Odour impact assessment of Hogsmill STW

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Odour impact assessment of Hogsmill STW
Odour impact assessment of Hogsmill STW
TWPS12C_21, November 2013
Odournet UK Ltd
page 1 of 42
title:
report number:
project code:
Odour impact assessment of Hogsmill STW
TWPS12C_21
TWPS12C
key words:
client:
contact:
contractor:
authors:
approved:
Royal London Borough of Kingston
LDF and Planning Policy Team, Guildhall 2, High Street
Kingston upon Thames, KT1 1EU
Surrey
020 8547 5418 phone
K Perry
Odournet UK Ltd
5, St. Margaret’s Street
Bradford on Avon
Wiltshire BA15 1DA
01225 868869 phone
01225 865969 fax
Companies House Cardiff 2900894
[email protected]
Andrew Meacham
on behalf of Odournet UK Ltd by
Mr. Nick Jones, director
date:
copyright:
13th November 2013
©2013, Odournet UK Ltd
Copyright and Non-Disclosure Notice
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extent that copyright has been legally assigned by us to another party or is used by Odournet UK Ltd under licence. To the extent that
we own the copyright in this report, it may not be copied or used without our prior written agreement for any purpose other than the
purpose indicated in this report.
The methodology (if any) contained in this report is provided to you in confidence and must not be disclosed or copied to third parties
without the prior written agreement of Odournet UK Ltd. Disclosure of that information may constitute an actionable breach of
confidence or may otherwise prejudice our commercial interests.
page 2 of 42
Non technical summary
Odournet UK Ltd were commissioned by the Royal Borough of Kingston to undertake an odour impac t
assessment to investigate odour exposure levels in the vicinity of Hogsmill Sewage Treatment Works
operated by Thames Water Utilities Limited (Thames Water), and the potential for adverse odour impact
on land potentially proposed for development by the Borough through the Hogsmill Valley Development
Plan Document (land packages A to J and I to V as set out in Figure 1).
Figure 1: Map of the area surrounding Hogsmill STW
The scope of the assessment was defined in the document entitled ‘Odour survey and dispersion
modelling spec. Hogsmill May 12’ which was issued on the 29 th May 2012.
The specific objectives of the study were as follows:
1.
To identify activities associated with the current sewage treatment operations conducted at the
site which have the potential to generate odour and to estimate the odour emissions released from
such operations.
2.
To assess the levels of odour exposure that are likely to be encountered around the works over the
long-term for the current operational conditions.
3. To assess the extent of potential impact of odour from the works upon the proposed
development as set out in Figure 1 and determine the areas where odour annoyance may occur.
The impact assessment was conducted using predictive odour dispersion modelling techniques that are
described in guidance issued by both DEFRA and the Environment Agency.
Odour sources were initially identified by an audit of the site conducted in August 2012. Odour emission
estimates were then defined on the basis of odour survey data collected during the spring and summer of
2012/13 (following completion of the recent Odour Improvement Scheme); and historic survey data
page 3 of 42
collected in summer of 2007/2006.a The use of both current and historic survey data enabled emission
rates to be defined that reflected the recent modifications conducted at the works by Thames Water,
but which also provided as robust an assessment as possible of the odour emission potential of elements
of the works that had not been altered as part of the works. The study took into account the following
improvements to the works that have been applied by Thames Water under the AMP5 improvement
scheme, which have resulted in a decrease in odorous emissions from the site:

Covering of the Worcester Park Tunnel inlet chamber, the inlet works channels and associated
screens and grit removal plant with extraction of odorous air to an odour treatment system.

Covering of the secondary digestors with extraction of odorous air to the site biogas system for
treatment.
The odour emission estimates were then fed into an odour dispersion model to assess how the odours
that are generated from the works disperse in the air following release and determine the odour
exposure levels that are likely to occur under the full range of varying weather conditions that occur at
the site. This was achieved using 5 years of recent meteorological monitoring data recorded from
Heathrow airport.
The results were then compared to a site specific odour impact criterion to define the extent of the
proposed development land that could be at risk of adverse odour impact under the current operating
conditions of the works. In the absence of a definitive odour impact criterion to use for assessing odours
from sewage treatments for planning purposes, the site specific criterion was chosen on the basis of
Odournet’s specialist experience, the findings of recent legal cases relating to odour and sewage works,
and a comparison of the model predictions to the location of odour complaints that have been reported
in the last year around the site.
Application of the criterion to the assessment results led to the following conclusions:
1. Under current operating conditions, odours from the works pose a risk of adverse odour impact
across all of the development land listed in Figure 1. This indicates that further significant odour
mitigation measures will be required at the works before any potential development of this land
can be realised.
2. Analysis of the effect of theoretical odour mitigation measures for the site indicates that there
is potential to reduce odour exposure levels at a selection of the development sites ( I (south),
III, IV, and V)) to below the site specific impact criterion. However, odour exposure levels at
land packages A to Jand II are predicted to remain at a level that could lead to adverse odour
impact. The cost of this theoretical mitigation is estimated by Thames Water to be £14.1m.
It is very important to note that the assessment applied in this case aims to assess the potential or
probability of people living in the area around the site to suffer from adverse odour impact. This is not
the same as assessing whether people will complain. This is because willingness to register a public
complaint of odour to authorities is a personal matter that is influenced by a range of factors that are
unrelated to actual odour exposure. As a result, although public complaints provide evidence that there
is a problem in a given area, they provide no real indication of the actual magnitude of the underlying
problem or the potential for impact on areas proposed for development.
a
Historic data (i.e. data collected pre implementation of the Odour Improvement Scheme) has only been utilised for aspects
of the works which are not influenced by the Odour Improvement Scheme.
page 4 of 42
Table of Contents
Non technical summary
3
Table of Contents
5
1
Introduction and scope
7
1.1 Introduction
7
1.2 Structure of report
7
1.3 Quality Control and Assurance
8
Description of approach
9
2.1 Identification of odour sources and estmation of odour emissions
9
2.2 Assessment of the odour impact
9
2
3
4
5
6
7
8
2.3 Assessment of impact risk
10
Overview of works operations and the proposed developement
12
3.1 Location of works and development site
12
3.2 Overview of current sewage treatment works operations
13
3.3 Overview of complaint history
14
Identification of odour sources
16
4.1 Overview of the mechanisms for odour generation from sewage treatment operations.
16
4.2 Identification of sources of odour emission.
16
Review of odour survey data
18
5.1 Summary of survey data
18
5.2 Review and discussion of survey results
19
Estimation of current site odour emissions
22
6.1 Assumptions applied to estimate emissions
22
6.2 Breakdown of emissions
24
Odour dispersion modelling
26
7.1 Dispersion modelling assumptions
26
7.2 Model outputs
27
7.3 Discussion of results
28
7.4 Assessment of the risk of odour impact at the development site for current operational
conditions.
28
Theoretical mitigation measures
30
page 5 of 42
9
8.1 Proposed theoretical mitigation measures
30
8.2 Assumptions applied to define odorous emissions
30
8.3 Review of changes in site emissions
30
8.4 Results of dispersion modelling
31
Summary of findings
34
Annex A Odour sampling and analysis techniques.
35
Annex B Odour measurement results – 2012/13
36
Annex C Odour measurement results - 2007
39
Annex D Odour measurement results - 2006
41
page 6 of 42
1 Introduction and scope
1.1 Introduction
Odournet UK Ltd were commissioned by the Royal Borough of Kingston to undertake an odour impact
assessment to investigate odour exposure levels in the vicinity of Hogsmill Sewage Treatment Works and
the potential for adverse odour impact on land potentially proposed for development by the Borough
through the Hogsmill Valley Development Plan Document (land packages A to J and I to V as set out in
Figure 1).
The overall aim of the survey was to determine the odour impact of the sewage works under current
operational conditions and to set out a remediation scenario to show odour exposure levels that would
be expected to occur on specific land packets in the area surrounding the works that have been
identified for potential development as set out in Figure 2.
The scope of the assessment was defined in the document entitled ‘Odour survey and dispersion
modelling spec. Hogsmill May 12’ which was issued on the 29th May 2012.
The specific objectives of the study were as follows;
1.
To identify activities associated with the current sewage treatment operations conducted at the
site which have the potential to generate odour and to estimate the odour emissions released from
such operations.
2.
To assess the levels of odour exposure that are likely to be encountered around the works over the
long-term for the current baseline.
3. To assess the extent of potential impact of odour from the works upon the proposed
development and determine the areas where odour annoyance may occur.
This report provides the findings of this assessment.
1.2 Structure of report
The report is structured as follows:

Section 2 describes the approach adopted.

Section 3 presents an overview of the sewage works site and proposed development.

Section 4 identifies the sources of odour at the works.

Section 5 presents a summary of odour measurement data from the 2006, 2007, 2012 and 2013
surveys.

Section 6 presents an estimation of odour emissions for the site under current baseline.

Section 7 describes the odour dispersion modelling study for the current baseline.

Section 8 describes the impact of a theoretical unfunded mitigation scenarios and provides
indicative costs of the mitigation.

