technical odour assessment – beca ltd · 2018. 5. 17. · st marys bay and masefield beach water...

Beca // 2 May 2018

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Appendix P

Technical Odour Assessment – Beca Ltd

Report

St Marys Bay and Masefield Beach Water Quality Improvement Project - Technical Odour Assessment

Prepared for Auckland Council

Prepared by Beca Limited

17 April 2018


Beca // 17 April 2018

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Contents

1 Introduction 2

1.1 Background 2

1.2 Purpose and Scope 2

1.3 Limitations 2

1.4 References 2

2 Project Description 4

3 Ventilation Sites and Receiving Environment 6

3.1 Ventilation Sites 6

3.2 Receiving Environment 8

3.3 Air Quality Status 9

3.4 Topography 10

3.5 Meteorology 11

3.6 Existing Odours 12

4 Discharges to Air 13

4.1 Overview of the Discharges to air 13

4.1 Ventilation Air Discharge Scenarios 13

4.2 Odour Emissions 15

5 Assessment of Potential Odour Effects 18

5.1 Assessment Method 18

5.2 How Odour Causes Adverse Effects 18

5.3 Duration and Frequency of Emissions 21

5.4 Assessment of Potential Exposure to Odour 21

6 Mitigation 25

7 Summary 25

Appendices

Appendix 1

Visualisation of London St / New St Air Exchange Points

Appendix 2

Storage Pipe Utilisation Rate



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1 Introduction

1.1 Background

The St Marys Bay and Masefield Beach Water Quality Improvement Project (the Project) is an Auckland

Council (AC) project being undertaken to reduce combined sewer overflows to the beaches of St Marys Bay

and Masefield Beach. The Project involves the installation of a new storage pipeline that will run from New

Street/London Street through to a new site within Pt Erin Park. The new pipeline will have the capacity to

store much of the combined sewer overflow (CSO) volume, so that when there is capacity in the sewer

network, the captured overflows can be pumped back to the sewer network for treatment at the Mangere

Treatment Plant.

During the filling, storage and discharge of the storage pipe, air will be displaced from the pipe and

discharged to atmosphere. The discharged air may contain odorous compounds generated by the stored

effluent.

Auckland Council (AC) has commissioned Beca to undertake a technical assessment of the effects of odours

emitted from the proposed Project. This report has been prepared as part of the technical work necessary to

support resource Consent Application for construction of the new infrastructure. The reports summarises the

methodology and results of the assessment.

1.2 Purpose and Scope

The purpose of this report is to detail methods, results and findings of the assessment of nuisance effects of

odour discharges from the storage pipe. In particular this report includes:

A brief description of the site and potential odour sources;

A description of the existing environmental setting in relation to sensitivity to odour and influences on

odour dispersion;

Details of the emission scenarios during the various fill and empty configurations of the storage pipe;

An assessment of the effects the odour emissions may have on nearby receivers;

Approaches to mitigation and,

A summary of conclusions of the assessment.

1.3 Limitations

This report has been prepared by Beca for Auckland Council. Beca has relied upon the information provided

by Auckland Council in completing this document. Unless otherwise stated, Beca has not sought to

independently verify the information provided. This document is, therefore, based upon the accuracy and

completeness of the information provided and Beca cannot be held responsible for any misrepresentations,

incompleteness, or inaccuracies provided within that information. Should any new or additional information

become available, this report will need to be reviewed accordingly.

1.4 References

This assessment is based on the concept design of the Project’s ventilation and odour control system

detailed in the following report (referred to as ‘Ventilation Concept Report’ in this document):

Aurecon, St Mary’s Bay and Masefield Beach Improvement – Storage Pipeline Air Displacement,

Ventilation & Odour Control Concept, Nov 2017.



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It is noted that the design at present is preliminary and has yet to be finalised.



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2 Project Description

The location of the proposed storage pipeline is shown in Figure 2-1. The underground pipeline will be

located in St Marys Bay, Auckland. The storage pipe will run between Pt Erin Park and the corner of London

St and New St. A weir structure, underground pump station and odour control system will be constructed at

the pipe’s lowest point in the Pt Erin Park.

The storage pipe will capture up to 2,500m3 of combined sewer overflows during wet weather events. The

mixed wastewater/stormwater will be stored in the storage pipe until capacity in the main sewer network is

available.

The residence time of the stored mixture will vary depending on the available capacity of the sewage

network, and the volume of combined sewer overflows captured. Aurecon estimates that, on average, once

per year the pipeline is expected to be 75% full for three days, 50% full for four days and 25% full for five

days.

In instances when overflow volume exceeds the volume of the storage pipe the excess overflows will be

discharged to the Waitemata harbour via a new marine pipeline.

Air will be displaced from the storage pipe during filling, storage and emptying operations. The discharged

ventilation air will potentially contain odorous compounds generated by the combined sewer overflows. The

discharged odour could potentially have adverse odour nuisance effects.

The displaced ventilation air and odours will be released to the atmosphere from three locations;

Pt Erin Park;

St Marys Road park ; and

Air exchange points located near the corner of London Street and New Street (in St Mary’s Bay).

At Pt Erin Park, a forced ventilation system connected to the pump station will evacuate air from the storage

pipe during the initial filling and final emptying stages. The displaced air from the storage pipe will be passed

through an activated carbon odour control system prior to being discharged to the atmosphere via vertical

stacks.

An activated carbon filter will also be installed at the St Marys Road park site to control odour emissions. At

this site, the system has been designed so that ventilation air can discharge passively through the filter to the

atmosphere near ground level via a vertical ventilation shaft.

