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.