Project Overview

A630 Deephams Sewage Works Upgrade

Project Overview Report

Public consultation version


P1 A630 Project overview report (public consultation version)

Project overview report Page 2 of 54 June 2012

Revision Date File Name

Public consultation version

June 2012 Description Project overview report, published as part of phase 1 public consultation

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Table of Contents 1 EXECUTIVE SUMMARY .................................................................................................................. 6 2 INTRODUCTION .............................................................................................................................. 7

2.1 Background to the Deephams Sewage Works upgrade ........................................................... 7 2.2 Structure of this report ............................................................................................................... 7

3 BACKGROUND TO THE DEEPHAMS SEWAGE WORKS ............................................................. 8 3.1 Section objectives ..................................................................................................................... 8 3.2 Location and history .................................................................................................................. 8 3.3 The Deephams Sewage Works catchment ............................................................................... 9 3.4 Overview of the Deephams sewage treatment processes ...................................................... 10

4 THE NEED FOR THE DEEPHAMS SEWAGE WORKS UPGRADE ............................................. 14 4.1 Section objectives ................................................................................................................... 14 4.2 Summary of the need .............................................................................................................. 14 4.3 The discharge consent ............................................................................................................ 15 4.4 Flow to the works .................................................................................................................... 16 4.5 Upgrading and improving assets ............................................................................................. 17

5 APPROACH TO SITE SELECTION ............................................................................................... 18 5.1 Section objectives ................................................................................................................... 18 5.2 The Assessment Methodology ................................................................................................ 18 5.3 Strategic options review .......................................................................................................... 19 5.4 Site assessment ...................................................................................................................... 20 5.5 Treatment options assessment ............................................................................................... 28 5.6 Identification of preferred site .................................................................................................. 33

6 DEVELOPMENT OF THE PREFERRED SITE .............................................................................. 45 6.1 Section objectives ................................................................................................................... 45 6.2 Phasing of development .......................................................................................................... 45 6.3 Illustrative layouts .................................................................................................................... 45 6.4 Sludge strategy ....................................................................................................................... 47 6.5 Odour management ................................................................................................................ 49 6.6 Protection of the environment and amenity ............................................................................. 50

7 FUNDING THE UPGRADE ............................................................................................................ 52 8 NEXT STEPS ................................................................................................................................. 52




List of Tables Table 1: Evolution of discharge consent standards ............................................................................... 15 Table 2: Longlist of sites ........................................................................................................................ 23 Table 3: Comparison of secondary treatment technologies .................................................................. 31 Table 4: Indicative timetable .................................................................................................................. 54

List of Figures Figure 1: Plan showing location of Deephams Sewage Works ............................................................... 8 Figure 2: Plan of Deephams Sewage Works catchment ......................................................................... 9 Figure 3: Existing Deephams Sewage Works (at March 2010) ............................................................. 11 Figure 4: Deephams Sewage Works processes ................................................................................... 12 Figure 5: The assessment methodology ............................................................................................... 18 Figure 6: Plan of site search area ......................................................................................................... 21 Figure 7: Plan of longlist sites ............................................................................................................... 24 Figure 8: Plan of shortlist sites .............................................................................................................. 28 Figure 9: Illustrative Deephams Conventional Activated Sludge plant layout ....................................... 35 Figure 10: Illustrative Deephams IFAS / Conventional Activated Sludge layout ................................... 36 Figure 11: Illustrative Deephams MBR / Conventional Activated Sludge layout ................................... 37 Figure 12: Illustrative Meridian Business Park IFAS layout ................................................................... 38 Figure 13: Illustrative layout for infrastructure required at Deephams Sewage Works site in association

with any of the off site development options ................................................................................. 39 Figure 14: Illustrative Lee Valley Leisure Complex (Southern part) Conventional Activated Sludge

layout ............................................................................................................................................ 40 Figure 15: Illustrative Ardra Road Conventional Activated Sludge layout ............................................. 42 Figure 16: Illustrative Lower Hall Conventional Activated Sludge layout .............................................. 43 Figure 17: Illustrative aerial view of Deephams Conventional Activated Sludge layout ........................ 46 Figure 18: Illustrative aerial view of Deephams IFAS / Conventional Activated Sludge layout ............. 46 Figure 19: Illustrative aerial view of Deephams MBR / Conventional Activated Sludge layout ............. 47 Figure 20: Illustrative potential Deephams THP Sludge Treatment layout ........................................... 48 Figure 21: Illustrative aerial view of potential Deephams THP Sludge treatment layout ....................... 48 Figure 22: Illustration of existing Deephams Sewage Works odour generation .................................... 49 Figure 22: Illustration of potential future Deephams Sewage Works odour generation ........................ 50 Figure 24: Illustration of potential odour reductions at Deephams Sewage Works ............................... 51




