The document provides information about sewer systems in Nepal. It discusses the historical development of sewage systems in Nepal from the 1920s to present day. It outlines the objectives of understanding sewer types, design criteria, construction, and appurtenances. The document describes various sewer shapes including circular, rectangular, egg-shaped, and others. It covers design criteria such as sewage flow calculations, velocity, gradient, and materials. Common sewer materials like concrete, brick, cast iron are explained. The importance of manholes and other appurtenances for maintenance and inspection is highlighted.

1. Group:- G2
Santosh Acharya 151
Santosh Panta 153
Santosh Sah 154
Saru Prajapati 156
Rashmi Pahari 193
Tulsiram Bhattarai 195
Tutor:
Asst. Prof. Shukra Raj Paudel
Department of Civil
Engineering
IOE, Pulchowk Campus
2017-06-15
1

2. Objectives
 To be familiar with the present status of sewer system in Nepal.
 To know about the type of sewers(on shape and material basis).
 To understand about the design criteria and construction of sewers.
 To know about the necessity and construction of sewer appurtenances.
2

3. Presentation Outline
 Historical Development of sewage system in Nepal.
 Shapes of sewers
 Design Criteria of sewers
 Sewer Materials
 Design of Sewers of separate and combined systems
 Numerical on designs of sewers
 Construction of sewers
 Necessity of sewer appurtenances
 Construction of sewer appurtenances
3

4. Historical Development of Sewage system
in Nepal
 Start of the development dates back to 1920’s with the construction of 55
km long brick channel for collection and disposal of combined sewer and
rainwater runoff in Ktm and Patan.
 DWSS formally established in 1972 ,before which the sector activities
were placed under department of Irrigation.
 In mid eighties, this sector succeeded in developing essential policies and
legislations after being reinstituted into the Ministry of Physical Planning
and Works(MoPPW)
4

5.  Major works include: Establishment of sewage treatment plant comprising oxidation
pond and activated sludge ditch at Hanumandhoka,Dhobighat,Kodku and Sallaghari .
 In 2011 MoPPW further splitted up and wash sector was formed under newly formed
Ministry of Urban Development which further splitted up and reinstituted as Ministry of
Water supply and Sanitation.
5

6. Shapes of Sewers
I. Circular Sewer
II. Non Circular
Sewer
 Triangular
 Rectangular
 Trapezoidal
 Parabolic
 Semi-Elliptical
Sewers are generally circular pipes laid below ground level, slopping continuously towards
outfall and designed to flow under gravity.
 U-shaped
 Standard Egg shaped
 Modified egg shaped
 Basket-Handle section
6

7. I. Circular Sewer
Advantages
1. Maximum less chance of deposition.
3. Easy to construct, transport and handle, utilizes minimum
materials of cohydraulic mean depth so discharge is maximum
2. No corners hence instruction.
4. Stable in laying and useful for separate sewers.
Disadvantages
1. Not useful in combine system because self cleansing velocity
cannot be maintained at DWF conditions.
7

8.  It comprises open and closed non-circular sewers.
 Open sewer is generally used for drinking storm water and lined with materials (PCC,
cement soil, clay, brick, stone) to protect drain surface from erosion and infiltration of
water.
 Closed type is normally used for sanitary sewage.
8

9. Merits
 More stability and easy in construction
Rectangular
Demerits
 Hydraulically not efficient
9

10. Egg Shaped
Merits
 Hydraulically efficient than circular section
 Maintains self-cleansing velocity even in DWF
Demerits
 Difficult in construction
 Less stable, requires good masonry
10

11. Standard Egg
shaped
Section of some non-circular sewers
Modified Egg shaped
Parabolic
Rectangular
11
Section of some non-circular sewers

12. Basket -Handle
shaped
Semi Circular
U shaped
Semi-elliptical
shaped
12

13. Design of Sewers
Sewers normally contains 99.9% water and 0.1% solid matters, so design of sewer is
similar to water mains except the following differences:-
 Sewers are designed to achieve non-silting or self-cleansing velocity to avoid clogging
problems due to solid deposition from sewage.
 Unlike pressurised flow in water supply pipes, sewers in most of the cases are
considered as open channel flow
and are laid at continuous downward gradient .
13

