Sewer Appurtenances

A drop manhole is a manhole in which the incoming sewer enters at a higher level than the outgoing sewer, and the sewage is led down to the lower sewer through a vertical pipe (drop pipe) built on the outside or inside of the manhole.

Necessity (need) of drop manhole

1. Steep ground: when a branch sewer joins a main sewer at a much lower level (difference more than about 0.6 m), a direct fall would splash the sewage, cause erosion of the manhole base and release foul gas.
2. Avoid high velocity: sewage falling freely produces a high velocity and scouring of the lower sewer.
3. Level difference at junction: when a shallow sewer must join a deep main sewer, the drop pipe avoids deep excavation of the entire branch and reduces cost.
4. Gradient control: it lets the sewer follow a flatter gradient than the ground slope.
5. Protection of workers: the sewage does not fall on a person who enters the manhole.

Purpose

To bring the sewage safely from a higher sewer to a lower sewer without splashing and damage, and to avoid deep trenching.

Construction

   Ground level
   ________________          Cover
  |  |          |  |   <- Manhole shaft
  |  |   Inlet  |  |
  |  |  sewer-->|  |
  |  |__ ____   |  |   Vertical drop pipe
  |        |  |    |   (outside the shaft, in
  |        |  |    |    concrete encasement)
  |        v  |    |
  |  Outlet ---> main sewer
  |____ Channel ___|

• It is built like an ordinary brick/RCC manhole (brickwork in 1:3 cement mortar, circular or rectangular chamber with steps and a cast iron cover).
• A vertical drop pipe of the same diameter as the branch sewer (CI or stoneware) is fixed on the outside wall of the manhole and connected to the branch sewer through a tee/bend at the top. A cleaning eye with a cover or plug is provided at the top for rodding.
• The bottom of the drop pipe discharges into the manhole base through a bend, in a half-round channel (benching) leading to the main sewer.
• The drop pipe is encased in concrete 1:2:4 to protect it from load and impact.
• The base is built of concrete with a cushion to take the impact of falling sewage.

A manhole is an access chamber, built on a sewer line, that allows a person to enter the sewer for inspection, cleaning, testing and repair.

Necessity of manholes

1. For inspection and cleaning of the sewer (removing silt and blockage).
2. For joining two or more sewers (junctions) and for changes in direction, gradient or size of sewers.
3. For ventilation of the sewer through the cover.
4. For testing (smoke, mirror and lamp) of the sewer.
5. For laying sewers in straight lengths between the points, so the work is easy.
6. For flushing and for house connections.

Manholes are provided at every change of alignment, gradient and diameter, at junctions, and at intervals of 30-90 m on straight lengths (smaller sewers need closer spacing, e.g. 30 m for 300 mm).

Construction (components)

        Cast iron cover & frame
         ________________
        |_______________|   <- Cover slab / ring
         | |         | |
         | |  Neck   | |    <- Shaft (min 0.9 m dia.)
  Steps  |-|         | |       with CI/GI steps at 0.3 m
         | |         | |
        /               \    <- Chamber widens (working
       /     Chamber     \      chamber about 1.2-1.5 m)
      |  Inlet-->____     |
      |        ( channel )--> outlet
      |__Benching____Base_|
          concrete base

1. Base (foundation): cement concrete 1:3:6, 15-30 cm thick, laid on a firm bed to carry the manhole.
2. Walls (chamber): brickwork in cement mortar 1:3 (or stone masonry, or RCC), 20 cm thick for shallow and 30-45 cm for deep manholes, plastered inside and out with 1:2 mortar to make it watertight.
3. Channels and benching: semicircular channels of half-round pipes (or cement concrete) in the base carry the flow smoothly from the inlet to the outlet. The sides (benching) are sloped at about 1 in 6 towards the channel, in 1:2:4 concrete, so that the workers can stand and solids do not stay.
4. Shaft (access shaft): above the chamber, the manhole narrows to a shaft of 0.9 m diameter or 0.9 x 0.6 m rectangle, to take a cover (corbelling at the top).
5. Steps: CI or galvanised iron steps (foot rests), fixed 30 cm apart in the wall, for going down.
6. Cover and frame: heavy cast iron (or RCC for light traffic) cover, with frame set in the top slab, circular of 50-60 cm diameter, kept flush with the road.
7. Drop pipe or lamp facilities may be added for deep or steep manholes.

