Wastewater Collection Systems

1
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

2
Wastewater Collection Systems
Sources of wastewater

• Domestic flows
• Industrial and trade wastes
• Urban stormwater
• Infiltration/inflow

3
Wastewater Collection Systems
Materials in wastewater
Impurities 0.01
Chemical
Biological
Physical form
inorganic
suspended
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

5
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
  non-biodegradability through oxygen demand
  exerted by microorganisms or chemicals

6
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

7
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
  non-biodegradable carbon, consequently COD values
  will be higher than BOD5

8
Solids in wastewater

• Total Solids, mg/L
• Total Suspended Solids or Non-Filterable Residue
  (TSS or NFR), mg/L
• Total Dissolved Solids, mg/L

9
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

10
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

11
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 lt 20 mg/L
  NFR lt 30 mg/L DO gt 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

18
House drain and connection

• House drain is the property sewer pipe that
  adjoins councils sewer
• Minimum house drain dia. is 100 mm with a min.
  slope of 160 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
21
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

22
Locations of Manholes

• Not more than 90 m apart for sewers 375 mm and
  less
• Not more than 150 mm apart for sewers gt 375 mm
• Manholes to be provided at end of each sewer and
  for changes in direction, grade and at
  intersections

23
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

24
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

25
Peaking factors for maximum flows
26
Proportional velocity and discharge
27
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?