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Odour impact assessment for a Digested Sludge Scheme at Finham STW

SETR12A12final, July 2012 Odournet UK Ltd

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2430

title: Odour impact assessment for a Digested Sludge Scheme at Finham STW

report number: SETR12A12final

project code: SETR12A

key words:

client: Severn Trent Water Ltd

Stratford Road LONGBRIDGE, WARWICK CV34 6QW 01926 496221 phone 01926 403965 fax

contact: Mr S Archer

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: Paul Ottley

approved: on behalf of Odournet UK Ltd by

Mr. Nick Jones, director

date: 10 July 2012

copyright: ©2012, Odournet UK Ltd

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Executive Summary

This report presents the findings of an odour assessment of a proposed upgrade to the sludge handling operations at Finham Sewage Treatment Works (STW). The assessment was undertaken by Odournet UK Ltd under instruction from Severn Trent Water Ltd (SETR).

The objectives of the study were as follows:

To identify the activities associated with sewage treatment operations conducted at the site which have the potential to generate odour and estimate the odour emissions released from such operations.

To identify the activities associated with the proposed digested sludge scheme operations which have the potential to generate odour and assess how the odour emission profile for the works is likely to change compared to the current baseline.

To assess and compare the odour exposure levels that are likely to occur around the works before and after completion of the proposed scheme.

The assessment was conducted using odour measurement (measurements of odour emission rates directly from the odour sources at the works) and impact assessment techniques, which have been described in guidance issued by DEFRA and the Environment Agency1.

The findings of the study can be summarised as follows:

1. The proposed digested sludge scheme involves construction of a range of plant which have the potential to generate odorous emissions. These include:

• 3 No. new open digested sludge buffer tanks with mixing system.

• New covered Centrate Balance Tank Feed Pump Station.

• 3 No. new covered centrate balance tanks.

• 3 No. new sludge thickening centrifuges.

• New sludge day pad.

• New 90 day sludge storage pad (divided into 8 No. separate bays).

2. Odour control measures will however be incorporated into the design which will ensure any emissions are minimised. The primary odour control systems will include:

• The use of airtight acoustic containers on the centrifuges to effectively eliminate the emissions of odours to atmosphere.

• Extraction of odorous air from the covered centrate balance tanks and Centrate Balance Tank Feed Pump Station and treatment within a dedicated odour control unit.

In addition, the following measures will be applied to the existing works assets:

• The addition of covers to sections of the Sherbourne inlet channels to provide some containment of the odorous emissions.

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

Code of Practice on Odour Nuisance from Sewage Treatment Works, published by DEFRA, 2006.

Odour Guidance for Local Authorities, published by DEFRA, March 2010.

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• Chemical dosing of iron at the primary settlement tank distribution cruciform.

3. The odour impact assessment for the works indicates that once the digested sludge scheme is completed and the odour control measures listed above have been installed, the odour emissions predicted to occur from the site in the summer months decrease slightly. The winter emissions, which are significantly lower than the summer baseline, increase as shown below. This increase is due to the storage of sludge cake during the autumn and winter months when emissions from the remainder of the site are comparatively low.

Change in emissions from the works (emission rates in ouE/s)

0

50000

100000

150000

200000

250000

300000

350000

Summer Winter

Current

Future

4. The results of the dispersion modelling exercise for the works indicate that once the digested sludge scheme has been completed the odour exposure levels predicted to occur in the area over the year surrounding the works increase slightly. This increase is due to the storage of sludge cake during autumn and winter months when emissions from the remainder of the site are comparatively low as described above.

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Table of Contents

Executive Summary 2

Table of Contents 4

1 Introduction 6 1.1 Scope 6 1.2 Structure of report 6 1.3 Quality Control and Assurance 6

2 Description of approach 8 2.1 Identification of odour sources and estimation of odour emissions 8 2.2 Odour assessment methodology 8 2.3 Odour exposure criteria 9

3 Description of baseline operations 11 3.1 Site location 11 3.2 Sewage treatment process 11

4 Review of odour sources for baseline operations 14 4.1 Overview of the mechanisms for odour generation. 14 4.2 Identification of potential odour sources 14 4.3 Assumptions applied to estimate odour emissions for baseline operations 15 4.4 Breakdown of estimated emissions for the current baseline 16

5 Review of changes to emissions as a result of the upgrade 17 5.1 Identification of odour sources associated with the proposed upgrade 17 5.2 Assumptions applied to define odorous emissions 18 5.3 Breakdown of emissions before and after the proposal 19

6 Review of changes to the odour exposure levels 22 6.1 Modelling assumptions 22 6.2 Dispersion modelling results 22 6.3 Discussion of model results 23

7 Conclusions 25

Annex A Odour sampling and analysis techniques. 27

Annex B Model assumptions 28

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Annex C Odour measurement results 30

Annex D Windrose for Coventry Airport weather station 2005 to 2007 33

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1 Introduction

1.1 Scope

This report presents the findings of an odour assessment of a proposed upgrade to the sludge handling operations at Finham Sewage Treatment Works (STW). The assessment was undertaken by Odournet UK Ltd under instruction from Severn Trent Water Ltd (SETR).

The objectives of the study were as follows:

To identify the activities associated with sewage treatment operations conducted at the site which have the potential to generate odour and estimate the odour emissions released from such operations.

