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

Q_{avg} = 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 m^{3}/sec

P.F = 2 (for discharges ≥ 5.66 m^{3}/sec

**4.** Calculate the allowance for industrial and commercial sewage at the rate of 3734 m^{3}/km^{2}/day

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

Q_{inf }= (0.05 – 0.1) Q_{avg}

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

Q_{design} = Q_{peak} + Q_{ind} + Q_{inf}

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**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)

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**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

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**According to Kutter Sanitary Sewer**

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**According to Bazin Sanitary Sewer**

Where

k = constant depending upon the nature of sewer

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**Design Procedure of Sanitary Sewer**

Calculate the design flow as already explained (Q_{d})

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.