14. Provision of freeboard in sewers
 Sewers with diameter less than 0.4 m are designed to run half full at maximum
discharge and sewers with diameter greater than 0.4 m are designed to flow 2/3 to ¾ full
at maximum discharge.
 The extra space provided in the sewers provides factor of safety to counteract against
the following factors:
1. Safeguard against lower estimation of the quantity of wastewater to be collected at the
end of design period due to private water supply by industries and public, which ensure
sewers will never flow full eliminating pressure flow inside the sewer.
14

15. 2. Large scale infiltration of storm water through wrong or illegal connection, through
underground cracks or open joints in the sewers.
3. Unforeseen increase in population or water consumption and the consequent
increase in sewage production
15

16. Design criteria of sewers
1.Determination of Sewage
and Storm water
a) Consideration of design period (25-30 years ).
b) Population forecast as described in water
supply engineering
c) Quantity estimate of sanitary sewage from dry
weather flow (DWF) and storm sewage from
hydraulic analysis.
a) Design quantity of sanitary sewage=P*DWF
b) Quantity of storm sewage at WWF=(CiA/360)
cumec.
16

17. 2. Selection of
the system
a) For separate system formula given in consideration no.1 is
used and design is done for Qsa and Qst for separate sewer or
drain.
b) For combined system, discharge (Q)= Qsa +Qst is taken and
only one sewer is designed.
c) For partial separate system, storm water only from roof
pavement and yards are with Qsa in one sewer is designed and
a drain is designed for remaining storm water.
3. Selection of shape and
type of sewer
a) Closed for sanitary sewage.
b) open drain for storm flow.
17

18. 4. Select size of
sewer
a) Not less than 15 cm but recommended is
20 cm.
b) Commercially available sizes(cm): 15,
20, 25, 30, . . ….60 etc. maximum
available size is 3 m.
5. Sewer gradient
Selected as per site condition
a) Minimum : Such that self cleansing velocity
. is achieved. Generally 1:100
b) Maximum : such that the velocity doesn’t
. exceed non scouring velocity
Generally 1:20.
18

19. Self-cleansing velocity/minimum velocity/non-silting velocity
 It is the velocity that do not permit solids to settle down and make them remain in
suspension.
 Should develop at least once in a day so as not to allow any deposition in the sewer
leading to blockage.
 Design velocity of flow should not be less than self-cleansing velocity.
 Normally (0.6-0.9)m/s for separate system and 0.7m/s for combined system.
 From Shield formula self-cleansing velocity Vs= √( 8K(Gs-1)gd/f)Where,
K= constant, for clean inorganic solids = 0.04 and for organic
solids = 0.06
f = Darcy Weisbach friction factor (for sewers = 0.03)
Gs = Specific gravity of sediments
g = gravity acceleration
d' = diameter of grain, m 19

20. Minimum velocity generated in sewers helps in following wa
 Adequate transportation of suspended solids preventing decomposition of sewage.
 Keeping sewer size under control.
 Preventing evolution of foul gases.
20

21. Non-scouring velocity/maximum velocity
 Velocity above which pipe’s interior gets scoured due to continuous abrasion caused by
suspended solids.
 Results wear and tear of pipes and reduced carrying capacity.
 Depends upon material of sewer.
 Severe in hilly areas where ground slope is very steep and is overcome by constructing
drop manholes at suitable places along the length of the sewer.
21

22. Limiting or non-scouring velocity for different sewer
material
Sewer material Limiting velocity
Vitrified tiles 4.5 – 5.5
Cast iron sewer 3.5 – 4.5
Cement concrete 2.5 – 3.0
Stone ware sewer 3.0 – 4.5
Brick lined sewer 1.5 – 2.5
22

23. Hydraulic formula for design of sewers
.
Discharge(Q) = AV
Velocity
Chezy’s Formula
V=C√(RS)
Hazen- Williams Formula
V = 0.849 C R0.63 S 0.54
Manning’s Formula
V= 1
𝑛
𝑟
2
3 𝑆
1
2
23