At the London Street/New Street site air will also be discharged passively to the atmosphere from a

ventilation shaft connected to four air exchange points. The discharged air will pass through an activated

carbon odour control system prior to being discharged to the atmosphere.

Discharges to air from the project are described in section 4.



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Figure 2-1 Location of the proposed storage pipe and ventilation points

Pt Erin Park

St Mary’s Bay Road park

London St/New St



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3 Ventilation Sites and Receiving Environment

3.1 Ventilation Sites

3.1.1 Pt Erin Park

The approximate location of the Pt Erin Park weir structure and odour control unit is shown in Figure 3-1.The

area surrounding the Pt Erin Park weir structure is an open recreational space. The Pt Erin Park and pool

facilities are located to the south, the Curran St on-ramp is located to the northwest, and State Highway 1

(SH1) is located to the east of the site. The site is zoned Strategic Transportation Corridor Zone under the

Auckland Unitary Plan Operational in part (AUP:OP).

Figure 3-1. Approximate location of the proposed Pt Erin Park weir structure and odour control unit

3.1.2 St Marys Road Park

Figure 3-2 shows the approximate location of the proposed St Marys Road Park ventilation shaft and odour

control unit. The St Marys Road park is located at the northern end of St Marys Road. The site is an open

park area and is relativity flat. SH1 is located on the north-eastern side of the park and a public walkaway is

located adjacent to SH1 on the north eastern boundary of the park. Residential properties are located to the

southeast through to the northwest of the site.

The proposed shaft and the passive odour control unit will be located on land zoned Strategic Transportation

Corridor Zone under the AUP:OP.

Pt Erin Park odour control unitPt Erin Park

weir structure



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Figure 3-2. Approximate location of the St Marys Road park shaft and odour control unit

3.1.3 London St and New Street

Figure 3-3 shows the approximate location of the London Street / New Street ventilation shaft and air

exchange points. The ventilation shaft will be below ground level. Four air exchange points will be connected

to the shaft through which the air from storage pipe will be discharged to the atmosphere. The ventilation air

will be emitted from the four air exchange points which will comprise poles approximately 8 - 10 m above

ground level. Each pole will have a diameter of approximately 0.3 m and will have a similar appearance to

the existing street lamp posts. Visualisations of the preferred option is shown in appendix 1.

The surrounding land use is predominantly residential in nature and zoned Residential – Single House in the

AUP:OP.

St Mary’s Bay Road park ventilation shaft and odour control



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Figure 3-3. Approximate location of the London Street / New Street shaft (shown as a red circle) and the preferred option air exchange points (shown as yellow circles)

3.2 Receiving Environment

Figure 3-4 shows the zoning of the surrounding land uses under the AUP:OP. The area surrounding the

ventilation sites is predominantly residential in nature, and includes single housing, terraced houses and

apartments1.

The closest residential properties are located approximately 195 m from the proposed Pt Erin Park weir

structure and odour control unit, and 35 m from the St Marys Road park shaft. Depending on the final design

of the ventilation system, the London Street/New Street air exchange points could be 7m from the nearest

dwelling.

The Pt Erin Park and pool are located to south the Pt Erin Park weir structure and odour control unit. The

park and pool areas are zoned Open Space - Sport and Active Recreation Zone and Open Space - Informal

Recreation Zone in the AUP:OP.

1 These areas are zoned a mixture of Residential – Single House Zone, Residential – Mixed Housing

Suburban Zone, Residential – Terrace Housing and Apartment Building Zone under the AUP:OP.

ventilation shaft

air exchange points



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St Mary’s College (zoned Special Purpose – School Zone) is located approximately 65 m to the south of the

London Street/New Street ventilation shaft and 18 m to the south of the nearest preferred option air

exchange point. The closest school building is located approximately 50 m to the south of the air exchange

point.

Areas zoned Business – Mixed Use are located approximately 170 m to the south east of the London

Street/New Street site.

SH1 runs along the north eastern boundary of the proposed storage pipe. The Westhaven Marina is located

to the east of SH1. Closest boat moorings at the marina are located approximately 150 m to the east of the

Pt Erin Reserve ventilation stacks, and 280m to the north of the St Mary’s Road park.

Figure 3-4 Map Showing the AUP: OP zoning in the vicinity of the storage pipe

3.3 Air Quality Status

Under the AUP:OP provisions the air quality Activity Status varies with respect to the land use zoning. The

Activity Status of an area recognises that the sources which contribute to air quality contaminant levels vary

with respect to land use; similarly the level of amenity will also vary between areas.

The Activity Status of the surrounding areas zoned Residential, School and Open Space - Recreation are

defined as a “High air quality – dust and odour area” under the AUP:OP. Therefore a relatively high level of

air quality amenity value is expected in these areas.

Pt Erin Park

St Mary’s Road park

London St /New St

Residential

Business –mixed use

Open Space



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3.4 Topography

The topography of the project area is shown in Figure 3-5. The dominant topographical features of the area

are the Waitemata Harbour and coastline.

The sites are located at the base of surrounding hills, which slope down towards the harbour. The terrain

elevation at the Pt Erin Park and St Marys Road park is approximately 4 - 5 m above sea level.

The terrain elevation at the Pt Erin Park and pool area is approximately 14m higher than the Pt Erin Park

weir structure and odour control unit site.

The residential areas which surround the St Marys Road park are in general located on higher terrain

elevations than the site of the proposed ventilation shaft and odour control unit.