To identify the activities associated with the proposed digested sludge scheme operations which have the potential to generate odour and assess how the emission profile for the works is likely to change compared to the current baseline.

To assess and compare the odour exposure levels that are likely to occur around the works before and after completion of the proposed scheme.

1.2 Structure of report

The report is structured as follows:

Section 2 describes the methodology undertaken to conduct the assessment.

Section 3 presents a description of the current operational configuration of Finham STW.

Section 4 identifies the odour sources associated with Finham STW under current operational conditions (current baseline) and summarises the assumptions applied to define emissions from these sources.

Section 5 identifies the odour sources associated with Finham STW after completion of the proposed digested sludge scheme and reviews how emissions will change compared to current baseline conditions.

Section 6 reviews the changes to odour exposure levels associated with the proposed changes to operations.

Section 7 summarises the conclusions of the study.

Supporting information is provided in the Annexes.

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

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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 estimation of odour emissions

The odour sources associated with each operational condition were defined on the basis of a review of the existing site operations, and operational and technical design data supplied by Severn Trent Water Ltd and their consultants. Emission estimates (expressed in terms of European odour units) for each source were then defined using data collected at Finham during odour surveys conducted in 2008 and 2011, and relevant library data from other operational sewage treatment facilities.

All of the data utilised was collected using UKAS accredited sampling and analysis techniques compliant with the British Standard for Olfactometry BSEN13725: 2003. Further details regarding the sampling and analysis techniques applied during these studies are presented in Annex A.

2.2 Odour assessment methodology

On the basis that odour annoyance or ‘nuisance’ is a symptom that develops through intermittent exposure to odours over extended time periods (see Section 2.3 below), 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 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 Coventry Airport for the years 2005 to 2007, which is located approximately 1.5km to the east 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 Severn Trent Water Ltd and their consultants.

The model was run to investigate the following:

Scenario 0: Current Baseline. The odour exposure levels which are predicted to be generated from the STW under the current operational regime.

Scenario 1: Upgraded works. The odour exposure levels which are predicted to be generated from the STW after completion of the proposed digested sludge scheme.

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2.3 Odour exposure criteria 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 criteria has been successfully applied during numerous planning and 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 originally introduced in the draft Horizontal Guidance Note for Odour H4 issued by the Environment Agency in 20022, which was finalised in March 20113. 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

High (most) 1.5 ouE/m3 98th percentile (hourly average)

Moderate 3 ouE/m3 98th percentile (hourly average)

Low 6 ouE/m3 98th percentile (hourly average)

2 2 IPPC H4 Technical Guidance Note “H4 Odour Management, Consultation Draft”, published by the Environment Agency, October 2002 3 IPPC H4 Technical Guidance Note “H4 Odour Management”, published by the Environment Agency, March 2011.

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These indicative criteria aim to differentiate between odours of different offensiveness, and range from C98, 1-hour > 1.5 ouE/m3 (for highly offensive odours) to C98, 1-hour > 6 ouE/m3 (for low offensive odours). It should be noted that the sewage treatment sector does not currently fall under the IPPC regime and that these criteria are based on relatively limited data and have not undergone any robust validation in terms of their applicability to the sewage treatment sector in the UK.

The comparison of odour exposure levels generated by the works before and after completion of the proposed sludge dewatering schemes was focused on the Newbiggin criterion (C98, 1-hour = 5 ouE/m3), and the most stringent EA criterion (C98, 1-hour = 1.5 ouE/m3).

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3 Description of baseline operations

3.1 Site location Finham Treatment Works (STW) is located on the southern extent of the city of Coventry. The site is located in a predominantly rural area. The nearest receptors are a residential area located approximately 400m directly north of the site, and a golf club located adjacent to the north eastern site boundary.

Figure 1 shows the location of the works relative to nearby settlements:

Figure 1: Location of site

© 2012 Getmapping plc, © 2012 Infoterra Ltd & Bluesky, © 2012 Google

3.2 Sewage treatment process Finham STW serves a population equivalent of approximately 360,000 and has a maximum consented flow of 360 ML/day (average daily flow of approximately 120Ml/day). The incoming flows are primarily made up of domestic sewage with a small percentage of trade effluents.

The layout of the treatment assets at Finham STW is shown in Figure 2 below:

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Figure 2: Layout of site assets

© 2012 Infoterra Ltd & Bluesky, © 2012 Europa Technologies, © 2012 Tele Atlas, © 2012 Google

3.2.1 Effluent treatment stream

Sewage arrives at 2 No. inlet works entitled Sherbourne and Sowe. At the Sherbourne inlet works rags are removed by 2 No. coarse screens and compacted and deposited into 2 No. open skips. Flows then pass through 2 No. detritors which are fitted with automatic degritting systems which deposit grit into 2 No. open skips. Following grit removal the flows pass through 1 No. fine screen and screenings are compacted and deposited into 1 No. skip. Flows greater than 3X Dry Weather Flow (3DWF) are diverted at the inlet through storm screens to primary (circular) and secondary (rectangular) storm tanks. Once storm conditions subside, the storm water is returned to the inlet via a storm returns channel.

At the Sowe inlet works screenings are removed by 2 No. coarse screens and then compacted and deposited into 1 No. screenings skip. The screened sewage flows then pass through 2 No. detritors (duty/standby) where grit is washed and deposited into 2 No. grit skips. Flows then pass to the Sowe transfer pumping station. Flows greater than 3DWF are diverted from the head of the inlet through storm screens and a detritor (with 1 No. screenings and 1 No. grit skip) to primary (circular) and secondary (rectangular) storm tanks. Storm flows are returned to the inlet via a storm returns channel.

