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
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


Pd   D
d/D
 radian
0.015
0.4911
28
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
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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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