# How to Design Sanitary Sewer using Manning's Equation

## Sanitary Sewer Design step wise procedure:

*Step 1: Finding the Design Flow*

- Calculate the anticipated future (ultimate) population of area served by the sewer using ultimate population density.
- Multiply the ultimate population by per capita average daily sewage flow to obtain the average daily sewage flow e.g.
- Select or calculate peaking factor (P.F) & Peak hourly waste water flow

P.F = *or *

*Or*

P.F = with max of 4

P.F = 4

Where P = Population in 1000 & Q = Flow in MGD

Now

Peak hourly domestic flow = P.F x Qw

4. Calculate the infiltration allowance at the rate of 10% of average daily waste water flow

or (3% to 5% of peak hourly domestic flow)

or 200 to 500 gallons/mile/inch-dia

or 700 to 1500 gallons/day/acre

or 30,000 (gallons per day) infiltration per mile of sewer

or 170,000 (liters per day) infiltration per km & house connections.* *

5. Calculate the allowance for industrial and commercial sewage at the rate of 3,734 m^{3}/km^{2}/day or 4,000 gal/acre/day.

*Design flow for sanitary sewer = *

*Peak hourly domestic + Infiltration + Commercial waste water flows*

*Step 2: Finding velocity using Manning formula*

(m/s, m, mm)

(ft/s, ft, ft/ft)

*Step 3: Check for minimum self cleaning velocity*

**V > V _{min }** (V

_{min}= 0.60 m/sec OR 2 ft/sec)

*Step 4: Finding the Diameter of Sewer pipe*

Design Flow = X-sec Area of pipe * Velocity

Q = A*V

Where A =

From the above you can find the diameter of the pipe