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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

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

Public Health Engineering


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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

Public Health Engineering


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Wastewater Collection Systems

Sources of wastewater

  • Domestic flows

  • Industrial and trade wastes

  • Urban stormwater

  • Infiltration/inflow

Public Health Engineering


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Wastewater Collection Systems

Materials in wastewater

Impurities 0.01%

Chemical

Biological

Physical form

inorganic

suspended

organic

dissolved

living

Non-living

bacteria, fungi, protozoa,

algae

Public Health Engineering


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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

Public Health Engineering


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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

Public Health Engineering


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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

Public Health Engineering


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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

Public Health Engineering


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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

Public Health Engineering


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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

Public Health Engineering


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Wastewater Collection Systems

Pressure system

Public Health Engineering


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Wastewater Collection Systems

Vacuum system

Public Health Engineering


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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

Public Health Engineering


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Wastewater Collection Systems

Sewer installation

Public Health Engineering


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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

Public Health Engineering


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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

Public Health Engineering


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Wastewater Collection Systems

Sewer installation

Typical sewer and housedrain connection

Public Health Engineering


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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.

Public Health Engineering


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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

Public Health Engineering


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Wastewater Collection Systems

Typical manhole configuration

Public Health Engineering


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Wastewater Collection Systems

  • 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

Public Health Engineering


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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.

Public Health Engineering


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Peaking factors for maximum flows

Public Health Engineering


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Proportional velocity and discharge

Public Health Engineering


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d/D

radian

Ad

Pd

v/V

q/Q

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

Proportional geometry elements

Public Health Engineering


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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

Public Health Engineering


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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 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

Public Health Engineering


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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

Public Health Engineering


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Effects of long detention times

Public Health Engineering


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Effects of long detention times

Public Health Engineering



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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?

Public Health Engineering



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