Section 9 summarises the findings of the study.
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1.3 Quality Control and Assurance
Odournet’s odour measurement, assessment and consultancy services are conducted to the highest
possible quality criteria by highly trained and experienced specialist staff. All activiti es are conducted in
accordance with quality management procedures that are certified to ISO9001 (Certificate No. A13725).
All sensory odour analysis and odour sampling services are undertaken using UKAS accredited procedures
(UKAS Testing Laboratory No. 2430) which comply fully with the requirements of the international
quality standard ISO 17025: 2005 and the European standard for olfactometry EN13725: 2003. Odournet
is the only company in the UK to have secured UKAS accreditation for all elements of the o dour
measurement and analysis procedure.
The Odournet laboratory is recognised as one of the foremost laboratories in Europe, consistently out
performing the requirements of the British Standard for Olfactometry in terms of accuracy and
repeatability of analysis results.
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2 Description of approach
2.1 Identification of odour sources and estmation of odour emissions
A site audit was conducted on the 15th August 2012 to clarify the current operational regime at the works
and identify the potential sources of odour associated with the sewage treatment operations.
Emission estimates (expressed in terms of European odour units) for each source were then defined using
data collected from Hogsmill STW during odour surveys conducted in 2008, 2009, September and
November 2012 and June 2013, alongside relevant library data from other operational sewage treatment
facilities.
All of the data utilised was collected using sampling and analysis techniques compliant with the British
Standard for Olfactometry BSEN13725: 2003. Further details regarding the sampling and analysis
techniques applied are presented in Annex A.
In defining emissions, the following factors were considered:

The dimensions of each odour source.

The frequency of use of each aspect of the plant.

The potential effects of sludge/sewage turbulence.

The potential influence of seasonal differences in terms of temperature and rainfall.
2.2 Assessment of the odour impact
On the basis that odour annoyance is a symptom that develops through intermittent exposure to odour s
over extended time periods, the study focused on assessing and comparing the long -term odour exposure
levels which may occur around the site under each operational condition.
This assessment was performed using mathematical atmospheric dispersion modelling techniques which
provided a statistical analysis of the odour exposure levels that are likely to occur around the site for a
typical meteorological year. The output of the model was presented as isopleths of equal odour
concentration and plotted on a plan of the area surrounding the STW, to enable comparison between
different operational scenarios.
The dispersion modelling was conducted using the US Environmental Protection Agency (US EPA) AERMOD
dispersion model. The model was run in accordance with recent guidance issued by the US EPA and
Environment Agency. The meteorological data used by the model to simulate the dispersion and dilution
effects generated by the atmosphere were obtained from Heathrow for the years 2007 to 2011, which is
located approximately 13 km to the north west of the site. Data describing the topography of the local
area was obtained from Ordnance Survey. The locations of the odour sources at the STW were defined
from maps of the site provided by Thames Water.
The model was run to investigate the following:

Scenario 1: Current baseline. The odour exposure levels which are predicted to be generated
from the STW under the current operational regime.

Scenario 2: Theoretical mitigated operations. The odour exposure levels which are predicted to
be generated from the STW with theoretical odour mitigation measures in place.
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An analysis of current complaint locations has been used alongside published odour impact criteria to
assess the risk of future impact on the proposed development land packages.
2.3 Assessment of impact risk
In general terms, odour impact is recognised as a symptom that develops as a result of intermittent but
regular exposure to odours that are recognisable and have an offensive character. The key factors that
contribute to the development of odour annoyance can be usefully summarised by the acronym FIDOL:

Frequency of exposure.

Intensity or strength of exposure.

Duration of exposure.

Offensiveness.

Location sensitivity.
In acknowledgement of these factors, a number of odour impact criteria have been developed that
enable the odour impact risk of proposed facilities to be predicted using dispersion modelling
techniques. These criteria are generally defined in terms of a minimum concentration of odour
(reflecting the intensity/strength element of FIDOL) that occurs for a defined minimum period of time
(reflecting duration and frequency element of FIDOL) over a typical meteorological year. The
concentration element of these criteria can be increased or lowered to reflect varia tions in the
offensiveness of the odours released from a specific type of facility, and the sensitivity of nearby
sensitive locations.
In the UK, odour impact criteria are generally expressed in terms of a European odour unit concentration
that occurs for more than 2% of the hours of a typical meteorological year, and have been designed for
application to permanent residential properties which are considered to be the most sensitive from an
impact risk perspective.
The most commonly applied criterion from this perspective is the ‘Newbiggin criterion’. This criterion
was originally introduced into a public inquiry for a new sewage works at Newbiggin-by-the-sea in 1995,
and equates to an odour exposure level of 5 European odour units per cubic meter (C 98, 1-hour> 5 ouE/m3).
The Newbiggin criterion has been successfully applied during numerous planning and odour nuisance
assessment studies since 1995 for sewage, waste, food and a range of other industrial and agricultural
activities.
Since 2002, a range of indicative odour annoyance criteria have also been applied to assess odour impact
risk from residential properties, which have supplemented the use of the Newbiggin criterion. These
criteria were introduced in the Horizontal Guidance Note for Odour Management H4 issued by the
Environment Agencyb and define three different levels of exposure at which odour impact or annoyance
could potentially be expected to occur, for odours with high, moderate and low offensiveness. The
indicative criteria are presented in the table below:
Table 1: Odour impact criteria
b
Relative offensiveness
Indicative criterion
Highest
1.5 ouE/m3 98th percentile (hourly average)
Medium
3 ouE/m3 98th percentile (hourly average)
Lowest
6 ouE/m3 98th percentile (hourly average)
IPPC H4 Technical Guidance Note “H4 Odour Management”, published by the Environment Agency, March 2011.
page 10 of 42
There is currently some debate on which of these odour criteria currently represents the most appropriate
level for assessing the risk of impact of sewage treatment works. Although complete scientific consensus
has not yet been reached, Odournet’s experience suggests that odour annoyance is a symptom that is most
likely to develop at exposure levels to sewage type odours of between C98, 1-hour = 3 ouE/m3 and C98, 1-hour = 5
ouE/m3.
This observation is supported to some extent by the findings of recent legal cases in relation to odours
generated from sewage treatment works, as well as a recent policy statement issued by the Chartered
Institute of Water and Environmental Management (CIWEM) as indicated below. Whilst odours from
sewage treatment facilities and waste handling facilities can clearly vary, both activities are generally
considered to fall within the moderate to highly offensive category applied by the Environment Agency.

Appeal by Abbey Homes against St Edmundsbury Borough Council (March 2012). The Borough
Council originally refused planning permission for the erection of 101 dwellings on land between
Upthorne Road and Hepworth Road, Stanton, Suffolk, for reasons including the proximity of the
site to an existing small rural sewage treatment works and the potential effects on the living
conditions of future residents of the dwellings. On the basis of odour dispersion modelling
submitted by experts acting for both parties, the inspector considered C98, 1-hour 3 - 5 ouE/m3 an
appropriate threshold, allowed the appeal and planning permission was granted.

Appeal by JS Bloor (Northampton) Ltd 2012. This appeal concerned a proposed residential
development on land near an existing sewage treatment works in Leighton Linslade. The
inspector noted that the water company used a standard of C 98, 1-hour > 5 ouE/m3 which they
indicated would be a “concentration level above which odour might be a potential nuisance”,
and stated that the approach seemed reasonable and had been accepted at a previous a ppeal.

Thames Water vrs Dobson 2009. This nuisance action was brought against Thames Water
Mogden Sewage Treatment Works by a group of residents claiming odour nuisance caused by this
large municipal sewage works in London. The inspector concluded that he would be reluctant to
find nuisance if the modelled odour concentration was only C98, 1-hour > 1.5 ouE/m3 but as the
odour concentration rises to C98, 1-hour = 5 ouE/m3 he considered that this was the area where
nuisance from the works would start and that by the time that C98, 1-hour > 5 ouE/m3 or above is
reached nuisance would certainly be established.

Extract from CIWEM policy statement. CIWEM issued a position statement on odour in 2012
stating that the following framework is the most reliable that can be defined on the basis of the
limited research undertaken in the UK at the time of writing:

C98, 1-hour >10 ouE/m3 - complaints are highly likely and odour exposure at these levels
represents an actionable nuisance;

C98, 1-hour >5 ouE/m3 - complaints may occur and, depending on the sensitivity of the locality
and nature of the odour, this level may constitute a nuisance;

