Conventional Sewerage Flashcards
Name 3 pollution regulation organisations
- EU Bathing Water Directive.
- EU Urban Wastewater Directive (min treatment).
- EU water Framework Directive (surface ground water).
Describe the piped sewerage system
Drains carry flow from individual properties.
sewers carry flow from groups of properties.
sewerage refers to the whole infrastructure (pipes, manholes, CSO’s, treatment).
What is the capacity of a typical water treatment plant?
6 * DWF
Why are separate sewer systems not the best alternative to combined?
- more costly (two networks)
- perfect separation impossible to achieve
- stormwater may be polluted
Name some components of a sewer
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.
What is the minimum (self-cleansing) velocity in a sewer?
1 m/s < V < 3 m/s (avoided)
Define DWF and how to calculate
“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)
What are some standard sewer design parameters used (if not given)?
Q(design) = 6PG
G = 200
P = 3
Ks = 1.5 mm
tc = 4 mins
Define time concentration in sewer design
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
What is the rational method?
Q(L/s) = 2.78 * C * i * A
C = runoff coefficient
i = intensity
A = catchment area
What is the modified rational method?
accurate up to 150 ha.
now, C = Cv * Cr
Cv = PR/PIMP (0.6-0.9)
Cr = 1.3 (recommended)
V>1m/s
What are the limitations of the modified rational method
- Assumes constant rate of rainfall, uniform over whole catchment.
- Assumes catchment imperviousness is constant.
- Assumes sewers flow at constant (pipe-full) velocity.
Summarise Foul sewer systems
- capacity based on DWF
- d/D=0.75 ; Q/Qf=0.9
- min velocity = 0.75 m/s
- pipe roughness, Ks = 1.5mm
Summarise Storm sewer systems
- capacity based of tc and modified rational method
- Qp = 2.781.3CviAi
- design to d/D=1
- min velocity=1m/s
- pipe roughness, Ks=0.6mm
How do you calculate CSO setting?
Traditionally: 6DWF
Formula A:
DWF + 1360P + 2*E
List some CSO types
- high side weir with screens
- stilling pond CSO
- vortex separator
Define Urban Pollution Management
“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”
Why is network modelling for UPM difficult?
Data collection is expensive:
-harsh environment
-complex network
-need DWF and storm samples
-hydraulic and water quality parameters
What are the main factors to consider when designing small sewers (foul)?
- Intermittent flows
- High sedimentation flows and deposition
- Self cleansing
- diameter >100mm to prevent blocking
What design storm return periods are typically used in the design of storm sewers (storm)?
1 or 2 years unless consequence of failure is high. 5 years in vulnerable locations
What is the function of a CSO?
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.
What is CSO consent?
Level of discharge permitted, agreed between sewer operator and environment agency. Specifies number of CSO’s and total volume/year.
Explain how CSO shape and design can be used to reduce the amount of pollution discharged into urban watercourses.
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.
How do you convert litres/s into m3/s?
1000 litres/s = 1 m3/s
What is formula A?
Setting = 1360P +2E (l/d)
DWF = + PG + I + E
Assume G = 200 l/hd/day
How will climate change impact storm sewer designs?
Increases in the occurrence of rainfall extreme must be accounted for.
Adapting to more frequent extreme rainfall generally means increasing pipe size.
What is the percentage runoff equation?
PR = 0.829PIMP + 25SOIL + 0.078UCWI - 20.7
PR = 0.4PIMP (PR<0.4PIMP)
