Sewer Appurtenances

SEWER APPURTENANCES

Sewer appurtenances are those structures which are constructed at suitable intervals along a sewerage system, which helps in efficient operation and maintenance. These include:

 (1) Manholes.

(2) Drop Manholes

 (3) Lampholes

(4) Clean-outs

 (5) Street inlets (Gullies)

(6) Catch Basins

(7) Flushing Tanks

(8) Grease and Oil Traps

(9) Inverted Siphons

(10) Storm Regulators.

(1) Manholes:

Manholes are Masonry (or) R.C.C chambers.

Purpose - Provides access into sewers for inspection, cleaning and maintenance works.

Location - Provided at every bend, junction, change of gradient (or) change of sewer diameter at suitable intervals along sewer line

Spacing -   Depends on sewer sizes (Larger diameter - greater spacing).

CLASSIFICATION OF MANHOLES: (BASED ON DEPTH) 

(i) Shallow Manhole:

❖ Used as inspection chambers.

❖ Depth = 0.7 m to 0.9 m.

❖Constructed at start of branch sewer and at places of minimum traffic.

❖Light cover is provided at top.

(ii) Normal (or) Medium Manhole:

❖ Depth = 1.5 m.

❖ Square (1 mx 1 m)/Rectangular (1.2 m x 1 m) shape.

❖ Heavy cover is provided at the top.

❖ Unlike shallow manholes, the section does not change with depth.

(iii) Deep Manhole:

❖ Depth > 1.5 m.

❖ Section normally varies, the upper portion is reduced.

❖ Steps are provided to enable workers entry to bottom.

❖ Heavy cover is provided at the top.

Component Parts of Manhole:

(i) Access Shaft:

❖ It is the upper portion of deep manhole.

❖ For rectangular manhole - size of access shaft is 0.75 m x 0.6 m.

❖ For circular manhole - size is 0.6 m to 0.7 m.

❖ Depth of access shaft depends on manhole depth.

(ii) Working Chamber:

❖ It is the lower portion of manhole.

❖ Provides working space for inspection and cleaning operations.

❖ For rectangular manhole - size of working chamber is 1.2 m × 0.9 m.

❖ For circular manhole - size of working chamber is 1.2 m diameter.

❖ The height of this chamber must not be less than 1.8 m.

(iii) Benching/Bottom/Invert Portion of Manhole:

❖ The bottom portion of the manhole is constructed in cement concrete.

❖ A semicircular (or) U-shaped channel is generally constructed and the sides are made to slope towards it.

❖ The concreting is known as benching and facilitate the entry branch sewers into the main sewer.

(iv) Side Walls:

❖ The side walls are made of brick/stone masonry/R.C.C.

❖ The minimum thickness of the brick walls should be 22.5 cm (9'').

❖ The approximate thickness may be computed by using emperical thumb-rule t = 10 + 4d (for brick walls).

t = Thickness of wall in cm.,

d = Depth of excavation in m.

❖ The thickness of R.C.C walls will however be much less as compared to that of brick wall, but RCC walls are costly than brick.

(v) Steps (or) Ladders:

❖ Steps are generally provided for descending into deep manholes.

❖ Steps are made of cast iron and are placed at a horizontal distance of about 20 cm and a vertical distance of about 30 cm. Steps are embedded in walls.

vi) Cover and Frame:

❖ Cover is generally made of cast iron material and are fixed at the top of the manhole embedded in pavements.

❖ The thickness of the frame is about 20 to 25 cm and is about 10 cm wide.

❖ Circular or rectangular shape cover is provided of size 0.5 to 0.6 m & or 0.6 m × 0.45 m respectively where & is the diameter of the cover.

❖ The top surface of the cover also carries an arrow mark to mark the direction of flow of sewage.

❖ The weight of the cover and frame varies between 90 to 270 kg which depends on traffic volume.

2. Drop Manhole:

❖ Branch line is provided 0.5 to 0.6 m above main sewers.

❖ The sewage from branch sewers is not allowed to fall directly into the manhole, but is brought into it through down pipe taken from the branch sewer to the bottom of the manhole.

❖ Either vertical down pipes (drop manhole) or inclined pipes (ramp) are provided. The vertical pipe is economical.

❖ A manhole in which a vertical down pipe is provided is called a drop manhole.

❖ A plug and inspection arm are provided at the point where the straight stretch of branch sewer intersects the wall of the manhole.