GLOSSARY OF TERMS/KEY TO ABBREVIATIONS

Amm. Abbrev. Ammonia – a pollutant within wastewater/effluent which can be harmful to fish

BAFF Abbrev. Biological Aerated Flooded Filter – a compact secondary sewage treatment process

BOD Abbrev. Biochemical Oxygen Demand – a measure of the pollutant potential of sewage

CHP Combined Heat and Power

CAS Abbrev. Conventional Activated Sludge – a common secondary sewage treatment process

EA Abbrev. Environment Agency. Environmental Regulator of the water companies.

Effluent

The final treated wastewater which is discharged from the sewage works into a watercourse

EIA Abbrev. Environmental Impact Assessment

ES Abbrev. Environmental Statement

FRA Abbrev. Flood Risk Assessment

FST Abbrev. Final Settlement Tank

GLA Abbrev. Greater London Authority

IFAS Abbrev. Integrated Fixed Film Activated Sludge – a secondary sewage treatment process

LB Enfield Abbrev. London Borough of Enfield

MBR Abbrev. Membrane Bio-Reactors – a secondary and tertiary sewage treatment process

NPS Waste Water Abbrev. National Policy Statement for Waste Water

NSIP Nationally Significant Infrastructure Project

P Abbrev. Phosphorus – – a pollutant within wastewater/effluent comes from household detergents and agricultural fertilisers. Phosphorus can lead to excessive plant growth and decay (eutrophication) and can damage fish and habitats.

PE Abbrev. Population Equivalent – the pollution load to a sewage works from various different sources such as industrial, commercial and residential areas, expressed in terms of the number of people that would generate the same polluting load in household sewage

PST Abbrev. Primary Settlement (or Sedimentation) Tank

RGF Abbrev. Rapid Gravity Filters – a tertiary sewage treatment process to remove suspended solids

SBR Abbrev. Sequencing Batch Reactors – a secondary sewage treatment process

SS Abbrev. Suspended Solids – a measure of the pollutant potential of effluent

THP Abbrev. Thermal Hydrolysis Plant – a sludge treatment process




1 Executive summary

1.1.1 Deephams Sewage Works, located in Enfield in north east London, is the ninth largest sewage works in England. It treats sewage collected from within its catchment and discharges treated effluent that flows into the Salmon’s Brook, a tributary of the River Lee, in accordance with a discharge consent set by the Environment Agency.

1.1.2 The Environment Agency has issued a new discharge consent that requires us to make improvements to the quality of the discharge. The Deephams Sewage Works Upgrade (the upgrade) will improve the quality of effluent (treated wastewater) discharged from the Deephams Sewage Works into the Salmon’s Brook. It will accommodate growth within the catchment to at least 2031, and improve infrastructure at the sewage works, much of which is now over 50 years old.

1.1.3 This report provides a project overview. It describes the existing Deephams Sewage Works and its catchment, and explains the background to the project and the need for the upgrade. The report then outlines the process that we have followed in assessing options for delivering the upgrade, cross referring to more detailed documents that explain the stages of work in more detail.

1.1.4 The upgrade has to be delivered whilst the existing sewage works remains operational to treat the sewage to the existing consent levels. This places significant engineering constraints on our ability to implement the upgrade within the boundaries of the existing sewage works site. We have therefore considered the use of a number of treatment technology options to deliver the upgrade both on the existing site, and on a new site, within these engineering constraints.

1.1.5 Having first considered strategic options for the upgrade, we identified and then assessed potential sites for the upgrade, including the existing Deephams Sewage Works site and other sites within an approximate radius of 3km of that site. A total of 22 potential sites were assessed. We also assessed a number of possible alternative treatment technology options that could be used for the upgrade. From this work we identified a shortlist of 5 sites, for which we assessed property, legal, planning, environmental and engineering issues in more detail.