C98, 1-hour <3 ouE/m3 - complaints are unlikely to occur and exposure below this level are
unlikely to constitute significant pollution or significant detriment to amenity unless the
locality is highly sensitive or the odour highly unpleasant in nature.
This statement therefore appears to support the position that a significant risk of adverse odour
impact may occur at exposure levels between C98, 1-hour = 3 ouE/m3 and C98, 1-hour =5 ouE/m3
depending upon local conditions.
page 11 of 42
3 Overview of works operations and the proposed developement
3.1 Location of works and development site
Hogsmill Sewage Treatment Works is located in an urban area to the south east of Kingston upon
Thames, Greater London.
The proposed areas for development are located to the north, east and south of the works. The land has
been divided into specific land packages defined by proposed usage. These land packages are as follow s:
Table 2: Proposed usage of development land
Land package
Proposed use
A
Housing
B/C
Expansion of football stadium
D
Primary school
E, F, G, H, J
Open spaces for recreation and allotments
I (north)
Nature conservation
I (south)
Housing
II
Open spaces for recreation
III
Extension to cemetery
IV
Undefined
V
Waste facility
The extent of the development in relation to the sewage works is presented in Figure 2 below. The site
boundary is shown in blue, and the proposed area for development in orange.
Figure 2: Map of the area surrounding Hogsmill STW
page 12 of 42
3.2 Overview of current sewage treatment works operations
The layout of the treatment assets at Hogsmill STW is shown in Figure 3 below.
Figure 3: Layour of treatment assets at Hogsmill STW
3.2.1
Sewage handling and treatment
Hogsmill Sewage Treatment Works (STW) primarily treats sewage of a domestic origin, althoug h some
industrial releases are also received.
Sewage is conveyed to the works via the Hogsmill sewer and Worcester Park tunnel to covered chamber
from which sewage is pumped into a covered elevated inlet reception chamber. The flow is then
screened (in 4 No. 6mm band screens) and gravitates via covered channels through a pair of detritors
before it is distributed to 4 No. primary settlement tanks. Prior to distribution to primary treatment
assets, ferric chloride is added. Screenings material from the screens is passed to a washpactor and then
deposited into an enclosed compactor prior to removal from the site for disposal. With the exception of
an open chamber to the west of the inlet works, the grit skip and screenings skips , the inlet works is
fitted with covers and odorous air extracted to an odour control system.
Under storm conditions, flows in excess of the treatment capacity of the works are directed to 8 No.
rectangular storm tanks, via a flow split downstream of the screens or via a screen bypas s channel.
These tanks are fitted with 2 No. Amajets each, which aim to keep any solid material in suspension so
that it can be returned back to the works when storm conditions subside.
There are 4 No. Primary Settlement Tanks (PSTs) in operation at Hogsmill STW. The tanks are fed with
iron dosed sewage by a common distribution chamber and each tank is fitted with scum traps (which
discharge to 2 no. open scum collection chambers) and a sealed desludge system. It is understood that
the tanks are operated with a minimal sludge blanket, using an automated desludging regime.
Following primary settlement, the settled sewage is mixed with Return Activated Sludge (RAS) in an open
chamber, and then distributed to the activated sludge Biological Nutrient Removal (BNR) plant. Each
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lane of this plant is divided into anaerobic, anoxic and aerobic stages. Under the current operational
regime, hypochlorite is dosed into the plant via the RAS to control the formation of filamentous algae.
Following treatment, the flow passes via open channels to distribution chambers which feed 8 No. Final
Settlement Tanks (FSTs). Following settlement, the flow undergoes tertiary treatment in sand filters,
and is then discharged to river. Activated sludge from the Final Settlement Tanks is collected in a
central open chamber and then pumped back to the head to the BNR plant (RAS). Surplus Activated
Sludge (SAS) is pumped into a buffer tank, to await thickening and disposal (see below).
3.2.2
Sludge handling and treatment
Sludge from the PSTs is conveyed through enclosed pipe work to 2 No. covered Picket Fence Thickeners
(PFTs). The thickened sludge is then directed to a covered Sludge Blending Tank, where it is mixed with
thickened SAS and feeds into 3 No. gas mixed fixed roof digesters. Supernatant from the PFTs is returned
back to the Worcester Park tunnel. Biogas from the digesters is collected into gas holders and either
used to power the Combined Heat and Power plant (CHP), or burnt off in an enclosed flare. The Picket
Fence Thickeners and Sludge Blend Tank are extracted continuously to a common biological odour
control unit.
Following digestion, flows are transferred under gravity to 2 No.converted PSTs for secondary digestion,
then pumped via an open sump to an open sludge holding tank. The two converted PSTs operate in series
for secondary digestion. The secondary digestion tanks are covered and air is extracted to be utilised
within the CHP biogas system.
The digested sludge is then pumped to the sludge processing building (via a digested sludge buffer tank)
where it is dewatered in 3 No. Klampresses. The resultant sludge cake is deposited onto an open pad,
where it is stored prior to removal offsite for application to land. The sludge processing building also
houses 2 No. Aquabelts which are used to thicken the surplus activated sludge prior to entry into the
blend tank.
3.2.3
Anticipated future baseline
It is understood that Thames Water is anticipating a potential requirement to provide additional plant at
Hogsmill to allow for a predicted increase in treatment capacity due to population growth in the
catchment. It should be noted that the plans are provisional and would need to be agreed through
Thames Water’s next 5 year Business Plan for the period between 2015 and 2020. Therefore any
additional new plant would not come forward before 2015 at the earliest.
It is anticipated that the works may require additional primary settlement tanks, sludge thickening,
digestion and storage plant.
Design of additional plant would include provision to ensure that there was no overall increase in odours,
for example by covering and treating vented air from the plant.
3.3 Overview of complaint history
It should be noted that limited complaint data is available for the current operational regime because of
the implementation of an odour reduction scheme, which became fully operational at the end of March
2012. The scheme was signed off for ‘beneficial use’ on the 31 st March and fully handed over on the 25 th
September 2012. As it is expected that odour annoyance will develop and dissipate over a period of
time, the full benefit of the odour reduction scheme may not have been seen at this point in time.
page 14 of 42
A total of 27 No. odour complaints were reported to either The Royal Borough of Kingston or Thames
Water between April and 21st September 2012 that have been attributed to Hogsmill STW. A summary of
complaint frequency per month is presented in Figure 4 below.
Figure 4: Number of odour complaints attributed to Hogsmill STW per month during 2012
The locations of complaints that have occurred since beneficial signoff of the odour control scheme are
displayed in Figure 5 below.
Figure 5: Location of complaints attributed to Hogsmill STW since April 2012
page 15 of 42
4 Identification of odour sources
4.1 Overview of the mechanisms for odour generation from sewage treatment
operations
The generation of odour from the processing of sewage is primarily associated with the release of
odorous Volatile Organic Compounds (VOCs) that are generated as a result of the anaerobic breakdown
of organic matter by micro-organisms. Anaerobic breakdown starts within the human bowel and may
continue within the sewerage network and treatment works if conditions (i.e. a lack of oxygen) allow.
The key objectives of the sewage treatment process are to remove solid organic matter which is
responsible for generation of the majority of sewage odours and to provide treatment to remove any
residual contaminants from the wastewater so that it can be returned back into the environment.
Since the main source of odour and VOCs is the solid organic matter, the most intense and offensive
odours tend to be generated from the operations involving the handling of sludge i.e. the proc esses
applied to dewater and store raw sludge. These processes are generally considered to present the
greatest risk of odour impact offsite, unless adequate controls are put in place. Depending upon the
quality of the sewage presented to the works, the aspects of the treatment process involved in the
handling of raw sewage (e.g. preliminary and primary treatment stages) may also generate significant
levels of offensive odours.
Odours generated from the sewage treatment processes downstream of the primary s ludge removal stage
(e.g. the activated sludge processes and final settlement) present a significantly reduced risk of odour
impact. This is due to the fact that the majority of odorous biogenic material has been removed from
the flow at this point, and the treatment processes applied to remove any remaining contaminants in the
sewage are aerobic which inhibits the formation of the majority of the reduced sulphur compounds
which are responsible for offensive sewage odours.
The rate of odour release from sewage and sludge sources is primarily dependent upon temperature of
the material, and the surface area exposed to the atmosphere. As a result, odorous emissions from
sewage treatment operations tend to be highest during the summer months. Furthermore, activi ties that
lead to increase in the surface area of odorous material exposed to the atmosphere (e.g. due to
turbulence generated by sewage handling processes and agitation of sludge) will inevitably lead to an
increase in the magnitude of odour released.
4.2
Identification of sources of odour emission
On the basis of the site audit and inspection of operational data supplied, a range of odour sources were
identified at Hogsmill STW. These sources are summarised in Table 3 below.
Table 3: Identification of open odour sources for current baseline
Stage of
treatment
Odour Source
Nature of odorous material/level of
enclosure/ turbulence
Preliminary
treatment
Inlet reception chamber
Open chamber
Continuous
Inlet OCU
Covered inlet works, extracted to OCU
Continuous
Screenings skip
Semi enclosed
Continuous
Grit handling
Open skip
Continuous
Storm water
Storm tanks
Open tank and channels
Intermittent
Primary
PST distribution chamber
Open chamber, moderate turbulence
Continuous
page 16 of 42
Frequency of
emission
treatment
Secondary
treatment
Sludge
processing and
handling
Primary Settlement Tanks
Open tanks, highly turbulent weirs
Continuous
PST descum chambers
Open chamber
Continuous
Anoxic zone of BNR plant
Open channels
Continuous
Anaerobic zone of BNR plant
Open channels
Continuous
Aerobic zone of BNR plant
Open channels
Continuous
RAS PS and collection chamber
Open chamber, moderate turbulence
Continuous
RAS / settled sewage mixing chamber
Open chamber, moderate turbulence
Continuous
Mixed liquor channels
Open channels
Continuous
Picket fence thickener OCU
Covered tanks, extracted to OCU
Continuous
Sludge screening skip
Open skip
Continuous
Digested sludge holding tank
Open tank
Continuous
Digested sludge transfer well
Open chamber, high turbulence
Continuous
Digested sludge klampress feedtank
Open tank
Continuous
Surplus activated sludge buffer tank
Open tank
Continuous
Sludge thickening building
Semi enclosed building
Continuous
Centrate return well
Open chamber
Continuous
Sludge cake storage
Open sludge cake storage pad
Continuous
Sludge cake handling
Sludge cake transfer and export
Intermittent
page 17 of 42
5 Review of odour survey data
5.1 Summary of survey data
5.1.1
Olfactometry data
The data obtained from the odour surveys is summarised in Table 4 to Table 7 below. Further details of
the measurements and conditions under which they were collected are provided in Annex B to D.
Table 4: Emission measurements from open sources from 2006, 2007 and 2012 surveys
Mean area odour emission rate [ouE/m2/s]
Statge of treatment process
Source
Preliminary
Inlet (pre-screen)
2012/2013 survey
Primary
Secondary
Sludge
Previous surveys
n/m
225
Grit skip
8.2
8.5 – 16.3
Primary Settlement tank
5.0
4.0 – 8.4
PST scum chamber
2.1
2.2 – 29.7
BNR Anaerobic zone
15.8
9.8 – 16.5
BNR Anoxic zone
12.9
1.6 – 15.8
BNR Aerobic zone
2.2
9.1 – 12.1
RAS/SAS chamber
36.6
2.3 – 21.1
Sludge screening skip
12.1
19.7 – 34.7
Digested sludge transfer well
553.5
-
Digested sludge holding tank
39.0
11.5 – 35.6
1.7
14.7
SAS buffer tank
Centrate return well
24.6
2.7
Surface of fresh sludge cake
2.3
2.5
Surface of aged sludge cake
9.8
2.1
Table 5: Emission measurements from odour control units from 2006, 2007 and 2012 surveys
Source
Mean odour concentration
Mean odour emission rate
[ouE/m3]
[ouE/s]
2006
2007
2012
2006
2007
2012
Inlet OCU inlet
-
-
8843
-
-
28298
Inlet OCU outlet
-
-
5256
-
-
16819
PFT OCU inlet
-
66484
1215505
-
15956
243101
PFT OCU outlet
-
11872
4145
-
2849
829
Table 6: Emission measurements from sludge agitation tests from 2006, 2007 and 2012 surveys
Source
Mean odour emission rate
[ouE/kg]
2006
2007
2012
Fresh sludge cake (agitated)
-
5.7
86
Aged sludge cake (agitated)
-
11.4
76
page 18 of 42
Table 7: Concentration measurements from sludge agitation tests from 2006, 2007 and 2012 surveys
Source
Mean odour concentration
[ouE/m3]
2006
2007
Sludge dewatering building
5.1.2
103
2012
198
178
Hedonic tone data
The data obtained from the hedonic tone analysis is presented in Table 8 below.
Table 8: Hedonic tone analysis data from 2006, 2007 and 2012 surveys
Stage of
process
Source
Concentration at which odours are perceived as ‘mildly offensive’
[ouE/m3]
Preliminary
Inlet works OCU
1.7
1.6
2.8
Primary
Primary settlement tank
2.0
1.1
2.5
Secondary
Aerobic zone of BNR plant
2.3
2.3
3.2
Sludge
Digested sludge holding tank
1.9
1.4
1.9
2006
5.2
2007
2012
Review and discussion of survey results
The following obvserations and conclusions can be drawn from review of the survey data:
1. No odour emission measurements were possible from the sewage within the preliminary
treatment stage of the works since these areas have been covered and extracted to an odour
control as part of the AMP4 improvement works.
2. At the time of measurement the residual outlet concentration measured from the inlet odour
control plant was towards the higher end of the range expected for the type of abatement
technique employed. It is understood that a Thames Water investigation into the OCU
performance is ongoing.
3. Enhanced cleaning procedures were adopted for the storm tanks for 2007 and effectively
removed sediment from the bottom of the tanks. The storm tanks are therefore not considered
to represent a significant source of odour under normal operating condition, except when they
are in use.
4. The odour emission measurement results obtained from the primary settlement tanks operating
under normal conditions are within the range indicated during previous surveys. Elevated
emissions rates were collected from one of the primary tanks and BNR plant in September 2012.
However, these were attributed to abnormal operation of the primary tank desludge systems at
the time and have therefore been discarded for the purposes of this assessment.
5. If the Septmeber 2012 results are discarded, then the odour emission measurement results from
the anoxic and anaerobic phases of the BNR plant are within the range expected based on
previous measurements. However, the odour emission rate measured from the aerobic zone in
2012/3 is lower than expected from previous measurements. Review of operational records does
not indicate any specific reason for this decrease and as a result it is assumned that this reflects
a natural variation in emission for this type of plant. This is consistent with observations made
by Odournet at other operational sewage treatment works.
page 19 of 42
6. The odour emission rates measured from the sludge treatment elements of the process are
generally within the range expected with reference to previous odour surveys at the works.
Issues of note are as follows:
5.2.1

