Odour impact assessment of Hogsmill STW - … · page 1 of 42 Odour impact assessment of Hogsmill...

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Odour impact assessment of Hogsmill STW

TWPS12C_21, November 2013

Odournet UK Ltd

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title: Odour impact assessment of Hogsmill STW

report number: TWPS12C_21

project code: TWPS12C

key words:

client: Royal London Borough of Kingston

LDF and Planning Policy Team, Guildhall 2, High Street

Kingston upon Thames, KT1 1EU

Surrey

020 8547 5418 phone

contact: K Perry

contractor: 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]

authors: Andrew Meacham

approved: on behalf of Odournet UK Ltd by

Mr. Nick Jones, director

date: 13th November 2013

copyright: ©2013, Odournet UK Ltd

Copyright and Non-Disclosure Notice The contents and layout of this report are subject to copyright owned by Odournet UK Ltd (©Odournet UK Limited 2012) save to the 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.

mailto:[email protected]

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Non technical summary

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

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 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 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

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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.

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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

2 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 Assessment of impact risk 10

3 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

4 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

5 Review of odour survey data 18

5.1 Summary of survey data 18

5.2 Review and discussion of survey results 19

6 Estimation of current site odour emissions 22

6.1 Assumptions applied to estimate emissions 22

6.2 Breakdown of emissions 24

7 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

8 Theoretical mitigation measures 30

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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

9 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

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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 activities 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 odour

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 odours

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 variations 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 (C98, 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

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)

b IPPC H4 Technical Guidance Note “H4 Odour Management”, published by the Environment Agency, March 2011.

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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 C98, 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 appeal.

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.

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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 follows:

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

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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, although 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 bypass 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 31st March and fully handed over on the 25th

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.

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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

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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 processes

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 sludge 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

Frequency of

emission

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

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treatment Primary Settlement Tanks Open tanks, highly turbulent weirs Continuous

PST descum chambers Open chamber Continuous

Secondary

treatment

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

Sludge

processing and

handling

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

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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

Statge of treatment process Source Mean area odour emission rate [ouE/m2/s]

2012/2013 survey Previous surveys

Preliminary Inlet (pre-screen) n/m 225

Grit skip 8.2 8.5 – 16.3

Primary Primary Settlement tank 5.0 4.0 – 8.4

PST scum chamber 2.1 2.2 – 29.7

Secondary 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 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

SAS buffer tank 1.7 14.7

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

[ouE/m3]

Mean odour emission rate

[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

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Table 7: Concentration measurements from sludge agitation tests from 2006, 2007 and 2012 surveys

Source Mean odour concentration

[ouE/m3]

2006 2007 2012

Sludge dewatering building 103 198 178

5.1.2 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]

2006 2007 2012

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

5.2 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.

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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:

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 ouE/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.

5.2.1 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

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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%.

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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.

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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

Stage of

treatment

Source Frequency and

duration of

release

Estimated odour

emission rate

(ouE/m2/s)

Turbulence

factors

applied

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

Secondary

treatment

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

processing and

handling

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

*Emissions from the PSTs were calculated from the geomean of all data excluding the 24.8 and 0.5 ouE/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

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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:

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 m3 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 m2 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.

6.2 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

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treatment Emission rate

[ ouE/s]

% of total

Preliminary

treatment

Inlet chamber 1170 0.7%

Inlet OCU 16819 10.1%

Screenings skip 40 0.0%

Grit handling 180 0.1%

Storm water Storm tanks 14950 9.0%

Primary

treatment

PST distribution lanes 7215 4.4%

Primary settlement tanks 51184 30.9%

PST descum chambers 31 0.0%

Secondary

treatment

Anoxic zone of AS plant 8237 5.0%

Anaerobic zone of AS plant 7207 4.3%

Aerobic zone of AS plant 25350 15.3%

RAS PS and collection chamber 4446 2.7%

RAS / settled sewage mixing chamber 751 0.5%

Mixed liquor channels 3096 1.9%

Sludge

processing and

handling

PFT OCU 829 0.5%

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%

SAS buffer tank 132 0.1%

Sludge thickening building 1155 0.7%

Centrate return well 24 0.0%

Sludge cake storage 6335 3.8%

Sludge cake handling 747 0.5%

Total 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.

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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 Guidec 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

Data describing the topography of the area surrounding the works was obtained from Ordnance

Survey in Landform PanoramaTM 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.

c AERMOD Implementation Guide, Published by the US EPA, Last Revised: March 19, 2009

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7.2 Model outputs

7.2.1 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

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7.3 Discussion of results

7.3.1 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 conditions,

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%

C98 1 hour > 2 ouE/m3 14 78% 22 81%

C98 1 hour > 1.5 ouE/m3 15 83% 23 85%

C98 1 hour > 1 ouE/m3 16 89% 25 93%

C98 1 hour < 1 ouE/m3 18 100.0% 27 100%

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:

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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

I (north) 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

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.