Depth classification: shallow (up to 0.9 m), normal (0.9-1.5 m) and deep (>1.5 m) manholes.

Definition

Sewer appurtenances are the structures and accessories built along a sewerage system, apart from the sewer pipes themselves, that help it work properly and make it easy to inspect, clean, ventilate and maintain.

Need (necessity) of sewer appurtenances

Sewers are underground, closed conduits. Without appurtenances it would be impossible to operate them safely. They are needed:

1. To inspect and clean the sewers and remove blockages.
2. To admit storm water and surface runoff (street inlets) in combined and storm systems.
3. To change direction, gradient or size of sewers and join several sewers (manholes).
4. To reduce velocity and deliver at the lower level on steep grounds (drop manholes).
5. To keep the sewer self-cleansing in flat areas through flushing devices.
6. To keep out harmful materials: catch basins retain grit and silt, and grease traps retain oil and grease which would block sewers.
7. To ventilate the sewers and prevent accumulation of explosive and poisonous gases (ventilating shafts).
8. To cross obstacles such as rivers and depressions (inverted siphons).
9. To discharge sewage safely to the receiving water (outfalls/outlets).

Examples and their function

Street inlets (storm water inlets) are openings, with a grating or a slot, built at the edge of a road or in the gutter to admit surface storm water into the storm drain or combined sewer.

Types of street inlets

Street inlets are of three main types: curb inlet, gutter inlet and combined inlet.

1. Curb inlet

A vertical opening in the curb (kerb) face through which water enters the chamber. It is suitable for roads with a steep gradient or little debris, and it is not blocked easily by leaves because the opening is vertical (but silt can still collect).

  Road   Gutter  Curb
  ______         |
        \_______ |  <- opening (10-15 cm high)
                 |_____ Inlet chamber
         ---------> to sewer

2. Gutter inlet

A horizontal opening in the gutter covered by a cast iron grating (cast iron bars parallel to the flow). Water falls through the grating into the chamber. It is more efficient on flat to moderate slopes and can handle large flow, but may be blocked by leaves, paper and debris and is a risk to cyclists.

   Curb |  Gutter
        |__[#######]__    <- grating
        |  |       |
        |  | chamber|----> to sewer
        |  |________|

3. Combined inlet

A curb inlet and a gutter inlet are provided together, so that water enters through both. If the grating is blocked, the curb opening continues to work. It has high capacity and is widely used on busy city roads.

Other points

• The inlet is connected to the sewer by a 150-300 mm pipe, and a catch basin (silt pit with 0.6-0.9 m deep sump) or a trapped inlet is often provided to stop grit and smell.
• Inlets are spaced at 30-60 m on a road, depending on road slope, width and rainfall, and they are placed at street corners, low points and before pedestrian crossings.

Why grease and oil must be excluded

1. They are lighter than water, float, and stick to the walls of the sewer, reducing its capacity and causing blockages.
2. They clog pumps, screens and treatment units and interfere with the oxygen transfer, so the biological treatment fails.
3. They form scum layers in the treatment tank and cause foul smell as they decompose.
4. They pollute rivers, forming a film which stops oxygen entering the water.
5. They can ignite and cause explosions if petroleum oils enter the sewer.

Sand, grease and oil trap

It is a small tank built in the line of the sewer from hotels, restaurants, garages, workshops and petrol stations to remove sand (grit), grease and oil from wastewater before it enters the sewer.

Principle of working

It works on gravity separation due to difference in density and a low velocity.

• The tank reduces the velocity of flow (to about 0.1-0.3 m/s), giving a detention time of a few minutes.
• Heavy particles like sand and grit (specific gravity about 2.65) settle to the bottom.
• Grease and oil, which are lighter than water (specific gravity about 0.9), float to the surface and are trapped by a baffle (a T pipe or a submerged outlet).
• The clear water from the middle layer flows out.

   Inlet               Baffle         Outlet
     |                   |              |
  -->|___________________|___           |-->
     |  grease/oil  ~~~~~|~~~~~          |
     |                   |  clear water  |
     |   . . . sand . . .|_______________|
     |___sand pocket_____|
        removable cover

Construction and operation

• A brick or RCC tank, with a cover and an inlet pipe well below the water level, an outlet with a dip pipe (baffle) that leaves the floating layer behind, and a sump at the bottom for sand.
• The trap is cleaned regularly: floating grease is skimmed off and the sand removed manually or by pump, otherwise it will be re-suspended and carried away.

