cbco17a 04 draft - fairfield parish council · contractor: odournet uk ltd 5, st. margaret’s...
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Report Review of odour sources and best practical means at Letchworth
Wastewater Treatment Works
Client: Central Bedfordshire Council
Priory House, Monks Walk, Chicksands, Shefford
SG17 5TQ
Report number: CBCO17A_10_FINAL
Project code: CBCO17A
Date: 23 March 2017 (March 2017)
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our commercial interests.
title: Review of odour sources and best practical means at Letchworth Wastewater Treatment Works
report number: CBCO17A_10_FINAL
project code: CBCO17A
key words:
client: Central Bedfordshire Council
Priory House, Monks Walk, Chicksands, Shefford
SG17 5TQ
contact: Jane Mann
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
authors: Paul Ottley
approved: on behalf of Odournet UK Ltd by
Mr. Nick Jones, director
date: 23 March 2017
copyright: ©2017, Odournet UK Ltd
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Executive Summary
Central Bedfordshire Council (CBC) have served an abatement notice under section 80 of the
Environmental Protection Act 1990 in relation to odour nuisance from the Anglian Water Letchworth
wastewater treatment works (WWTW) near Letchworth Garden City.
Odournet UK Ltd were commissioned by CBC to undertake a review of the odour sources and control
measures at the WWTW and to review the existing measures against best practicable means (BPM) to
control odour.
The specific scope of the study was defined as follows:
1. To review the current works operations undertaken at the WWTW and identify the odour sources
which are most likely to contribute to offsite odour impact.
2. To review the measures currently in place to control odorous emissions from the works against
the requirements of BPM.
3. To identify any additional mitigation measures which are available to Anglian Water and
compare these to BPM.
The study involved a site inspection and review of existing operations at the works, followed by a review of
these operations in comparison to BPM. The study draws upon Odournet’s experience as recognised experts
in the field of odour impact assessments for sewage treatment facilities which has been developed over a
period of more than 20 No. years.
The key findings of the study are summarised as follows:
1. The following sources of odour at Letchworth WWTW are considered most likely to contribute to
offsite odour impact:
Source Estimated contribution to offsite odour impact
Material in base of storm tank Low
Primary settlement tanks Low to moderate
Picket fence thickeners Low to moderate
Sludge holding tank High
Centrifuge buffer tank Low to Moderate
Centrate wells and returns tank High
Discharge points of cake conveyors Moderate
2. Most of the techniques employed to control odours from the wastewater reception and
preliminary treatment operations at the works are compliant with indicative baseline best
practice for the industry. The main exceptions are the absence of covers on the screenings and
grit skips, and the absence of grit washing. These deficiencies are however unlikely to have any
significant detrimental effect in terms of odour generation or offsite odour impact risk.
3. For the storm water handling operations, the procedure for undertaking prompt cleaning of the
tanks following emptying should be implemented to ensure that indicative baseline best practice
for the industry is achieved in this area.
4. For the primary treatment operations, the measures to control odours comply with indicative
baseline best practice with the following exceptions:
o Co-settlement of secondary solids (SAS) in the PSTs: In the absence of odour emissions
data for the tanks it is unknown whether this practice results in increased odour
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emissions. However based on onsite observations and the fact that Anglian Water say
that this practice will cease in Summer 2017, it is not considered that this is an area
requiring immediate focus.
o Scheduling tank drawdown to avoid odour impact: A procedure for scheduling tank
draining for maintenance/cleaning to avoid odour impact by taking into account
forecast weather conditions should be implemented.
5. The techniques employed to control odours from the secondary treatment operations at the
works are compliant with indicative baseline best practice for the industry.
6. The approach to control odours in relation to sludge handling and treatment at the works falls
short of indicative baseline best practice for the industry in the following respects:
o The storage of large quantities (and surface area) of odorous raw sludge in the open
sludge holding tank is contrary to best practice.
o It appears that Anglian Water did not develop an effective and sustainable odour
management system/procedure to prevent odour impact resulting from the change in
operational regime in summer 2015 (cessation of sludge digestion and implementation of
the current sludge storage and handling operations).
o The use of perfumed odour surfactant spray systems (in use around some of the sludge
plant) is not providing an effective level of abatement of odours from the sludge area of
the works and is not considered to represent best practice.
o Substantial odour emissions are associated with the unloading of sludge tankers as a
result of the discharge of air from the tankers into the sludge holding tank once the
tanker is empty of sludge.
o The containment provided to the sludge centrate chambers does not provide a
sufficiently high level of containment to prevent the emission of substantial and highly
offensive odours.
7. The following baseline measures should be adopted in order to achieve indicative best practice
from the sludge handling and treatment operations:
o Minimisation of the quantity of raw sludge stored onsite and the contact time with the air.
To assess the measures that would be required to achieve a suitable level of odour control
a detailed assessment of the odour impact of the works and the relative contribution of
the various odour sources would be required. The required measures are likely to involve
reducing or eliminating the odours from the sludge holding tank through changes to the
operational practices and sludge storage/handling strategy, or through enhanced odour
control techniques such as ‘cover and treat’ solutions.
o Implementation of a procedure to prevent venting of tanker air into the sludge holding
tank.
o Minimisation of the odour emissions from the raw sludge centrate chambers and centrate
storage tank.
8. It is possible that the implementation of further enhanced odour control measures to the sludge
handling and treatment area of the works (over and above the baseline measures) would be
justified.
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9. The odour management plan at the works fall short of indicative best practice for the industry in
a number of respects, and the following optimisation measures are recommended:
o Update and expand the odour management plan, ensuring that the requirements
outlined in section 4.7 of this report are met.
10. Once the baseline best practice measures in place at the works have been optimised, it is
recommended that the operator should evaluate the odour emissions and impact of the works
and then assess the need for enhanced measures.