1.1.6 The conclusion of this work is the identification of our preferred option for the upgrade, which is to develop the sewage treatment works within the boundaries of the current Deephams Sewage Works site.

1.1.7 We anticipate building the upgrade in two or three main phases in order to keep the works running throughout the construction period. With the development of any solution on the existing Deephams Sewage Works site, we will endeavour to implement the most sustainable and value added solution. Therefore, wherever possible, we will look to reuse any existing assets within the development.

1.1.8 The upgrade is likely to take up to six to seven years to complete and, subject to securing permission, the main construction work is expected to start in 2015.

1.1.9 No decisions have been made about the upgrade and this is the first of two phases of consultation where you can have your say. Your feedback will help us find out about any local issues that we need to be aware of and allow us to amend our plans, where we can, for how we will design and build the upgrade.




2 Introduction 2.1 Background to the Deephams Sewage Works upgrade

2.1.1 This report provides an overview of the Deephams Sewage Works Upgrade project.

2.1.2 Deephams Sewage Works, located in Enfield in north east London, is the ninth largest sewage works in England. It treats sewage collected from within its catchment and discharges treated effluent into the Salmon’s Brook, a tributary of the River Lee, in accordance with a discharge consent set by the Environment Agency.

2.1.3 The Deephams Sewage Works Upgrade (the upgrade) will improve the quality of effluent (treated wastewater) discharged from the sewage works into the Salmon’s Brook. The Environment Agency has issued a new discharge consent that requires us to make improvements to the quality of the discharge. These improvements are necessary to help meet the requirements of three European Directives – the Urban Wastewater Treatment Directive, the Freshwater Fish Directive and the Water Framework Directive.

2.1.4 In addition to delivering water quality improvements, the upgrade will cater for population growth within the catchment to at least 2031, and improve current infrastructure at the sewage works, much of which is now over 50 years old. It will provide sewage treatment for north London that is sustainable in the long term, responding to the challenges of climate change.

2.1.5 Parliament approved the National Policy Statement for Waste Water in March 2012. This establishes the need for the upgrade, and identifies it as a potential Nationally Significant Infrastructure Project (NSIP).

2.1.6 This Project Overview report, published as part of the Phase 1 public consultation process, gives an overview of the process by which we have identified our preferred site for the upgrade and assessed potential treatment technologies.

2.1.7 This report provides an overview of the upgrade for readers who wish to understand the background to the project, and the process by which sites and treatment options have been considered to date. This report refers the reader to other consultation documents if they wish to gain more detailed background information on a particular aspect of the process.

2.1.8 The comments and feedback that we receive through the consultation process will inform and influence our final selection of a preferred site and treatment option for the upgrade. It is important to recognise that potential treatment options are subject to further work as part of our ongoing scheme design development, including input from potential contractors and suppliers.

2.2 Structure of this report

2.2.1 Section 3 of this report provides a brief overview of the Deephams Sewage Works and the treatment processes, and section 4 explains the ‘need’ for the project. Section 5 explains the assessment methodology that we have followed, and summarises the outcomes of each stage of our work to date, through setting a strategy, reviewing possible site and technology options, and concluding on a preferred site.




2.2.2 Section 6 provides a brief explanation of the proposed development of the preferred site, including phasing, indicative layouts, and our approach to odour management and environmental considerations. Section 7 outlines how the project would be funded. Finally, section 8 explains the anticipated future timeline and proceeding consultation process.

3 Background to the Deephams Sewage Works

3.1 Section objectives

3.1.1 This section provides a brief background to the existing Deephams Sewage Works and the process that it undertakes in the treatment of sewage.

3.2 Location and history

3.2.1 Deephams Sewage Works is one of our six main sewage works that serve London. It is located off Picketts Lock Lane in Edmonton, as shown in Figure 1 below. It is the ninth largest sewage works in England and serves population equivalent of 885,000 people (as of 2010).

Figure 1: Plan showing location of Deephams Sewage Works




3.2.2 Sewage treatment has been undertaken in this part of Edmonton since the 1870s when the first Edmonton sewage farm was developed on adjoining land. The Deephams Sewage Works was largely constructed on the current site in the 1950s and 1960s.