The terrain elevation of the London Street / New Street shaft is approximately 21 – 22m above sea level. The

surrounding land slopes down toward St Marys Bay in a north east direction. The residential areas to the

south west of London Street and St Marys College are located at a higher elevation to the shaft site and the

residential areas to the north east of the site are at a lower elevation.

Figure 3-5 Topography of the area surrounding the proposed storage pipe project area

Pt Erin Park

St Mary’s Road Park

London St / New St



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3.5 Meteorology

The meteorological parameters that will have the most influence on the dispersion of odour emitted from the

ventilation outlets are wind speed, wind direction and the thermal stability of the atmosphere.

The North Shore meteorological monitoring station (Site number 12328) is located approximately 6.4 km

north of the site. Wind conditions at the monitoring station are expected to be broadly indicative of wind flows

in the project area, although the topography of the site will also influence local conditions.

Figure 3-6 shows the distribution of hourly average wind speeds and wind directions observed at the North

Shore meteorological monitoring site for the period January 2012 – December 2016. The wind rose has been

superimposed on an aerial photograph of the project area.

The wind rose distribution shows the predominance of winds from the west and the southwest. Low wind

speeds (less than 1.5 m/s), which are the worst case wind conditions for the dispersion of odours, occurred

predominantly during west, northwest and southwest winds. The wind rose indicates that the nearby

residential areas are mostly located upwind of the ventilation sites during the predominant wind conditions.

Figure 3-6 Distribution of hourly average wind speed and directions observed at the North Shore meteorological monitoring station (2012-2016) (Sourced from NIWA Cliflo database)

Wind flows in the project area will also be influenced by the channelling effects of the surrounding hills and

the coastal location of the site. Winds from the north-east could be expected to be channelled by the hills

along the coast line in the northwest and southeast direction. The surrounding hills are also likely to partly

shelter the St Mary’s Bay site from southwest winds (i.e. the predominant wind direction).



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Land breezes (from land towards the harbour) during warm days and sea breezes (from the harbour towards

the land) during the evening and night would also expected to have an influence on local wind flows.

Similarly drainage of the cooler air flowing down from the surrounding hills down towards the harbour is

expected to occur during cool calm winter evenings. These wind flows would tend to transport any odours

emitted from the London St / New St air exchange points to the north east and away from the residential

properties and St Mary College located on the higher terrain elevations.

3.6 Existing Odours

There are no significant industrial or commercial odour emission sources in the vicinity of the ventilation sites

that are expected to contribute to existing ambient air quality levels. Background odours are therefore

expected to be typical of urban residential areas.

Naturally occurring odours associated with the coastal location of the project would also be expected to

contribute to ambient air quality levels at the Pt Erin Park and St Marys Road park sites.



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4 Discharges to Air

4.1 Overview of the Discharges to air

The proposed configuration of the storage pipe ventilation and odour treatment concept is shown in Figure

4-1. The figure shows the relative location of the three ventilation sites (i.e. Pt Erin Park, St Marys Park and

London Street/New Street). The primary air quality concern is the discharge of odours generated by the

combined sewer overflows (being a mixture of untreated sewage and stormwater).

Odours will potentially be emitted to the atmosphere with the ventilation air displaced during filling, storage

and emptying operations. The ventilation air discharge rate and odour emission rates from the three

ventilation sites will vary between sites and the operational phase of the storage pipe. The ventilation air

emission scenarios are described in section 4.1.

Figure 4-1 General storage pipe configuration (Image adapted from Aurecon, 20172)

4.1 Ventilation Air Discharge Scenarios

4.1.1 Filling

The method of ventilation from the storage pipe will change as the storage pipe fills with the combined sewer

overflows. There are three possible emission scenarios when the storage pipe is filling which are described

below.

Phase 1: Forced ventilation

Initially the pipe will be forced ventilated by the fan units located at the Pt Erin Park. The fan units will draw

ventilation air along the length of the tunnel and fresh air will enter the tunnel through the London Street/New

Street air exchange points.

2 Aurecon, St Mary’s Bay and Masefield Beach Improvement – Storage Pipeline Air Displacement,

Ventilation & Odour Control Concept, Nov 2017.

ST MARY’S ROAD PARK

POINT ERIN PARK

LONDON STREET / NEW STREET



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The ventilation air will be discharged to the atmosphere from the Pt Erin Park odour control unit air exchange

points. The ventilation air will be treated by an activated carbon odour filter to minimise the emission of

odours prior to discharge.

Provided the fan units are suitably sized and the air extraction rate is equal to or greater than the fill rate of

the combined sewer overflows, no other discharges to air are expected to occur from the other ventilation

sites during this phase of filling. The ventilation concept report estimates the fan units should be sized to

achieve a maximum ventilation rate of 3.0 m3/s.

Phase 2a: Passive Ventilation

If the storage pipe continues to fill with combined sewer overflows it will at some point create a seal in the

pipe which will prevent air from flowing between the London Street/New Street shaft and the Pt Erin Park

weir structure. When this occurs the forced ventilation of the pipe will be stopped. At this point the air

displaced by the rising combined sewer overflows will be passively discharged through the St Marys Road

park and London Street/New Street air exchange points.

The volume of combined sewer overflows which is captured before the forced ventilation ceases will be

determined by the final gradient of the pipe. Estimates presented in the ventilation concept report3 for a

storage pipe installed with a 1 to 600 gradient indicates this cut off point would likely occur when the storage

pipe is at 52 – 56% of its maximum storage capacity (i.e. when 1300m3 to 1400m3 of combine sewer

overflow is captured in the storage pipe).

