3. Table of Contents
Topic___________________________________________Page no.
Introduction 03
Sources of wastewater 03
Types of sewer 03
Types of sewerage systems 04
Steps of design 04
Preliminary investigation 05
Design criteria consideration 06
Actual design of sewer 08
Preparation of drawing & BOQs 09
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4. Related Theory:
Sewage:
It is the liquid waste of wastewater produced as a result of water use.
Sewer:
It is a pipe or conduit for carrying sewage. It is generally closed and
flow tables place under gravity.
Sewerage:
It is a comprehensive term. This term is applied to the art of the
collection of wastewater and conveying it to the point of final disposalwith
or without treatment.
Sources of wastewater:
1. Domestic:
It is waste water from residential buildings, offices, other buildings and
institutions etc.
2. Industrial:
It is liquid waste from industrial processeslike dying, paper
making, fertilizers, chemicals, leather etc.
3. Storm water:
It include surface run off generated by rainfalls and street wash.
Types of sewers:
1. Sanitary sewer:
Sewer which carries sanitary sewage i.e., wastewater originating from
municipality including domestic and industrial wastewater.
2. Storm sewer:
It carries storm sewage including surface run off and street washes.
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5. 3. Combined sewer:
It carries domestic, industrial and storm sewage.
4. House sewer:
Pipe conveying sewage, industrial and storm sewage.
5. Lateral sewer:
It receives discharge from house sewer.
6. Sub main sewer:
It receives discharge from one or more laterals.
7. Main / Trunk sewer:
Receives discharge from two or more sub mains.
8. Outfall sewer:
Receives discharge from all collecting system and convey it to the point of
final disposal(e.g., a water bodyetc.)
Types of sewage system:
1. Separate System:
If storm water is carried separately from domestic and industrial waste, the
system is called separate system.
When favored:
(i) There is an immediate need for collection of sanitary sewage but not
for storm sewage.
(ii) Where sanitary sewage need treatment but storm water does not.
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6. 2. Combined System:
A system in which sewer carries both sanitary as well as storm sewage.
When favored:
(i) When combined sewage can be disposed off without treatment.
(ii) When both need treatment.
(iii) When streets are narrow and two separate sewers cannot be laid.
3. Partially Combined:
If some portion of storm or surface run off (from roads, open spaces etc.) is
allowed to be carried along with sanitary sewage, the system is known as
partially combined system.
Steps for designof sewer:
1) Preliminary investigation
2) Design consideration / Formulation of design criteria
3) Actual design
4) Preparation of drawings and BOQ
5) Subsequent modifications
1. Preliminary
Investigation:
It includes:
- If map of the area is not already available, the step is to carry out
survey to draw a map of the project area.
Different details are marked out on the map like:
*Streets
*Railway lines
*Streams
Location of underground utilities like gas, water mains etc.
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7. - Establish bench marks throughout the area and make
contour profiles
- Soil conditions should be investigated for the type of
structure, location of water table, presenceof any
underground rock etc.
- Collection of rainfall and run off data
- Study of natural slopes of the area and selection of a
suitable disposalpoint
2. Designconsideration/ Formulation of designcriteria:
(i) DesignFlow:
(a) Sanitary sewer = Peak sewage flow + Infiltration +
Industrial flow
(b) Partially combined = 2 * Peak sewage flow + Infiltration
+ Industrial flow
(WASA criteria)
(ii) DesignEquation:
Sewers are designed on the basis of open channel flow not at
under pressure
V = 1/n * R^2/3 * S^1/2 (Manning’s formula)
Where:
V =velocity, m/sec
R = hydraulic mean depth = Area / wetted perimeter
S = D/4, when pipe is flowing full or ½ full
N = Co-efficient of roughness for pipe (0.012 for R.C.C pipe)
(iii) Minimum Velocity:
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8. Min. velocity also called as self-cleaning velocities must be
maintained in sewers to avoid deposition of suspended solids
and sub sequent choking of sewers.
Sanitary sewers = 0.6 m/sec (2ft/sec) {organic particles sp.qs =
1.61}
Storm sewer = 1 m/sec {inorganic particle sp.qs =
2.65} Partially combined = 0.7 m/sec
The slopes with which will give the velocity in pipe 0.6
m/s when flowing full when n = 0.013
A cleaning velocity will achieve only when pipe flow full or
78% full
(iv) Maximum Velocities:
Should not be greater than 2.4 m/sec
A limit on higher velocity is imposed due to abrasive character
of solids in wastewater.