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Table of Contents
Executive Summary 3
Table of Contents 6
1 Introduction and scope 7
1.1 Introduction 7
1.2 Scope 7
1.3 Structure of report 7
1.4 Quality Control and Assurance 7
2 Description of approach 9
3 Site inspection findings and identification of odour sources 11
3.1 Site inspection findings 11
3.2 Identification of key odour sources 13
4 Review of best practice/BPM 16
4.1 Overview 16
4.2 Wastewater reception and preliminary treatment operations 16
4.3 Storm water handling operations 17
4.4 Primary treatment operations 18
4.5 Secondary treatment operations 19
4.6 Sludge handling and treatment operations 20
4.7 General housekeeping and odour management plan 23
4.8 Enhanced measures 24
5 Summary of findings 25
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1 Introduction and scope
1.1 Introduction
Central Bedfordshire Council (CBC) have served an abatement notice under section 80 of the
Environmental Protection Act 1990 in relation to odour nuisance from the Anglian Water Letchworth
wastewater treatment works (WWTW) near Letchworth Garden City.
Odournet UK Ltd were commissioned by CBC to undertake a review of the odour sources and control
measures at the WWTW and to review the existing measures against best practicable means (BPM) to
control odour.
1.2 Scope
The specific scope of the study was defined as follows:
1. To review the current works operations undertaken at the WWTW and identify the odour sources
which are most likely to contribute to offsite odour impact.
2. To review the measures currently in place to control odorous emissions from the works against
the requirements of BPM.
3. To identify any additional mitigation measures which are available to Anglian Water and
compare these to BPM.
The study has drawn upon Odournet’s experience as recognised experts in the field of odour impact
assessments for sewage treatment facilities which has been developed over a period of more than 20 No.
years and includes several hundred studies on behalf of public bodies, developers and water companies.
This report presents the findings of the study.
1.3 Structure of report
The structure of the report is as follows:
Section 2 provides a description of the approach adopted for the study.
Section 3 presents the findings of the site inspection and identifies the sources of odour at the
site.
Section 4 presents a review of BPM for the site.
Section 5 summarises the key findings of the study.
Supporting information is provided in the Annex.
1.4 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. Where
required, Odournet are accredited to conduct odour sampling from stacks and ducts in accordance to ISO
17025: 2005 and EN13725: 2003 under the MCERTS scheme. Odournet is the only company in the UK to
have secured UKAS accreditation for all elements of the odour measurement and analysis procedure.
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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
The approach adopted for the study comprised of the following two key stages:
1. Stage 1: Review of odour sources and odour control measures.
2. Stage 2: Review of existing measures against BPM and identification of additional options.
The approach which was adopted for each of these stages is defined below.
2.1.1 Stage 1: Review of odour sources and odour control measures
The first stage of the assessment involved a site inspection. The inspection was undertaken by Paul
Ottley, an experienced senior consultant at Odournet UK Ltd with a detailed understanding of the
wastewater treatment process and the factors that influence odour emissions.
During the inspection the current works configuration and odour control measures were reviewed, the
current operational regime (and how this had changed in recent years) was established, and the
potential sources of odour were identified. The findings of the inspection were used to identify which
odour sources were considered most likely to contribute to offsite odour impact.
2.1.2 Stage 2: Review of existing measures against BPM and identification of additional options
The findings of the first stage of the assessment were used to undertake a review of the current odour
abatement and management provisions at the WWTW against best practice/BPM.
There is currently no UK guidance which definitively sets out best practice/BPM for odour control for all
elements of sewage treatment processing. In practice what constitues best practice/BPM will vary from
site to site and will be influenced by the specific circumstances of the works in question. Experience and a
degree of interpretation is therefore required to identify the specific parameters which are relevant in
terms of odour generation and control at any given site.
The review was therefore conducted on the basis of Odournet’s extensive UK and European experience
within the wastewater treatment sector, and information within the following primary references:
Code of Practice on Odour Nuisance from Sewage Treatment Works, DEFRA, 2006.
Best Practical Means (BPM) – A guidebook for odour control at wastewater treatment works, UK
Water Industry research (UKWIR), 2006.
Odour control in wastewater treatment – A technical reference document, UK Water Industry
research (UKWIR), 2000/2001.
The DEFRA Code of Practice considers odour control provisions in terms of ‘baseline’ and ‘enhanced’
measures:
Baseline measures: Proactive odour control measures which should be thought of as minimum
day-to-day operating standards to be used as part of good management practice.
Enhanced measures: In some cases the baseline measures may not be enough to avoid statutory
odour nuisance, and other further measures to prevent, reduce or control statutory nuisance will
need to be put in place. These further (enhanced) measures will be required if the site is
creating or likely to create a statutory odour nuisance. If the source of odour nuisance is
identified, suitable measures should be considered in the context of BPM, cost-benefit and cost-
effectiveness to find the optimum solution.
The specific measures that are required to achieve a suitable reduction in odour emissions to eliminate
odour impact risk associated with Letchworth WWTW cannot be defined within this report. In order to
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define the specific requirements in any particular case detailed information regarding the odour
emissions from the key sources and the level of odour impact risk is required.
The focus of this review was therefore to initially review the measures adopted on site in comparison to
‘baseline’ best practice, and then where additional mitigation measures which are available to Anglian
Water were identified, an opinion provided as to whether these are likely to represent BPM for the site
(based on the information available).
Ultimately the operator is required to demonstrate that the methods they apply represent BPM for their
specific case, and for the court to decide whether it agrees.
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3 Site inspection findings and identification of odour sources
3.1 Site inspection findings
The site inspection of the WRC was undertaken on 9th March 2017 by Mr Paul Ottley (senior consultant at
Odournet) in the company of Jane Mann (Environmental Health Officer with Central Bedfordshire Council),
Neil Chamberlain (Letchworth Area Water Recycling Manager with Anglian Water) and Omid Shafibeik
(odour specialist with Anglian Water). The sewage and sludge treatment operations undertaken at the WRC
under the current operations are described below.