24. Hydraulic elements of Circular sewer for partial flow condition
24

25. • Sewer Materials
• Design of sewers of separate and combined systems
25

26. 3.3 Sewer Materials
26

27. Resistance to Corrosion
Resistance to Abrasion
Strength and Durability
Imperviousness Economy and Cost
Weight of material
Hydraulically efficient
27

28.  Asbestos Cement Sewer
 Brick Sewer
 Cement Concrete Sewer
 Cast Iron (CI) Sewers
 Steel Sewers
 Plastic Sewers
 Wooden Sewers
 Stoneware Sewers
28

29.  Asbestos Cement Sewer
 Smooth
 Lightweight
 Can easily be cut, fitted, drilled
 Durable against soil corrosion
29
Source:kiev.all.biz

30.  Brick Sewer
 Applicable in large size sewer
 Deformation and leakage can take
place
 A lot of hard work is required
 Plaster is necessary
30
Source:
nationalgunite.com

31.  Cement Concrete Sewer
 Resistant to heavy load, corrosion
and high pressure
 Very heavy and difficult to transport
31
Source:cementpipemachi
ne.blogspot.com

32.  Cast Iron (CI) Sewers
 High strength and durability
 Water tight
 Applicable in high pressure
 Can withstand heavy external load
 No adverse effect on temperature change
32
Source:Balkanplumbin
g.com

33.  Steel Sewers
 Impervious
 Light
 Resistant to high pressure
 Flexible
 Suitable to cross rivers and railway track
33
Source:armtec.com

34.  Plastic Sewers
 Light weight
 Smooth
 Flexible
 Resistance to corrosion
 High thermal expansion
34
Source:youtube.com

35. Wooden Sewers
 Earliest sewer disposal method
 Expensive and difficult to
construct
35
Source:pinterest.com

36. Stoneware Sewers
 Highly resistant to sulphide corrosion
 Resistant to erosion
 High compressive strength
 Cheap and easily available
 Weak in tension
 Brittle
 Quite heavy and bulky
36
Source:pinterest.com

37. Chezy’s Formula
V=C√(RS)
Manning’s Formula
V=(1÷N)R2/3s1/2
Hazen and William’s Formula
V=0.85CR0.63
S0.54
Discharge(Q)=AV
Formulas for
Design
37

38. 1) A combined sewer was designed to serve an area of 60 sq. km with an average
population density of 185 persons/hectare. The average rate of sewage flow is 350
L/Capita/day. The maximum flow is 50% in excess of the average sewage flow. The
rainfall equivalent of 12 mm in 24 h can be considered for design, all of which is
contributing to surface runoff. What will be the discharge in the sewer? Find the
diameter of the sewer if running full at maximum discharge.
Solution:
Total population of the area = population density x area
= 185 x 60 x 102
= 1110 x 103 persons
Average sewage flow = 350 x 11.1 x 105 Liters/day
= 388.5 x 106 L/day
= 4.5 m3/sec
Storm water flow = 60 x 106 x (12/1000) x [1/(24 x 60 x 60)]
= 8.33 m3/sec
Maximum sewage flow = 1.5 x average sewage flow
= 1.5 x 4.5 = 6.75 m3/sec
Total flow of the combined sewer = sewage flow + storm flow
= 6.75 + 8.33 = 15.08 m3/sec
Hence, the capacity of the sewer = 15.08 m3/sec
Hence, diameter of the sewer required at the velocity of 0.9 m/s can be calculated as 38

39. 2)
39

40. 40

41. 41

42. 3.6.2 Alignment and gradient of sewer
• Sewer are laid at correct
alignment
and gradient with the help of
boning
rods or mechanical cranes.
42

43. 3.6.3 Excavation, timbering and dewatering of trenches
• Excavation is done such that minimum
depth is 0.9 m and width is 0.6 m.
• When depth increases to 1.5m to 2m
then timbering of trenches is to be done.
• When the excavation is done below the
ground table then dewatering is to be
done either manually or with the help of
pump.
43

44. 3.6.6 Laying and joining of pipe
• Before laying of the sewer
the gradient of bottom is to be
checked.
• Pipes are laid such that
sockets end faces at up
gradient and joining are done
properly.
44

