ventilating Shaft

Ventilating shaft helps to remove the foul, and explosive gases produced in the sewer. They provide fresh air to the workers working in the manholes. They also help to prevent the formation of airlocks in the sewage and thereby ensure the continuous flow of sewage inside the sewer.

Construction of Ventilating Shaft

The ventilating shaft consists of a vertical shaft made by joining, cast iron or steel pipes. A foundation block is provided at the bottom end of the shaft to keep it in a vertical position. A cowl is provided at the top end to allow the escape of sewer gases. The shaft is connected to the sewer by an underground pipe. The height of the ventilating shaft should be more than the height of the nearby structures.  

7) Inverted Siphons

When an obstruction is met by a sewer line, the sewer is constructed lower than the adjacent section to overcome the obstruction. Such a section of a sewer is termed as an inverted siphon or depressed sewer or a sag pipe. The sewage through such section flows under pressure.

Inverted Siphons - Location, Function & Construction

Location of Inverted Siphons

The inverted siphon is constructed at the place where a sewer pipe has to be dropped below the hydraulic gradient line for passing it beneath a valley, a road, a railway or any other obstruction.

Function of Inverted Siphons

The main purpose of the installation of inverted siphons is to carry the sewer line below the obstruction such as road, railway, stream, river, etc.

Construction of Inverted Siphons

An inverted siphon usually consists of cast iron or concrete siphon tubes or pipes. The inverted siphon is constructed between inlet and outlet chambers. It is generally made up of two sloping pipe lengths joined by a flat pipe length. If the length of the siphon is more, a ventilating shaft should be provided in the siphon to prevent air locking.  

8) Flushing Tank

The cleaning operation of a small sewer is generally done by flushing tanks. The flushing tank is a device that stores water temporarily and throws it into the sewer for the purpose of flushing and cleaning the sewer.

Sewer Appurtenances - Flushing Tank
Sewer Appurtenances – Flushing Tank

Location of Flushing Tank

It is installed at places where there are chances of blockage of sewer pipes. In case of sewer laid on flat topography not producing self-cleaning velocities or near the dead end points of the sewers, flushing tanks are installed.

The Function of Flushing Tank

It helps in flushing and cleaning of sewers. It is also used to store sewage temporarily at some places.

Construction of Flushing Tank

The flushing tanks are of two types: 

  • a) Hand operated flushing tank
  • b) Automatic flushing tank

In a hand-operated flushing tank, the flushing and cleaning operation is carried out at suitable intervals by manual labour. It is carried out by operating the sluice valve fitted at the outlet end and the inlet end of the manhole suitably. 

In automatic flushing tank, the flushing and cleaning operation is carried out automatically at regular intervals. It consists of a U-tube encased in a compartment. An overflow pipe is also provided to drain away excess water. This tank functions automatically by siphon action.   

Read Also: 

Dead End System – And Their Advantages & Disadvantages

Grid-iron System- And their advantages & Disadvantages

Types of Sewer Pipe Joints