3.2.3 The works was improved in the late 1980s when the inlet works (where raw sewage enters the works) was upgraded to provide screening for storm flows into the works. A new sludge digestion facility was added in the mid 1990s. Improvements constructed in 2011-12 will further improve the inlet works and increase storm storage capacity. They will also improve the water quality in the effluent discharged into the Salmon’s Brook from the site.

3.2.4 However, despite the various improvements over the last 30 to 40 years, and those under construction, the current sewage works is predominately the works that was first constructed in the 1950s and 1960s. The ageing plant is under increasing pressure to meet and maintain modern treatment standards.

3.3 The Deephams Sewage Works catchment

3.3.1 The catchment which Deephams Sewage Works serves extends over large parts of north east London and northwards beyond the M25, as indicated in Figure 2 below.

Figure 2: Plan of Deephams Sewage Works catchment




3.3.2 We have an obligation to take away used water from domestic and commercial premises within the catchment. This includes discharges from toilets, wash hand basins, baths, washing and dish washing machines. These discharges are termed ‘sewage’ or ‘wastewater’ and are conveyed from their source to a site for treatment in pipes called ‘sewers’. We also take away some discharges from industrial premises, though these flows may require specialist pre-treatment at source prior to discharge to a sewer.

3.3.3 In some areas, rainwater falling on roofs, roads and pavements is collected in surface water sewers which discharge directly to local watercourses. However, in some areas, including in parts of the Deephams Sewage Works catchment, surface water and sewage are mixed together in the sewers before flowing to the treatment works. These date from before 1936 and are called combined sewers. Many of them were originally watercourses that were turned into pipes or culverts and incorporated into the sewerage system.

3.3.4 When a toilet is flushed or a sink emptied, the wastewater goes down the drain and into a pipe which takes it to a larger sewer under the road. This sewer then joins our network of other sewers that take the wastewater to the sewage treatment works. Usually this involves pumping. At the treatment works the sewage is treated in a variety of cleaning processes to remove the solids and treat the sewage so that the effluent can be put safely back into the river system.

3.3.5 Raw sewage arising in the Deephams sewerage catchment area arrives at the sewage works via three main gravity trunk sewers. The Lee Valley Sewer (serving Waltham Abbey, Cheshunt, Cuffley and north east Enfield), the Barnet High Level Sewer (serving east Barnet and south and west Enfield) and the Tottenham Low Level Sewer (serving Tottenham, Wood Green and south east Enfield). There is also a further Chingford Branch Sewer serving the western side of the London Borough of Waltham Forest, which connects into the Tottenham Low Level Sewer just south of the Deephams Sewage Works.

3.4 Overview of the Deephams sewage treatment processes

3.4.1 The Deephams Sewage Works is a conventional sewage treatment works. Sewage enters the works through an inlet works, where rags and grit are removed from the incoming sewage, followed by Primary Settlement Tanks (PSTs), where solids are settled out and sent to the sludge treatment plant.

3.4.2 The remaining sewage is then treated using a Conventional Activated Sludge process. This is a suspended-growth system where the biomass (activated sludge) is mixed with the sewage and then dissolved oxygen is used to promote the growth of bacteria that substantially removes organic pollution. This process takes place in aeration tanks, followed by Final Settlement Tanks (FSTs). The treated effluent from the FSTs is then discharged to the final effluent channel and the Salmon’s Brook.

3.4.3 At Deephams, primary and secondary treatment takes place in three separate and independent process streams. Streams A and B each consists of six PSTs, four aeration lanes and sixteen FSTs. Stream C consists of four PSTs, four aeration lanes and sixteen FSTs. Stream ‘A’ was installed in the early/mid 1950s and extended in the 1960’s, Stream ‘B’ was installed in 1956 and extended in 1966, Stream ‘C’ was installed from 1966. Figure 3 below identifies the location of the main processes on the Deephams Sewage Works site.




Figure 3: Existing Deephams Sewage Works (at March 2010)

3.4.4 Solids from the primary and final settlement tanks are pumped to the sludge treatment plant, where they are treated by anaerobic digestion (broken down by bacteria which live without oxygen) in digestor tanks. After further treatment and removal of water from the sludge, the treated sludge solids are recycled as fertiliser on agricultural land and the methane produced during the digestion process is reused as a renewable energy source on site, as fuel in Combined Heat and Power (CHP) engines.

