This document provides design considerations for a sewerage system for a small township. Key assumptions are made about sewage generation rates and peak flows. Manning's formula and the rational method are used to size sewer pipes and drainage channels. Population calculations are made to determine sewage flows from different areas. Pipes with a diameter of 15cm and slope of 1:85 are designed to collect sewage from individual units. A 22.5cm pipe with slope of 1:100 is designed to collect the total sewage flow. A rectangular concrete channel with 30cm width and depth is designed to collect rainfall runoff and discharge it based on calculations using Manning's formula.

1. Design of sewerage system for small
township… A Practical Approch
Mota Manoj H.
Assistant Professor,
SITCOE, Yadrav

2. Some important assumptions….
• Av. Daily sewage generated…..75-80%water supplied
• Max. daily flow……2times the av. daily flow
• Max. hourly flow……1.5 times the max. daily flow
……3times av. daily flow
Min. dia for sewage pipe……15cm.
Computing the peak drainage discharge
• Rational formula……
• Qp….peak runoff(m3/sec)
• K….coeff. of runoff
• pc…. Critical rainfall intensity(cm/hr)
• A…. Catchment area(hectares)

3. Manning’s formula
• Q=A.V
• V=(1/n) .R2/3 .S1/2
Where…
•V…velocity in m/s
•n…rugosity coeff.(0.011-0.013)
•R…hydraulic mean depth
•S…hydraulic gradient (slope)
•Always check for maximum velocity i.e. non
scouring limiting velocity.

4. Design the sewerage system for given township.
Design separate system for drainage and sewage.
Assume following data…
Per capita water supplied…..135 lpcd.
Critical rainfall intensity….4.5cm/hr.
K for roof surface….0.85
K for lawns and garden….0.25
K for paved roads….0.65
n…Rugosity coeff….0.011
Limiting velocity….3.0 m/sec.
No. of units –A to D wing….28
No. of units –E(E1 & E2)wing….32

6. Population calculations…
• 3BHK units….. 7person per unit
• 2BHK units….. 5 person per unit
Total population…..
• Wings A to D….28*7=196 per wing=784
• Wings E1 &E2….64*5=320
• Total population=1104

7. • Wing wise discharge of sewage…
• Wing A to D….196*135*0.8=21.168 m3/day
• Design flow(A to D)……
21.168/(24*3600)*3=0.735*10-3 m3/sec
• Wing E….320*135*0.8=34.65 m3/day
• Design flow(E)……
34.65/(24*3600)*3=1.2*10-3m3/sec

8. Assuming pipes are running at 0.3d i.e 4.5cm
• r/R…..0.684
• a/A…..0.252
Capacity of 15cm dia. pipe laid at 1:85
• V….(1/n) .R2/3 .S1/2
• V=(1/0.011)*(0.684*0.15/4)2/3*(1/85)1/2
• V=0.857 m/s
• Q=V*A=0.857*{(∏/4)*0.152 *0.252}=3.82*10-3 m3/s
• Therefore providing the 15cm dia. pipes for
collection from each unit

9. • Design of pipe to collect total flow and to
discharge it in municipal sewer line….
• Cumulative flow=(1.2*10-3)+(4*0.735*10-3)
=4.14*10-3 m3/sec.
•Assuming pipe dia to be 22.5cm and pipe
running at 0.3depth i.e 6.75cm
Q= A*V
A = {(∏/4)*0.2252 *0.252}=0.01
V =(1/0.011)*(0.684*0.225/4)2/3*(1/100)1/2
V=1.03m/sec
Q=10.3*10-3 m3/sec > Cumulative flow

10. Design of drainage……
• Area of garden=2318.25m2
• Area of bldg.=3492m2
• Area of paving=2789.75m2
Calculate total runoff…..
• Qd=(1/36)*(4.5)*{(2318.25*0.25) +
(3492*0.85) + (2789.75*0.65)}*10-4
• Qd=66.91*10-3 m3/sec

11. • Providing rectangular concrete lined smooth
channel , covered with steel grating, having 30cm
width and 30cm depth.
• Slope =1:100
• Assuming 10cm top clearance
• Effective depth=20cm
• A/p=(0.3*0.2/0.7)=0.0857
• Capacity of channel….
• Qc=(0.30*0.2)*(1/0.011)*(0.0857)2/3*(1/100)1/2
• Qc=0.159m3/s > Qd…..safe
• Draw neat fig. of c/s of concrete lined channel.

12. Summery of design…
• Pipes to collect the sewage from each
wing…dia. 15cm and slope 1:85
• Pipe to collect total sewage and discharge it in
municipal sewerage system….dia. 22.5cm and
slope 1:100
• Rectangular concrete lined smooth channel ,
covered with steel grating, having 30cm width
and 30cm depth to collect rain water and
discharge it to municipal sewerage system.

13. References…
• Environmental Engineering. Vol. II …. S K Garg