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Settled and degritted flows from each inlet works are combined and conveyed to 2 No. banks of 3 No. circular Primary Settlement Tanks (PSTs). Each of these tanks is fitted with automatic scum and sludge removal systems.

Following primary settlement, the settled sewage is directed to 3 No. banks of aeration plant (ASP 1-3), each divided into 4 No. lanes and comprising anoxic and aerobic zones (12 No. lanes in total). Following secondary treatment, the flow is settled in 12 No. final tanks prior to discharge to the River Sowe.

3.2.2 Sludge Treatment operations

Indigenous sludge from the PSTs is directed through 2 No. odour controlled elevated primary sludge screens to 2 No. odour controlled primary sludge buffer tanks. Screenings from the primary sludge screens are deposited into a skip at ground level.

Sludge from the buffer tanks is then thickened within the sludge thickening building by up to 5 No. odour controlled centrifuges.

Thickened sludge then passes to the odour controlled thickened sludge holding tank and then to the odour controlled primary sludge dewatering tanks.

Imported sludge is delivered by road tanker to the imported sludge reception area. Here tankers discharge the imported sludge into a below ground reception tank prior to screening through a below ground screen (screenings are deposited into an above ground open screenings skip). The screened sludge is then pumped to the primary sludge dewatering tanks. All aspects of the sludge reception facility (with the exception of the screenings skip) are odour controlled.

The dewatered sludges are then fed into 8 No. digesters. The gas generated during the digestion process is collected and directed to a Combined Heat and Power (CHP) plant. The digested sludge is then conveyed by pipeline (via a semi-enclosed and odour controlled digested sludge sump) to the Rock Farm sludge facility.

The following odour control units serve the sludge processing aspects of the works:

Enclosed biofilter unit serving the primary sludge screens, sludge buffer tanks, centrifuges and thickened sludge holding tank.

Peacemaker odour control unit serving the primary sludge dewatering tanks.

Carbon filter unit serving the import sludge reception facility.

Peacemaker odour control unit serving the digested sludge sump.

3.2.3 Odour complaints history

It is understood that in the last 12 months there have been 2 No. complaints of odour relating to Finham STW. The first was from a local property (the farm located opposite Gate 1) and is believed to relate to an import tanker (rather than the works itself). The second was from a motorist using the nearby A46.

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4 Review of odour sources for baseline operations 4.1 Overview of the mechanisms for odour generation.

The generation of odour from the processing of wastewater 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.

The key objectives of the wastewater treatment process is to remove solid organic matter which is responsible for generation of the majority of wastewater 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. Depending upon the quality of the wastewater presented to the works, the aspects of the treatment process involved in the handling of raw wastewater (e.g. preliminary and primary treatment stages) may also generate significant levels of offensive odours.

Odours generated from the wastewater treatment processes downstream of the primary sludge removal stage (e.g. the activated sludge processes and final settlement) tend to be less offensive and smaller in magnitude. 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 wastewater are aerobic which inhibits the formation of the majority of the reduced sulphur compounds responsible for offensive wastewater odours.

The rate of odour release from wastewater 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 wastewater treatment operations tend to be highest during the summer months. Furthermore, activities that lead to an increase in the surface area of odorous material exposed to the atmosphere (e.g. due to turbulence generated by wastewater handling processes and agitation of sludge) will inevitably lead to an increase in the magnitude of odour released.

4.2 Identification of potential odour sources There are a range of potential odour sources associated with the sewage and sludge treatment activities that are currently conducted at Finham STW.

These odour sources are summarised in Table 2 below:

Table 2: Summary of principal odour sources identified at Finham STW

Stage of treatment Odour Source Nature of source Frequency and duration of release

Influent Open inlet channels Continuous Preliminary treatment

Screenings and grit Open skips Continuous

Storm water Open storm tanks and channels Intermittent (see 4.3 below) Storm water

Residual sediment Open tanks (rectangular tanks) Continuous

Influent Open tanks and channels Continuous Primary treatment

Settled sewage Open channels Continuous

Secondary treatment Anoxic/aerated sewage Open aeration tanks and distribution channels

Continuous

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Sludge screenings Open skips Continuous

RAS Open channels/distribution chambers

Continuous

Treated air from sludge handling plant

Odour control units Continuous

Thickened sludge Fugitive emissions from thickening building

Continuous

Sludge handling and treatment

Digested sludge Fugitive emissions from digested sludge sump

Continuous

4.3 Assumptions applied to estimate odour emissions for baseline operations

A summary of the key assumptions applied to derive emission estimates are presented below. Further details are provided in Annex B.

The odour emission rate for the majority of open odour sources for worst-case dry, summer conditions were calculated by multiplying the plan area of the treatment process by the geometric mean area odour emission rate measured during the 2008 (and 2011 for the inlet channels) odour sampling programmes. Consideration was also given to emission estimates obtained from a broader range of studies conducted on sewage treatment processes by the Odournet group. Further details of the data obtained during the surveys of Finham STW are presented in Annex C.