❖ Sewage from the branch sewer dips through vertical pipe and trickles over the main sewer.

Advantages:

(i) Steep gradients of branch sewers is avoided by providing vertical drop manholes. The cost of excavation is reduced.

(ii) Sewage entering the manhole directly from branch sewers that are provided at certain height is likely to fall on the working personnel, which is prevented.

3. Lamp Holes (now become obsolete):

❖ Openings or hole extending from ground to sewer line through T-junction.

❖ Purpose:

(i) For inspection (obstructions)

(ii) For flushing sewers

(iii) For ventilation of foul gases.

❖ These holes permit insertion of lamps into sewers. The lamp light is viewed at upstream and downstream ends to find any obstructions.

❖ Lamphole consists of a vertical pipe (20 to 30 cm ) made of cast iron or stone-ware, where o is the diameter of the vertical pipe.

❖ The vertical pipe is surrounded by concrete.

❖ A manhole cover is provided at the top.

❖ Location:

(i) At bends, change in direction or gradient of sewers.

(ii) Where manhole construction is difficult.

(iii) Where manhole spacing is more.

4. Clean-Outs

❖ An inclined pipe extending from the ground to the underground sewer.

❖ A clean out is used for cleaning sewer pipes - flushing sewers (laterals) to remove obstacles.

❖ A clean out is provided at the upper ends of lateral sewers in place of manholes. The functioning of a clean out is very simple. Removing the top cover and forcing water through the clean out pipe to lateral sewers to remove obstacles in the sewer line.

❖ If the obstructions are large, a flexible rod may be inserted and pushed forward and backward to remove the obstacles.

5. Street Inlets (Gullies):

❖ Openings on road provided at lowest point which drains rain water into storm/ combined sewers.

❖ Provided at spacing of 30 m to 60 m.

❖ Gullies are connected to manholes by pipe lines.

❖ A street inlet is a simple concrete box with gratings (or) openings in vertical (or) horizontal direction.

❖ The inlet having vertical openings is vertical inlet and inlet having horizontal openings is horizontal inlets.

6. Catch Basins (or) Catch Pits

❖ Catch basins are street inlets provided with small settling basins.

❖ Grit, sand, debris etc., do settle in these basins and their entry into the sewer is prevented

❖  A hood is provided which prevents the escape of foul gases.

❖  Catch basins need periodical cleaning.

❖ Catch basins are provided only in old combined sewerage system.

❖ Catch basins are not required in modern separate sewerage system as the streets are paved and not much debris enters the sewers. Further the foul storm water drains is also less

7. Flushing Tanks:

❖ Device that stores water temporarily and then throws it into the sewer for flushing and cleaning.

❖ It can be operated either manually or automatically.

❖ Sewers that are laid on flat gradient or dead ends may not produce self cleansing velocity and may be frequently blocked. This can be cleaned with the help of flushing tanks.

❖ Quantity of water added in one flush is about 1600 litres.

Types:

 (a) Hand operated flushing operation (Manual) Lino

(b) Automatic Flushing Tanks

(a) Hand Operated Flushing Operation: (Manual)

❖ A hose pipe is provided with one end connected to the nearest fire hydrant and another end is placed in the manhole to achieve flushing action.

❖ The outlet and inlet ends are closed by sluice or gate valve and the manhole is filled with water. When sufficient head builds up, both the inlet and outlet valves are opened and flushing is achieved. (or)

❖ The outlet end of the manhole is closed with sluice or gate valve. The incoming sewage collects in manhole and when sufficient head builts up, the gate valve is suddenly opened to achieve flushing.

b) Automatic Flushing Tanks:

❖ Flushing action is achieved automatically at regular intervals.

❖ Entry of water is so regulated that the tank is filled in a period equal to the flushing interval.

❖ An overflow pipe is also provided to drain away excess water in case the tank overflows before the flushing action starts.

❖ The tank consists of a masonry (or) concrete chamber fitted with a water

❖ A U-tube connects the chamber with the enlarged end of the sewer pipe.

❖ The bell cap has a sniff hole.

❖ When water level is below the sniff hole, water level in the U-tube is at level [Y₁-Y₂] (long arm, short arm).

❖ As the water level in the tank rises above the sniff hole, the air caught up in the bell (compressed air) exerts pressure on water surface Y, and the water level in the long arm is depressed.