3.1.1 Sewage treatment
Letchworth WWTW serves a population equivalent of approximately 44,000. The works treats up to 330 l/s
of incoming sewage which is made up primarily of domestic influent (there are no notably odorous trade
discharges). The majority of the influent received at the works is delivered via gravity sewer, although a
small proportion of the influent is delivered via pumped rising mains from the nearby Fairfield area.
Sewage arrives at Letchworth WWTW into an open inlet channel via gravity sewer and a covered onsite
pumping station (which receives flows from the Fairfield area). 3 No. enclosed fine screens remove rag
from the influent which is compacted prior to deposit in an open skip. The skip is removed from site when
full and replaced within an empty one (approximately once per week on average).
Following screening the flows pass through an open channel into an open circular detritor (which is
currently not in operation). When the detritor is operational grit is removed from the sewage and
deposited via a grit elevator into an open skip which is also typically replaced approximately once per
week on average.
A new inlet works comprising 2 No. open inlet channels, 2 No. screens and 2 No. rag compactors and skips,
is currently under construction. This will replace the existing inlet works upstream of the detritor.
After the detritor the flows are conveyed along an open channel and then underground pipework to the
primary treatment stage. Ferrous chloride is dosed into the sewage at the end of the open channel to
ensure that the works achieves its phosphate consent limit.
Storm flows received at the works (those above 330 l/s) are diverted over a storm weir located along the
open post-detritor channel and diverted into 2 No. sequential fill open radial storm tanks via enclosed
pipework. Once the incoming flow rate into the works subsides the storm water within the tanks is
returned to the inlet works downstream of the detritor for treatment. Once emptied the first fill tank
typically retains a quantity of sediment/sludge on the base of the tank which requires manual cleaning.
From discussions with the site operator during the site inspection it is apparent that the storm tanks are
rarely used.
Flows from the inlet works are conveyed via an open distribution chamber into 4 No. radial primary
settlement tanks (PSTs) for solids (sludge) settlement and removal. Each tank is fitted with rotating sludge
scrapers and scum removal plant. Within the base of each tank sludge is collected in a central sump from
where it is transferred via an airlift system into a common open two stage open desludge chamber.
Following primary treatment, the settled sewage is conveyed via an open distribution chamber into one of
2 No. secondary treatment streams (stream A and B). At the head of each stream settled sewage from the
PSTs and return activated sludge (RAS – which is pumped via 2 No. Archimedes screw pumps) are mixed in
open channels and then these mixed liquors are treated in open anoxic and aerobic sections of the
activated sludge plant (ASP). Treated sewage from the ASPs is transferred via open outlet channels into 6
No. radial final treatment tanks (FSTs) for final clarification. The final effluent is then discharged to the
Pix Brook.
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3.1.2 Sludge handling and treatment
Sludge from the PST desludge chamber flows by gravity via enclosed pipework to 4 No. picket fence
thickeners (PFTs). Supernatant liquors from the PFTs are transferred to a ‘washout well’ which pumps
them back to the PST distribution chamber.
Thickened sludge from the base of the PFTs is pumped via enclosed pipework to an enclosed strainpress
which removes rags from the sludge and deposits them in an open skip. The skip is removed from site
once full and replaced with an empty one (typically once every 2 or 3 weeks).
Raw sludge from other Anglian Water sewage treatment works is brought to Letchworth WWTW by road
tanker (approximately 3 to 4 loads per day). During delivery the sludge is also passed through the
strainpress.
Sludge which has passed through the strainpress is conveyed via enclosed pipework through a discharge
pipe into the first section of a large open rectangular sludge holding tank which is divided into 3
sections. The discharge pipe extends below the surface of the sludge. Sludge spills from the first section
of the sludge holding tank into the second and third sections in turn. At any given time all 3 sections of
the tanks typically contain sludge (a mixture of indigenous and imported raw sludge).
Sludge from these tanks is pumped into an above ground circular centrifuge buffer tank which is fitted
with an automatic mixing system. The sludge is then dewatered in 2 No. centrifuges which can be run
together or separately depending on demand. Below each centrifuge is a well which receives centrate
from the centrifuge above. These wells have recently been fitted with enclosures in an attempt to
reduce the magnitude of odour release.
Centrate from these wells is pumped via enclosed pipework in to an open circular above ground centrate
return tank. From here centrate is returned to the inlet works channel downstream of the detritor and
storm weir.
The centrifuges generate a sludge cake which is conveyed via enclosed conveyors into 2 No. sludge cake
trailers. The trailers are uncovered during filling, and then covered with tarpaulin covers once full
(typically at the end of the working day). The covered trailers are kept onsite overnight prior to being
replaced with empty ones the next morning.
Surplus activated sludge (SAS) is pumped from the final settlement tanks to the washout well. In this way
SAS is co-settled in the PSTs with the primary sludge.
3.1.3 Planned operational changes
The site operator has indicated that at some point in 2017 the co-settlement of SAS and indigenous raw
sludge in the PSTs will cease and SAS will be thickened using a belt thickener and processed in the same
manner as the raw sludge.
3.1.4 Odour control measures
Under the current operational conditions at the works the following odour control measures are in place :
Use of enclosures and washwater to attempt to reduce odours released from centrate within
centrate chambers.
Use of tarpaulin covers on full cake trailers.
Use of ‘Airborne 2’ perfumed odour surfactant sprays around the centrifuge buffer tank and in
the centrate wells and cake discharge areas.
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3.1.5 Recent operational history
It is understood that the works has been operating in its current configuration since March/April 2015
when a change to the sludge treatment operations was implemented. Prior to this time the sludge
operations undertaken onsite differed in that indigenous and raw sludges were digested (a process that
results in the production of less odorous digested sludge, sludge cake and centrate). During this period
raw sludge from the picket fence thickeners was pumped directly into 2 No. anaerobic digesters prior to
being stored in a sludge consolidation tank, followed by one of 4 No. open circular ‘21 day’ batch tanks
(one of which is the current centrifuge buffer tank), and then centrifuged as per the current operations.