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 for six months of the year (October to March) to reflect the reduction in emissions due to lower sewage/ambient temperature and the general dilution effects of rainwater. Emissions from aspects of the operations involving handling of sludge, screenings or grit material were assumed to remain constant throughout the year. For turbulent sources, a multiplier was also applied to reflect the increase in emissions associated with the increased surface area of liquid exposed to the atmosphere at such locations. The following factors were applied to account for varying levels of turbulence:

Table 3: Turbulence factors

Level of turbulence Factor

Quiescent 1

Low 3

Moderate 6

High 12

Extreme 20

Emissions from storm channels and storm tanks during storm conditions were estimated by dividing the measured odour emission rate from the incoming influent by 3 (based on the operation of the storm weirs at three times Dry Weather Flow).

For the Sherbourne inlet storm events it was assumed that the circular primary storm tank and the secondary rectangular storm tanks are both full for 25 occasions per year for a duration of 3 days per occasion.

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For the Sowe inlet storm events it was assumed that the circular primary storm tanks are full for 25 occasions per year for a duration of 1.5 days per occasion and that the secondary rectangular tanks are full for 20 times a year for a duration of 1.5 days per occasion.

Emissions from the Sowe and Sherbourne primary circular storm tanks were assumed to be zero once the tanks were empty due to the efficiency of the tank cleaning systems.

4.4 Breakdown of estimated emissions for the current baseline

A breakdown of the odour emissions generated during ‘summer’ and ‘winter’ conditions (‘winter’ figures in brackets) from each aspect of the sewage treatment process under the current baseline conditions is presented in the table 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 4: Breakdown of time weighted summer odour emissions for the baseline conditions (winter emissions shown in brackets)

Scenario 0: Current operations Stage of treatment Component

Time weighted emission [ou/s]

% of total

Inlet channels 35729 (7146) 11.0 (8.9)

Screenings and grit handling 7260(7260) 2.2 (9.1)

Preliminary treatment (Sherbourne)

Storm water handling 29337 (5867) 9.0 (7.3)

Inlet channels 19654 (3931) 6.0 (4.9)

Screenings and grit handling 3586 (3586) 1.1 (4.5)

Preliminary treatment (Sowe)

Storm water handling 14447 (2889) 4.4 (3.6)

PST distribution channels 40589 (8118) 12.5 (10.5)

Primary settlement tanks 152701 (30540) 46.9 (38.1)

Primary treatment

Settled sewage / ASP distribution 5676 (1135) 1.7 (1.4)

Aeration plant 8737 (1747) 2.7 (2.2)

RAS handling 1386 (1386) 0.4 (1.7)

Secondary treatment

LLT pumping station 195 (39) 0.1 (<0.1)

Final treatment Final settlement tank distribution 150 (30) <0.1 (<0.1)

Sludge thickening building 6195 (6195) 1.9 (7.7)

Sludge screenings handling 47 (47) <0.1 (0.1)


Digested sludge sump (emissions from uncovered section) 35 (35) <0.1 (<0.1)

Sludge imports OCU 30 (30) <0.1 (<0.1)

Main sludge biofilter OCU 153 (153) <0.1 (0.2)

Sludge dewatering OCU 2 (2) <0.1 (<0.1)

Sludge odour control units

Digested sludge holding tank OCU 8 (8) <0.1 (<0.1)

TOTAL 325916 (80145) 100 (100)

10 July 2012page 17 of 32

5 Review of changes to emissions as a result of the upgrade

5.1 Identification of odour sources associated with the proposed upgrade

The proposed digested sludge scheme will see the rationalisation of sludge handling sites in the Coventry area to Finham STW, which will facilitate the future remediation of the current Rock Farm site.

The aspects of the proposed scheme which are relevant from an odour perspective are as follows:

3 No. new open digested sludge buffer tanks with mixing system.

New covered Centrate Balance Tank Feed Pump Station.

New covered centrate balance tank.

3 No. new sludge thickening centrifuges.

New sludge day pad.

New 90 day sludge storage pad (divided into 8 No. separate bays).

A range of measures will however be incorporated into the design to control emissions generated from the new plant:

The centrifuges will be contained within airtight acoustic containers to effectively eliminate the emission of odours to atmosphere.

Extraction of odorous air from the covered centrate balance tanks and Centrate Balance Tank Feed Pump Station and treatment within a dedicated odour control unit.

The addition of covers to sections of the existing Sherbourne inlet channels to improve containment of the odorous emissions.

Chemical dosing of iron at the primary settlement tank distribution cruciform.

A plan indicating the location of the proposed assets (in blue) is presented below.

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Figure 3: Proposed assets layout

© 2012 Getmapping plc, © 2012 Infoterra Ltd & Bluesky, © 2012 Google

5.2 Assumptions applied to define odorous emissions

The following assumptions were applied to quantify the emissions from the scheme:

The air emitted by the odour control unit serving the covered centrate and drainage pumping station and centrate balance tanks will have a flow rate of 3835 m3/hour and an odour concentration of 1000 ouE/m3.

The new centrifuges were assumed to operate for 16 hours per day, 5 days per week, and to produce 238 No. (peak) and 172 No. (average) tonnes of sludge cake per day.

The emissions from the sludge storage areas were calculated by multiplying the plan area of sludge cake stored during worst case conditions by representative area odour emission rates measured from comparable sources, including that measured at the Rock Farm facility during the 2008 odour survey.

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For the sludge day pad 100m2 surface area of fresh cake (stacked 1.5m high) was assumed to be stored.