Most of the displaced air in the pipe during this phase is expected to be discharged through the London

Street/New Street air exchange points.

The ventilation air at the London Street /New Street site will be released from the four proposed air exchange

points, which will comprise of poles approximately 8 – 10 m above ground level. The maximum combined air

discharge rate from these sites are expected to be less than 3.0 m3/s during rapid fill conditions.

An activated carbon filter will be installed at the London Street /New Street site which will minimise any

odours present in the ventilation air. The height of the air exchange points will also assist in the dispersion

and dilution of any residual odours emitted in the ventilation air.

An activated carbon filter is also proposed for the St Marys Road park site to minimise the emission of any

odours. The installation of an odour control system is considered appropriate for this site as the ventilation air

will be discharged near to ground level. Air contaminants and odours from ground level sources can be

poorly dispersed during some meteorological conditions.

Phase 2b: Passive Ventilation

If the pipe continues to fill, eventually the St Marys Road park shaft will also be sealed off by the inflowing

combined sewer overflows. When this occurs the air (and any odour) displaced in the pipe by the combined

sewer overflows will only be discharged through the London Street/New Street air exchange points.

4.1.2 Storage

The combined sewer overflows will be stored in the pipe until capacity is available in the main sewer

network. The residence time is expected to vary between rainfall events (refer Appendix 2).

3 Figure 7, p11



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During the storage period the pipe will be either forced ventilated if air is able to flow along the length of the

pipe, or passively ventilated through St Marys Road park and London Street/New Street sites.

If forced ventilation is possible the only discharges to air are expected to be from the Pt Erin Park odour

control unit air exchange points. The ventilated air will be treated by the proposed activated carbon filter prior

to being discharged to the atmosphere. Fresh air will enter the pipe through the London Street /New Street

and St Marys Road park air exchange points.

Otherwise the storage pipe will be passively ventilated through the London Street/New Street and St Marys

Park air exchange points. The ventilation air discharged at the both sites will be treated by activated carbon

filters.

Passive ventilation rates will vary over the storage period. Differences between the in-pipe and ambient air

temperature and relative humidity will influence relative buoyancy of in-pipe air and ventilation rates4.

Similarly the venturi effect of wind flows over the top of the ventilation stacks (i.e. educt suction) will also

influence ventilation rate.

4.1.3 Emptying

When the storage pipe is emptied, fresh air will be drawn into the pipeline through the London Street/New

Street air exchange points and from the St Marys Road park site as the level of the wastewater/stormwater

mixture drops. As a consequence no discharges to air (including odours) are expected to occur from these

sites during the emptying of the storage pipe.

Once the combined sewer overflow level has dropped to a level that allows air to flow along the length of the

storage pipe, the pipe will once again be force ventilated by the Pt Erin Park fan units. The treated ventilation

air will be discharged to the atmosphere through the Pt Erin Park air exchange points.

Based on a continuous pumping rate of 20 l/s the storage pipe will take approximately 35 hours to empty

when full.

4.1.4 Empty pipe

Some of the residual combined overflow will remain in the Pt Erin Park pump station sump after emptying.

The residual overflow will be a potential source of odour. However, any odours are expected to be contained

in the pump station and discharged to the atmosphere after being treated by the odour control unit.

Provided the forced ventilation system is in operation no discharges are expected to occur from the other

ventilation sites when the pipe is empty.

4.2 Odour Emissions

The captured wastewater/stormwater mixture has the potential to generate odour. The rate odour is emitted

and the character of the emitted odour will vary over time depending on the quantity, concentration and age

of the material stored in the pipe.

When the storage pipe is filling the wastewater will be comparatively fresh and diluted by the captured

stormwater. The ventilation concept report indicates that the level of dilution could vary significantly between

4 The air within storage pipe can often expected to be warmer and moister than the ambient air and therefore

less dense and more buoyant



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rainfall events (between 0% and 100%), however, the average percentage of wastewater in the captured

mixture is expected to be approximately 6.5%.

Therefore, during the filling of the storage pipe the captured wastewater/stormwater mixture is expected to

be well oxygenated (i.e. not anaerobic) and generally have a low odour emission potential. Odour from

fresh/aerobic wastewater is often described as having a ‘musty’ character. A similar but less intense odour is

expected to be generated from the freshly captured wastewater/stormwater mixture. In general any observed

odours are not expected to be offensive in nature.

Typically discharges to air during filling will occur over a relatively short duration and during heavy rainfall

when fewer people are expected to be outdoor and therefore potentially exposed to emissions.

The primary air quality concern is most likely to occur when anaerobic conditions have developed in the

storage pipe.

The combined wastewater/stormwater has the potential to become anaerobic (i.e. ‘septic’) if stored for more

than 12 hours5. When anaerobic, the wastewater has a much higher potential to generate odour. The

character of odour emitted from anaerobic wastewater is commonly described as having a ‘rotten egg’,

‘rotten cabbage’, or ‘sewage’ like character. The emitted odour is also generally considered to be unpleasant

and often offensive.

Anaerobic conditions can be onset by elevated temperatures, high biological oxygen demand, high sulphate

levels and the presence of reducing chemicals, however, the dilution of the raw sewage by the stormwater

will tend to delay the onset of anaerobic conditions, due to the higher level of dissolved oxygen in the

mixture.

