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
P.F = with max of 4
P.F = 4
Where P = Population in 1000 & Q = Flow in MGD
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 m3/km2/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 > Vmin (Vmin = 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