3.4.5 The following text and Figure 4 (below) explain the treatment processes at the Deephams Sewage Works.

Inlet works and screening

3.4.6 The three main sewers arriving at the site are located deep underground so the flows need to be pumped up to the inlet works. The inlet works provide screens to remove coarse and finer debris including paper, plastics, cloth and larger items from the incoming raw sewage. These large objects are removed as they might otherwise block or damage equipment or be unsightly if allowed back into the watercourse. Materials removed by screening are washed to remove faecal matter and discharged into skips for disposal to a landfill site.




Figure 4: Deephams Sewage Works processes

3.4.7 Sewage flows into the works normally contain a lot of grit, especially in areas with combined sewers that receive rainwater flows from roads. Grit is removed in a specialist physical treatment process and then washed to remove faecal matter and the grit is then either recycled for use as building material or sent to a landfill site. The screened and degritted sewage is then passed into the works for further treatment.

Storm storage

3.4.8 The amount of sewage arriving at a treatment plant increases significantly during periods of wet weather as it includes rainwater. During storms or at times of high flow when the capacity of the treatment works is exceeded the excess storm sewage flows are separated upstream of the inlet works screens and a proportion passes through dedicated storm screens prior to flowing into storm tanks. Once the flows in the incoming sewer have reduced to normal levels, the content of the storm tanks is returned to the inlet works, and is then treated.

3.4.9 Should the storm continue such that the storm tanks become full, they are then permitted to discharge an overflow of partially treated (screened, degritted and settled) sewage via a culvert to the Salmon’s Brook.




3.4.10 Whilst these flows are not fully treated, they are significantly diluted by the high volumes of rainwater that enter the sewage works during storm events. Hydrogen peroxide dosing is also used to ensure that the overflow is well-oxygenated and so does not have a significant effect on Salmon's Brook. The discharge of these storm flows to the Salmon’s Brook is regulated by the Environment Agency under the terms of the discharge consent for the sewage works.

Primary treatment

3.4.11 At the primary treatment stage the sewage receives physical treatment to remove organic solid matter or human waste. This is achieved by passing the flow into large tanks where the solid material is allowed to settle to the floor of the tanks. This settled material is called ‘sludge’. Chemicals may be added to encourage the sludge to settle quickly. The sludge is collected from the floor of the tanks by mechanical scrapers which travel along the base of the tank to push the sludge into hoppers set into the floor where it is stored prior to transfer to the sludge treatment plant.

3.4.12 Fats and greases, which float in the sewage, collect on the surface of these tanks. This material, which is termed ‘scum’ is removed by skimming it from the surface of the tank. After an appropriate period of storage, to allow the majority of the organic solids to settle to the tank floor, treated sewage is discharged over a weir at the outlet end of the tank and is taken to the next stage of the treatment process.

Secondary treatment

3.4.13 The primary treatment stage removes most of the larger solid organic material from the flow, however there will still be an amount of fine suspended and some dissolved solid material. This material is broken down and removed by biological action in the secondary treatment stage.

3.4.14 Bacteria which break down the pollution in the incoming flow are grown in tanks which are called ‘aeration lanes’. These tanks have air pumped through them to encourage the bacteria to grow and consume the pollution. The aeration lanes are sized such that the bacteria have sufficient time to consume the pollution before flows are passed to the next treatment stage.

3.4.15 Following treatment in the aeration tanks the biomass mixed with the partially treated sewage is discharged into the Final Settlement Tanks (FSTs) There the activated sludge is settled out for recycling to the head of the aeration tanks leaving the liquid fraction of the flow to be discharged over weirs around the tank perimeter. This treated effluent is then discharged to the final effluent channel and into the Salmon’s Brook. Periodically, part of the biomass from the FSTs is drawn off as surplus activated sludge which is dewatered and treated with the rest of the sewage sludge.

Tertiary treatment

3.4.16 Where a high quality of treatment is required before effluent can be discharged to a watercourse or river, it is necessary to provide additional treatment to the flow, known as tertiary treatment. This involves filtration of the flow to remove fine particles using either sand filters or a membrane with a fine woven mesh. Effluent passing through the sand filter or membrane is discharged to the river. Periodically the sand filter or membrane is taken out of service and washed to remove accumulations of sediment which has been collected. A disc filter tertiary treatment plant is currently under construction to improve effluent quality, and will be completed in 2012.