Odour emitted from anaerobic wastewater is a complex mixture of different chemical species including

hydrogen sulfide (H2S), ammonia (NH3), thiols (mercaptans), other reduced sulphur compounds, and

amines6. However, H2S is commonly used as an indicator species for wastewater odours.

H2S (and other chemical compounds emitted by anaerobic wastewater) can be detected at very low ambient

air concentrations (i.e. in the order of parts per billion (ppb))7. Within the storage pipe the concentrations of

these air contaminants are likely to be many magnitudes higher than their odour detection limits if the

wastewater/stormwater becomes anaerobic (i.e. in the order of parts per million (ppm)). Contaminant levels

in the storage pipe will vary depending on the storage temperature, retention time and wastewater

composition.

5 The captured wastewater is expected to become septic within approximately 12 hours of storage

6 Defra, Code of Practice on Odour Nuisance from Sewage Treatment Works, 2006

7 The H2S odour detection limit is between 0.2 – 1.4 ppb



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The ventilation air discharged from the three ventilation sites will be treated by activated carbon filters to

minimise the discharge of odour. If the proposed control systems are appropriately sized and maintained an

odour control efficiency of more than 99% for sewer type odours can be expected8910.

Activate carbon filters are commonly used technology to control odour emitted from municipal wastewater

emission sources, including pump station and combined sewer storage facilities. The performance of these

filters are therefore well understood.

8 Holyoake, K.M. and Kotze, K.V. Odour Control for Pump Stations. NZWWA Conference, Christchurch,

September 2010. (The authors reported a H2S removal rate of more than 99.99%)

9 Sydney Water. 2011. Odour Control Unit Standard Specification.

10 Scottish environmental Protection Agency. 2010. Odour guidance 2010



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5 Assessment of Potential Odour Effects

5.1 Assessment Method

The Ministry for the Environment “Good Practice Guide for Assessing and Managing Odour” (2016, ‘GPG

Odour’) provides guidance on methods for assessing the effects of odour discharges.

For a new activity, the GPG Odour recommends using dispersion modelling where reliable odour emissions

data is available. Where reliable data is not available, the GPG Odour recommends that past experience with

the same type of activity in other locations is the best method of assessment.

Although the proposed storage pipe will be a new emission source, no reliable odour emission rate

information is available. Therefore, dispersion modelling is not considered to be an appropriate assessment

method in this instance.

Therefore, for this assessment the potential odour nuisance effects have been based on the following:

The sensitivity of the receiving environment;

Consideration of separation distances and meteorological influences on pollutant dispersion;

The strength and character of the odour emitted from the storage pipe and the proposed odour control

methods; and

Frequency and duration of discharges.

5.2 How Odour Causes Adverse Effects

The effects of odour discharges depend on a number of features of the odour exposure which are

collectively known as the “FIDOL” factors:

Frequency How often an individual is exposed to odour

Intensity The strength of the odour

Duration The length of a particular event

Offensiveness/character The character relates to the “hedonic tone” of the odour, which may be pleasant, neutral or unpleasant

Location The type of land use and nature of human activities in the vicinity of an odour source.

As part of the “location” factor, the sensitivity of the receiving environment must be taken into account.

In assessing whether a particular activity is likely to generate “offensive or objectionable‟ discharges of odour

– and, in consequence, whether significant adverse effects are likely, each of the FIDOL factors must be

taken into account. Different combinations of these factors are significant when assessing adverse effects.

Depending on the severity of an event, one single occurrence may be significantly adverse and this is known

as an “acute” effect. However, in other situations, where there is a higher frequency of events, the threshold

level would be lower. This longer term impact is known as a “chronic” effect.

Different locations have different sensitivities to odour and can be classified as having high, moderate or low

sensitivity. The degree of sensitivity in any particular location is based on characteristics of the land use,

including the time of day and the reason people are at the particular location (e.g. for work, home living or



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recreation). In a residential area, an acceptable odour frequency is likely to be much lower than would be

expected in a rural or industrial area.

5.2.1 Sensitivity to Odour in Residential Areas

People living in residential zoned areas typically have a high sensitivity to both industrial and non-industrial

odours, because of the following factors:

People can be present at all times of day and night, both indoors and outdoors;

Higher densities of population are generally resident in residentially zoned areas;

All sectors of the population, including those of high sensitivity to odours, are more likely to be present in

residentially zoned areas;

People tend to carry out activities at residences, which are highly sensitive odours, such as dining,

entertaining, outdoor living, sleeping;

People usually expect a high level of air quality including the absence of odours, and have a low

tolerance of even typical rural odours; and,

Visitors to the area who are unfamiliar with an odour are more likely to be sensitive to odours they are not

used to, and may raise awareness of a problem.

Amenity conflicts between residential and industrial zones, in terms of incompatible neighbouring land uses

and odour presence, are recognised as an issue in many district and regional plans in New Zealand.

Under the AUP:OP areas zoned residential are considered to have a “High air quality – dust and odour area”

air quality status reflecting their high sensitivity to dour nuisance effects.

5.2.2 Sensitivity to Odour in Recreational Areas

People in recreational areas generally have a higher expectation of amenity than those engaged in other

industrial and commercial activities. People of high sensitivity may be exposed in these areas. However,

open fields and parks usually have a lower occupancy than residential areas and often are only be used

during the daytime or early evening. These areas also do not tend to be utilised during poor weather

conditions.

The GPG Odour considers recreational areas to have a moderate to high sensitivity to odour.

