Wastewater Collection Systems On completion of this module you should be able to: Discuss the sources of wastewater Understand the relevant.
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Transcript Wastewater Collection Systems On completion of this module you should be able to: Discuss the sources of wastewater Understand the relevant.
Wastewater Collection Systems
On completion of this module you should be able to:
Discuss the sources of wastewater
Understand the relevant sections of the legislation
relating to sewer collection systems and wastewater
Plan and design a wastewater collection system
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Wastewater Collection Systems
Some definitions on wastewater systems
Sewerage – a system comprising of collection and
treatment facilities
Sewage – spent water or wastewater
Sewers – a collection system of pipes to convey
wastewater to a central point of treatment
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Wastewater Collection Systems
Sources of wastewater
Domestic flows
Industrial and trade wastes
Urban stormwater
Infiltration/inflow
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Wastewater Collection Systems
Materials in wastewater
Impurities 0.01%
Physical form
suspended
Chemical
Biological
inorganic
organic
dissolved
living
Non-living
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bacteria, fungi, protozoa,
algae
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Wastewater Collection Systems
Infiltration/inflow (I/I)
Ingress of groundwater or rainwater from pipe
defects, joints etc
Ground condition also dictates I/I
I/I peaks during and after storms and varies with
season
Extraneous water from illegal connections
Qld’s guidelines allow 14 – 28 m3/d.km
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Wastewater Collection Systems
Wastewater presents a unique design problem
Hydraulic loading (ML/d)
Organic loading (kg/m3.d)
Concept of equivalent person or
population (ep) for design
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Wastewater Collection Systems
Relevant legislations
Plumbing & Drainage Act 2002, and Standard
Plumbing & Drainage Regulation 2003, that relate to
licensing and assessing of work
Environmental Protection Act 1994 that relates to
quantity and quality of flows into the environment
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Wastewater Collection Systems
Discharge into sewers
Governed by the local authority (trade
officers)
Industrial and trade effluent are considered
on a case by case basis
Land discharge is subject to the
Environmental Protection Act 1994
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Wastewater Collection Systems
Quality of treated effluent
Subject to the Environmental Protection Act
1994
Administered by the Environmental Protection
Agency
Generally licence conditions of BOD5 < 20
mg/L; NFR < 30 mg/L; DO > 2 mg/L
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Wastewater Collection Systems
Types of wastewater systems
Separate versus combined systems
Gravity and pumped flows
Small collection systems using pressure or
vacuum
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Wastewater Collection Systems
Pressure system
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Wastewater Collection Systems
Vacuum system
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Wastewater Collection Systems
Sewer installation
Sewer alignment
Depth of sewer
House connection
Location of manholes
Testing of sewers and house drains
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Wastewater Collection Systems
Sewer installation
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Factors that control the depth of
sewers
Self-cleansing velocity ie. minimum slope
Minimum cover to protect the sewer
Required depth to drain properties serviced
Sufficient depth to avoid other services
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House drain and connection
House drain is the property sewer pipe that
adjoins council’s sewer
Minimum house drain dia. is 100 mm with a min.
slope of 1:60 allowing 0.5 invert depth at the head
House drains must be vented at the head
House connection is made at the lowest point
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Wastewater Collection Systems
Sewer installation
Typical sewer and housedrain connection
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Testing of sewers
Water test - apply a pressure equal to 2 m
head at the higher section of the length under
test. Loss of water shall not exceed 1 L/m
diameter. m length in 30 minutes
Air test – apply a pressure of 30 kPa and hold
for 3 mins. Time taken for a drop from 25 kPa
to 20 kPa shall be not less than 90 secs for
pipes less than 225 mm.
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Use of manholes and location
Inspection and maintenance
Changes in vertical and horizontal alignment
Intersections
Spaced not greater than 90 m for 375 mm pipes
Spaced not more than 150 m for larger pipes
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Wastewater Collection Systems
Typical manhole configuration
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Wastewater Collection Systems
Design parameters
System must drain all points of the catchment
Peak wet weather flow capacity
Self-cleansing flow velocity
gravity flow at minimum slope
Pressure mains where necessary
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Wastewater Collection Systems
Design for maximum flow (Queensland
Planning Guidelines for Water Supply and
Sewerage Schemes)
Average dry weather flow (ADWF) 275 L/c.d
Peak dry weather flow, PDWF = C1 x ADWF
Peak wet weather flow, PWWF = C1 .ADWF + I/I
Maximum flow at 3/4 pipe depth
150 mm min. dia gravity flow & min. slope dependent
on pipe dia.
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Peaking factors for maximum flows
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Proportional velocity and discharge
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Proportional geometry elements
sin 2
Ad
D
8
Pd D
d/D
radian
0.015
0.4911
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5.48648E-05
0.0368
0.1164
0.00036
0.016
0.5073
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6.04235E-05
0.0380
0.1215
0.00042
0.017
0.5230
30
6.61557E-05
0.0392
0.1265
0.00047
Ad
Pd
v/V
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q/Q
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Wastewater Collection Systems
About 60 – 80% of the daily water demand appears
as spent water
There is a diurnal pattern in the collection system
Minimum 150 mm pipe with a minimum slope of
1:150 in the collection system
Minimum slopes relate to self-cleansing velocity
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Wastewater Collection Systems
Owing to a mixture of solids and liquids,
sewage flow velocities must be self-cleansing
0.75 PDWF at least once a day to promote selfcleansing flow
Generally, self-cleansing velocity is achieved at 0.6
– 0.75 m/s
Use of 0.15 kg/m2 shear stress for organic solids
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Wastewater Collection Systems
Effects of long detention times
Owing to the high O2 demand of biodegradable organic
matter, long HRT will deplete dissolved oxygen (DO)
Slime growth under waterline will promote anaerobic
bacteria and reduce sulfate to sulfides
Downstream turbulence will release H2S into the air
space
Moist film above the waterline and aerobic bacteria will
oxidise H2S to H2SO4
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Effects of long detention times
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Effects of long detention times
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Effects of detention times in rising mains
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Wastewater Collection Systems
Finally do we question:
Is it still environmentally responsible to use 50 - 80
kg/day of drinking water to transport 1 - 1.5 kg/d of
human waste to a treatment plant?
Do we have to continue improving the wrong solution
or do we have the intelligence for new solutions?
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End of Module 6 Wastewater
Collection Systems
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