Public Health Engineering1 Wastewater Collection Systems Discuss the sources of wastewater Understand the relevant sections of the legislation relating

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:



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



Domestic flows

Industrial and trade wastes

Urban stormwater

Infiltration/inflow

Sources of wastewater



Materials in wastewater

Impurities 0.01%

Physical form

suspended

dissolved

Chemical

inorganic

organic

Biological

living

Non-living bacteria, fungi, protozoa,algae



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)



Hydraulic loading (ML/d)

Organic loading (kg/m3.d)

Concept of equivalent person or population (ep) for design

Wastewater presents a unique design problem



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



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



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



Separate versus combined systems

Gravity and pumped flows

Small collection systems using pressure or vacuum

Types of wastewater systems

11Public Health Engineering


Pressure system



Vacuum system



Sewer alignment

Depth of sewer

House connection

Location of manholes

Testing of sewers and house drains

Sewer installation



Sewer installation


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


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


Wastewater Collection SystemsSewer installation

Typical sewer and housedrain connection


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


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



Typical manhole configuration



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



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)


Peaking factors for maximum flows


Proportional velocity and discharge


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



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



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



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)


Effects of long detention times


Effects of long detention times


Effects of detention times in rising mains



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:


End of Module 6 Wastewater Collection Systems

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Public Health Engineering1 Wastewater Collection Systems Discuss the sources of wastewater Understand the relevant sections of the legislation relating