This exercise includes the estimation of the quantity of wastewater for separate, Partially combined, and combined sewer systems. The use of rational formula and calculation of weighted runoff coefficient is also discussed for actual storm discharge.

1. Design Discharge of
Various Sewer Systems
Resource Person
Engr. Sidra Rashid
sidradar9@gmail.com

2. Textbook
WATER SUPPLY AND SEWERAGE
By
E.W. Steel and T.J. McGhee
6th Edition
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3. Sample Problem
Determine the runoff coefficient for an area of 0.2 Km2.
Out of this 3000 m2 is covered by buildings, 5000 m2 by
paved driveways and walks and 2000 m2 by Portland
cement streets. The remaining area is flat, heavy soil,
covered by grass lawns. If population density in the area
is 600 persons per Km2 and rate of water supply is 200
LPCD, and maximum intensity of rainfall is 40 mm/hour,
calculate design discharge for separate system, partially
separate system, and combined system.
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4. Solution
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Estimation of sewage discharge for separate system
Separate System  Sanitary Sewage & Storm Sewage (Separately)
Discharge in Sanitary Sewer
Quantity of sanitary sewage
Population of area = 600 * 0.2 = 120 persons
Water Consumption = 120x 200 = 24000 lpd
Wastewater generation = 19200 l/d = 19.2 m3/d
= 0.0002 m3 /s
Considering peak factor of 2, the design discharge for sanitary sewers
= 0.0002 x 2
= 0.0004 m3/sec

5. Cont.
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Separate System  Sanitary Sewage & Storm Sewage (Separately)
Discharge in Storm Sewer
Storm Discharge
Area =0.2 km2 = 200000 m2
Cavg =0.183875
I =40 mm/hr
Qstorm =CIA
= 0.4086 m3/s
Surface Type C Area A*C
Roof 0.825 5000 4125
Driveways & walks 0.8 3000 2400
Portland Cement Streets 0.875 2000 1750
Flat, heavy soil grass lawns 0.15 190000 28500
Cavg 0.183875

6. Cont.
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Estimation of sewage discharge for combined system
Combined System  Sanitary Sewage + Storm Sewage (Collectively)
Sanitary sewage = 0.0004 m3/sec
Storm sewage = 0.4086 m3/s
Sewage in Combined Sewer System
= 0.0004 + 0.4086 m3/s
= 0.409 m3/s

7. Cont.
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Estimation of discharge for partially Combined system
Storm water discharge falling on roofs and driveways/walks will be added to the sanitary
sewer. This quantity can be estimated as:
If Qstorm* is portion of storm discharge going into the sanitary sewer then
Area =8000 m2 ; Cavg =0.815625 ; I =40 mm/hr
Qstorm*= CIA= 0.0725 m3/s
Discharge in partially combined system = 0.0004 m3/sec + 0.0725 m3/s
= 0.0729 m3/sec
Discharge in storm water drains = Qstorm – Qstorm*
= 0.4086 - 0.0725
= 0.336 m3/s
Surface Type C Area A*C
Roof 0.825 5000 4125
Driveways & walks 0.8 3000 2400
Cavg 0.815625

8. Problem 1
Drainage Area: 80 acres
30% - Rooftops (24 acres)
10% - Streets & Driveways (8 acres)
20% - Lawns @ 5% slope (16 acres) on sandy soil
40% - Woodland (32 acres)
Time of Concentration (TC) = 15 min.
Location: Tallahassee, Florida (Leon County), Zone 2
Peak runoff rate from 10-year frequency storm
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9. Cont.
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10. Cont.
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11. Cont.
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12. Problem 2
Determine designed discharge for a combined system
serving population of 50000 with rate of water supply
of 135 LPCD. The catchment area is 100 hectares, and
the average coefficient of runoff is 0.60. The time of
concentration for the design rainfall is 30 min and the
relation between intensity of rainfall and duration is I=
1000/(t + 20).
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13. Problem 3
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