Under the AUP:OP areas zoned Open Space - Sport and Active Recreation Zone and Open Space -

Informal are also considered to have a “High air quality – dust and odour area” air quality status.

5.2.3 Sensitivity to Odour in the Project Area

In the vicinity of the proposed ventilation sites the high sensitivity receptors which may be impacted by the

proposed discharges are considered to be the following:

Residential properties close to the ventilation sites;

Pt Erin pool and park;

St Mary’s College

Westhaven Marina

The Point Erin Park and St Marys Road park are considered to be moderate to high sensitivity areas.

The sensitivity of the outdoor recreational areas to nuisance odour effects will typically be higher during the

daytime when a greater number of people will be present in these areas. Few people are expected to use



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any of these areas during bad weather, including high rainfall events when the storage pipe will be in

operation.

St Mary’s College is also considered to be high sensitivity receptor. Although the sensitivity of the site will

generally be greater during the day during school hours. The school tennis courts would expected to the

most impact by any emitted odour due to their proximity to the air exchange points.

Odours emitted from the ventilation sites will disperse and dilute with increasing distance downwind of the

sites. Therefore, the potential nuisance effect of any emitted odours will also decrease with increasing

downwind distance. Table 5-1 summarises the estimated separation distances between the ventilation

sources and the closest offsite high sensitivity receptors.

Table 5-1 Estimated separation distance between the ventilation sites and sensitive receptors

Ventilation outlet Sensitive receptor Separation Distance

Point Erin Park (odour control unit) Point Erin Pool 14-16 m to boundary of park

45 m to edge of pool picnic area

70 m to the pool edge

Nearest dwelling 195 m

Westhaven Marina 150 m to closest boat mooring

St Marys Road park Nearest dwelling 35 m

Corner of London St / New St Nearest dwelling 7 m*

St Marys College 18 m* to tennis courts

53* m to closest school building

*Dependent on the final configuration of the vent stacks

The residential properties located near the London Street/New Street site are the closest sensitive receptors

to any of the ventilation sites. Depending on the final configuration of the air exchange points discharges to

air may occur within 7 m of residential dwellings.

A number of the dwellings located on London Street are two storey houses. The upper level windows and

door of these dwellings will be approximately 2 – 4 m lower than the top of the proposed air exchange point

vent stacks.

The predominant wind direction in the project area is expected to be from the southwest (refer Figure 3-6).

As a consequence most nearby sensitive receptors are expected to be up wind of the ventilation sites for the

majority of time. Therefore, any odour emitted from the sites would generally be expected to be transported

away from these areas.

The exception to this are the residential properties to the north-east of the London Street/New Street site.

These properties are located in the predominant downwind location. However, these properties will be at a

lower terrain elevation than the air exchange points. The lower terrain elevation of these properties will

increase the relative height difference between upper levels of these dwellings and the top of the air

exchange points which will tend to any reduce potential exposures.

The Westhaven Marina is also located in the predominant downwind distance from the Pt Erin Park site.

However, the closest mooring are located more 150 m from the ventilation stacks. Any odours emitted from

the air exchange points are expected to be well dispersed in the atmosphere before they reach the marina

and are not expected to be observable.



4216571 // NZ1-15021348-36 0.36 // page 21

The secondary predominant wind flow is expected to be from the northeast direction. During these wind

flows any emitted odour from the St Marys Road park and London Street/New Street sites would be

transported towards nearby residential properties.

5.3 Duration and Frequency of Emissions

In general anaerobic conditions can be expected to occur when the wastewater is stored for approximately

12 hours. The frequency that anaerobic conditions are expected to occur in the pipe, and the duration that

the pipe will be ventilated under these conditions have been estimated from the hydraulic modelling results

presented in Figure 4 of the Ventilation Concept Report. The figure shows the predicted storage time of the

captured combined sewer overflows with respect to the estimated average recurrence interval (ARI) when

the pipe is at 25%, 50% and 75% of its maximum storage capacity. Figure 4 is reproduced in Appendix 2 of

this report.

The hydraulic modelling predictions indicate the combined sewer overflows will on average be stored for one

day, once every 26 days (i.e. ARI of 0.07 yrs), and for approximately 5 days, once every year (an ARI of 1.0)

when the pipe is 25% full. During these storage periods the captured combined sewer overflows is likely to

be anaerobic and the storage pipe is expected to be continuously force ventilated.

The storage pipe is expected to be passively ventilated when it is approximately 52% to 56% full (assuming a

1 in 600 gradient in the pipe). The frequency and duration of discharges during passive ventilation have been

estimated from the hydraulic model predictions of times when the pipe is approximately 50% full.

The results of the hydraulic modelling indicate that the captured mixture will be stored on average for one

day once every 37 days (i.e. ARI of 0.1 yrs), and for 3.5 - 4 days once every year (an ARI of 1.0) when the

pipe is 50% full. During these storage periods the wastewater would be expected to be anaerobic and the

pipe will be passively ventilated through the St Marys Road park and the London Street/New Street air

exchange points.

5.4 Assessment of Potential Exposure to Odour

5.4.1 Pt Erin Park

Discharges to air will occur from the Pt Erin Park site when the pipe is force ventilated during filling, storage

and emptying operations.

The hydraulic modelling indicates that discharges from the site could occur relatively frequently in response

to sewer overflow events. Discharges to air from the site could also occur for several days at a time during

and after each event.

However, the proposed activated carbon odour filters are expected to provide a high level of odour treatment

provided it is adequately sized for the ventilation air flows and appropriately maintained. Activated carbon

filters have been widely used and shown to be effective in other wastewater applications. As a consequence,

only comparatively low levels of odour are expected to be discharged with the ventilation air.

