Sewer Design & Design flow in Sanitary Sewer

Sewer Design

To find the design flow in sanitary sewer the following steps are followed

1. Forecast the design population (P) of the area;

2. Find the sewage flow per day by multiplying population with flow per day per capita of sewage. The sewage is taken as (70-80)% of average water supply

If (q) is the average per capita per day water consumption of water then

Qavg = Average sewage flow = (0.7 – 0.8) *q*P

Note: average value of 75% i.e. 0.75 can be used

3. Select a peaking factor (P.F) to find the peak sewage flow (Note. Sewers are designed for peak flow)

According to WASA (Water and Sanitary Agency)

P.F = 4 (for discharge upto 0.0283 m3/sec

P.F = 2 (for discharges ≥ 5.66 m3/sec

4. Calculate the allowance for industrial and commercial sewage at the rate of 3734 m3/km2/day

5. Calculate infiltration allowance of (5-10)% of average sewage flow as given by WASA

Qinf = (0.05 – 0.1) Qavg

6. Find the design sewage flow by adding Peak Flow industrial allowance and infiltration flow

Qdesign = Qpeak + Qind + Qinf

Manning's Formula of Sanitary Sewer Equation

Normally, Manning’s formula is used for design of sanitary sewer, which is


V = Velocity of sewage flow


R = Hydraulic radius   


S = Slope of sewer

n = roughness co-efficient (0.013 – 0.015)

n = 0.01 (for smooth inside surface like PVC etc)

n = 0.013 (for brick masonry, concrete, RCC Pipe)

n = 0.017 (for stone masonry)

n = 0.025 (for earthen channels)

Chezy’s Formula for Sanitary Sewer


C= Chezy constant value of c can be obtained by using either Kutter’s formula or Bazin’s formula

According to Kutter Sanitary Sewer

According to Bazin Sanitary Sewer


k = constant depending upon the nature of sewer

Design Procedure of Sanitary Sewer

Calculate the design flow as already explained (Qd)

Select value of self cleansing velocity and compute area of pipe (sewer) by A = Q/V and diameter

Find the slope of sewer by Manning's formula

Check the velocity, it should be greater than the minimum self cleansing velocity.