public health engineering1 wastewater collection systems discuss the sources of wastewater...
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Public Health Engineering 1
Wastewater Collection Systems
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
On completion of this module you should be able to:
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Wastewater Collection 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
Some definitions on wastewater systems
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Wastewater Collection Systems
Domestic flows
Industrial and trade wastes
Urban stormwater
Infiltration/inflow
Sources of wastewater
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Wastewater Collection Systems
Materials in wastewater
Impurities 0.01%
Physical form
suspended
dissolved
Chemical
inorganic
organic
Biological
living
Non-living bacteria, fungi, protozoa,algae
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Wastewater Collection Systems
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
Infiltration/inflow (I/I)
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Wastewater Collection Systems
Hydraulic loading (ML/d)
Organic loading (kg/m3.d)
Concept of equivalent person or population (ep) for design
Wastewater presents a unique design problem
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Wastewater Collection Systems
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
Relevant legislations
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Wastewater Collection Systems
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
Discharge into sewers
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Wastewater Collection Systems
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
Quality of treated effluent
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Wastewater Collection Systems
Separate versus combined systems
Gravity and pumped flows
Small collection systems using pressure or vacuum
Types of wastewater systems
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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 alignment
Depth of sewer
House connection
Location of manholes
Testing of sewers and house drains
Sewer installation
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Sewer installation
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Self-cleansing velocity ie. minimum slope
Minimum cover to protect the sewer
Required depth to drain properties serviced
Sufficient depth to avoid other services
Factors that control the depth of sewers
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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
House drain and connection
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Wastewater Collection SystemsSewer installation
Typical sewer and housedrain connection
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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.
Testing of sewers
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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
Use of manholes and location
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Wastewater Collection Systems
Typical manhole configuration
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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
Design parameters
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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.
Design for maximum flow (Queensland Planning Guidelines for Water Supply and Sewerage Schemes)
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Peaking factors for maximum flows
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Proportional velocity and discharge
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Proportional geometry elements
2
8
sinDAd
2DPd
0.015 0.4911 28 5.48648E-05 0.0368 0.1164 0.00036
0.016 0.5073 29 6.04235E-05 0.0380 0.1215 0.00042
0.017 0.5230 30 6.61557E-05 0.0392 0.1265 0.00047
d/D radian Ad Pd v/V 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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0.75 PDWF at least once a day to promote self-cleansing 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
Owing to a mixture of solids and liquids, sewage flow velocities must be self-cleansing
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Wastewater Collection Systems
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
Effects of long detention timesOwing to the high O2 demand of biodegradable organic matter, long HRT will deplete dissolved oxygen (DO)
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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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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?
Finally do we question:
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End of Module 6 Wastewater Collection Systems