Sludge from the day pad was assumed to be moved to the main cake pad by loading shovel during a 30 minute period in the morning and a 30 minute period in the afternoon (40 No. loads per day, 5 No. days per week).

The main cake pad was assumed to be divided into 8 No. bays, with a total surface area of sludge of 10670m2 (1334m2 in each bay) of sludge (stacked 1.5m high). It was assumed that at any given time all 8 No. bays will be full.

Once every 11 No. days a sludge bay is assumed to be unloaded, with unloading taking 6 No. hours per day over a 4.5 No. day period.

Emissions from the covered sections of the Sherbourne inlet channels have been reduced by 50% to reflect the containment offered by the covers.

The dosing of the PSTs with iron at the distribution stage will reduce emissions by the PSTs from 25 ouE/m2/s to 15 ouE/m2/s.

5.3 Breakdown of emissions before and after the proposal A breakdown of the time weighted odour emissions generated from each aspect of the sewage treatment process during ‘summer’ and ‘winter’ conditions (‘winter’ figures contained brackets), for the current baseline and after completion of the proposed scheme are presented in Table 5 below:

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Table 5: Breakdown of time weighted summer odour emissions (winter emissions shown in brackets)

Scenario 0: Current operations

Scenario 1: Digested sludge scheme

Stage of treatment Component

Odour emission [ouE/s]

% of total Odour emission [ouE/s]

% of total

Inlet channels 35729 (7146) 11.0 (8.9) 33101 (6620) 10.3 (5.3)

Screenings and grit handling 7260(7260) 2.2 (9.1) 7260 (7260) 2.3 (5.8)

Preliminary treatment (Sherbourne)

Storm water handling 29337 (5867) 9.0 (7.3) 29337 (5867) 9.2 (4.7)

Inlet channels 19654 (3931) 6.0 (4.9) 19654 (3931) 6.1 (3.1)

Screenings and grit handling 3586 (3586) 1.1 (4.5) 3586 (3586) 1.1 (2.9)

Preliminary treatment (Sowe)

Storm water handling 14447 (2889) 4.4 (3.6) 14447 (2889) 4.5 (2.3)

PST distribution channels 40589 (8118) 12.5 (10.5) 40589 (8118) 12.7 (6.5)

Primary settlement tanks 152701 (30540)

46.9 (38.1) 91621 (18324) 28.6 (14.6)

Primary treatment

Settled sewage / ASP distribution 5676 (1135) 1.7 (1.4) 5676 (1135) 1.8 (0.9)

Aeration plant 8737 (1747) 2.7 (2.2) 8737 (1747) 2.7 (1.4)

RAS handling 1386 (1386) 0.4 (1.7) 1386 (1386) 0.4 (1.1)

Secondary treatment

LLT pumping station 195 (39) 0.1 (<0.1) 195 (39) 0.1 (<0.1)

Final treatment Final settlement tank distribution 150 (30) <0.1 (<0.1) 150 (30) <0.1 (<0.1)

Sludge thickening building 6195 (6195) 1.9 (7.7) 6195 (6195) 1.9 (4.9)

Sludge screenings handling 47 (47) <0.1 (0.1) 47 (47) <0.1 (<0.1)


Digested sludge sump 35 (35) <0.1 (<0.1) 35 (35) <0.1 (<0.1)

Sludge imports OCU 30 (30) <0.1 (<0.1) 30 (30) <0.1 (<0.1)

Main sludge biofilter OCU 153 (153) <0.1 (0.2) 153 (153) <0.1 (0.1)

Sludge dewatering OCU 2 (2) <0.1 (<0.1) 2 (2) <0.1 (<0.1)

Sludge odour control units

Digested sludge holding tank OCU 8 (8) <0.1 (<0.1) 8 (8) <0.1 (<0.1)

Centrate OCU - - 1065 (1065) 0.3 (0.8)

Digested sludge buffer tanks - - 2477 (2477) 0.8 (2.0)

Sludge cake handling – day pad - - 423 (423) 0.1 (0.3)

Sludge digestion scheme assets

Sludge cake handling – 90 day pad - - 54262 (54262) 16.9 (43.2)

TOTAL 325916 (80145)

100 (100) 320436

(125631) 100 (100)

The change in emissions from the works is presented graphically in Figure 4 below:

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Figure 4 Change in emissions from the works (emission rates in ouE/s).

0

50000

100000

150000

200000

250000

300000

350000

Summer Winter

Current

Future

From review of the table and figure above it can be seen that, following commissioning of the digested sludge scheme at Finham STW, the summer emissions from the site are predicted to decrease by approximately 2% (5480 ouE/s) in comparison to current operational conditions.

Although the new sludge handling assets will add approximately 58,000 ouE/s to the overall site emissions, the odour control measures to be applied to existing assets (Sherbourne inlet channel covering and iron dosing prior to the PSTs) are predicted to more than offset this increase in summer months.

In comparison during winter conditions the emissions from the works are predicted to increase by approximately 45,500 ouE/s (57%). This increase is due to the emissions from the new sludge cake pad, which in comparison to the lower emissions from the remainder of the site is comparatively high.

