Quiz No. 3
Wastewater
Collection
SOLUTION
d/D |
v/V |
q/Q |
Mannings Formula |
1.0 |
1.000 |
1.000 |
V = Velocity in m/s R = Hydraulic Radius, m S = Slope in m/1000m Assume, n = 0.013 Assume circular sewer |
0.9 |
1.124 |
1.066 |
|
0.8 |
1.140 |
0.968 |
|
0.7 |
1.120 |
0.838 |
|
0.6 |
1.072 |
0.671 |
|
0.5 |
1.000 |
0.500 |
|
0.4 |
0.902 |
0.337 |
|
0.3 |
0.776 |
0.196 |
|
0.2 |
0.615 |
0.088 |
|
0.1 |
0.401 |
0.021 |
Solution:
Present Population (in Year 2003) = 10,000
Present Average Water Supply (in Year 2003) = 10,000.(180) = 1.8 million liters per day
Assuming that 80 percent water becomes wastewater,
Present Average Wastewater Generation = 1.8.(0.8) = 1.44 million liters per day
Assuming a Peaking Factor of 3,
Present Peak Wastewater Generation = 3.(1.44) = 4.32 million liters per day
Design Population (in Year 2023) = 45,000
Design Average Water Supply (in Year 2023) = 45,000.(240) = 10.8 million liters/day
Assuming 80 percent water becomes wastewater,
Design Average Wastewater Generation = 10.8.(0.8) = 8.64 million liters per day
Assuming a Peaking Factor of 3,
Design Peak Wastewater Generation = 3.(8.64) = 25.92 million liters per day
For a pipe flowing full:
At q = 0.3 m3/s, the pipe is flowing 0.8 full
When, . Therefore,
Assuming, n = 0.013,
Solving, D = 0.628 m, say, 0.700 m or 700 mm
Corresponding to D = 700 mm,
Therefore, the new value of
Corresponding , and
;
Therefore, vDesign = Velocity at design peak flow = (1.076).(1.1) = 1.184 m/s
(which is within the recommended range of 0.8 m/s to 3 m/s)
At present peak flow, ; corresponding
Therefore, vpresent = velocity at present peak flow = (0.65).(1.076) = 0.699 m/s
(which is greater than the recommended minimum value of 0.6 m/s)
Hence use 700 mm circular concrete pipe at a slope 0f 2 m per 1000 m for accommodating the above flow.
2. Consider the sewer system shown in the figure
below. A main sewer (MS5) with manholes,
M5.4 (starting manhole) to M 5.1 discharges into manhole M5 of the trunk
sewer. Branch sewers discharge into
manhole M5.1 M5.4 of the main sewer (MS5) as shown in the figure. Invert levels of the branch sewers
discharging into manhole M5.4 M5.1 are 97.223, 98.775, 98.311, and 96.334 m
respectively. Invert level of the trunk
sewer leaving manhole M5 is 93.700 m. Based on this information explain how
values in the last three columns of the design table for main sewer (MS5) have
been calculated. All branch sewers have 400 mm diameter. The ground level in the area under question
is between 101 and 102 m.
Table 1. Tabular Calculations for Design of main Sewer M5
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
Manhole |
Length, m |
Peak Flow, (2003) m3/s |
Peak Flow, (2023) m3/s |
Diameter, mm |
Slope |
Discharge, (2023) Litre/s |
Velocity, (2023) m/s |
Present Peak Flow, Year 2020 Litre/s |
Self-Clensing Velocity (2000) m/s |
Total Fall, m |
Invert Elevation, m |
||||
From |
To |
Full |
Actual |
Full |
Actual |
Upper |
Lower |
||||||||
M5.4 |
M5.3 |
88 |
0.05 |
0.15 |
500 |
0.002 |
0.169 |
0.15 |
0.860 |
0.972 |
0.05 |
0.688 |
0.176 |
96.773 |
96.597 |
M5.3 |
M5.2 |
77 |
0.09 |
0.28 |
700 |
0.002 |
0.414 |
0.28 |
1.076 |
1.152 |
0.09 |
0.829 |
0.154 |
96.397 |
96.243 |
M5.2 |
M5.1 |
101 |
0.12 |
0.33 |
700 |
0.002 |
0.414 |
0.33 |
1.076 |
1.184 |
0.12 |
0.861 |
0.202 |
96.213 |
96.011 |
M5.1 |
M5 |
122 |
0.15 |
0.45 |
800 |
0.002 |
0.591 |
0.45 |
1.176 |
1.235 |
0.15 |
0.929 |
0.244 |
95.734 |
95.490 |
Solution:
Elevation of the centerline of branch sewer at manhole M5.4 = 97.223 + 0.2 = 97.423
Elevation of centerline of the upper end of the main sewer from M5.4 to M5.3
= 97.423 0.4 = 97.023
Invert level of the upper end of the main sewer for M5.4 to M5.3
= 97.023 0.25 = 96.773
Fall from M5.4 to M5.3 = 88.(0.002) = 0.176
Invert level of the lower end of the main sewer for M5.4 to M5.3
= 96.773 0.176 = 96.597
Invert level of the upper end of the main sewer for M5.3 to M5.2
= 96.597 0.2 = 96.397
Considering the above invert level, minimum invert level of the branch sewer at M5.3 should be 96.397 + 0.35 + 0.4 0.2 = 96.957, while the actual invert level is 98.775.
Hence okay.
Fall from M5.3 to M5.2 = 77.(0.002) = 0.154
Invert level of the lower end of the main sewer for M5.3 to M5.2
= 96.397 0.154 = 96.243
Invert level of the upper end of the main sewer for M5.2 to M5.1
= 96.243 0.03 = 96.213
Considering the above invert level, minimum invert level of the branch sewer at M5.2 should be 96.213 + 0.35 + 0.4 0.2 = 96.763, while the actual invert level is 98.311.
Hence okay.
Fall from M5.2 to M5.1 = 101.(0.002) = 0.202
Invert level of the lower end of the main sewer for M5.2 to M5.1
= 96.213 0.202 = 96.011
Invert level of the upper end of the main sewer for M5.1 to M5
= 96.011 0.1 = 95.911
Considering the above invert level, minimum invert level of the branch sewer at M5.1 should be 95.911 + 0.4 + 0.4 0.2 = 96.511, while the actual invert level is 96.334.
Hence not okay.
Hence, actual invert level of the upper end of the main sewer for M5.1 to M5
= 96.334 + 0.2 0.4 0.4 = 95.734
Fall from M5.1 to M5 = 122.(0.002) = 0.244
Invert level of the lower end of the main sewer for M5.1 to M5
= 95.734 0.244 = 95.490