Conventional Sewerage

Question 1

Name 3 pollution regulation organisations

Answer 1

1. EU Bathing Water Directive.
2. EU Urban Wastewater Directive (min treatment).
3. EU water Framework Directive (surface ground water).

Question 2

Describe the piped sewerage system

Answer 2

Drains carry flow from individual properties.
sewers carry flow from groups of properties.
sewerage refers to the whole infrastructure (pipes, manholes, CSO’s, treatment).

Question 3

What is the capacity of a typical water treatment plant?

Answer 3

6 * DWF

Question 4

Why are separate sewer systems not the best alternative to combined?

Answer 4

1. more costly (two networks)
2. perfect separation impossible to achieve
3. stormwater may be polluted

Question 5

Name some components of a sewer

Answer 5

Materials- vitrified clay, concrete, PVC, brick.
Invert refers to lowest point in pipe.
Soffit refers to highest point in pipe.
Manholes at junctions where change in direction for access.

Question 6

What is the minimum (self-cleansing) velocity in a sewer?

Answer 6

1 m/s < V < 3 m/s (avoided)

Question 7

Define DWF and how to calculate

Answer 7

“Average rate of wastewater not immediately influenced by rainfall including domestic, commercial and industrial wastes, and infiltration.”
DWF = PG + I + E
= (population)(average consumption) + (Infiltration) + (average industrial effluent)

Question 8

What are some standard sewer design parameters used (if not given)?

Answer 8

Q(design) = 6PG
G = 200
P = 3
Ks = 1.5 mm
tc = 4 mins

Question 9

Define time concentration in sewer design

Answer 9

tc is defined as the time required for surface runoff to flow from the remotest part of the catchment area to the point under consideration.
= overland flow time, te + channel flow time, tf

Question 10

What is the rational method?

Answer 10

Q(L/s) = 2.78 * C * i * A
C = runoff coefficient
i = intensity
A = catchment area

Question 11

What is the modified rational method?

Answer 11

accurate up to 150 ha.
now, C = Cv * Cr
Cv = PR/PIMP (0.6-0.9)
Cr = 1.3 (recommended)
V>1m/s

Question 12

What are the limitations of the modified rational method

Answer 12

1. Assumes constant rate of rainfall, uniform over whole catchment.
2. Assumes catchment imperviousness is constant.
3. Assumes sewers flow at constant (pipe-full) velocity.

Question 13

Summarise Foul sewer systems

Answer 13

1. capacity based on DWF
2. d/D=0.75 ; Q/Qf=0.9
3. min velocity = 0.75 m/s
4. pipe roughness, Ks = 1.5mm

Question 14

Summarise Storm sewer systems

Answer 14

1. capacity based of tc and modified rational method
2. Qp = 2.781.3CviAi
3. design to d/D=1
4. min velocity=1m/s
5. pipe roughness, Ks=0.6mm

Question 15

How do you calculate CSO setting?

Answer 15

Traditionally: 6DWF
Formula A:
DWF + 1360P + 2*E

Question 16

List some CSO types

Answer 16

1. high side weir with screens
2. stilling pond CSO
3. vortex separator

Question 17

Define Urban Pollution Management

Answer 17

“the management of wastewater that discharges from sewer and sewage treatment systems under wet weather conditions such that the requirements of the receiving water are met in a cost effective way”

Question 18

Why is network modelling for UPM difficult?

Answer 18

Data collection is expensive:
-harsh environment
-complex network
-need DWF and storm samples
-hydraulic and water quality parameters

Question 19

What are the main factors to consider when designing small sewers (foul)?

Answer 19

• Intermittent flows
• High sedimentation flows and deposition
• Self cleansing
• diameter >100mm to prevent blocking

Question 20

What design storm return periods are typically used in the design of storm sewers (storm)?

Answer 20

1 or 2 years unless consequence of failure is high. 5 years in vulnerable locations

Question 21

What is the function of a CSO?

Answer 21

Hydraulic function to divide the inflow between continuation flow and spill flow. Acts as mini treatment works, protecting the receiving water body from pollutants and retain them within the sewer system.

Question 22

What is CSO consent?

Answer 22

Level of discharge permitted, agreed between sewer operator and environment agency. Specifies number of CSO’s and total volume/year.

Question 23

Explain how CSO shape and design can be used to reduce the amount of pollution discharged into urban watercourses.

Answer 23

Shape can cause flow paths that carry pollution away.
Stilling zones allow sediments to settle.
Weir CSO’s reverse flow at surface to remove floating material.
Scumboards hold floating material back from the weir
Screens keep large solids within CSO chamber.

Question 24

How do you convert litres/s into m3/s?

Answer 24

1000 litres/s = 1 m3/s

Question 25

What is formula A?

Answer 25

Setting = 1360P +2E (l/d)
DWF = + PG + I + E
Assume G = 200 l/hd/day

Question 26

How will climate change impact storm sewer designs?

Answer 26

Increases in the occurrence of rainfall extreme must be accounted for.
Adapting to more frequent extreme rainfall generally means increasing pipe size.

Question 27

What is the percentage runoff equation?

Answer 27

PR = 0.829PIMP + 25SOIL + 0.078UCWI - 20.7
PR = 0.4PIMP (PR<0.4PIMP)