Two commonly provided appurtenances are the manhole and the catch basin (or street inlet). Their working is explained below.

1. Manhole

A manhole is a vertical chamber built over a sewer to let a worker go down for inspection, cleaning and repair.

      ___ cover
     |   |
     | S |   shaft with steps
     | H |
    /  A  \
   |Inlet  |
   |--->===|---> Outlet
   |_______|
   Half-round channel

Working: the sewage from the inlet sewer flows through a half-round channel built in the base to the outlet sewer without splashing; side benching sloped to the channel stops solids from settling. The cast iron cover also ventilates the sewer. When a blockage occurs, workers enter and remove it or insert rods from the manhole. Manholes are provided at changes of direction, gradient and size, at junctions and every 30-90 m in straight length.

2. Catch basin

A catch basin is a small chamber with a silt pit below the level of outlet, provided at the inlet of a storm or combined sewer.

   Street inlet
        |
     ___v___
    |       |  <- cover
    |       |_ outlet to sewer (hooded)
    |_______|
    silt/grit settles here

Working: storm water enters from the street inlet and falls in the chamber. Its velocity is reduced, so heavy silt, sand, grit and rubbish settle to the bottom pit, and only the clear water flows out through the outlet pipe which is higher than the bottom and often has a hood or elbow dipping under water to hold back floating matter and foul gas (water seal). The deposit is removed periodically by hand or by a bucket. This protects the sewer from silting and blockage.

Drop manhole

A drop manhole is a manhole in which a branch sewer entering at a high level is connected to the lower main sewer by a vertical drop pipe, usually outside the manhole shaft, so that the sewage does not fall directly into the chamber.

   Ground
  ___________
  |  |    |  |
  |  |<---|--+--- branch sewer (high)
  |  |    |
  |  |  drop pipe (concrete encased)
  |  |    |
  |__|____v____ main sewer ---->

Importance

• Prevents splashing, noise, erosion of the manhole base and release of foul gases when sewage falls from more than about 0.6 m.
• Keeps workers safe and makes entry dry.
• Avoids deep excavation of the branch sewer on steep ground and so reduces cost.
• Reduces velocity and protects the downstream pipe from scour.

Alternative structural options

1. Steep sewer or cascade (stepped) gradient: the sewer itself is laid on a steep slope with cascades/step drops in the manhole, with the energy dissipated by a stilling basin. Applicable where large flows come on very steep ground, or for large trunk sewers where a drop pipe would not be big enough.
2. Ramp (inclined) drop: a sloping channel is built in the manhole to bring the sewage down smoothly; suitable for large sewers with a drop of about 1-3 m.
3. Backdrop / vortex drop structures in major trunk sewers.
4. Deeper excavation of the branch (laying at lower level) when the difference is small (<0.6 m), where an ordinary manhole with a steep channel can be used.

A drop manhole is chosen for small to medium branch sewers; cascades or ramps are used for large sewers.

Street inlets

Street inlets are openings in the road kerb or gutter that let storm water from the road surface enter the underground storm drain or combined sewer. Types: curb inlets (vertical opening in the kerb), gutter inlets (horizontal grating in the gutter) and combined inlets (both together).

 Curb inlet          Gutter inlet
   |                 ________
   |____ opening     |=grating=|
   |   |             |         |
   |   v chamber     |  chamber|

Inlets are placed at 30-60 m spacing, at corners and at low points of the road, and they connect to the sewer by a 150-300 mm pipe.

Catch basin

A catch basin is a small masonry or RCC chamber, usually 0.6-0.9 m in diameter and 0.9-1.5 m deep below the outlet, built in the line from a street inlet to the sewer to collect silt, sand, grit and floating rubbish which would otherwise settle in the sewer.

      Street inlet grating
          |
     _____v_____
    |           |  ground level
    |           |
    |      ___  |-- hooded outlet (dips in water)
    |     |   | |
    |_____|silt|_|

Construction: brick or concrete walls plastered inside, a concrete floor, a cast iron cover, and an outlet pipe placed 0.3 m or more above the floor so that the heavy matter settles below it. A hood (bend) on the outlet traps floating matter and gives a water seal against foul gas. Working: the velocity falls in the basin, the silt settles, and the clear water goes into the sewer. The basin is cleaned regularly (every few weeks, and more often in the rainy season). It is not provided in modern sewers where velocity can carry the silt and where cleaning is difficult; instead gully traps are used.

