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How to Estimate the Design Flow for a Sanitary Sewer

Sanitary Sewer Design Flow

Design flow for sanitary sewer can be find as shown in steps below.

Sanitary Sewer Flow Estimation

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 = 1+\frac{14}{4+\sqrt{P}}or 1+\frac{22}{4+\sqrt{P}}\ngtr 4


P.F = \frac{2.5}{Q^{0.145}} 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

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