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6 Review of changes to the odour exposure levels

6.1 Modelling assumptions The key assumptions applied in the dispersion modelling for the current operations and digested sludge scheme operations scenarios were as follows:

The effect of buildings on the dispersion of odours from stack sources (i.e. the odour control plant) was considered using the embedded BPIPPRM building effect module. The following buildings were considered:

Table 6: Buildings included in the model

Building Height (m)

Power house 7.0

Sludge thickening building 9.2

Sowe transfer pumping station 4.3

Thickened sludge pumping station 4.0

Sherbourne offices 9.3

TVD building 10.0

Sludge buffer tanks 7.8

Thickened sludge holding tank 5.2

Centrate holding tank 5.0

Sludge digesters 3.0

Finham house 5.8

Sherbourne storm control 3.6

Digested sludge buffer tanks (scenario 1 only) 8.0

Centrate balance tanks (scenario 1 only) 12.0

The meteorological data applied was derived from 3 years of recent sequential hourly average data obtained from Coventry airport for the years 2005 to 2007. The meteorological data was adjusted to reflect the surface characteristics of the study site in accordance with the guidelines issued in the AERMOD User Guide issued by the US EPA.

The study area was defined as rural, in line with land use classification techniques described in the AERMOD User Guide issued by the US EPA.

Data describing the topography of the area surrounding the works was obtained from Ordnance Survey in Landform PanoramaTM format. A 200m resolution receptor grid, centred on the site, was utilised in the model.

The model only considered emissions generated under the normal running conditions for the treatment plant.

Further details are provided in Annex B.

6.2 Dispersion modelling results The output of each of the dispersion models for the current baseline and works following commissioning of sludge dewatering scheme are presented in

Figure 5 below. The figures present isopleths defining the area where the odour exposure level will exceed C98, 1-hour > 5 ouE/m3 and 1.5 ouE/m3 respectively.

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Figure 5: Results of Scenarios 0 & 1; Current and proposed operations model output

Contours of the areas are presented in which odour concentrations of 1.5 ouE/m3 and 5 ouE/m3 are exceeded for more than 175 hours per year for the current baseline (solid lines and green/yellow shading) and proposed digested sludge scheme (dashed lines and grey shading)

6.3 Discussion of model results The results of the dispersion modelling exercise for the facility indicate that once the upgrade has been completed the odour exposure levels predicted to occur in the area to the south, east and north of the works increase slightly in comparison to the baseline conditions despite the predicted 2% reduction in time weighted summer odour emissions. This is because although the time weighted summer emissions from the works are predicted to decrease under the proposal, the winter emissions are predicted to increase primarily as a result of the sludge cake.

Review of the extent of the isopleth for the most stringent EA odour impact criteria (C98 1 hour =1.5 ouE/m3 - the area where there is likely to be a risk of odour impact) indicates that following completion of the schemes an increased proportion of the land surrounding the works will be encompassed within this isopleth. This includes an additional proportion of the residential areas to the north east, north and north west of the works (in addition to that already encompassed under the baseline conditions).

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It is however interesting to note that no complaints have been reported from these densely populated areas where long term odour exposure levels between approximately C98, 1-hour = 1.5 and >5 ouE/m3 are predicted under current operations. With this in mind review of the change in extent of the C98 1 hour = 5 ouE/m3 isopleth is likely to be of greater relevance when assessing any potential change in the level of potential odour impact or annoyance posed by the schemes.

In overall terms, the proposal is predicted to result in a slight increase in the risk of odour impact in the area surrounding the works.

The changes in impact in terms of area and maximum distance of the C98 1 hour =1.5 and 5 ouE/m3 isopleths from the site boundary are summarised below:

Table 7: Summary of the extent and area of potential impact for existing and modified baselines

Scenario Area exposed to odour concentrations > C98 1 hour = 5 ouE/m3 [km2]

Area exposed to odour concentrations > C98 1 hour = 1.5 ouE/m3 [km2]

Sc0: Current baseline 2.3 7.3

Sc1: Upgraded works 3.0 9.4

Difference (Scenario 2 to 1) +0.7 +2.1

Difference (%) (Scenario 2 to 1) +30% +29%

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7 Conclusions

The findings of the study can be summarised as follows:

1. The proposed digested sludge scheme involves construction of a range of plant which have the potential to generate odorous emissions. These include:

• 3 No. new open digested sludge buffer tanks with mixing system.

• New covered Centrate Balance Tank Feed Pump Station.

• 3 No. new covered centrate balance tanks.

• 3 No. new sludge thickening centrifuges.

• New sludge day pad.

• New 90 day sludge storage pad (divided into 8 No. separate bays).

2. Odour control measures will however be incorporated into the design which will ensure any emissions are minimised. The primary odour control systems will include:

• The use of airtight acoustic containers on the centrifuges to effectively eliminate the emissions of odours to atmosphere.

• Extraction of odorous air from the covered centrate balance tanks and Centrate Balance Tank Feed Pump Station and treatment within a dedicated odour control unit.

In addition, the following measures will be applied to the existing works assets:

• The addition of covers to sections of the Sherbourne inlet channels to provide some containment of the odorous emissions.

• Chemical dosing of iron at the primary settlement tank distribution cruciform.

3. The odour impact assessment for the works indicates that once the digested sludge scheme is completed and the odour control measures listed above have been installed, the odour emissions predicted to occur from the site in the summer months decrease slightly. The winter emissions, which are significantly lower than the summer baseline, increase as shown below. This increase is due to the storage of sludge cake during the autumn and winter months when emissions from the remainder of the site are comparatively low.