The levels of variation are within the range expected based on experience at other
operational sewage works.

The survey did indicate a high odour emission rate from the new digested sludge well. This
is likely to be due to the high levels of turbulence present in this area rather than an
change in the odour emission potential of the sludge.

Emissions from the RAS/SAS chambers also demonstrate significant variation between
surveys. No specific operational reasons have been identified for this variation, although
variations in BNR emissions are likely to have an influence on the magnitude of RAS/SAS
emissions.

Emissions from the sludge screening skips range between 12.2 ou E/m2/s and 34.7 ouE/m2/s
and are within the normal variation anticipated bearing in mind the nature of this source.

The odour emission rates generated from the scum chambers serving the primary
settlement tanks, and the SAS holding tank are substantially lower in 2007 and 2012
compared to 2006. The reduction in emissions from these areas is likely to be due to
improvements in site housekeeping since the 2006 survey (scum chambers are now cleaned
out on a daily basis) and the adaptations which have been applied to the polymer used for
SAS thickening, which has led to a decrease in the residence time of sludge prior to
thickening.

The secondary digesters have been covered since the 2007 survey, with odorous air directed
to an existing biogas utilisation system.

Emissions from the centrate well are an order of magnitude higher in 2012 than those
measured in 2007. No specific operational variations have been identified and the results
indicate that emissions from this source can be variable.

Surface measurements from the digested sludge cake for both fresh and aged sludge are
towards the lower range anticipated but broadly comparable between surveys. Despite the
comparability between surface measurements, the agitated bag emissions are an order of
magnitude higher in 2012 compared to 2007.

Emissions from the odour control system serving the picket fence thickener are broadly
comparable and are indicative of a well operating system.
Hedonic tone results
Hedonic tone analysis provides a measure of the relative offensiveness of odour generated from a given
odour source, by identifying the concentration at which the odour is perceived as ‘mildly’ offensive by a
panel of trained observers. The primary objective of this analysis was to assess whether there are any
significant differences between the odours released from the different aspects of the treatment process,
to ensure that the impact assessment and assessment of significance of emissions discussed in the
remainder of the study was performed on a consistent basis.
It is evident from review of the results of the analysis, presented in Table 5 above, that aspects of the
works which handle raw sewage, and those that handle digested sludge cause a mildly offensive reaction
from the odour panellists at broadly comparable concentrations during each survey. This suggests that
page 20 of 42
such odours are likely to be comparable in their offensiveness and pose an equal risk in terms of odour
impact.
In contrast, the concentrations at which a mildly offensive reaction is observed in an odour panel for
odours generated from the activated sludge plant was consistently higher for each of the three surveys
compared to other treatment plant monitored. This would suggest that such odours are likely to be less
offensive than those generated from the other treatment plant at the works, by at least 20%.
page 21 of 42
6 Estimation of current site odour emissions
6.1 Assumptions applied to estimate emissions
The assumptions applied to estimate odour emissions from the works are presented below:

The emission rate of odour from open aspects of the works involved in handling liquid sewage
(e.g. the preliminary, primary and secondary treatment stages) were calculated by multiplying
the surface area of each element of the works by the surface odour emission rate defined in the
table below. For sources directly influenced by the Odour Improvement Scheme, measurements
from the 2012/13 survey have been used to define emission estimates. For odour sources not
influenced by the Odour Improvement Scheme the odour emission rates applied have been
defined by use of the average measurements collected during the 2006, 2007, and 2012/13
odour measurement surveys, with the following exceptions:

The PSTs and the anoxic plus anaerobic zones of the BNR plant. During the September
2012 sampling there was an operational problem associated with the PST desludge
systems on a number of primary settlement tanks. This resulted in unusually high
emissions from the affected tanks and is also thought to have resulted in elevated
emissions in the initial stages of the BNR plant. These elevated emissions have been
excluded from the emission estimate calculations.

The aerobic zone of the BNR plant. The emission rate from the aerobic stage of the BNR
plant has been decreased by 20% to reflect the reduced offensiveness of the emissions
compared to the remainder of the treatment processes.

The SAS tank, centrate well and PST scum chambers. Between the 2006 and 2007 surveys
Thames Water implemented a number of operational improvements. Results from the
2007 survey show that these improvements led to decreases in odour emission levels
when compared to the previous year. Results form the 2006 survey have therefore been
discounted from emissions estimate calculations for such sources.

The inlet reception chamber. Emissions from the inlet reception chamber have been
estimated from measurements from the inlet works during 2006 and 2007 surveys. Access
is not available to this open chamber and the Odour Improvement Scheme should not
have any direct affect on the quality of the influent received at Hogsmill STW (the
scheme collects and treats emissions from the inlet works downstream of this location).

The storm tanks. Emissions from the storm tanks have been estimated assuming a
threefold dilution of dry weather flow conditions (flows divert to storm at three times
dry weather flow conditions). Emissions from dry weather flow have been estimated
from 2006/2007 inlet measurements as no access is now available to these channels.

The PST distribution chambers. The channels used to measure PST distribution have been
covered as part of the Odour Improvement Scheme. Although the majority of PST
distribution is now covered, there is an open well in the centre of the PST area. Access
is not available to allow safe sampling from this source and measurements from the
2006/2007 surveys have been used to calculated emissions estimates for this source.
Although the preceeding channels have been covered, the Odour Improvement Scheme
should have no affect on the quality of the effluent passing beneath the covers.
page 22 of 42
The model reflects recent changes to the works made under the recent Odour Improvement
Scheme. This has included the following:

Covering of the majority of the preliminary treatment stage of the works and extraction
of these areas to a new odour treatment system.

