Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environment or is reused for various purposes (called water reclamation).

1. Wastewater Engineering
DESIGN OF SEWERAGE SYSTEMS
1

2. General Consideration
SEWER LINES
Designed to carry the maximum quantity of
sanitary sewage likely to be produced from the area
contributing to the particular sewer.
laid at steeper gradients falling towards the outfall
point with circular pipe cross section.
STORM WATER LINES
Designed to carry the maximum storm runoff that
is likely to be produced by the contributing
catchment area from a rain of design frequency and
of duration equal to the time of concentration
constructed as surface drains at suitable gradient,
either rectangular or trapezoidal section.
.
2

3. Requirements of Design and Planning
of Sewerage System
oAdequate in size - avoid overflow and possible health hazards.
oCorrect estimation of sewage discharge - proper diameter
oFLOW VELOCITY
o NOT BE large as to require heavy excavation and high lift pumping
o NOT BE VERY SMALL small causing deposition of the solid in the sewers.
oLaid - 2 to 3 m deep
oFlow under gravity i.e. at the maximum estimated discharge.
oConveyed - lying area, where the treatment plant is located.
oQuality of raw sewage to be taken into consideration
3

4. Manning’s Formula
4

5. VELOCITY IN SEWERS
SEWER MATERIAL NON SCOURING/
LIMTING
VELOCITY(m/s)
Vitrified Tiles 4.5 - 5.5
Cast iron sewers 3.5 - 4.5
Stoneware sewers 3.0 - 4.0
Cement Concrete Sewers 2.5 – 3.0
Ordinary Brick lined
Sewers
1.5 – 2.5
Earthen Channels 0.6 – 1.2
TYPES OF VELOCITY
 MINIMUM VELOCITY – SELF CLEANSING
 MAXIMUM VELOCITY – NON SCOURING
5

6. 6

7. EFFECT OF VARIATION OF FLOW
OF SEWAGE ON VELOCITY
PRESENT PEAK
FLOW(l/s)
SLOPE PER 1000
2 6.0
3 4.0
5 3.1
10 2.0
15 1.3
20 1.2
25 1.0
RECOMMENDED SLOPES FOR MINIMUM VELOCITY
7

8. DESIGN OF SEWERS:
Calculate the discharge and grade of a sewer running at full depth having a of 300 mm diameter and
a self cleansing velocity of 0.90 m/sec. Assume Manning's constant as 0.013.
1. Velocity(V) =
1
𝑁
∗ 𝑅
2
3 * 𝑆
1
2
2. Diameter (D) = 300 mm = 0.3 m
3. Hydraulic Mean Depth ®=
𝐷
4
=
0.3
4
= 0.075 m
4. Velocity(V) =
1
𝑁
∗ 𝑅
2
3 *
5. 𝑆
1
2 =
0.90 ∗ 0.013
0.178
6. S = 0.004 = 4.3% (Slope is considered as 1 in 1000)
7. Discharge Q = A*V =
3.14 ∗0.32
4
∗ 0.90 = 0.064 𝑚3
/sec
8

9. 9

10. 10

11. 11