Change in emissions from the works (emission rates in ouE/s)

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0

50000

100000

150000

200000

250000

300000

350000

Summer Winter

Current

Future

4. The results of the dispersion modelling exercise for the works indicate that once the digested sludge scheme has been completed the odour exposure levels predicted to occur in the area over the year surrounding the works increase slightly. This increase is due to the storage of sludge cake during autumn and winter months when emissions from the remainder of the site are comparatively low as described above.

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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 50 l Nalophane 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 [ouE/m3] at standard conditions.

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Annex B Model assumptions

B.1 Assumptions applied to estimate emissions for current operations:

Table 8 Emissions assumptions for open sources

Stage of treatment

Source Specific area emission rate (excluding turbulence multiplier) [ouE/m2/s]

Turbulence multiplier

Sherbourne inlet channels pre-screens

60 1-12

Covered section of the Sherbourne inlet channels pre-screens

30 1

Sherbourne coarse screens 90 12

Sherbourne detritor 50

Sherbourne grit skips 3.5 1

Sherbourne inlet channels post-detritor

50 1

Sherbourne fine screens 50 12

Preliminary treatment – Sherbourne

Sherbourne screenings skips 61 -

Sowe inlet channels pre-screens 35 1

Sowe coarse screens 35 12

Sowe detritor 87

Sowe grit skips 3.5 1

Sowe inlet channels post-detritor 35 1

Sowe screenings skips 61 -

Preliminary treatment – Sowe

LLT Pumping station 1 1-20

Sherbourne storm screens 17 12

Sherbourne storm channels 17 1

Sherbourne storm tanks (full) 17 1

Sherbourne secondary storm tank residual sediment

0.5 1

Storm – Sherbourne

Sherbourne storm returns channel 12 1-20

Sowe storm screens 12 12

Sowe storm screenings skip 61 1

Sowe storm grit skip 3.5 1

Sowe storm channels 12 1

Sowe storm tanks (full) 12 1

Sowe storm tank residual sediment 2.4 1

Storm – Sowe

Sowe storm returns channel 12 1

Combined flows to PSTs

Flow merge/PST distribution 50 1-20

PST distribution channels 50 1

Southern PST 25 1

Northern PST 25 1

Primary settlement

Settled sewage channels/chamber 10 1-10

ASP Distribution chamber 10 1-6 Secondary treatment Aeration lanes – anoxic zone 2.5 1

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Aeration lanes – aerobic zone 0.5 1

Aeration lanes outlet channel 0.5 1-10

FST distribution 0.5 1-6

Sludge imports screen skip 4 1

Primary Sludge Screen Skip 2 1

Sludge

RAS sources 1 1-20

Table 9 Summary of odour emission estimates for OCUs and volume sources.

Stage of treatment Source Airflow [m3/hour] Odour concentration

[ouE/m3]

Emission rate [ouE/s]

Main sludge biofilter OCU 3024 182 153

Sludge imports OCU 252 423 30

Primary sludge dewatering tanks OCU

2.16 4069 2

Digested sludge sump OCU 432 <64 8

Sludge thickening building 14,490 (estimated based on 3 air changes per hour)

1539 6195

Sludge

Digested sludge sump fugitive emissions

576 (estimated based on 10 air changes per hour)

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B.2 Assumptions applied to estimate emissions for upgraded works

The following emission rates were applied for the additional sources associated with the sludge scheme:

Table 10: Summary of odour emission estimates for area sources within the sludge scheme.

Stage of treatment

Source Odour emission rate (excluding turbulence multiplier) [ouE/m2/s]

Turbulence multiplier

Primary treatment Primary settlement tanks 15 1

Digested sludge buffer tanks 5 1

Fresh sludge cake 8 1


Aged sludge cake 5 1

Table 11 Summary of odour emission estimates for OCUs and volume sources.

Stage of treatment Source Airflow [m3/hour] Odour concentration

[ouE/m3]

Emission rate [ouE/s]


Centrate OCU 3835 1000 1065

Table 12: Summary of odour emission estimates from agitated sludge source

Treatment stage Source Odour emission rate per kilogram of sludge [ouE/kg]

Fresh sludge cake 15 Sludge cake handling

Aged sludge cake 60

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Annex C Odour measurement results

The results of the odour sampling surveys are presented in full below:

C.1 Odour emission measurement results for open sources (2008) Table 13: Emission measurements from open sources

Mean area emission rate Source / Location Date

Geomean odour [ouE/m2/s]

H2S [mg/m2/s] NH3 [mg/m2/s]

Sherbourne inlet pre-screens 14/10/08 0.7 ND ND

Sherbourne inlet post-screens (detritor) 14/10/08 11.9 0.002 ND

Storm returns channel (Sherbourne) 30/10/08 12.0 0.002 ND

Storm tank residual sludge (Sherbourne) 06/11/08 0.5 ND ND

Grit Skip (Sherbourne inlet) 29/10/08 3.5 < 0.001 ND

Screenings Skip (Sherbourne inlet) 29/10/08 61.0 0.001 ND

Sowe Inlet Pre-screens 14/10/08 35.1 0.012 ND

Sowe Inlet Post-screens (detritor) 14/10/08 86.9 0.021 ND

Storm tank residual sludge (Sowe) 30/10/08 2.4 < 0.001 ND

Combined inlet channel 14/10/08 1.9 < 0.001 ND

Primary settlement tank (South) 15/10/08 1.4 < 0.001 ND

Primary settlement tank (North) 15/10/08 3.2 0.001 ND

Settled sewage 14/10/08 1.3 ND ND

Aeration plant – anoxic zone 15/10/08 3.3 < 0.001 ND

Aeration plant – aerobic zone 15/10/08 0.5 ND ND

RAS 30/10/08 0.8 < 0.001 ND

Sludge imports screen skip 29/10/08 3.6 0.001 ND

Primary sludge screen skip 06/11/08 2.1 < 0.001 ND

ND= not detected

C.2 Odour measurement results for buildings

Table 14 Odour measurement results from buildings/enclosed spaces

Mean concentration Source / Location Date

Geomean odour [ouE/m3]

