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
Where
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
Where
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
Where
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.