The discharges will occur in a public park (although the area is zoned Strategic Transportation Corridor

Zone), which is considered to have a moderate to high sensitivity to odour nuisance effect. It is likely the

public would be able to approach within a few metres of the ventilation stack and therefore could be exposed



4216571 // NZ1-15021348-36 0.36 // page 22

to odour emitted from the ventilation stack if it is not adequately controlled or dispersed in the environment by

the ventilation stacks11.

The closest offsite high sensitivity receptors are the Pt Erin pool and recreational park area. These high

sensitivity receptors are located approximately 45 m from the ventilation stack. The pool and park are also

located at a higher terrain elevation than the stack. Therefore, the emission plume from the air exchange

point stack could impinge on these elevated receptors during poor dispersion conditions12.

However, these recreational areas will only be downwind of the ventilation stacks infrequently (refer Figure

3-6). Therefore any people in these areas will only be exposed rarely to any emitted odours from the site.

These areas would also only expected to be used by the public during the day.

Based on the expected lower odour emission rates from the site, the prevailing meteorological conditions

and separation distances between the site and nearby sensitive receptors the risk of an adverse nuisance

odour effects is expected to be minimal.

5.4.2 St Marys Road Park

Discharges from the St Marys Road park site will occur during filling and storage operations, and when the

pipe is not being forced ventilated from the Pt Erin Park odour control unit. Discharges are expected to be

intermittent during storage conditions and vary with changes in ambient temperature and wind speed.

No discharges to air are expected to occur when the storage pipe is emptying or empty. During these periods

the St Marys Road park shaft will act as an air inlet to the storage pipe.

A passive ventilation activated carbon filter system is proposed for the St Marys Road park site. The filter is

expected to provide a high level of odour treatment if appropriately sized and maintained and only low levels

of odour are expected to be discharged from the filter unit during ventilation.

However, during rapid filling of the pipe it is possible that ventilation air will be discharged from the filter at a

greater rate than the unit is sized for. In these instances, the concentration of any discharged odour would be

expected to relatively low and generally inoffensive in character due to the freshness of the combined sewer

overflows captured and the level of dilution with stormwater, but some level of treatment would still be

achieved by the control unit. The park is also likely to be less utilised by the public during a rainfall event

when the pipe will be filling and passive ventilation occurs.

St Marys Road park is considered to have a relatively high sensitivity to odour as people use the space for

recreation and therefore a high amenity value would be expected to be maintained. The maximum

downwind concentration of odour emitted from the filter is expected to occur close to the odour filter and the

concentration of odour in the plume is expected to decrease rapidly with distance from the source.

The nearest residential property is located approximately 35 m to the south of the ventilation shaft. Based on

the proposed odour control and separation distance between the site and the receptors no offensive or

objectionable odour is expected to be observed at any of the nearby residential properties. The nearby

11 The height of the ventilation stack has not been specified in the ventilation concept report.

12 Based on the characteristics of the proposed discharge the worst case dispersion conditions are expected

to be relatively low wind speeds and stable atmospheric conditions, typical of cool calm wind nights and early

morning periods.



4216571 // NZ1-15021348-36 0.36 // page 23

dwellings are also located at a higher elevation than the reserve which will tend to channel any odour away

from the nearby residential areas when downwind of the site.

The prevailing winds at the site would also tend to transport any emitted odours away from the nearby

residential properties and towards the harbour. Drainage flows from the surrounding hills and land breezes

would also tends to transport odours away the residential areas.

Overall, based on the proposed level of treatment, the separation distances between the air exchange point

and sensitive receptors, the infrequency of the discharge, the prevailing wind conditions and the expected

low level of odour emitted through the filter, the risk of adverse odour nuisance effects resulting from the

discharges from the site is expected to be minimal.

5.4.3 London Street / New Street Site

Discharges from the London Street / New Street site will occur during the filling of the storage pipe and when

the pipe is not being force ventilated. No discharges are expected to occur when the pipe is being emptied or

empty.

The air emitted from the air exchanged point will pass through an activated carbon filter before being

discharge to the atmosphere. The odour filter is expected to provide a high level of odour treatment if

appropriately sized and maintained. Therefore only lower level of odour are expected to be emitted from the

vent stacks. These odours are expected to rapidly dilute in the atmosphere to below odour detection limits.

Activated carbon filters are widely used to control odour emitted from wastewater emission sources,

including combined sewer storage facilities. The proposed odour control is considered to be appropriate due

to the high sensitivity of the receiving environment.

During filling of the pipe, the wastewater will be comparatively fresh and diluted by stormwater. The hydraulic

modelling indicates that the percentage of wastewater in the mixture would generally be less than 10% when

the storage pipe is being filled and is passively ventilated through the site’s air exchange points.

During this period the wastewater will have a comparatively low potential to generate odour. Any odour that

is discharged is not expected to be particularly offensive in nature.

Higher odour emission rates could potentially occur during the rapid filling of the storage pipe if the

ventilation rate were to exceed design capacity of the odour filter13. However, these discharges conditions

would occur for a relatively short period of time. Any emitted odour would also be well dispersed in the

environment by the vent stacks and unlikely to be offensive in character. The discharged air will also

undergo some level of treatment by the odour filter.