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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 ouE/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.

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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

[ ouE/s]

% of total Emission rate

[ ouE/s]

% of total

Preliminary

treatment

Inlet chamber 1170 0.7% 1170 1.2%

Inlet OCU 16819 10.1% 3828 3.8%

Screenings skip 40 0.0% 40 0.0%

Grit handling 180 0.1% 180 0.2%

Storm water Storm tanks 14950 9.0% 11976 11.9%

Primary

treatment

PST distribution lanes 7215 4.4% - 0.0%

Primary settlement tanks 51184 30.9% - 0.0%

PST descum chambers 31 0.0% 31 0.0%

Secondary

treatment

Anoxic zone of AS plant 8237 5.0% 8237 8.2%

Anaerobic zone of AS plant 7207 4.3% 7207 7.1%

Aerobic zone of AS plant 25350 15.3% 25350 25.1%

RAS PS and collection chamber 4446 2.7% 4446 4.4%

RAS / settled sewage mixing chamber 751 0.5% 751 0.7%

Mixed liquor channels 3096 1.9% 3096 3.1%

Sludge

processing and

handling

PFT OCU 829 0.5% 829 0.8%

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%

SAS buffer tank 132 0.1% 132 0.1%

Sludge thickening building 1155 0.7% 1155 1.1%

Centrate return well 24 0.0% 24 0.0%

Sludge cake storage 6335 3.8% 6335 6.3%

Sludge cake handling 747 0.5% 747 0.7%

Mitigation

scheme

Primary settlement tank OCU - - 8030 7.1%

Storm tank OCU - - 1413 1.2%

Total 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.

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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 With mitigation

Min boundary 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

I (north) 11.4 18.6 43.9 4.7 7.4 32.1

J 4.3 6.2 22.2 3.1 4.7 15.8

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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.

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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 C98 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.

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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 (m3/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 (Esp) 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 (m2) of the hood divided by the covered area (m2).

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 to

determine the dilution factor at the 50% detection threshold (D50). 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.

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Annex B Odour measurement results – 2012/13

Table 18 Emission measurements from open sources from 2012/13 survey

Source Date of

sampling

Mean area odour emission rate [ouE/m2/s]

Geomean 1 2 3

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

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

BNR Anoxic zone 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

BNR Aerobic zone 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

Table 19 Emission measurements from open sources from 2012 survey

Source Date of

sampling

Mean area H2S emission rate [µg/m2/s]

Mean 1 2 3

Grit skip 05/09/2012 0.2 0.2 0.3 0.3

Primary Settlement tank 06/09/2012 0.6 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

BNR Anoxic zone 05/09/2012 0.5 0.5 0.5 0.4

08/11/2012 0.6 0.7 0.8 0.1

13/06/2013 0.06 0.04 0.08 0.05

BNR Aerobic zone 05/09/2012 0.07 0.06 0.10 0.03

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13/06/2013 0.1 0.1 0.1 0.1

RAS/SS chamber 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


SAS buffer tank 12/09/2012 0.09 0.14 0.09 0.04


Surface of fresh sludge cake 12/09/2012 0.09 0.08 0.09 0.11

Surface of aged sludge cake 12/09/2012 0.072 0.080 0.075 0.070

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


Source Date of

sampling

Mean H2S emission rate [µg/kg]


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]


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


Source Date of

sampling

Mean H2S concentration [µg/m3]


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

[mm]

Sewage tempmerature

[oC]

05/09/2012 0, 0, 0 21

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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

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Annex C Odour measurement results - 2007


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


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


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


Scum collection chamber 11/07/2007 2.2 2.6 2.2 2.0


Odour source

Date

Area hydrogen sulphide emission rate

[µg/m2/s]


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

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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


Source Date Mean odour emission rate

[ouE/kg]


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


Source Date Mean H2S emission rate

[µg//kg]


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


Source

Date Odour concentration

[ouE/m3]


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


Source

Date Hydrogen sulphide concentration

[µg/m3]


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



prior to sampling

[mm]

Sewage tempmerature

[oC]

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

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Annex D Odour measurement results - 2006


Odour source

Date

Area odour emission rate [ouE/m2/s]


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



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


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


Odour source Date Area hydrogen sulphide emission rate [µg/m2/s]


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


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

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RAS/SAS chamber 07/08/2006 <1 3 3 <1


Source

Date Odour concentration

[ouE/m3]


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


Source

Date Hydrogen sulphide concentration [µg/m3]


Press house 10/08/2006 9 8 7 13

Digester gas 10/08/2006 - - - -

Sludge blend tank 20/10/2006 558000 586000 502000 586000



prior to sampling

[mm]

Sewage tempmerature

[oC]

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

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