Street inlets

Street inlets are openings or structures that admit storm water from the street surface into the storm drain or combined sewer.

Location

• At the street corners (upstream of pedestrian crossings) and along the kerb.
• At low points and sag points of the road where water collects.
• At intervals of about 30-60 m on a continuous grade, closer when the road is flat and the area is large; also at the end of a catchment length where gutter flow exceeds its capacity.
• In the side of the road at the downhill side of junctions, so that water does not cross the intersection.

Difference between curb inlet and gutter inlet

A catch basin is a small chamber built in the line between a street inlet and a storm or combined sewer, to collect and hold silt, sand, grit and floating refuse so that they do not enter and block the sewer.

Construction

         Cast iron cover
       __________________
      |                  |
      |                  |  hooded outlet
      |            ______|--->  to sewer
      |   water   |
      |   level   |  Outlet 0.3-0.6 m
      |___________|  above floor
      |  silt pit |
      |___________|

• Circular or rectangular, 0.6-1.0 m in diameter, depth 0.9-1.5 m, built of brickwork or concrete, plastered inside.
• The floor of concrete is below the outlet invert to give a sump (silt pit).
• A cast iron frame and cover or the street inlet grating at the top.
• The outlet is connected to the sewer through a trapped or hooded elbow dipping below the water level, which retains floating matter and gives a water seal against foul gas.

Working

Storm water from the inlet falls into the basin, velocity drops, and heavy particles settle in the silt pit while clear water overflows through the outlet. Floating matter stays behind the hood.

Maintenance

Periodic cleaning is needed (silt removed by buckets or a suction machine, especially before the monsoon), otherwise the basin fills up, loses its function and breeds mosquitoes.

Merits: keeps silt out of the sewer, prevents blockages, reduces sewer size by avoiding silting. Demerits: requires frequent cleaning, can give foul smell and breed mosquitoes; not needed if the sewer is self-cleansing.

Why sand, grease and oil traps are necessary

• Sand and grit settle in sewers, reducing the capacity and causing blockages, wear pumps and fill treatment tanks.
• Grease and oil float and stick on the sewer wall and on screens, blocking sewers, choking pumps, forming scum in tanks, interfering with biological treatment and causing odour. Petroleum oils may cause fire or explosion in the sewer. So the traps are fitted on the wastewater from kitchens, hotels, garages, petrol pumps, workshops and slaughter houses.

Description (with sketch)

Sand trap (grit chamber): a chamber which reduces the velocity of flow to about 0.2-0.3 m/s so that sand and grit settle in a sump, and the lighter organic matter remains in the flow.

Grease and oil trap: a rectangular tank with an inlet deep below the water level, a baffle and an outlet. Flow velocity is reduced, grease and oil rise to the top and are held back by the baffle; the clean water goes out from lower level.

  inlet            baffle        outlet
   -->|_____________|___          |-->
      | oil/grease ~~|~~~~~~~     |
      |              |  water ----|
      |  sand . . . .|__________  |
      |__sand pocket_|

A combined sand, grease and oil trap is used where all are present.

Operation and maintenance

• Frequency: grease traps of restaurants and hotels should be cleaned weekly or when 25% of the volume is filled (daily in heavy kitchens); sand traps of garages and road traps at least monthly, and before and after the monsoon. Larger traps may be cleaned every 3-6 months.
• Skimmed grease is removed with a ladle and disposed to a landfill or used for soap making. Sand is shovelled out or removed by suction.
• Never use hot water or detergents to wash out the grease, as this only moves the problem downstream.

Locations in a road network

• At petrol stations, workshops, garages and car washes (oil traps).
• At hotels, restaurants and canteens (grease traps, inside the premises).
• Along roads with heavy silt (sand traps) at the inlets and at the entry to a pumping station or the treatment plant (grit chamber).
• At the junctions where storm drains from unpaved areas join the sewer.

An inverted siphon (depressed sewer) is a sewer section which dips below the hydraulic gradient line to pass under an obstruction such as a stream, a railway, a road or a depression, and then rises to the original level. It runs full under pressure; it is not a true siphon.