Anaerobic conditions are likely to occur only when the combined wastewater and stormwater is stored for

extended periods of time. During these conditions the generated odour is expected to have a higher odour

intensity and is likely to be more offensive in character. Therefore, the potential risk of an odour nuisance

effect is considered to be greatest during this period.

13 Beca understands the design of odour filter has yet of be finalised



4216571 // NZ1-15021348-36 0.36 // page 24

The hydraulic modelling indicates that discharges to air from the air exchange points when the wastewater is

anaerobic would on average occur at least 10 times per year. During these events the pipe could be

passively ventilated for up to 4 days before being emptied.

The discharge of ventilation air, and any odours, from the pipe will vary over the storage period. The air

ventilation rate during this period is expected to be significantly lower than during this period compared to

when the pipe is undergoing rapid filling. However, the concentration of odour in the storage pipe, prior to

treatment, will potentially be higher.

The London Street /New Street air exchange points will be located in a residential area. The surrounding

area is considered to have a high sensitivity to adverse odour effects. The locations of the four proposed air

exchange points have yet to be finalised, but could potentially be located within 7 m nearby dwellings.

Residents living in close proximity to the air exchange points are expected be the high sensitivity receptors

most impacted by any adverse odour effect. Potential exposures to odour are expected to higher on the top

level of the nearby two storeyed dwellings (which also have openable windows or doors that face the air

exchange points) compared to the ground level.

The meteorological monitoring data indicates that the dwellings located to the east and north east of the

vents could frequently be downwind of the vents during storage of the CSO. Therefore, these properties

could be frequently be exposed to odours emitted from the storage pipe if they are not adequately controlled.

However, due to the proposed odour control, only low levels of odours are expected to be emitted from the

air exchange points during both during filling and storage periods. The height of air exchange points (i.e. 8 –

10 m above ground level) is expected to provide a good level of dispersion for any residual odours emitted to

the atmosphere.

The top of the exchange points will also be above the roofs of nearby dwellings, therefore the dispersion of

any emitted odour is not expected to be significantly impacted by building downwash effects.

Overall based on the level of odour control, the frequency of discharges to air, and the level of dispersion

achieved by the vent stacks the risk of adverse odour nuisance effects resulting from the discharges from the

site is expected to be minimal.



4216571 // NZ1-15021348-36 0.36 // page 25

6 Mitigation

The potential for offensive odours discharged from the Pt Erin Park, St Mary Road park and London

Street/New Street ventilation sites is considered to be adequately mitigated by the proposed activated

carbon filters, provided they are appropriately sized and maintained.

The height of the Pt Erin Park and Street/New Street air exchange points will also assist in the dispersion of

any residual odours which are emitted from the storage pipe. If required the height of the site air exchange

points could be increased to improve disperse and reduce downwind odour levels.

The proposed mitigation is considered to be appropriate for the expected magnitude and frequency of

discharges to air and the sensitivity of the receiving environments. The proposed odour control system is

consistent with the odour control system used to control at other combined sewer storage facility projects.

7 Summary

AC is proposing to construct a storage pipe as part of the St Marys Bay and Masefield Beach Water Quality

Improvement Project. During the filling, storage and emptying of the storage pipe ventilation air will be

discharged to the atmosphere. Odour generated by the combined sewer overflows will be discharged to the

atmosphere with the ventilation air.

Ventilation air will be discharged to air at three locations; Pt Erin Park, St Marys Road park and near the

corner of London Street and New Street.

Discharges from all three location will be treated by activated carbon odour filters. Provided these filters are

appropriately sized and maintained, the risk of odour creating adverse effect as consequence of the

discharges from these sites is considered to be minimal.



4216571 // NZ1-15021348-36 0.36 // page 26

Appendix 1

Visualisation of London St / New St Air Exchange Points

Filen

ame:

Plot

Date:

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ST MARY'S BAY - MASEFIELD BEACH WATER QUALITY IMPROVEMENT PROJECT

SKETCH 1 : 300mm DIAMETER x 10m HIGH AIR EXCHANGE POINT VISUALISATION: VIEW DOWN NEW STREET

N

E

W

S

T

R

E

E

T

ST MARY'S BAY - MASEFIELD BEACH WATER QUALITY IMPROVEMENT PROJECT

SKETCH 2 : 300mm DIAMETER x 10m HIGH AIR EXCHANGE POINT VISUALISATION: VIEW DOWN LONDON STREET

Filen

ame:

Plot

Date:

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e:P:

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4216571 // NZ1-15021348-36 0.36 // page 28

Appendix 2

Storage Pipe Utilisation Rate



4216571 // NZ1-15021348-36 0.36 // page 29

Figure 4 from the Ventilation Concept Report is reproduced below. The figure shows the utilisation of the

storage pipe when 25%, 50% and 75% full. The figure shows the storage period with regards to the

estimated ARI for these conditions.

Figure 4: Chart 3: Duration CSO stored within pipeline for various tunnel storage utilisation scenarios.

appendix covers.pdfAppendix P Final Odour ReportNZ1-15021348-St Mary-s Bay and Masefield Beach Water Quality Improvement Project - Technical Odour AssessmentSC6540105415021816120NZ1-15021348-St Mary-s Bay and Masefield Beach Water Quality Improvement Project - Technical Odour Assessment255303-AIR EXCHANGE POINT SKT 1 255303-AIR EXCHANGE POINT SKT 2 NZ1-15021348-St Mary-s Bay and Masefield Beach Water Quality Improvement Project - Technical Odour Assessment

technical odour assessment – beca ltd · 2018. 5. 17. · st marys bay and masefield beach water...

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