3.2 Identification of key odour sources
3.2.1 Overview of the mechanisms for odour generation from sewage treatment operations
The generation of odour from the processing of sewage and sludge 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 the 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. 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 typically responsible for offensive odours.
The rate of odour release from sewage and sludge sources is primarily influenced by the surface area of
material exposed to the atmosphere and the temperature of the material. As a result, odorous emissions
from sewage 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 sewage handling processes and agitation of sludge) will inevitably lead to an
increase in the magnitude of odour released.
3.2.2 Identification of sources of odour emission at Letchworth WWTW
Based on the findings of the site inspection, the following likely sources of odour from the WWTW under
the current operational regime were identified:
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Table 1 Identification of odour sources for Letchworth Wastewater Treatment Works
Stage of
treatment
Source Nature of odorous material/level of enclosure Frequency and duration
of release
Preliminary
Treatment
Inlet works channels and
detritor
Raw sewage / open (covered pumping station) Continuous
Screenings plant and skips Screenings / enclosed plant and open skips Continuous
Grit elevator and skip Grit storage / open Continuous
Storm water Storm channel and tanks Raw sewage (storm water) / open Intermittent (very
infrequent)
Storm tanks (sediment) Storm sediment / open Intermittent, dependent
on cleaning
Primary
Treatment
Distribution chamber Raw sewage plus supernatant and SAS / open Continuous
Primary settlement tanks Raw sewage / open Continuous
Settled sewage
distribution chamber
Raw sewage / open Continuous
Secondary
Treatment
Distribution/mixing
channels
Settled sewage and return activated sludge /
open
Continuous
Activated sludge plant –
anoxic & aerobic sections
Mixed liquors / open Continuous
Sludge storage
and handling
RAS RAS / open Archimedes screws Continuous
PST desludge chamber Raw sludge / open chamber Continuous
Picket fence thickeners Raw sludge / open Continuous
Sludge strain press and
skip
Raw sludge and screenings / semi enclosed
screen and open skip
Continuous
Sludge holding tank Raw sludge / open Continuous
Centrifuge buffer tank Raw sludge / open Continuous
Centrate wells Raw sludge centrate / semi enclosed Intermittent
Centrate return tank Raw sludge centrate / open Continuous
Raw sludge cake conveyors Raw sludge cake / semi-enclosed Continuous
Raw sludge cake trailers Raw sludge cake / open/covered Continuous
Washout well Supernatant liquors and SAS / covered Continuous
No odour measurement survey (odour sampling and analysis of samples using olfactometry) has been
undertaken at Letchworth WWTW in recent years. However, based on observations made during the site
inspection, in combination with Odournet’s experience of odour from sewage and sludge sources at sewage
treatment works, the following aspects of the operations are considered most likely to contribute to offsite
odour impact from Letchworth WWTW:
Table 2 Estimated contribution of odour sources to offsite odour impact
Source Estimated contribution to offsite odour impact
Material in base of storm tank Low
Primary settlement tanks Low to moderate
Picket fence thickeners Low to moderate
Sludge holding tank High
Centrifuge buffer tank Low to Moderate
Centrate wells and returns tank High
Discharge points of cake conveyors Moderate
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3.2.3 Odour complaint data
Review of odour complaints data which was provided by CBC indicates that between 2010 and 2014 no
complaints of odour relating to the site were received. 1 complaint was received in 2015, 129 were
received in 2016, and 9 were received in 2017 up to the 22nd Febuary. The vast majority of the
complaints appear to have been received from the Fairfield area. Review of complaint data reported by
Anglian Water in the document “170119 Odour Abatement timeline plan” indicates that between May
2016 and January 2017 inclusive, 60 odour complaints were made directly to them.
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4 Review of best practice/BPM
4.1 Overview
This review focuses on the following key areas of operation at Letchworth WWTW:
Wastewater reception and preliminary treatment operations.
Stormwater storage and handling operations.
Primary treatment operations.
Secondary treatment operations.
Sludge handling and treatment operations.
General housekeeping and odour management plan provisions.
4.2 Wastewater reception and preliminary treatment operations
4.2.1 Indicative best practice
The main expectations for baseline measures for the control of odours from wastewater reception and
preliminary treatment operations are summarised in Table 3 below:
Table 3 Indicative best practice (baseline measures) for odour control during wastewater reception and preliminary treatment
Requirement Reference
Screenings and grit skips should be covered, and removed as soon as practicable
for disposal
Defra COP on STW odour nuisance,
UKWIR BPM guidebook
Screenings washing and compacting should be employed, and screenings should
be stored in covered skips and removed on a weekly basis as a minimum
UKWIR Technical reference document
Grit should be washed and stored in covered skips and removed on a weekly basis
as a minimum
UKWIR Technical reference document
Control/reduce septicity (if relevant) by adding chemicals upstream of the works UKWIR BPM guidebook
Consider the use of odour reducing chemicals if influent is septic UKWIR Technical reference document
Limit receipt of odorous trade wastes UKWIR BPM guidebook
Avoid cascades and other areas of turbulence through design and/or operational
measures
UKWIR Technical reference document
4.2.2 Overview of current provisions
The following measures are currently employed in relation to the control of odorous emissions from the
wastewater reception and preliminary treatment areas at Letchworth WWTW:
The inlet pumping station is covered.
Screenings are washed and compacted prior to storage.
The screenings skip is removed from site approximately once per week once full and replaced
with an empty one. The grit skip is also removed from site once full (when the grit removal
plant is operating) and replaced with an empty one at approximately the same frequency.
The current configuration of the inlet works is such that substantial cascades and turbulence are
avoided.
With regards septicity risk of the incoming sewage, from discussions with site operational staff it is
understood that the influent received at the works is not typically septic.
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4.2.3 Review of current arrangements in comparison to best practice
Based on the above points it is considered that in general terms most of the techniques employed to
control odours from the wastewater reception and preliminary treatment operations at the works are
compliant with indicative baseline best practice for the industry. The main exceptions are the absence
of covers on the screenings and grit skips, and the absence of grit washing. These are discussed below.