Circumstances for providing it

• To cross a river, canal, stream or drainage channel below the bed.
• To pass under railway lines, highways, tunnels or underground utilities.
• To cross a deep valley or depression where the ground drops below the sewer's gradient and a bridge would be costly.

Purposes served

1. Lets the sewer cross an obstruction without pumping, using only the head available.
2. Avoids the cost of an aqueduct or bridge.
3. Keeps the sewage flowing by gravity with only a small loss of head.

Construction

 Inlet                          Outlet
 chamber                        chamber
  __                              __
 |  |-\                        /-|  |-->
 |  |  \  Ground level         /  |  |
 ----    \______________/ ------
         depressed sewer (below stream)

• The sewer is made of cast iron or steel (or RCC encased in concrete) pipes, laid below the obstruction.
• An inlet chamber and an outlet chamber are built at both ends. The inlet chamber has an overflow, and sluice gates or stop planks so that one of the pipes can be shut.
• Usually two or more pipes (dry weather pipe and wet weather pipe) are provided, so that the velocity in the dry weather flow is not less than 0.9-1.0 m/s, which prevents silting.
• The pipes are also provided with cleaning access (manholes and washouts).
• The head loss is equal to $\dfrac{v^2}{2g}$ plus friction, and the levels are fixed so that the loss is available.

Purpose

An automatic flushing device (flushing tank) is a storage tank that releases a large quantity of water at intervals, to flush the sewer. It is provided at the upper dead ends of sewers and in flat sewers, where the sewage flow is small and the velocity is below the self-cleansing value (0.6 m/s), so solids tend to settle. Flushing avoids blockages and removes deposits.

Features

• Works automatically without manual operation or attention.
• Discharges a fixed quantity of water at set intervals (e.g. once every 24 hours, or every 6 h) with a velocity high enough to scour the sewer for a distance of up to 150-200 m.
• Water is taken from the public water supply (or a tank, a river, a stream) by a small supply pipe with a control valve.
• It consists of a siphon, so there is no moving part.

Construction

        supply pipe (valve)
              |
        ______v______        manhole cover
       |   Water     |
       |   level  ___|___ bell mouth
       |         | cap  |
       |   U-siphon     |
       |_____|__________|---> sewer

1. A flush tank (brick or concrete) of 1-2 m³ capacity, built in a special manhole at the head of the sewer. A small tank has a capacity of about 1,000-2,000 litres.
2. A siphon pipe (inverted U) with the long leg connected to the sewer and a bell (cap) fitted over the short leg.
3. A supply pipe with a stop valve admits water slowly; the supply rate sets the interval of flushing.

Working

Water enters slowly and the level rises in the tank and in the siphon. When it reaches the crown of the siphon, the siphon starts, the air is expelled and the full tank flows out rapidly into the sewer. The siphonic action stops when the water level has dropped to the bell mouth and air enters. The cycle then repeats.

Modern practice avoids flushing tanks by giving proper slope, because the cost of water and the risk of cross connection with water supply are high.

A ventilating shaft (vent stack) is a vertical pipe or shaft connected to the sewer or to a manhole, which lets the sewer gases escape to the atmosphere at a safe height and lets fresh air in.

Need

Sewage decomposes and produces gases such as hydrogen sulphide, methane, carbon dioxide, ammonia and carbon monoxide. These are

• poisonous (H₂S, CO) and dangerous to sewer workers,
• explosive or inflammable (methane), and
• corrosive; H₂S changes into sulphuric acid and attacks the concrete crown of the sewer. Ventilation also reduces sewer odour and lets air in for the aerobic condition.

Arrangement

        ___ Cowl
       |   |
       | V |  vertical pipe, 100-150 mm CI,
       | e |  fixed on a building wall or a pole
       | n |  at least 3 m above ground (above roofs)
       | t |
   ____|   |____
  Manhole / sewer

• A 100-150 mm cast iron or PVC pipe is fixed along a building wall and connected to the sewer; the top has a cowl to keep rain out. Its height is above the roofs (at least 2-3 m above the highest window).
• Ventilation is also obtained through the perforated manhole covers (which let air in), and the house vent pipes (soil pipes carried up above the roof), and ventilating columns at the end of the sewers.
• Ventilating shafts are provided at the ends of branch sewers, at the highest point of the network, and spaced at regular distances along trunk sewers; if the sewer is long they may use forced draught or fans.

Precautions

Workers must still test for gases and ventilate by blowers before entering a sewer.