Transcript Wastewater Collection Systems

Wastewater Collection Systems
On completion of this module you should be able to:
 Discuss the sources of wastewater
 Describe wastewater characteristics
 Understand the relevant sections of the legislation
relating to sewer collection systems and wastewater
 Design a wastewater collection system
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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
dead
bacteria, fungi, protozoa,
algae
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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 population (ep)
for design
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Wastewater Collection Systems
Organic measurements
 There are many different sources, types and
complexities of aqueous organic matter
 It is not possible to quantitatively measure or
determine every organic constituents
 Use of concept of biodegradability and nonbiodegradability through oxygen demand
exerted by microorganisms or chemicals
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Biochemical Oxygen Demand (BOD5)
Defined as the oxygen demand for a mixed population of
aerobic heterotrophs bacteria in oxidising biodegradable
organic carbon present in a sample in 5 days at 20o C
 Free oxygen is used as the terminal electron acceptor
 [(C,H,O), N,P,S] + O2  CO2 + H2O + NH4+ + S2- + PO43-+ energy
 Reproducibility is  20% but reflects actual biodegradation
 There are some limitations
 In addition to organic carbon, reduced nitrogen I.e. ammonia can also
be oxidised by nitrification
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Chemical Oxygen Demand (COD)
The test uses a strong oxidising chemical agent to
completely oxidise organics
 The oxygen equivalent of the organic matter is determined by the
amount of K2Cr2O7 used in boiling acid at 150o C in the presence of a
catalyst (silver sulfate)
 The test takes 2 – 3 hours and reproducibility is  10%
 The test cannot discern between biodegradable and nonbiodegradable carbon, consequently COD values will be higher than
BOD5
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Solids in wastewater
 Total Solids, mg/L
 Total Suspended Solids or Non-Filterable Residue
(TSS or NFR), mg/L
 Total Dissolved Solids, mg/L
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Wastewater Collection Systems
Relevant legislations
 Sewerage and Water Supply Act is replaced
with the Plumbing and Drainage Act 2002
and the Standard Plumbing & Drainage
Regulation 2003
 Environmental Protection Act 1994
that relates to quantity and quality of flows into
the environment
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Plumbing and Drainage Act 2002
The act and its supporting Standard Plumbing &
Drainage Regulation 2003 generally deals with
 Assessing plumbing and drainage work
 Product certification
 Installation on premises
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Environmental Protection Act 1994
The act relates to quantity and quality of flows into the
environment
 All discharges are 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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Wastewater Collection Systems
Typical manhole configuration
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Testing of Sewers and House-drains
To ensure the integrity of connections
 Water test at a pressure equal to 2 m head
at the highest point. Loss of water less than
1 L/m dia. m length in 30 minutes
 Air test at 30 kPa for 3 minutes. Pressure
drop to be monitored
 Manholes and inspection chambers also to
be tested for water tightness
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Locations of Manholes
 Not more than 90 m apart for sewers 375 mm
and less
 Not more than 150 mm apart for sewers > 375
mm
 Manholes to be provided at end of each sewer
and for changes in direction, grade and at
intersections
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Design of gravity sewers
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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Design for maximum flow
Queensland Planning Guidelines for Water Supply
and Sewerage
 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
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Peaking factors for maximum flows
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Proportional velocity and discharge
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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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