CBCO17A 04 draft - Fairfield Parish Council · contractor: Odournet UK Ltd 5, St. Margaret’s Street Bradford on Avon Wiltshire BA15 1DA 01225 868869 phone 01225 865969 fax Companies

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

[email protected]

authors: Paul Ottley

approved: on behalf of Odournet UK Ltd by

Mr. Nick Jones, director

date: 23 March 2017

copyright: ©2017, Odournet UK Ltd

mailto:[email protected]

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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









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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


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


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


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


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


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


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


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.


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.


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


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


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


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


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.


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.


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.





4.5 Secondary treatment operations


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


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


Ensure continuous recycling of activated sludge within the aerobic treatment

process


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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


4.5.2 Overview of provisions


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.


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.





4.6 Sludge handling and treatment operations


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


Minimise the quantity of sludge stored, particularly raw (untreated) sludge if

stored uncovered



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


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.


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.


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.

Page 23 of 26

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


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


Avoid spillages of any odorous material and ensure they are cleaned up without

delay, maintain the site in a clean condition


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.


The OMP should include operation/management procedures for odour-critical

plant, details of operator training and plant maintenance/inspection


The OMP should include spillage and maintenance procedures, emergency

breakdown and incident response planning procedures.


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.


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.


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.


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


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:

Page 24 of 26

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.

Page 25 of 26

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:









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.

CBCO17A 04 draft - Fairfield Parish Council · contractor: Odournet UK Ltd 5, St. Margaret’s Street Bradford on Avon Wiltshire BA15 1DA 01225 868869 phone 01225 865969 fax Companies

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