Screenings skip. The absence of covers on the screenings skip is not considered likely to have
any significant detrimental effect in terms of odour generation or offsite odour impact risk.
Whilst a cover would ensure that the materials are kept dry and hence reduce the potential for
odour generation due to waterlogging and decomposition of the materials within the skips, the
fact that the rags are washed and compacted prior to deposit (to minimise the retention of
organic material), and that only 1 skip is used and frequently removed from site, means that this
risk is likely to be minimal.
Grit skip. For the grit skip the absence of grit washing plant means that the potential for the
accumulation and decomposition of organic material within the skip is increased. However given
the prompt removal and short residence time of the skip, this again is considered unlikely to
have any significant detrimental effect in terms of odour generation or offsite odour impact
risk.
Based on the information available and the observations made during the site inspection, the
implementation of enhanced odour control measures from this area of the works is considered unlikely
to be justified. Further clarification could be obtained through a detailed odour measurement survey
and assessment of offsite impact.
4.3 Storm water handling operations
4.3.1 Indicative best practice
The main expectations for baseline measures for the control of odours from storm water handling
operations are summarised in Table 4 below:
Table 4 Indicative best practice (baseline measures) for odour control during storm water handling
Requirement Reference
Minimise turbulence during filling UKWIR Technical reference document
Empty tanks as soon as possible after filling to prevent unnecessary
retention and degradation of odorous storm water
UKWIR Technical reference document
Remove sludges/sediment by automatic mechanical means and wash tanks
following emptying to ensure the retention of materials within the base of
the tanks and the associated odour generation are minimised
UKWIR Technical reference document
Avoid the return of highly odorous sludge liquors upstream of storm weirs UKWIR BPM guidebook, UKWIR
Technical reference document
4.3.2 Overview of current provisions
The following measures are currently employed in relation to the control of odorous emissions from the
storm water handling areas at Letchworth WWTW:
Storm flows are returned to the inlet works once incoming flows subside.
Regular cleaning of the bases of the storm tanks is undertaken (stated on page 7 of the site
odour management plan).
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Potentially odorous return liquors from the sludge dewatering are returned to the inlet works
downstream of the storm weir.
4.3.3 Review of current arrangements in comparison to best practice
It is apparent that some of the indicative baseline best practice measures outlined above are adopted
onsite. However, at the time of the site inspection a substantial quantity of material was present on the
base of one of the storm tanks and it was clear that the tank had not been cleaned following the last storm
event.
From discussions with the site operator and review of the odour management plan it is understood that
once emptied the normal procedure is to undertake manual cleaning of the tank bases as necessary to
remove any potentially odorous sludge/sediment. It was clear this procedure had not been implemented on
this occasion.
To ensure that indicative baseline best practice for the industry is achieved, the following baseline
measure should be adopted:
1. Ensure that the procedure for undertaking cleaning of the storm tanks following emptying is
promptly implemented.
Based on the information available and the observations made during the site inspection, the
implementation of further enhanced odour control measures from this area of the works is considered
unlikely to be justified. Further clarification could be obtained through a detailed odour measurement
survey and assessment of offsite impact.
4.4 Primary treatment operations
4.4.1 Indicative best practice
The main expectations for baseline measures for the control of odours from Primary Settlement Tanks
(PSTs) and associated plant are summarised in Table 5 below:
Table 5 Indicative best practice (baseline measures) for odour control during primary treatment
Requirement Reference
Avoid the build-up of odorous scum or foam on the surface of the tank Defra COP on STW odour nuisance
Minimise sludge retention time in the PSTs, prevent solids build up and reduce
contact time between sewage and sludge
Defra COP on STW odour nuisance,
UKWIR BPM guidebook
Regularly monitor PST sludge depth levels to ensure sludge build up is prevented
and sludge retention times within the tanks are minimised as far as possible
General best practice measure
Avoid the return of secondary solids (i.e. co settling) to primary treatment stage UKWIR BPM guidebook
Ensure retention time of sludge in PSTs is not excessive, desludge frequently UKWIR Technical reference document
Schedule tank draining for maintenance/cleaning to avoid odour impact Defra COP on STW odour nuisance
4.4.2 Overview of current measures
The following measures are currently employed in relation to the control of odorous emissions from the
primary treatment areas at Letchworth WWTW:
Use of scum removal systems on the surface of the PSTs.
Minimisation of the retention time and quantity of sludge in the PSTs through the following
measures:
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o Use of rotating sludge scrapers on base of tanks and enclosed sludge pump out process
which operates frequently (approximately 6x per day per tank on automatic timer and
twice per week manually).
o Regular monitoring of PST sludge blanket depths and maintenance of sludge depth at a
low level (<1m).
Restriction of tank drain down to times when absolutely necessary (for repair for example). In
this case the tank would be promptly emptied of all sewage and sludge to ensure that odour
impact risk at this time is minimised.
4.4.3 Review of current arrangements in comparison to best practice
Based on the above points it is considered that the measures to control odours in the primary treatment
stage of operations at Letchworth WWTW comply with indicative baseline best practice for the industry,
with the following exceptions:
Co-settlement of secondary solids (SAS) in the PSTs. It is generally considered best practice to
avoid co-settlement of sludges in the PSTs, but in the absence of odour emissions data for the
tanks it is unknown whether this practice is likely to be causing an increase in odour emissions
from the tanks. Based on onsite observations, and the fact that Anglian Water say that this
practice will cease in the summer of 2017, it is not considered that this is an area requiring
immediate focus.
Scheduling tank drawdown to avoid odour impact. The scheduling of this operation where
possible to ensure that odours disperse away from nearby sensitive receptors is a widely adopted
measure and considered a best practice measure.
To ensure that indicative baseline best practice for the industry is achieved, the following baseline
measure should be adopted:
1. Include an undertaking in the OMP to schedule tank draining for maintenance/cleaning to avoid
odour impact by taking into account forecast weather conditions, and ensure that this procedure
is implemented.