H2S [mg/m3] NH3 [mg/m3]

Sludge thickening building 15/10/08 1539 0.037 ND

ND= not detected

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Table 15 Odour measurement results from point sources

Mean concentration Source / Location Date Flow rate [Nm3/s] @ 101.3 kPa and 293K

Geomean odour [ouE/m3]

H2S [mg/m3] NH3 [mg/m3]

Main Sludge Biofilter OCU – Inlet 15/10/08 0.99 9659 1.12 ND

Main Sludge Biofilter OCU – Outlet 15/10/08 0.84 182 ND ND

Sludge Import Area OCU – Inlet 1 29/10/08 NM4 37282 6.55 ND

Sludge Import Area OCU – Inlet 2 29/10/08 NM 58741 5.94 ND

Sludge Import Area OCU – Inlet 3 29/10/08 NM 24006 6.27 ND

Sludge Import Area OCU – Outlet 29/10/08 0.07 423 0.01 ND

Primary sludge dewatering tanks OCU – Inlet

30/10/08 0.13 56848 2.00 ND

Primary sludge dewatering tanks OCU – Outlet

30/10/08 0.001 4069 0.12 ND

Digested sump OCU – Inlet 06/11/08 0.09 220 ND 1.64

Digested sump OCU – Outlet 06/11/08 0.12 <64 ND 1.52

ND = not detected

C.3 Efficiency assessment results

Table 16: Odour abatement efficiency results Odour Emission Rate [ouE/s]

H2S Emission Rate [mg/s] OCU unit Flow rate [Nm3/s] @ 101.3 kPa and 293K Untreated

Airstream Treated Airstream

Odour Abatement Efficiency [%] Untreated

Airstream Treated Airstream

H2S Abatement Efficiency [%]

Main sludge biofilter OCU

0.84 9562 153 98.4 1.033 ND >99.9

Import sludge OCU 0.07 24795 30 98.8 4.3 < 0.001 >99.9

Primary sludge dewatering tanks OCU

0.001 7390 4069 >99.9 0.240 0.015 93.8

Digested sludge sump OCU

0.12 20 <8 >60.0 ND ND NA

ND= not detected

C.4 Odour emission measurement results for open sources (2011) Table 17: Emission measurements from open sources

Mean area emission rate Source / Location Date

Geomean odour [ouE/m2/s]

H2S [mg/m2/s] NH3 [mg/m2/s]

Sherbourne inlet pre-screens 16/11/11 59.4 0.009 ND

4 Airflows at the inlets to the import sludge OCU were not measurable due to malfunction of the flow measurement equipment. 5 The inlet odour emission rate presented to the sludge imports area OCU has been calculated by multiplying the geometric

mean inlet odour concentration by the measured outlet airflow.

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Sherbourne inlet post-screens (detritor) 16/11/11 49.2 0.042 0.006

Sowe Inlet Pre-screens 16/11/11 27.3 0.012 ND

Sowe Inlet Post-screens (detritor) 16/11/11 63.5 0.067 ND

Combined inlet channel 16/11/11 56.3 0.008 ND

ND= not detected

C.5 Discussion of results The following observations can be made from review of the odour survey data:

• It is evident from review of the results above that there are a range of odour sources associated with the sewage treatment operations conducted at Finham STW which have varying characteristics depending upon the stage in the process and the nature of the material handled. This is expected for an operational sewage treatment works.

• The emission rates measured from the preliminary stage of the treatment processes at the Sherbourne inlet in 2008 were very low in comparison to similar sources measured at other sewage treatment works in the UK. In 2011 the measured emission rates were typical of those observed at comparable sites, and indicative of an influent of moderate to high odour emission potential.

• Emissions from the screenings skip are at the higher range of emissions typically observed for such sources.

• The measured emission rates from the preliminary stage of the treatment processes at the Sowe inlet during both surveys are indicative of an influent of moderate to high odour emission potential. The slightly elevated H2S levels may be indicative of the development of septicity within the sewerage system upstream of the works. It is notable that the emissions from the screened inlet flows measured within the detritor are greater than those measured from the pre-screened flows. The reason for this is unclear, although the elevated emissions may be indicative of the accumulation of grit within the detritor.

• The odour emission rate of the sewage within the primary settlement tanks was lower than would typically be expected from such sources. The low result may have been due in part to the low odour emission potential of the influent at the Sherbourne inlet.

• Emissions from the secondary treatment stage of the process are at the lower range of measured emissions observed at comparable sewage treatment works in the UK.

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Annex D Windrose for Coventry Airport weather station 2005 to 2007

odour impact assessment for a digested sludge scheme at ... · contractor: odournet uk ltd 5, st....

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