Covering of the secondary digestors and extraction of odorous air to the site biogas
system.
Table 9: Estimated emission rates from open odour sources
Source
Frequency and
duration of
release
Preliminary
treatment
Inlet chamber
Continuous
45
1
Grit handling
Continuous
10
1
Storm water
Storm tanks full
Intermittent
15
1
Primary
treatment
PST distribution chamber
Continuous
60
1-6
Primary settlement tanks
Continuous
6
1 (12 weir)
PST descum chambers
Continuous
2.5
1
Anoxic zone of BNR plant
Continuous
8
1
Anaerobic zone of BNR plant
Continuous
14
1
Aerobic zone of BNR plant
Continuous
4
1
RAS PS and collection chamber
Continuous
12
1-6
RAS / settled sewage mixing chamber
Continuous
12
1-6
Mixed liquor channels
Continuous
12
1-6
Sludge screening skip
Continuous
20
1
Digested sludge holding tank
Continuous
40
1
Digested sludge transfer well
Continuous
550
1
Digested sludge klampress feedtank
Continuous
40
1
Surplus activated sludge buffer tank
Continuous
5
1
Centrate return well
Continuous
8
1
Fresh sludge cake
Continuous
2.5
1
Aged sludge cake
Continuous
3.5
1
Stage of
treatment
Secondary
treatment
Sludge
processing and
handling
Estimated odour
emission rate
(ouE/m2/s)
Turbulence
factors
applied
*Emissions from the PSTs were calculated from the geomean of all data excluding the 24.8 and 0.5 ou E/m2/s
measurements form 2012.
The effect of sewage or sludge turbulence has been taken into account by applying a factors
applied were based on Odournet’s broader experience in the wastewater sector and the findings
of research:
Table 10: Turbulence factors

Level of turbulence
Turbulence multiplier
Low
3
Medium
6
High
12
Extreme
20
The emission rate of odour from all aspects of the works involved in handling liquid sewage (e.g.
the preliminary and primary treatment) were reduced by a factor of 5 during autumn/winter to
reflect the reduction in emissions due to lower sewage/ambient temperature and dilution
page 23 of 42
effects of rainwater. Emissions from aspects of the operations involving handling of sludge,
screenings or grit material were assumed to remain constant during summer and winter
conditions.

It was assumed that the 7 No. storm tanks were in use once per week throughout the year.
Storm water will be retained for 1 No. day for each storm event and no residual sediment is
retained. The emission rate for the storm water has been estimated by assuming a 3 fold dilution
from dry weather flow conditions.

Emission rates from odour control plant and buildings have been calculated by multiplying the
assumed airflow rate by measured concentration.
Table 11: Estimated emission rates from odour control plant and buildings
Stage of
treatment

Source
Frequency
and
duration of
release
Conc.
(ouE/m3)
Assumed
airflow
(m3/s)
Estimated
odour emission
rate
(ouE/s)
Preliminary
Inlet OCU
Continuous
5256
3.2
16819
Sludge processing
and handling
PFT OCU
Continuous
4145
0.2
829
Sludge dewatering building
Continuous
150
7.7
1155
Allowance was also made for the emissions which are likely to be generated during sludge
handling and storage operations:
6.2
o
The emission rate of sludge during agitation has been taken from the 2012 survey.
o
The sludge press building is assumed to have an airflow rate of 27720 m3/hr
(approximately six times the building volume per hour).
o
12 kg/s of sludge is assumed to be discharged from the sludge dewatering plant, over an
8 hour period on weekdays.
o
Transfer of sludge to the sludge cake storage pad is assumed to take place over a period
of one hour per day. 250 m 3 of sludge is assumed to be transferred in this period.
o
Sludge export is assumed to take place via 20 ton lorries. Up to 12 No. lorries are
assumed to export sludge per day. Each lorry is assumed to fill over a 15 minute period.
o
An area of 252 m 2 of fresh sludge is stored in the area of the klampresses prior to
transfer to the main sludge cake storage pad.
o
2 bays or 1,630 m2 of sludge cake is assumed to be stored on the cake pad throughout
the year.
Breakdown of emissions
A breakdown of odour emissions generated during summer conditions from each aspect of the sewage
treatment process is presented in the Table 12 below.
The emission rates presented in the table have been adjusted to reflect the frequency of occurrence of
each odour source, and are thus time weighted.
Table 12: Breakdown of time weighted summer emissions for current baseline operational conditions
Stage of
Odour Source
Scenario 1a: Current baseline
page 24 of 42
treatment
Emission rate
% of total
[ ouE/s]
Preliminary
treatment
Inlet chamber
Inlet OCU
Screenings skip
Grit handling
Storm water
Storm tanks
Primary
treatment
PST distribution lanes
40
0.0%
180
0.1%
14950
9.0%
7215
4.4%
30.9%
31
0.0%
Anoxic zone of AS plant
8237
5.0%
Anaerobic zone of AS plant
7207
4.3%
25350
15.3%
4446
2.7%
751
0.5%
3096
1.9%
829
0.5%
Aerobic zone of AS plant
RAS PS and collection chamber
RAS / settled sewage mixing chamber
Mixed liquor channels
Sludge
processing and
handling
0.7%
10.1%
51184
Primary settlement tanks
PST descum chambers
Secondary
treatment
1170
16819
PFT OCU
Sludge screen skip
200
0.1%
Digested sludge holding tank
10179
6.1%
Digested sludge transfer well
4400
2.7%
Digested sludge klampress feedtank
1057
0.6%
132
0.1%
1155
0.7%
24
0.0%
6335
3.8%
SAS buffer tank
Sludge thickening building
Centrate return well
Sludge cake storage
Sludge cake handling
Total
747
0.5%
165734
100.0%
The table above indicates that under current operational conditions 11% of the total time weighted site
emissions are estimated to be generated by the preliminary treatment stage, 35% from the primary
treatment stage, 30% from the secondary treatment stage, 9% from storm water handling and the
remainder (15%) from sludge handling and treatment processes.
The large surface area of the BNR plant alongside relatively high emission rate for this type of plant
accounts for the significance of this stage of treatment within the emission inventory. The large surface
area of the primary settlement tanks, combined with high turbulence levels associated with the outlet
weirs, results in primary treatment operations also being a major contributor towards total emissions.
The odour control plant serving the majority of the inlet works at Hogsmill results in a relatively minor
contribution from preliminary treatment operations to total emissions compared to site prior to
construction of the odour control scheme, although the measured outlet concentration of the inlet odour
control plant was higher than expected at the time the sample was taken.
page 25 of 42
7 Odour dispersion modelling
7.1
Dispersion modelling assumptions
An odour dispersion model was constructed to produce predictions of odour exposure levels from the
existing site operations in relation to the proposed developments around the site.
The following assumptions have been applied to the dispersion modelling study;

Meteorological data utilised within the study was derived from 5 years of recent sequential
hourly average data obtained from Heathrow meteorological station for the years 2007 to 2011.
This meteorological recording station is located approximately 20 km to the west of the site.
The meteorological data was adjusted to reflect the surface characteristics of the study site in
accordance with the guidelines issued in the AERMOD Implementation Guide c issued by the US
EPA. The wind rose for the meteorological data utilised in the study is presented below.
Figure 6: Wind rose of Heathrow metereological for 2007 to 2011
c

Data describing the topography of the area surrounding the works was obtained from Ordnance
Survey in Landform Panorama TM format.

A 3.4 km by 3.4 km uniform Cartesian receptor gird was defined for the study area with a
spacing of 100 m.

To assess exposure levels at land packet boundaries, boundary receptors were defined for each
area with a maximum spacing of 50 m between receptors.

The model only considers normal operational occurrences. Short term events such as plant
breakdown, maintenance and repair may impact considerably on the odorous emissions from time
to time. Such short term variations have not been considered within the model.
AERMOD Implementation Guide, Published by the US EPA, Last Revised: March 19, 2009
page 26 of 42
7.2
7.2.1
Model outputs
Predicted odour exposure levels
The output of the dispersion model for the current baseline scenario is presented in Figure 7 below.
The figures present the isopleths (i.e. lines of equal odour exposure) which correlate to the following
odour exposure levels:

C98, 1-hour = 1, 2, 3, 5 and 10 ouE/m3
The site boundary is shown in blue, complaint locations as pale blue stars, and the proposed areas of
development are shown in orange.
Figure 7: Output of current baseline dispersion model
page 27 of 42
7.3
7.3.1
Discussion of results
Identification of odour criteria for Hogsmill STW
The discussion on odour impact criteria presented in Section 2.3 indicates that the odour exposure level
that is likely to generate odour annoyance (which is the underlying symptom that may or amy not lead to
a complaint depending upon an individuals willingness to voice their concerns publically) ranges between
3 and 5 ouE/m3 for more than 2% of the hours in a typical year. This is based on experience, the findings
of research, and recent precedent set by planning and private nuisance cases.
For Hogsmill, a clearer picture on the site specific odour exposure level that is likely to generate adverse
odour impact can be gained by comparing the outputs of the model for current operational condition s,
with the reported complaint locations for the period April to September 2012 (i.e. since Thames Water
were handed ‘beneficial use’ of the 2012 odour mitigation scheme). This analysis is presented in tabular
form in Table 13 below:
Table 13: Analysis of predicted odour exposure levels at complaint locations
Criteria
Number of
complaint locations
Cumulative % of
total
Number of
complaints
Cumulative % of
total
C98 1 hour > 10 ouE/m3
1
6%
1
4%
C98 1 hour > 5 ouE/m3
11
61%
19
70%
C98 1 hour > 3 ouE/m3
13
72%
21
78%
14
78%
22
81%
15
83%
23
85%
C98 1 hour > 2 ouE/m
3
C98 1 hour > 1.5 ouE/m3
3
16
89%
25
93%
C98 1 hour < 1 ouE/m3
18
100.0%
27
100%
C98 1 hour > 1 ouE/m
The analysis indicates that if all complaints reported in this period are considered, odour complaints
have been reported at odour exposure levels as low as C98 1 hour < 1 ouE/m3.
However, it is important to note that a number of complaints that have occurred at exposure levels
below C98 1 hour = 3 ouE/m3 occurred over a relatively short period during September 2012, when known
issues at the works associated with the primary tank desludge system were likely to have led to
abnormally high odour emissions from both the primary and secondary treatment stages of the works.
These complaints are therefore unlikely to be representative of normal operating conditions of the
works.
If these complaints are removed, the analysis indicates that all but four of the odour complaints have
occurred at exposure levels that exceed C98 1 hour = 3 ouE/m3, with the majority (70%) occurring in areas
that exceed C98 1 hour = 5 ouE/m3.
This result is very consistent with the picture that Odournet have seen at other works in the UK as
discussed above. On this basis, Odournet consider the application of a C98 1 hour > 3 ouE/m3 criterion to
represent a robust and reasonable approach for assessing odour impact risk in the future.
7.4 Assessment of the risk of odour impact at the development site for current
operational conditions
Review of the odour exposure levels that are predicted to occur at the development sites under current
baseline conditions are presented in Table 14 below:
page 28 of 42
Table 14: Analysis of predicted odour exposure levels for proposed development plots
Land package
Odour exposure level predicted to occur for proposed development
plots
(C98 1 hour = x ouE/m3)
Current baseline
Min boundary
Midpoint
Max boundary
A
3.4
4.2
5.1
B
4.9
7.3
14.6
C
4.8
7.5
11.9
D
6.9
10.5
17.7
E
7.9
12.1
18.2
F
8.8
10.4
11.5
G
5.3
7.3
10.2
H
8.3
18.2
38.0
11.4
18.6
43.9
J
4.3
6.2
22.2
I (south)
3.2
3.8
4.5
II
4.3
7.9
16.2
III
2.1
2.4
3.0
IV
2.1
2.6
3.4
V
1.9
2.6
3.7
I (north)
Review of the table indicates that the predicted odour exposure levels that occur across all development
locations exceed the C98 1 hour = 3 ouE/m3.
page 29 of 42
8 Theoretical mitigation measures
8.1 Proposed theoretical mitigation measures
The odour impact assessment presented in the previous chapter demonstrates that there is likely to be a
risk of adverse odour impact at all of the potential proposed development locations (as set out Figure 2)
under the current operational conditions of the works, unless further significant odour mitigation
measures are applied that would require significant capital investiment.
A theoretical odour mitigation scheme with the aim of reducing odour exposure levels in the vicinity of
Hogsmill STW has been analysed to assess what reduction in odour exposure levels is likely to be
achievable. It should be noted that these are theoretical proposals and it is understood that there is no
Thames Water funding available for these measures.
The specific odour mitigation measures considered for the theoretical scheme are as follows:

The Primary Settlement Tanks will be covered and odorous air extracted to an odour control
system. Treated emissions will be discharged through a 15 m stack with an efflux velocity of
approximately 12 m/s.

2 No. of the storm tanks will be covered and odorous air extracted to an odour control system.
Treated emissions will be discharged through a 3 m stack with an efflux velocity of
approximately 12 m/s.

The inlet OCU will be serviced to ensure it is acheiving an outlet concentration of <1000 ou E/m3.
8.2 Assumptions applied to define odorous emissions
The assumptions applied to define odorous emissions are detailed as follows:

Emission rates from odour control plant and buildings have been calculated by multiplying the
assumed airflow rate by maximum design outlet concentration.
Table 15: Estimated emission rates from odour control plant and buildings
Stage of treatment
Source
Frequency
and duration
of release
Outlet conc.
(ouE/m3)
Assumed
airflow
(m3/s)
Estimated
odour
emission
rate
(ouE/s)
Primary
Primary settlement tank OCU
Continuous
1000
8.03
8030
Storm water
Storm tank OCU
Continuous
1000
1.41
1413
8.3 Review of changes in site emissions
A breakdown of odour emissions generated during summer conditions from each aspect of the sewage
treatment process under the current baseline conditions and with the theoretical mitigation scheme in
place is presented in Table 16 below.
The emission rates presented in the table have been adjusted to reflect the frequency of occurrence of
each odour source, and are thus time weighted.
page 30 of 42
Table 16: Breakdown of time weighted summer odour emissions for the anticipated future operational and mitigated future
operational conditions
Stage of
treatment
Odour Source
Current operations
Scenario 1: With mitigation
Emission rate
Emission rate
% of total
[ ouE/s]
Preliminary
treatment
Inlet chamber
Inlet OCU
1170
0.7%
1170
1.2%
16819
10.1%
3828
3.8%
Screenings skip
40
0.0%
40
0.0%
180
0.1%
180
0.2%
14950
9.0%
11976
11.9%
Grit handling
Storm water
Storm tanks
Primary
treatment
PST distribution lanes
7215
4.4%
-
0.0%
51184
30.9%
-
0.0%
31
0.0%
31
0.0%
Anoxic zone of AS plant
8237
5.0%
8237
8.2%
Anaerobic zone of AS plant
7207
4.3%
7207
7.1%
25350
15.3%
25350
25.1%
4446
2.7%
4446
4.4%
751
0.5%
751
0.7%
3096
1.9%
3096
3.1%
829
0.5%
829
0.8%
Primary settlement tanks
PST descum chambers
Secondary
treatment
Aerobic zone of AS plant
RAS PS and collection chamber
RAS / settled sewage mixing chamber
Mixed liquor channels
Sludge
processing and
handling
PFT OCU
Sludge screen skip
200
0.1%
200
0.2%
Digested sludge holding tank
10179
6.1%
10179
10.1%
Digested sludge transfer well
4400
2.7%
4400
4.4%
Digested sludge klampress feedtank
1057
0.6%
1057
1.0%
132
0.1%
132
0.1%
1155
0.7%
1155
1.1%
24
0.0%
24
0.0%
Sludge cake storage
6335
3.8%
6335
6.3%
Sludge cake handling
747
0.5%
747
0.7%
-
-
8030
7.1%
SAS buffer tank
Sludge thickening building
Centrate return well
Mitigation
scheme
% of total
[ ouE/s]
Primary settlement tank OCU
Storm tank OCU
Total
-
-
1413
1.2%
165733
100.0%
100812
100.0%
The table indicates that the total time weighted summer odour emissions from the site are likely to
decrease by approximately 36% as a result of the theoretical mitigation measures.
8.4 Results of dispersion modelling
The odour exposure levels that are predicted to occur around the site and at each proposed development
packet before and after application of further mitigation to the works are presented in Figure 8 below.
The figure presents the following isopleths:

C98, 1-hour = 3 and 5 ouE/m3.
The site is shown in blue and the proposed areas of development are shown in orange.
page 31 of 42
Figure 8: Output of dispersion model following theoretical mitigation scheme
Table 17: Analysis of predicted odour exposure levels for development plots
Land package
Odour exposure level predicted for proposed development plots
(C98 1 hour = x ouE/m3)
Current baseline
Min boundary
With mitigation
Midpoint
Max boundary
Min boundary
Midpoint
Max boundary
A
3.4
4.2
5.1
2.5
3.1
3.8
B
4.9
7.3
14.6
3.6
5.1
9.7
C
4.8
7.5
11.9
3.4
4.8
7.5
D
6.9
10.5
17.7
4.2
6.4
11.3
E
7.9
12.1
18.2
4.2
6.8
11.2
F
8.8
10.4
11.5
5.1
6.1
6.9
G
5.3
7.3
10.2
3.0
4.1
5.2
H
8.3
18.2
38.0
4.8
10.2
27.1
11.4
18.6
43.9
4.7
7.4
32.1
4.3
6.2
22.2
3.1
4.7
15.8
I (north)
J
page 32 of 42
I (south)
3.2
3.8
4.5
2.0
2.5
2.9
II
4.3
7.9
16.2
2.8
4.2
6.4
III
2.1
2.4
3.0
1.4
1.7
2.1
IV
2.1
2.6
3.4
1.4
1.7
2.1
V
1.9
2.6
3.7
1.3
1.8
2.6
The results of the study show that following application of the theoretical mitigation measures the odour
exposure levels on proposed development sites are predicted to decrease. Following implementation of
the measures the following observations can be made:

Review of the table indicates that predicted odour exposure levels that occur for parts of, or all
of potential development sites A, B, C, D, E, F, G, H, I (north), J, and II are predicted to exceed
C98 1 hour = 3 ouE/m3 under the theoretical mitigation conditions, which would imply that there is
a continued risk of adverse impact at these locations even after completion of the proposed
mitigation scheme.