Based on the information available and the observations made during the site inspection, the
implementation of enhanced odour control measures from this area of the works is considered unlikely
to be justified. Further clarification could be obtained through a detailed odour measurement survey
and assessment of offsite impact.
4.5 Secondary treatment operations
4.5.1 Indicative best practice
The main expectations for baseline measures for the control of odours from secondary treatment
operations are summarised in Table 6 below:
Table 6 Indicative best practice (baseline measures) for odour control during secondary treatment
Requirement Reference
Avoidance of the build-up of odorous scum or foam on tank surfaces Defra COP on STW odour nuisance
Ensure effective grit removal is achieved prior to treatment to prevent the build-
up of grit in the tanks resulting in increased odour emissions
UKWIR Technical reference document
Ensure continuous recycling of activated sludge within the aerobic treatment
process
UKWIR Technical reference document
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Aerobic zones: Monitor MLSS and DO to ensure optimal treatment conditions are
maintained and any problems with aeration levels can be identified so remedial
action can be taken
General best practice measure
4.5.2 Overview of provisions
The following measures are currently employed in relation to the control of odorous emissions from the
secondary treatment areas at Letchworth WWTW:
The plant is operated in a manner that appears to avoid substantial accumulation of scum or
foam on the surface of the secondary treatment tanks.
Grit removal plant is understood to effectively prevent the build up of grit in the base of the
tanks.
Return activated sludge is continuously returned to the head of the secondary treatment plant.
Measurements of mixed liquor suspended solids (daily) and dissolved oxygen (continuous) are
undertaken in each aeration lane and set points used to achieve optimal conditions for
treatment.
4.5.3 Review of current arrangements in comparison to best practice
Based on the above points it is considered that the techniques employed to control odours from the
secondary treatment operations at the works are compliant with indicative baseline best practice for the
industry.
Based on the information available and the observations made during the site inspection, the
implementation of enhanced odour control measures from this area of the works is considered unlikely
to be justified. Further clarification could be obtained through a detailed odour measurement survey
and assessment of offsite impact.
4.6 Sludge handling and treatment operations
4.6.1 Indicative best practice
The main expectations for baseline measures for the control of odours from sludge handling and
treatment operations are summarised in Table 7 below:
Table 7 Indicative best practice for odour control (baseline measures) during sludge handling and treatment
Requirement Reference
Minimise the quantity of sludge stored, particularly raw (untreated) sludge if
stored uncovered
Defra COP on STW odour nuisance,
UKWIR Technical reference document
Minimise raw sludge contact with open air UKWIR Technical reference document
Minimise turbulence during raw sludge handling General best practice measure
Maintain a consistent throughput of sludge and prevent sludge stocks building up UKWIR BPM guidebook
Sludge screens and centrifuges should be contained to minimise the release of
odour to the atmosphere
UKWIR BPM guidebook
Prompt removal of sludge screenings skips once full General best practice measure
Minimise raw sludge centrate contact with open air General best practice measure
Cover sludge cake trailers General best practice measure
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4.6.2 Overview of provisions
The following measures are currently employed in relation to the control of odorous emissions from the
sludge handling and treatment areas at Letchworth WWTW.
The strainpress screenings skip is removed from site once full (approximately once every 2 to 3
weeks) and replaced with an empty one.
The two sludge centrifuges are primarily enclosed units, each of which is located within a
separate container. During the site inspection no substantial centrifuge odours were notable in
the immediate vicinity of the containers.
Enclosures have been installed over the sludge centrate chambers, and a system of pipework
designed to spray/cascade washwater over the centrate is used within each chamber to attempt
to reduce odours released from the centrate.
Sludge cake trailers are covered once full to ensure the sludge is kept dry and reduce odour
emissions to the atmosphere. Full trailers are promptly exported, typically within 24 hours of
filling.
Perfumed odour surfactant sprays are used around the sludge holding tank, centrifuge buffer
tank and centrate wells/cake discharge areas.
4.6.3 Review of current arrangements in comparison to best practice
Based on the above points and findings of the site inspection it is considered that the approach to
control odours in relation to sludge handling and treatment at the works falls short of indicative baseline
best practice for the industry in the following respects:
Raw sludge is stored onsite in the picket fence thickeners, sludge holding tank and centrifuge
buffer tank, all of which are open tanks. The largest of these tanks is the sludge holding tank
with a large surface area of approximately 750 m2. As indicated in section 3.2.2 the storage of
raw sludge in the sludge holding tank is considered likely to contribute substantially to offsite
odour impact based on observations made during the site inspection and Odournet’s experience
of odour from sewage and sludge sources. The odour management plan for the site identifies the
sludge holding tank as the possible main cause of odour onsite. The storage of such a large
quantity (and surface area) of odorous raw sludge in the open sludge holding tank is clearly
contrary to best practice.
As discussed in section 3.2.3 substantial numbers of odour complaints were received in 2016 and
2017. Based on the information available it is probable that the increase in complaints resulted
from the change in operations at the works (namely the cessation of sludge digestion and the
storage of substantial quantities of raw sludge in the uncovered sludge holding tank and
centrifuge buffer tank, and the production of raw sludge cake). Given that raw sludge is more
odorous than digested sludge, it is considered that the increased level of odour generation could
have been reasonably anticipated by Anglian Water. It is unknown whether an assessment of the
likely change in odour impact risk was undertaken prior to the change in operations, but the
increased odour impact risk should have been assessed and the opportunities to implement
suitable solutions should have been reviewed and implemented. It is considered that such an
approach represents best practice for avoiding odour nuisance when a substantial change in
operations is planned. Given the substantial numbers of complaints of odour that have been
received since the change in operations, it appears that Anglian Water did not develop an
effective and sustainable odour management system/procedure to prevent odour impact.
