Mar 23, 2017 - which are most likely to contribute to offsite odour impact. 2. To review the ... of odour emissions data
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)
title: report number: project code:
Review of odour sources and best practical means at Letchworth Wastewater Treatment Works CBCO17A_10_FINAL CBCO17A
key words: client:
contact: contractor:
authors: approved:
Central Bedfordshire Council Priory House, Monks Walk, Chicksands, Shefford SG17 5TQ
Jane Mann 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] Paul Ottley on behalf of Odournet UK Ltd by
Mr. Nick Jones, director date: copyright:
23 March 2017 ©2017, Odournet UK Ltd
Copyright and Non-Disclosure Notice The contents and layout of this report are subject to copyright owned by Odournet UK Ltd (©Odournet UK Limited 2017) save to the extent that copyright has been legally assigned by us to another party or is used by Odournet UK Ltd under licence. To the extent that we own the copyright in this report, it may not be copied or used without our prior written agreement for any purpose other than the purpose indicated in this report. The methodology (if any) contained in this report is provided to you in confidence and must not be disclosed or copied to third parties without the prior written agreement of Odournet UK Ltd. Disclosure of that information may constitute an actionable breach of confidence or may otherwise prejudice our commercial interests.
2430 Page 2 of 26
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:
7.
8.
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.
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.
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
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
Summary of findings
25
4
5
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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 a ccredited 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 nuisan ce 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 costeffectiveness 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) wi ll 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 channel and tanks
Raw sewage (storm water) / open
Intermittent (very infrequent)
Storm tanks (sediment)
Storm sediment / open
Intermittent, dependent on cleaning
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
Distribution/mixing channels
Settled sewage and return activated sludge / open
Continuous
Activated sludge plant – anoxic & aerobic sections
Mixed liquors / open
Continuous
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
Storm water
Primary Treatment
Secondary Treatment
Sludge storage and handling
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 UKWIR Technical reference document as a minimum 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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4.3.3
Potentially odorous return liquors from the sludge dewatering are returned to the inlet works downstream of the storm weir.
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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4.4.3
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 (