The odour exposure levels that are predicted to occur on potential development sites (I (south),
III, IV, and V) to below the impact criterion of C98 1 hour > 3 ouE/m3.
page 33 of 42
9 Summary of findings
The findings of the assessment are as follows:
1. The survey identified a range of odour sources at the site associated with the treatment and
storage of sewage and sewage derived sludge. These included the raw sewage reception and
preliminary treatment (screening and grit removal) activities; primary settlement and secondary
(biological) treatment plant; and sludge collection, dewatering and storage operations.
2. Under the current operational regime, the total time weighted odour emissions from the site is
estimated at 166,000 ouE/s.
3. Analysis of complaints linked to the site since completion of the recent odour mitigation scheme
indicate that the majority occur at exposure concentrations of C98 1 hour > 3 ouE/m3. This analysis
implies that a criterion of C 98 1 hour > 3 ouE/m3 is likely to be an appropriate criterion for assessing
the current and future risk of adverse odour impact of the works operations on the proposed
development land.
4. Under current operational conditions, the study indicates that there is a risk of adverse odour
impact on all of the development land listed in Figure 1. This result indicates that further
significant odour mitigation measures will be required at the works before any potential
development of this land can be realised.
5. Analysis of the effect of theoretical (unfunded) odour mitigation measures for the site indicates
that there is potential to reduce odour exposure levels at a selection of the development sites ( I
(south), III, IV, and V) to below the impact criterion of C98 1 hour > 3 ouE/m3. However, odour
exposure levels at land packages A to J, and II are predicted to remain at a level that could lead
to adverse odour impact. The cost of this mitigation is estimated by Thames Water to be
£14.1m.
page 34 of 42
Annex A Odour sampling and analysis techniques
A.1
Collection of odour samples from odour control plant and ventilated
buildings
Collection of samples from ducts or vents was conducted using the ‘Lung’ principle. A 60 l Nalophan
sample bag was placed in a rigid container and connected to the duct containing odorous gas using a
PTFE sample line. Air was withdrawn from this container using a pump which caused a sample of the
odorous air to be drawn through the line into the bag.
If necessary, samples were pre-diluted with nitrogen at the point of collection to prevent condensation
from forming in the sampling lines and odour bag, which may influence the odour concentration prior to
analysis. Pre-dilution was conducted using Odournet’s patented Sample Master stack sampling system.
The temperature and velocity of the airflow at each point was also determined using suitable monitoring
techniques.
The emission rate of odour was then calculated by multiplying the measured odour concentration by the
volume flow rate (m 3/s) as measured in the duct.
A.2
Collection of odour samples from sources with no measurable flow
Collection of samples from area sources where there is no measurable flow were conducted using a
ventilated canopy known as a ‘Lindvall hood’. The canopy was placed on the odorous material and
ventilated at a known rate with clean odourless air. A sample of odour was collected from the outlet
port of the hood using the Lung principle.
The rate of air injected into the hood was monitored for each sample and used to calculate a specific
odour emission rate per unit area per second (E sp) as follows:
Esp = Chood x L x V
Where,
Chood is the odour concentration measured from the sample bag.
L is the hood factor, which is equal to the path length (m 2) of the hood divided by the covered area (m 2).
V is the velocity (m/s) of air presented to the hood.
A.3
Measurement of odour concentration using olfactometry
Odour measurement is aimed at characterising environmental odours, relevant to human beings. As no
methods exist at present that simulates and predict the responses of our sense of smell satisfactorily,
the human nose is the most suitable ‘sensor’. Objective methods have been developed to establish odour
concentration, using human assessors. A British standard applies to odour concentration measurement:

BSEN 13725:2003, Air quality - Determination of odour concentration by dynamic olfactometry.
The odour concentration of a gaseous sample of odorants is determined by presenting a panel of selected
and screened human subjects with that sample, in varying dilutions with neutral gas, in order t o
determine the dilution factor at the 50% detection threshold (D 50). The odour concentration of the
examined sample is then expressed as multiples of one European Odour Unit per cubic meter [ou E/m3] at
standard conditions.
page 35 of 42
Annex B Odour measurement results – 2012/13
Table 18 Emission measurements from open sources from 2012/13 survey
Mean area odour emission rate [ou E/m2/s]
Source
Date of
sampling
Grit skip
05/09/2012
8.2
15.3
15.3
15.3
Primary Settlement tank
06/09/2012
5.7
6.1
3.5
8.5
06/09/2012
24.8
12.9
25.1
47.2
08/11/2012
0.5
0.6
0.4
0.4
Geomean
1
2
3
13/06/2013
4.3
5.5
1.9
7.4
PST scum chamber
05/09/2012
2.1
1.6
2.1
2.6
BNR Anaerobic zone
05/09/2012
50.6
75.0
32.5
53.2
08/11/2012
8.3
7.3
14.8
5.3
13/06/2013
30.3
23.3
43.3
27.5
05/09/2012
54.6
58.4
52.8
52.8
08/11/2012
30.1
20.6
30.0
44.3
13/06/2013
5.5
18.3
2.8
3.3
05/09/2012
2.8
7.6
2.2
1.3
13/06/2013
1.7
1.5
1.5
2.1
RAS/SS chamber
05/09/2012
36.6
30.4
85.9
18.9
Sludge screening skip
06/09/2012
12.2
4.6
18.4
21.4
Digested sludge transfer well
12/09/2012
554
538
656
481
Digested sludge holding tank
11/09/2012
39.0
40.9
43.1
33.7
SAS buffer tank
12/09/2012
1.7
2.7
1.3
1.4
Centrate return well
12/09/2012
24.6
13.7
27.3
39.6
Surface of fresh sludge cake
12/09/2012
2.3
10.7
11.4
7.8
Surface of aged sludge cake
12/09/2012
9.8
1.7
1.4
4.7
BNR Anoxic zone
BNR Aerobic zone
Table 19 Emission measurements from open sources from 2012 survey
Source
Date of
sampling
Mean area H2S emission rate [µg/m 2/s]
Mean
1
Grit skip
05/09/2012
0.2
Primary Settlement tank
06/09/2012
0.6
2
3
0.2
0.3
0.3
0.5
0.6
0.7
06/09/2012
0.7
1.3
0.3
0.5
08/11/2012
0.06
0.05
0.06
0.07
13/06/2013
0.2
0.2
0.2
0.3
PST scum chamber
05/09/2012
0.1
0.1
0.1
0.1
BNR Anaerobic zone
05/09/2012
0.4
0.3
0.3
0.5
08/11/2012
0.2
0.1
0.2
0.1
13/06/2013
0.4
0.5
0.4
0.3
05/09/2012
0.5
0.5
0.5
0.4
BNR Anoxic zone
BNR Aerobic zone
08/11/2012
0.6
0.7
0.8
0.1
13/06/2013
0.06
0.04
0.08
0.05
05/09/2012
0.07
0.06
0.10
0.03
page 36 of 42
13/06/2013
0.1
0.1
0.1
0.1
05/09/2012
0.2
0.2
0.2
0.3
Sludge screening skip
06/09/2012
0.9
1.0
0.9
0.9
Digested sludge transfer well
12/09/2012
4.3
5.2
2.9
4.9
Digested sludge holding tank
11/09/2012
14.5
18.8
13.3
11.4
SAS buffer tank
12/09/2012
0.09
0.14
0.09
0.04
Centrate return well
12/09/2012
0.5
0.5
0.6
0.5
0.09
0.11
0.075
0.070
RAS/SS chamber
Surface of fresh sludge cake
12/09/2012
0.09
0.08
Surface of aged sludge cake
12/09/2012
0.072
0.080
Table 20 Sludge agitation tests from 2012 survey
Source
Date of
sampling
Mean odour emission rate [ouE/kg]
Geomean
Sample 1
Sample 2
Sample 3
Fresh sludge cake (agitated)
14/09/012
86
52
108
113
Aged sludge cake (agitated)
14/09/2012
76
47
72
131
Table 21 Sludge agitation tests from 2012 survey
Source
Date of
sampling
Mean H2S emission rate [µg/kg]
Geomean
Sample 1
Sample 2
Sample 3
Fresh sludge cake (agitated)
14/09/2012
1.4
1.4
1.4
1.4
Aged sludge cake (agitated)
14/09/2012
0.8
0.7
0.8
0.8
Table 22 Concentration measurements from buildings and odour control units from 2012 survey
Source
Date of
sampling
Mean odour concentration [ouE/m3]
Geomean
Sample 1
Sample 2
Sample 3
Sludge dewatering buidling
06/09/2012
178
150
151
251
Inlet OCU inlet
06/09/2012
8843
10079
8710
7877
Inlet OCU outlet
06/09/2012
5256
4475
5682
5709
PFT OCU inlet
12/09/2012
20339
1083259
1451104
1136826
PFT OCU outlet
12/09/2012
4145
5312
3234
4146
Table 23 Concentration measurements from buildings and odour control units from 2012 survey
Source
Date of
sampling
Mean H2S concentration [µg/m3]
Geomean
Sample 1
Sample 2
Sample 3
Sludge dewatering buidling
06/09/2012
13
11
9
17
Inlet OCU inlet
06/09/2012
4275
4326
4402
4098
Inlet OCU outlet
06/09/2012
34
29
33
39
PFT OCU inlet
12/09/2012
7438
4326
4629
5464
PFT OCU outlet
12/09/2012
7
9
5
7
Table 24 Summary of conditions during the 2012 survey
Date
Daily average rainfall for 3 days
prior to sampling
Sewage tempmerature
[oC]
[mm]
05/09/2012
0, 0, 0
page 37 of 42
21
06/09/2010
0, 0, 0
19
11/09/2012
0, 0, 0
n/m
12/09/2012
0, 0, 0
n/m
08/11/2012
0.2, 0.2, 0.2
13
page 38 of 42
Annex C Odour measurement results - 2007
Table 25 Emission measurements from open sources from 2007 survey
Odour source
Date
Area odour emission rate
[ouE/m2/s]
Mean
Sample 1
Sample 2
Sample 3
Inlet works (pre screen)
09/07/2007
224.8
311.5
284.9
128.0
Detritors (prior to cleaning)
09/07/2007
56.5
52.1
44.8
77.0
Detritor (following cleaning)
06/08/2007
60.0
56.5
68.4
55.8
Grit skip
06/08/2007
8.5
9.1
9.4
7.1
PST distribution
09/07/2007
70.0
72.0
63.0
75.6
Primary settlement tank
09/07/2007
8.4
3.8
7.0
21.9
Primary settlement tank
09/07/2007
4.0
4.6
2.8
5.0
Anaerobic zone (re-test)
06/08/2007
16.5
19.0
14.0
16.7
Anoxic zone
10/07/2007
1.6
1.8
1.1
2.0
Aerobic zone
10/07/2007
9.1
4.1
7.6
23.8
RAS/SAS Chamber
06/08/2007
2.3
2.3
2.2
2.3
SAS sludge buffer tank
10/07/2007
14.7
20.3
18.6
8.4
Digested sludge holding tank
10/07/2007
35.6
32.8
28.2
48.5
Sludge screen skip
06/08/2007
34.7
29.6
49.5
28.6
Fresh sludge cake
11/07/2007
2.5
2.4
2.8
2.2
Aged sludge cake
11/07/2007
2.1
3.0
1.9
1.7
Secondary digestion tanks
06/08/2007
9.7
8.6
9.5
11.0
Centrate return well
11/07/2007
2.7
2.5
2.7
2.8
Scum collection chamber
11/07/2007
2.2
2.6
2.2
2.0
Table 26 Emission measurements from open sources from 2007 survey
Odour source
Area hydrogen sulphide emission rate
[µg/m2/s]
Date
Mean
Sample 1
Sample 2
Sample 3
Inlet works (pre screen)
09/07/2007
97
59
87
143
Detritors (prior to cleaning)
09/07/2007
36
31
27
51
Detritor (following cleaning)
06/08/2007
10
17
14
15
Grit skip
06/08/2007
n/d
n/d
n/d
n/d
PST distribution
09/07/2007
9
12
9
6
Primary settlement tank
09/07/2007
2
2
2
1
Primary settlement tank
09/07/2007
n/d
n/d
n/d
n/d
Anaerobic zone (re-test)
06/08/2007
1
1
1
1
Anoxic zone
10/07/2007
n/d
n/d
n/d
n/d
Aerobic zone
10/07/2007
n/d
n/d
n/d
n/d
RAS/SAS Chamber
06/08/2007
n/d
n/d
n/d
n/d
SAS sludge buffer tank
10/07/2007
n/d
n/d
n/d
n/d
Digested sludge holding tank
10/07/2007
n/d
n/d
n/d
n/d
Sludge screen skip
06/08/2007
1
1
1
1
page 39 of 42
Fresh sludge cake
11/07/2007
<1
<1
<1
<1
Aged sludge cake
11/07/2007
n/d
n/d
n/d
n/d
Secondary digestion tanks
06/08/2007
2
2
2
2
Centrate return well
11/07/2007
n/d
n/d
n/d
n/d
Scum collection chamber
11/07/2007
n/d
n/d
n/d
n/d
Table 27 Sludge agitation tests from 2007 survey
Source
Date
Mean odour emission rate
[ouE/kg]
Geomean
Sample 1
Sample 2
Sample 3
Fresh sludge cake (agitated)
25/07/2007
5.7
12.4
5.1
2.9
Aged sludge cake (agitated)
10/08/2007
11.4
8.2
41.6
15.9
Table 28 Sludge agitation tests from 2007 survey
Source
Date
Mean H2S emission rate
[µg//kg]
Geomean
Sample 1
Sample 2
Sample 3
Fresh sludge cake (agitated)
25/07/2007
n/d
n/d
n/d
n/d
Aged sludge cake (agitated)
10/08/2007
n/d
n/d
n/d
n/d
Table 29 Concentration measurements from buildings and odour control units from 2007 survey
Source
Date
Odour concentration
[ouE/m3]
Mean
Sample 1
Sample 2
Sample 3
Sludge dewatering building
11/07/2007
198
235
204
162
PFT/blend tank OCU inlet
11/07/2007
66484
81516
51101
70546
PFT/blend tank OCU outlet
11/07/2007
11872
11751
8069
17646
Table 30 Concentration measurements from buildings and odour control units from 2007 survey
Source
Date
Hydrogen sulphide concentration
[µg/m3]
Mean
Sample 1
Sample 2
Sample 3
Sludge dewatering building
11/0720/07
11
10
17
6
PFT/blend tank OCU inlet
11/07/2007
14561
12381
14468
16833
PFT/blend tank OCU outlet
11/07/2007
4011
3895
3895
4243
Table 31 Summary of conditions during the 2007 survey
Date
Daily average rainfall for 3 days
prior to sampling
Sewage tempmerature
[oC]
[mm]
09/07/2007
0, 0, 0.3
22
10/07/2007
0, 0.3, 8
20
11/07/2007
0.3, 8, 0
19
06/08/2007
0, 0, 0.3
21
page 40 of 42
Annex D Odour measurement results - 2006
Table 32 Emission measurements from open sources from 2006 survey
Odour source
Area odour emission rate
Date
Mean
Sample 2
[ouE/m2/s]
Sample 3
Sample 1
Inlet (pre-screen)
07/08/2006
312.0
211.2
312.8
459.9
Detritor
07/08/2006
684.1
702.5
871.0
523.1
Grit skip
07/08/2006
16.3
23.3
14.0
13.3
PST Distribution
07/08/2006
48.0
49.4
39.8
56.1
Primary settlement tank
07/08/2006
7.3
9.1
6.5
6.7
Primary settlement tank
18/10/2006
5.5
4.4
5.0
7.5
Anaerobic ASP
08/08/2006
9.8
7.1
12.4
10.7
Anoxic ASP
08/08/2006
15.8
29.9
6.4
20.3
Aerobic ASP
08/08/2006
12.1
7.5
13.8
17.1
Storm residue
08/08/2006
17.9
25.8
14.6
15.3
SAS Buffer Tank
09/08/2006
90.9
120.7
78.8
78.9
Digested sludge holding tank
09/08/2006
11.5
10.8
10.3
13.8
Sludge screen skip
09/08/2006
19.7
27.2
18.7
15.1
Fresh sludge
09/08/2006
2.5
3.5
2.6
1.8
Stored sludge
10/08/2006
2.0
3.0
1.7
1.5
Secondary digestion tanks
10/08/2006
5.4
3.2
6.3
7.8
Scum collection chamber
07/08/2006
29.7
35.7
27.6
26.6
RAS/SAS chamber
07/08/2006
21.1
27.6
26.6
22.7
Table 33 Emission measurements from open sources from 2006 survey
Odour source
Area hydrogen sulphide emission rate [µg/m2/s]
Date
Mean
Sample 1
Sample 2
Sample 3
Inlet (pre-screen)
07/08/2006
96
74
98
115
Detritor
07/08/2006
237
225
249
237
Grit skip
07/08/2006
n/d
n/d
n/d
n/d
PST Distribution
07/08/2006
15
17
14
14
Primary settlement tank
07/08/2006
2
2
2
2
Primary settlement tank
18/10/2006
0.1
0.1
0.1
0.1
Anaerobic ASP
08/08/2006
n/d
n/d
n/d
n/d
Anoxic ASP
08/08/2006
n/d
n/d
n/d
n/d
Aerobic ASP
08/08/2006
n/d
n/d
n/d
n/d
Storm residue
08/08/2006
4
6
4
4
SAS Buffer Tank
09/08/2006
22
21
21
23
Digested sludge holding tank
09/08/2006
4
5
4
4
Sludge screen skip
09/08/2006
0.004
0.004
0.004
4
Fresh sludge
09/08/2006
n/d
n/d
n/d
n/d
Stored sludge
10/08/2006
n/d
n/d
n/d
n/d
Secondary digestion tanks
10/08/2006
1
<1
1
1
Scum collection chamber
07/08/2006
4
5
3
3
page 41 of 42
RAS/SAS chamber
07/08/2006
<1
3
3
<1
Table 34 Concentration measurements from buildings and odour control units from 2006 survey
Source
Date
Odour concentration
[ouE/m3]
Mean
Sample 1
Sample 2
Sample 3
Press house
10/08/2006
103
95
100
114
Digester gas
10/08/2006
832157
973731
876546
675154
Sludge blend tank
20/10/2006
5634236
5744323
5439350
5724259
Table 35 Concentration measurements from buildings and odour control units from 2006 survey
Source
Date
[µg/m3]
Hydrogen sulphide concentration
Mean
Sample 1
9
Sample 2
8
Sample 3
Press house
10/08/2006
7
Digester gas
10/08/2006
-
-
-
-
Sludge blend tank
20/10/2006
558000
586000
502000
586000
Table 36 Summary of conditions during the 2006 survey
Date
Daily average rainfall for 3 days
prior to sampling
Sewage tempmerature
[oC]
[mm]
07/08/2006
0, 0, 0
18.2
08/08/2006
0, 0, 1
19.1
09/08/2006
0, 1, 1
19.4
10/08/2006
1, 1, 1
19.7
18/10/2006
0, 0, 0.5
19.5
page 42 of 42
13

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