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The performance of the perfumed odour surfactant sprays used around the sludge holding tank,
centrifuge buffer tank and centrate wells/cake discharge areas is unknown. There is little
objective evidence available to demonstrate how effective these products and systems are, and
their likely effectiveness at reducing long term odour impact risk from an odorous site is
questionable. The use of such products as the primary odour control measure is not considered
to represent best practice. During the site inspection high intensity odours from the sludge
holding tank and the centrate wells were noted downwind of the tanks/wells. This in
combination with the odour complaint data implies that the spray systems are not providing an
effective level of abatement of odours from the sludge area of the works.
The discharge pipe which delivers imported and indigenous raw sludge into the sludge holding
tank extends below the sludge surface (a measure designed to reduce odour release through
turbulence during discharge). However during the site inspection substantial turbulence (and
odour release) was noted during the discharge of imported sludge from a tanker. The site
operator explained that this was likely to be a result of the discharge of air from the tanker
once it was empty of sludge.
The containment provided to the sludge centrate chambers does not provide a sufficiently high
level of containment to prevent the emission of substantial and highly offensive odours. During
the site visit high magnitude and highly offensive odours were noted in the area around the
chambers and centrate storage tank, and as outlined in section 3.2.2 it is considered likely that
odours from these sources contribute to offsite odour impact risk.
To ensure that indicative baseline best practice for the industry is achieved, the following baseline
measures should be adopted:
Minimisation of the quantity of raw sludge stored onsite and the contact time with the air.
The level of reduction in odours required from this area of the works and the way in which this
should be achieved cannot be defined within this report. In order to assess this a detailed
assessment of the odour impact of the works as a whole and the relative contribution of the
various odour sources would be required. It is clear that a key aspect of this is however likely to
involve reducing or eliminating the odours from the sludge holding tank.
Depending on the level of odour reduction required Anglian Water may determine that it is
feasible to implement sufficient changes to the operational practices and sludge
storage/handling strategy at the works to substantially reduce the quantity of raw sludge which
is stored and which is in contact with the air (and hence odour emissions).
However changing the operational practices and sludge storage/handling strategy may not
provide a sufficient reduction in emissions in which case the implementation of enhanced odour
control techniques such as ‘cover and treat’ solutions may be required. In Odournet’s
experience the use of enclosed storage tanks (of substantially smaller size than the existing
sludge holding tank) and the extraction of the captured air to dedicated odour abatement plant
for treatment is commonly applied within the industry.
Implementation of a procedure to prevent venting of tanker air into the sludge holding tank.
To avoid the turbulence and odour release from the tank following the delivery of sludge by
tanker a procedure should be implemented to ensure that this is avoided.
Minimisation of the odour emissions from the raw sludge centrate chambers and centrate
storage tank. As for the sludge holding tank, it is likely that a substantial reduction in odours
from the centrate chambers and centrate storage tank will be required, and ‘cover and treat’
solutions may be required.
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Based on the information available it is considered possible that the implementation of further enhanced
odour control measures to the sludge handling and treatment area of the works (over and above the
baseline measures) would be justified. Once the shortcomings in the baseline measures identified have
been resolved, an assessment of the odour impact of the works would be required and the need for
enhanced measures could then be assessed.
4.7 General housekeeping and odour management plan
4.7.1 Indicative best practice
The main expectations for baseline measures for the control of odours in relation to general
housekeeping and the odour management plan are summarised in Table 8 below:
Table 8 Indicative best practice for odour control (baseline measures) in relation to general housekeeping and the OMP
Requirement Reference
Avoid spillages of any odorous material and ensure they are cleaned up without
delay, maintain the site in a clean condition
Defra COP on STW odour nuisance,
UKWIR BPM guidebook
The OMP should provide sufficient detail to allow operators to understand
operational procedures for normal and abnormal conditions
Defra COP on STW odour nuisance
The OMP should include a summary of operations, odour sources and receptors,
and details of site management responsibilities for faults/maintenance/
consumables etc.
Defra COP on STW odour nuisance
The OMP should include operation/management procedures for odour-critical
plant, details of operator training and plant maintenance/inspection
Defra COP on STW odour nuisance
The OMP should include spillage and maintenance procedures, emergency
breakdown and incident response planning procedures.
Defra COP on STW odour nuisance
The operator should prepare an Odour Response Procedure for odour critical
systems/plant, documenting the response for emergency breakdown. The
procedure should also cover the possibility of unusual or extreme conditions that
could affect odour impacts.
Defra COP on STW odour nuisance
The competence and training of staff in relation to odour relevant issues should
be confirmed and documented. Training should include awareness of
responsibilities for avoiding nuisance and action to minimise emissions during
abnormal conditions.
Defra COP on STW odour nuisance
A complaints procedure should be implemented to define the actions that should
be taken in the event of an odour complaint, including details of who is
responsible for specified actions and what is being done.
Defra COP on STW odour nuisance
The OMP may consider factors which have the potential to affect the process and
generation of odour (material inputs, process parameters, throughput,
development of anaerobic conditions)
UKWIR BPM guidebook
The OMP may consider factors which affect the ability to abate/minimise odour
(start-up/shut down, power failure, mechanical breakdown or other failures in
abatement plant)
UKWIR BPM guidebook
Provide details of all routine monitoring and inspections undertaken in relation to
odour and critical plant
General best practice measure
4.7.2 Review of housekeeping and the current OMP in comparison to best practice
During the site inspection no notable areas of spillage of odorous materials or housekeeping issues were
noted, and the spillage management provisions detailed within the OMP appear to be effective.