❖ As water level rises in tank (upto discharge level) Y, level in long arm of U- tube depresses more and more and the compressed air caught in bell is released through shorter arm. The siphonic action starts releasing water from the tank into the sewer till water level falls below sniff hole.

❖ When water level in tank falls down, air enters the bell cap and breaks the siphonic action.

8. Grease and Oil Traps:

→ Trap chambers are used to remove oil/grease from sewage, before entering sewer line.

→ These are located near sources which generate grease and oil (automobile repair workshops, garages, kitchens of hotels, oil/grease indusstries etc.)

Necessity:

(i) Grease/oil stick to sewer sides and causes obstruction or reduce sewer capacity. (ii) Increases the possibilities of explosion in sewer lines.

(iii) Presence of oil and grease causes difficulties in treatment.

(iv) Presence of oil and grease on surface of waste water prevents oxygen penetration

Principle:

❖ The grease and oil traps consists of two chambers interconnected through a pipe.

❖ The Inlet is provided with a grating on the top.

❖ The oil and grease float on surface. The outlet is provided submerged to exclude oil and grease.

❖ Combined sand, grease and oil trap is also available.

9. Inverted Siphons: (Depressed Sewer) (Sag Pipe)

❖ Inverted siphons are sewer sections that are provided lower than the adjacent sewers, (or) that drops below the hydraulic gradient line (HGL).

❖ They are provided under the obstructions such as roadway, railway, stream, valley, etc.

❖ Inverted siphons should be provided only in areas where other means of passing the obstructions are not feasible.

❖ They are siphon tubes or pipes made of cast iron or concrete.

A Self cleansing velocity of 0.9 m/s is achieved at minimum discharge.

❖ In the combined flow system, three channels or pipes are provided:

(a) Pip Pipe 1: carry minimum sanitary sewage (Channel 1)

(b) Pipe 2: carry maximum sanitary sewage (Channel 2)

(c) Pipe 3: carries combined flow during monsoons (Channel 3).

When pipe/channel 1 overflows, the sewage enters channel 2 and 

For a separate sewerage system, two channels are provided,

(a) Channel 1: For carrying minimum dry weather flow.

(b) Channel 2: For carrying maximum dry weather flow.

Designing Siphon - Important Points.

(i) The construction should be simple.

(ii) Direction change should be easy and gradual.

(iii) Self cleansing velocity should be atleast 1 m/s during minimum flow. galynas Jonnado

(iv) Two or three parallel pipes are provided such that average flow happens in first pipe and any excess discharge (flow) goes to the second and third pipes. 1911sm gritsoll to be invig et bobivong

(v) Selection of pipe size depends on both average and maximum flows.

(vi) Total pipe length should consist of straight lengths, and the lengths of falls, bends and rise.

(vii) Head loss at bends, friction, silting and roughness should also be considered.

(viii) Possibility of silting should be avoided by providing screens and grit (detritus) pits.

(ix) Minimum diameter of siphon should be 15 to 20 cm.

(x) Chambers should be provided with sufficient room for cleaning and maintenance. In the outlet chambers, any backflow of sewage should be prevented.

Demerits of Siphons:

(i) Silting occurs because the down gradient pipe is not continuous and is difficult to cleaning.

(ii) Improper design of inlet chamber cause accumulation of floating matter and reduce efficiency.

10. Storm Water Regulator / Storm Relief Works:

❖ Storm water regulators are provided in combined sewerage system to divert excess storm water into stream.

❖ These are overflow devices to prevent overloading of sewers, pumping stations, treatment plants, etc.

Types:

(i) Overflow Weirs (Side Flow Weirs):

❖ Masonary chamber with overflow weir is the common type.

❖ Overflow weirs are located where sewer approaches a water course.

❖ Single acting (single side) (or) double acting (double side) overflow weirs can be provided.

❖ Openings are provided at suitable height of combined sewer and joined to storm drains. Excess sewage flows to drains that are discharged to water course.

❖ Excess sewage overflows the combined sewer and enters channel, carrying storm water drain or into a stream.

❖ Adjustable plates are provided to prevent escape of floating matter.

(ii) Leaping Weir:

❖ Opening is provided at invert of storm drain (combined sewer).

❖ Normal storm water flows in intercepting sewer.

❖ Excess flows are diverted to nearby stream by the leap over combined sewers.