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TRANSCRIPT
Week-3
Environmental Engineering – II
Design of Sanitary Sewers
Dr.Amir Farooq
PhD. Environment Management
MSc. Environmental Engineering
BSc. Civil Engineering
STEPS FOR DESIGN OF SEWER SYSTEMS
1. Preliminary Investigations
2. Design consideration/Formulation of design criteria
3. Design Calculations
4. Preparation of drawings and BOQ
5. Subsequent modifications.
1. Preliminary Investigations
i. Obtain MAPS and DRAWINGS that furnish the following
information about the area;
Population Density, water Consumption, Soil Characteristics,
Natural Slopes, Disposal Points, Groundwater Table, Rocks,
Underground Structure, Rainfall Data, Location of Water and
Gas Pipes, Electric Conduits, Cellars,
MAPS should also show the location of Streets, Railways,
Parks, Buildings, Ponds, Streams etc
Generally MAPs are available with sufficient details.
These consist of the following activities:
ii. No previous information/ maps, carry out a
DETAILED SURVEY of the area.
iii. An aerial strip photography is also a probably least
expensive method
iv. Establish BENCH MARKS through out the area
and make contour profiles with contour intervals
ranging from 250 mm to 3 m.
v. Mark surface elevations at street intersections.
vi. Make profile of the street through which sewer has
to run
2. Design Criteria Considerations:
i. Design Flow:
Find out the average sewage flow on the basis of Water
Consumption and the Population at the end of design
period.
a) Sanitary Sewers:
Qdesign = Peak Sewage Flow + Infiltration
b) Partially Combined Sewers:
Qdesign = (2 x Peak Sewage Flow ) + Infiltration
ii. Design Equation:
MANNING’S FORMULA is used for sewers (flowing
under gravity)
Where;
V = velocity, m/sec
R = hydraulic mean depth = D/4, when pipe is
flowing full or half full
S = slope of sewer
n = Co-efficient of roughness for pipe (0.013 for
R.C.C pipes)
Design Criteria Considerations (Cont…
V = 1/n R2/3 S1/2
iii. Minimum (Self Cleansing) Velocity:
Flow in sewer is due to gravity
Self cleansing velocities must be maintained in sewers, at all
times, to avoid deposition of suspended solids and subsequent
choking of sewers.
“Self Cleansing Velocity is the minimum velocity that ensures non-
settlement of suspended matter in sewers”.
Following self cleansing velocities are generally employed.
Sanitary Sewer = 0.6 m/s [Org. particles, sp. gr =1.61]
Storm Sewer = 1 m/s [Inorg. Particles, sp. gr = 2.65]
Partially Combined
Sewer = 0.7 m/s
2. Design Criteria Considerations (Cont…
iv. Maximum Velocity:
Maximum velocity of flow in sanitary sewer should
not be greater than 2.4 m/s.
v. Minimum Sewer Size:
Minimum sever size is 225 mm. [WASA, PHED]
vi. Minimum Sewer Cover:
At least 1 m soil cover to avoid damage from live
loads.
2. Design Criteria Considerations (Cont…
vii. Manholes:
Purpose
Cleaning and Flushing
Inspection
House connections
Serve as ventilators for sewer(perforated man-hole covers
Facilitate laying of sewers in convenient length
House connections
Where provided
At every change in direction
Where two pipes having different sizes or slopes are to be
connected.
Spacing
Sewer Size (mm) Spacing b/w Manholes (m)
255 -350 100
460-760 120
>760 150
NOTE: For plots, one manhole be provided for 2 - 4 plots
2. Design Criteria Considerations (Cont…)
2. Design Criteria Considerations (Cont…)
viii. Qd/Qf Ratios:
WASA recommends the Qd/Qf ratios in order to provide
air space in the upper portion of sewers for ventilation
purpose.
WASA recommended minimum size to be used is 9”
There is no limit of maximum size.
Qd represents design flow & Qf is discharge when sewer
is flowing full.
Sewer Size (mm) Qd/Qf Ratio
225-380 0.7
460 – 1220 0.75
1370 & Larger 0.8
ix Minimum Slope
The slope of the sewer line depends on the size of the sewers.
WASA suggests following slopes for different sizes of sewers
Size of
Sewer
(inches)
Slope Size of Sewer
(inches)
Slope
9 0.0028 30 0.0006
12 0.002 36 0.00045
15 0.0015 42 0.00037
18 0.0012 48 0.00031
21 0.001 54 0.00026
24 0.0008 60 0.00023
27 0.0007 66 0.00020
72 0.00018
3. Design Calculations
By design of sewer, we mean the following two things
The size of sewer &
Slope of Sewer
Equation of continuity is used to find size (diameter).
Manning’s equation is used to find slope/grade.
Nomograms are available for sizing the pipes by using
manning’s equation
Problem: Calculate the size and slope of a sanitary sewer system
serving a population of 5000 persons. How many extra persons it can
serve if slope is doubled. Per Capita water consumption is 400 lpcd.
Sol:
Population = 5000
Water
consumption
= 400 lpcd
Wastewater Flow = 0.7 x 400 = 280 lpcd
Ave. Wastewater
Flow
= 5000 X 280 = 1400 000 l/day
= 1400 m3/day
Peak Flow = 4x1400 = 5600 m3/day
Q = 0.065 m3/Sec
Velocity
V
= 0.6 m/sec Minimum self-Cleansing
Velocity
Q = AV
d = [(4 x0.065)/(3.14x0.6)]1/2
= 0.3708 m = 370.8 mm
= 380 mm Contd..
For Slope
V = 1/n R2/3 S1/2
Where n = 0.013 (new sewer)
And R = d/4
0.6 = 1/0.013(0.380/4)2/3 x S1/2
Thus S = 0.0014
Now if we doubled the slope i.e
S = 2x 0.0014 = 0.0028
V = 1/n R2/3 S1/2
V = 1/0.013(0.380/4)2/3 x (0.0028)1/2
Q = AV
Q =
3.14/4 X (0.38)2 x0.85 =
= 8328.94 m3/day =
Qave = 8328940.2 /4 =
Additional
Discharge
= 2082235 – 1400000 =
Extra person = 682235.04 / 280 =
A sewer is discharging 3.5 m3/min under a grade of 3m /1000m. At what
grade its discharge will be doubled
Sol:
Wastewater Flow Q = 3.5 m3/min = 0.058 m3/Sec
S = 3 / 1000 = 0.003
Velocity V = 0.6 m/sec (Minimum self-Cleansing Velocity
Q = AV
d = [(4 x0.058)/(3.14x0.6)]1/2
= 0.352 m = 380 mm
Now Q = 2 x 0.058 = 0.116 m3/Sec
V = (4Q)/3.14 d2 =
= (4 x 0.116) / [3.14 (0.352)2
= 1.192 m/Sec
For Slope
V = 1/n R2/3 S1/2
Where n = 0.015 (Existing Sewer)
And R = d/4
1.192 = 1/0.013(0.352/4)2/3 x S1/2
Thus S = 0.0082
Prob:
4. Preparation of Drawings and BOQs
Typical drawings include;
Sewer joints
Manholes
Disposal station
Sewer profiles or L - sections
5. Subsequent Modifications
Mostly done due to some unforeseen incident, to
accommodate some additional demand/requirement of the
client.