However from review of the odour management plan (Revision 12, dated 15th February 2017) it is clear that
it does not consider a number of the issues identified in the table above. On this basis the following
measures would be required to ensure that indicative baseline best practice is achieved:
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1. Update and expand the odour management plan, ensuring that the requirements outlined in the
table above are considered, including but not limited to:
o Provide a description of all operational procedures and tasks that are in place for
minimising odour impact risk during normal and abnormal conditions.
o Include a description of all nearby receptors.
o Expand section 8 (emergency breakdown and incident response planning) by including
all plant for which breakdown would have potentially substantial odour implications
(including the primary settlement tanks scrapers and desludge equipment and the
centrifuges). Consider incidents/factors which would affect the ability to
abate/minimise/manage odour (including the possibility of unusual or adverse
conditions that could affect odour dispersion and impacts). Consider factors which have
the potential to affect the process and generation of odour (material inputs, process
parameters, throughput, development of anaerobic conditions etc.).
o Include further detail of what odour-specific training is provided to site operatives (it is
unclear from section 5 whether the training listed includes relevant odour related
content). If required expand operator training to include relevant odour related
content, and ensure responsibilities for avoiding nuisance and action to minimise
emissions during abnormal conditions are considered.
o Include procedures for implementing and documenting the maintenance/servicing/
inspection/monitoring for all odour-critical plant.
o Include details of all monitoring undertaken under routine operations and following
complaints (such as onsite and offsite sniffing).
o Include of a clear step by step complaints response procedure defining the actions that
should be taken in the event of an odour complaint, including the specific investigations
that will be carried out and details of who is responsible for the specified actions.
2. Review and update Appendix 2, which states that there are currently no odour issues with
Letchworth WWTW.
3. Ensure that the odour management plan becomes a ‘live’ operational document that is updated
routinely and used by operational staff as a reference document in the day-to-day management
of odours at the works.
4.8 Enhanced measures
Once the baseline best practice measures in place at a site have been optimised, a key element of BPM
is that additional measures over and above the optimisation of the ‘baseline’ measures may be required
for if a nuisance is still caused (as discussed in section 2.1.2).
It is recommended therefore that once the operator has addressed the shortcomings in the ‘baseline’
measures identified in this report, the odour impact of the works should be evaluated at that stage and
the need for enhanced measures should then be assessed.
The DEFRA Code of Practice describes a procedure (‘Good Practice Approach’) that can be followed by
an operator if an odour nuisance is still being caused once the baseline measures have been
implemented. The procedure is designed to allow the operator to demonstrate that the choice of
abatement and control measures has been arrived at in a way that is technically justifiable and
practicable, having regard to financial implications.
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5 Summary of findings
The key findings of the study are summarised as follows:
1. The following sources of odour at Letchworth WWTW are considered most likely to contribute to
offsite odour impact:
Source Estimated contribution to offsite odour impact
Material in base of storm tank Low
Primary settlement tanks Low to moderate
Picket fence thickeners Low to moderate
Sludge holding tank High
Centrifuge buffer tank Low to Moderate
Centrate wells and returns tank High
Discharge points of cake conveyors Moderate
2. Most of the techniques employed to control odours from the wastewater reception and
preliminary treatment operations at the works are compliant with indicative baseline best
practice for the industry. The main exceptions are the absence of covers on the screenings and
grit skips, and the absence of grit washing. These deficiencies are however unlikely to have any
significant detrimental effect in terms of odour generation or offsite odour impact risk.
3. For the storm water handling operations, the procedure for undertaking prompt cleaning of the
tanks following emptying should be implemented to ensure that indicative baseline best practice
for the industry is achieved in this area.
4. For the primary treatment operations, the measures to control odours comply with indicative
baseline best practice with the following exceptions:
o Co-settlement of secondary solids (SAS) in the PSTs: In the absence of odour emissions
data for the tanks it is unknown whether this practice results in increased odour
emissions. However based on onsite observations and the fact that Anglian Water say
that this practice will cease in Summer 2017, it is not considered that this is an area
requiring immediate focus.
o Scheduling tank drawdown to avoid odour impact: A procedure for scheduling tank
draining for maintenance/cleaning to avoid odour impact by taking into account
forecast weather conditions should be implemented.
5. The techniques employed to control odours from the secondary treatment operations at the
works are compliant with indicative baseline best practice for the industry.
6. The approach to control odours in relation to sludge handling and treatment at the works falls
short of indicative baseline best practice for the industry in the following respects:
o The storage of large quantities (and surface area) of odorous raw sludge in the open
sludge holding tank is contrary to best practice.
o It appears that Anglian Water did not develop an effective and sustainable odour
management system/procedure to prevent odour impact resulting from the change in
operational regime in summer 2015 (cessation of sludge digestion and implementation of
the current sludge storage and handling operations).
Page 26 of 26
o The use of perfumed odour surfactant spray systems (in use around some of the sludge
plant) is not providing an effective level of abatement of odours from the sludge area of
the works and is not considered to represent best practice.
o Substantial odour emissions are associated with the unloading of sludge tankers as a
result of the discharge of air from the tankers into the sludge holding tank once the
tanker is empty of sludge.
o The containment provided to the sludge centrate chambers does not provide a
sufficiently high level of containment to prevent the emission of substantial and highly
offensive odours.
7. The following baseline measures should be adopted in order to achieve indicative best practice
from the sludge handling and treatment operations:
o Minimisation of the quantity of raw sludge stored onsite and the contact time with the air.
To assess the measures that would be required to achieve a suitable level of odour control
a detailed assessment of the odour impact of the works and the relative contribution of
the various odour sources would be required. The required measures are likely to involve
reducing or eliminating the odours from the sludge holding tank through changes to the
operational practices and sludge storage/handling strategy, or through enhanced odour
control techniques such as ‘cover and treat’ solutions.
o Implementation of a procedure to prevent venting of tanker air into the sludge holding
tank.
o Minimisation of the odour emissions from the raw sludge centrate chambers and centrate
storage tank.
8. It is possible that the implementation of further enhanced odour control measures to the sludge
handling and treatment area of the works (over and above the baseline measures) would be
justified.
9. The odour management plan at the works fall short of indicative best practice for the industry in
a number of respects, and the following optimisation measures are recommended:
o Update and expand the odour management plan, ensuring that the requirements
outlined in section 4.7 of this report are met.
10. Once the baseline best practice measures in place at the works have been optimised, it is
recommended that the operator should evaluate the odour emissions and impact of the works
and then assess the need for enhanced measures.