1. Sanitary Engineering Lecture 5

2. Qdes = Qmax + QI/l ( if found)
Design of W.W. Collection System
Design criteria:
Waste water flow: Flow varies according to:
The season (monthly variations)
Weather conditions
Week of the month , day of the week, time of the day.
Estimation of the design flow Qdes: Data needed:
Average daily water consumption per capita for domestic areas (L/c/d), (Qavg).
Average daily water consumption per capita for institution ( school, offices, ….etc. ), (Qavg).
Average daily water consumption for commercial and industrial areas.
Qdes = Qmax + QI/l ( if found)
Infiltration, inflow:
QI/I = QInfil + QInflo
QInfil is taken as [24-95 m3/day/km] or [0.5 m3/day/diameter (cm)], take the bigger value of the two.
QInflo is taken as [m3/ha/day]. (hectare = 10,000 m2)
Qmax = [0.80* Qavg] * Pƒ (0.8 > 80% return from water supply).
This equation is for domestic users only. Q max for institutions, commercial activities, and industries are calculated according to the type of industry, and cannot be calculated from this equation. Each industry has its specific average wastewater production and peaking factor that can be taken from published references or from the records of these industries or institutions.

3. Sewage flow diagram for a small town
149 86 43
hour
1.8
1.5
1.0
0.5
0.0
Flow coefficient
Flow (L/s)
Peak coefficient
Average day flow
Average 24 hr flow
Average night flow
Sewage flow diagram for a small town

4. Note: [Qavg]w = 0.8 Qavg , which is the average domestic wastewater production , while Qavg is the average water consumption.

5. Example
Solution

7. Runoff-coefficient-Estimation
Rational Method Equations:
C = runoff coefficient
Example 13-2: Determine the runoff coefficient for an area of 0.2 km2, of which 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.
Solution
From Table 13-2, one may obtain values of C for each area:

8. Runoff-coefficient-Estimation

9. The fraction of the area with each surface is
Runoff-coefficient-Estimation
Roofs 0.70 to 0.95
Driveways and walks 0.7to 0.85
Street 0.80 to 0.95
Lawn 0.13 to 0.17
The fraction of the area with each surface is
Roofs 3000/200,000 = 0.015
Driveways and walks 5000/200,000 = 0.025
Street 2000/200,000 = 0.010
Lawn 190,000/200,000 = 0.95
The average value of C, depending on the specific values chosen
for the individual areas, will thus lie between 0.16 and Conservative practice, in the absence of local knowledge, would be to use the higher values in the table, resulting in a runoff
coefficient of 0.21

10. Application
Given for a gravity pipe serving in a community of inh. the following:
Pipe length km
Qavg L/c.d
Infiltration rate m3/km.d
Inflow m3/d
The waste water production rate is 80% of the water supply
Find Qmax, Qdes and Qmin