45. Sewer are testing for water tightness of the joint. The tests are of two types:
Water test : Water is stored for 30 minutes and then the water level is measured. For
consideration ,the water loss should not be more than 2 liter per cm diameter of pipe per
kilometer length.
Air test : From one end, air pressure of 100 mm of water is given by hand pump. If
pressure is maintained at 75 mm of water, the joint is said to be air tight.
45

46.  Backfilling of the trenches is done immediately after testing.
 Backfilling is done in every 15 cm layer with watering for 60 cm above crown and
remain left for one week.
46

47.  Necessity of sewer appurtenances
 Construction of sewer appurtenances
47

48.  Efficient operation of the system.
 For maintainance of the system.
 For regular observation of the system.
 During change in alignment, gradient and size of sewer
48

49. 2.1 Manhole
2.2 Drop Manhole
2.3 Lamphole
2.4 Street inlets
2.5 Catch basins
2.6 Flusing device
2.7 Sand, grease and oil traps
2.8 Inverted siphon
2.9 Sewer outlet
2.10 Ventilating shaft
49

50.  helps in inspection, cleaning and
maintenance of sewer
 provided at every bend, junction,
change of gradient or change of
diameter of the sewer
 Lay sewer in convenient length
 Achieve the ventilation of sewers
Pipe Diameter Spacing
Small sewers 45
0.9 to 1.5 m 90 to 150 m
1.5 to 2.0 m 150 to 200 m
Greater than 2.0 m 300 m
Table 8.1 Spacing of Manholes
50

51. Shallow manhole :
 0.7 to 0.9 m depth
 constructed at the start of the branch
sewer or at a place not subjected to
heavy traffic conditions
 Provided with light cover at top
Fig : shallow manhole
51

52. 52
Normal manhole :
 1.5 m deep with dimensions 1.0 m x
1.0 m square or rectangular with 1.2
m x 0.9 m
 provided with heavy cover at its top
to support the anticipated traffic load

53. Deep manholes :
 depth is more than 1.5 m
 size in upper portion is reduced by providing an offset
 provided with heavy cover at its top to support the traffic
load
53Fig : Rectangular deep manhole Fig : Circular deep manhole

54. Construction of Manhole
 Top cover and frame
Depth of frame is 20-25 cm and width is 10cm. The clear cover of the opening should be at
least 50 cm. The top cover with frame weights 90-270 kg.
 Access Shaft
The upper portion of the manhole is called the access shaft which provides an access to the
working chamber. Its size is 0.6m X0.75m for rectangular and 0.6 to 0.75m diameter for
circular manhole.
54

55.  Working Chamber
It is the lower portion of the manhole, which provides working space and should have
minimum 0.9m X 1.2m for rectangular and 1.2m diameter for circular manhole.
 Bottom or Invert or Benching
The bottom of the manhole is made of concrete bed of 150-300 mm thick and top slope
of 1:6 towards the centre and called benching which may be semi- circular or U-shaped.
 Steps or Ladder
It is the CI or steel steps provided for all deep manholes and made of and placed 30 cm
apart vertically for up and down.
55

56.  The drop manhole is required in the
same sewer line in sloping ground,
when drop more than 0.6 m is
required to control the gradient and to
satisfy the maximum velocity i.e., non-
scouring velocity
 The main purpose of the drop
manhole is to avoid splashing of
sewage on the man working in
manhole as well as for smooth fall of
sewage
Fig : Drop Manhole
(Source : nptel.ac.in/courses/105105048/M8L10.pdf)
56

57.  It is an opening or hole constructed in a
sewer to permit the insertion of lamp into
the sewer for the purpose of inspection of
sewer and detecting the presence of any
obstruction in the sewer line.
 It is connected to sewer line through a T-
junction
 It consists of a vertical shaft of 20-30 cm
diameter connected to the sewer and the
top opening is covered by CI or RCC cover
Fig : lamphole
(Source: nptel.ac.in/courses/105105048/M8L10.pdf)
57

