How to Estimate the Design Flow for a Sanitary Sewer
Design flow for sanitary sewer can be find as shown in steps below.
Step 1: Future Population
Calculate the anticipated future (ultimate) population of area served by the sewer using ultimate population density.
Step 2: Avg. Daily Sewage Flow
Multiply the ultimate population by per capita average daily sewage flow to obtain the average daily sewage flow e.g.
|Anticipated Population (P) =||200||Persons|
|Water Consumption (q) =||400||Liters per capita per day|
|Waste water flow per capita per day (qw )=||60% to 80%||of water consumption (q)|
|Future anticipated average waste water flow (Qw) =||P*q*qw||Liters per day|
|140,000||Liters per day|
Step 3: Peaking Factor
Select or calculate peaking factor (P.F) & Peak hourly waste water flow
P.F = or
P.F = with max of 4
Where P = Population in 1000 & Q = Flow in MGD
P.F = 4 ..................For sewage flow upto 0.0283 m3/sec
P.F = 2 ..................For sewage flow upto 56,000 m3/
Peak hourly domestic flow = P.F x Qw
Step 4: Infiltration
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.
Step 5: Industrial and Commercial Sewage
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 [step 3] + Infiltration [step 4] + Commercial [step 5] waste water flows