Scraping or wearing away
(v) Min. sewersize:
225 mm is taken as min sewer size {WASA}
(vi) Min Cover:
1 m is taken as min cover over sewers to avoid damage from
live loads coming on sewers.
(vii) Manholes:
Purpose:
1. Cleaning
2. Inspection
3. House connection
When provided:
1. At every change in direction
2. Where two different dia pipes are to be connected
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9. Spacing:
225 mm - 350 mm spacing not greater than 100 m
460 mm – 760 mm spacing not greater than 120 m
{WASA}
>760 mm spacing not greater than 150 m
For plots, one manhole be provided for 2 plots
(viii) Qd / Qt Ratios:
WASA recommends the Qd/Qt ratios in order to provide air
space in the upper portion of sewer for ventilation purpose.
Qd represent design flow and Qt is flow when flow sewer is
flowing full.
Sewer size Ratio (Qd/Qt)
225-380 mm 0.7
460 – 1220 mm 0.75
1370 mm and larger 0.8
3. Designof sewer:
By design of sewer, we mean the following two things:
(i) To find the size of sewer Q
= AV is used to find size
(ii) To find required slope to maintain a min. velocity
in sewers
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10. V = 1/n * R^2/3 * S^1/2 is used to find slope.
4. Preparationof drawings and
BOQ:Typical drawings includes:
- Sewer joints
- Manholes
- Disposal station
- Sewer profiles or L-sections
5. Subsequent Modifications:
Mostly done due to some unforeseen incidents, to
accommodatesome additional demand / requirement
of the client etc. There may be several other reasons.
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11. Project
Sewerage system for housing society (through gravity flow).
Location:
Jhelum road, Bank of upper Chenab canal, Jhelum.
Projectdescription:
No. of plots 281
No. of apartments 3
No. of flats 3
No. of dispensary 1
No. of parks 2
No. of graveyards 1
No. of school 1
Design period 20 years
.
Distribution system:
Grid system
Population forecast:
Geometric method
Pipe material:
Concrete pipes
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12. DESIGN CALCULATIONS
POPULATION FORCASTING:
Population Present Future
(2017) (2037)
Persons/plot 7 11
Persons/app 400 618
Persons/flat 200 309
Pop. of school 300 464
Park 200 309
Graveyard 50 77
Dispensary 150 232
Presentpopulation:
(No. of plots * no. of persons per plot) + other population
Design future population:
Pi + [1+ % rate of growth]
Here n = no. of years for design
Per capita waterconsumption:
400 lpcd
Avg. daily flow:
(Population*per capita consumption*factor)/1000
Here, Factor= 0.8 to 0.9
Peak factor:
1+ (14/4+P.05
)
Here, P = in thousands
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13. Peak flow:
Peak factor* Avg. daily flow
Slope of sewer:
S =
((V*n)/R2/3
)2
Here, n=0.013
R=D/4
Min. velocity:
0.7m/s
Min. sewersize:
225mm
Cycle time:
20 minutes
Depth of wetwell:
2m
Infiltration:
10% of Qavg
Minimum cover:
1m
Sewageflow:
90% of Qavg
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14. Calculations:
Manhole-1 to manhole-2
Total no. of plots = 29
Other population = 0
Initial population = 7*29+0
=203 persons
Future population = 203*(1+2.3%) ^20
=319 persons
Average daily flow = (0.9*319*400)/1000
=115 m3
d-1
Peak factor = 1+ (14/4+P0.5
)
=4.066
Peak sewage flow = 4.066*115
=467.05 m3
d-1
Storm sewage flow = 467.05 m3
d-1
Infiltration = 0.1*319*0.4
= 12.76 m3
d-1
Total sewage flow = 946.86 m3
d-1
=0.011 m3
s-1
Min. velocity = 0.7 ms-1
Area of sewer = Q/V =0.016 m2
Dia. of sewer = ((0.016*4)/3.14)0.5
=142 mm
Adopted dia. = 225mm
Thickness = 42 mm
Slope of sewer= ((0.7*0.012)/
(0.225/4)2/3
)2
=0.0033
QFull = (0.7*3.14*0.2252
)/4
=0.0278 m3
d-1
Q/QFull = 0.40
d/D = 0.49 (From graph)
d = 0.49*225
=110 mm
V/VFull = 0.79 (From graph)
V = 0.553 ms-1
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15. Length of pipe = 79 m (Map scale: 1cm=10m)
Ground level upper = 101.5 m
Ground level lower = 100.5 m
Invert level upper = 101.5-1-42-0.225
=100.233 m
Invert level lower = 100.5-1-42-0.225
=99.23 m
Height of manhole = 101.5-100.23
=1.267 m
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