58. Fig : curb inlet
(Source : nptel.ac.in/courses/105105048/M8L10.pdf)
 Street inlets are the devices or opening in the
street constructed to intercept the storm
water and surface wash along the street and
to convey it into the sewer.
 They are provided either depressed or flush
with respect to the elevation of the pavement
surface.
 A maximum spacing of 30 m is recommended
between the inlets, which depends upon the
road surface, size and type of inlet and
rainfall. 58

59. Fig : curb inlet
(Source : nptel.ac.in/courses/105105048/M8L10.pdf)
Types of street inlets
 Curb inlets :
These are vertical opening in the road curbs through which
stormwater flow enters the stormwater drains. These are
preferred where heavy traffic is anticipated .
 Gutter Inlets:
These are horizontal openings in the gutter which is
covered by one or more grating through which stormwater
is admitted.
 Combined Inlets:
In this, the curb and gutter inlet both are provided to act
as a single unit. The gutter inlet is normally placed right in
front of the curb inlets.
59

60. Fig : catch basins
(Source : nptel.ac.in/courses/105105048/M8L10.pdf)
 Catch basins are provided to stop the entry of
heavy debris present in the storm water into
the sewers.
 It has depth of 600 to 750 mm may be
square, rectangular or square of dimension
600-900mm.
 After entering the water the sand , girt etc
settles down in the pit and only water enters
the outlet pipe
 The outlet pipe is fixed about 60 cm above
the bottom with trap to prevent from out
coming of odour.
60

61.  Arrangement which holds water and then throws it into sewer for flushing it
 Why do sewers need flushing when certain reaches of sewer have unfavorable
combination of low slope and low tributary sewage flow i.e. have chances of blockage
also placed at dead end point of sewer
 Two methods of flushing
1. hand operated flushing operation
2. automatic flushing tanks
61

62. Automatic flushing tank
•Carried out automatic at regular intervals
•entry of water so regulated as to fill the tank
up to the discharge point in a period equal to
flushing point
•overflow pipe to drain away water incase the
tank fails to discharge and there by overflow
Source: www.google.com.np
62

63. .
Manual flushing arrangement
Source: www.google.com.np
63

64. Grease and oil traps
 Trap chambers which are constructed in a sewerage system to exclude grease and oil
from the sewage before it enters the sewer line
 Such traps are located near the sources contributing grease and oil to the sewage
The following are the reasons for excluding grease and oil from the sewage
 They stick to sides of sewer and consequently the capacity of sewers are reduced
 Possibility of explosion in sewers
64

65.  Makes treatment complicated
 Suspended matter which would otherwise have been conveyed also sticks to the side of
sewer
 If sand is also desired to be excluded from the sewage, dead space should be kept at
the bottom of the chamber for sand to be deposited
Source: www.google.com.np
Sand, Grease And Oil Traps
65

66.  In areas having more depressions in the ground, the sewer line can be made above the
ground supported by piers but when sewer is crossed through roads, canals etc lying at
the lower level than sewer , it is depressed below them is called depressed sewer or
inverted siphon.
 A true siphon flows full with the flow line above HGL and hence flows under less then
atmospheric pressure but inverted siphon flows full with the flow line below the HGL
and flows under greater then atmospheric pressure.
66

67. Inverted siphon
67
Sewer outlet
Source: dreamtimes.com
Ventilating shaft
Source : www.slideshare.net

68.  Storm water and treated water may be discharged into small or large water bodies.
 Outlets to small streams are similar to the culverts, consisting of a simple concrete
head wall and apron to prevent erosion.
 Sewers discharging into larger bodies of water are usually extended beyond the banks
into fairly deep water where dispersion and diffusion will aid in the mixing sewage with
the surrounding water.
68

69.  During decomposition of organic matters within the sewer the foul gases is produced
and causes harm to human heath as well as reduce the life of sewer by corroding the
sewers.
 The produced gases are explosive and poisonous and may cause explosion and
accidents to the maintenance person.
 Hence to help free flow of sewage with relief pressure the ventilation is provided to
sewer lines.
69

70. 70

71. 1. Er. Arun Prasad Parajuli, Sanitary Engineering, Kathmandu Nepal
2. nptel.ac.in/courses/105105048/M8L10.pdf
3. www.civilsolutions.com
4. www.google.com.np
5. dreamtimes.com
6. www.slideshare.ne
71