Sludge treatment

3.4.17 Sludge and scum removed from the primary treatment stage is processed to remove water and stored on site in tanks to allow bacteria and pathogen levels to reduce naturally to acceptable levels. This process is accelerated by the digestion of sludge in heated tanks, which uses bacteria to break down the cell structure of the sludge. This helps to remove water from the sludge, increasing the rate at which bacteria and pathogens are destroyed and producing sludge gas, which can be collected for use as a fuel. The ‘biogas’ recovered and used on site at Deephams in a Combined Heat and Power (CHP) plant generates approximately 2.5MW of renewable energy.

3.4.18 Sludge from secondary treatment processes is collected in the final settlement tanks and recycled back into the aeration lanes to maintain a healthy level of bacteria. Periodically it is necessary to remove a proportion of these bacteria to prevent bacteria concentrations rising too much. These surplus bacteria are thickened in a similar manner to that used for sludge arising from primary treatment stages and then subjected to further treatment (digestion).

3.4.19 Following treatment, the resulting sludge is transported to farmland where it is recycled as a nutrient-rich fertiliser - known as biosolids.

4 The need for the Deephams Sewage Works upgrade


4.1.1 This section provides details on the ‘need’ for the project and the approach we are undertaking to provide a solution to this requirement.

4.2 Summary of the need

4.2.1 The need for the upgrade is set out in the National Policy Statement for Waste Water, and can be summarised as the requirement to respond to:

• an increase in the discharge consent requirements

• an increase in flow to the works:

• a requirement to provide sufficient treatment capacity to meet population growth within the catchment already served by the works.

• a requirement to respond and adapt to the challenges of climate change.

• ageing infrastructure under pressure to meet and maintain treatment standards

4.2.2 The upgrade will also deliver wider social or environmental benefits, for example reduced odour emissions and, depending on the option selected for implementation, increased renewable energy generation.

4.2.3 Sections 4.3 to 4.5 below provide further detail on the need for the upgrade.




4.3 The discharge consent

4.3.1 The Deephams Sewage Works operates under the terms of a discharge consent issued by the Environment Agency. This sets standards for water quality and identifies the volumes of treatment and storm storage capacity that must be provided. Since the first discharge consent was issued for the Deephams Sewage Works in 1957, the consent has been tightened a number of times by the Environment Agency (or its predecessors).

4.3.2 The environmental need for the proposed upgrade is driven by the requirement to improve water quality in the River Lee by meeting a tighter effluent discharge consent. Improving the quality of treated effluent discharged to the Salmon’s Brook will help to improve the natural environment in and around the Brook and further downstream in the River Lee.

4.3.3 This requirement comes from three European Directives – the Urban Wastewater Treatment Directive, the Freshwater Fish Directive, and the Water Framework Directive. It is being implemented by a new discharge consent set by the Environment Agency.

4.3.4 The new discharge consent requires us to develop an improved sewage treatment process that will deliver identified improvements to the quality of the effluent discharged to the Salmon’s Brook and the River Lee. The new consent requires the treated effluent to achieve the quality standards set out in Table 1 – shown in the context of current and previous discharge consents for the Deephams Sewage Works.

Table 1: Evolution of discharge consent standards

Timeline of consent changes

Suspended solids (mg/l 95%ile)

BOD (mg/l 5d-ATU 95%ile)

Total Ammonia (mgN/l 95%ile)

Total Phosphate (mg/l annual average)

Original since 1957 30 20 - -

1999 30 20 8 -

2004 30 10 4 -

2010 30 10 3 -

2012 30 10 3 1

New Consent 10 5 1 1

4.3.5 The new consent standard requires a step change in treated effluent water quality, compared to the preceding standards. Suspended solids and Ammonia must be reduced by two thirds and Biochemical Oxygen Demand (BOD) must be halved. The upgrade will ensure that these new standards can be achieved.




4.4 Flow to the works

4.4.1 The second requirement for the upgrade is to meet the demands of a growing population. The Deephams Sewage Works catchment area is extensive, including land within the London Boroughs of Enfield, Haringey, Waltham Forest and Barnet and also extending into the counties of Hertfordshire (Borough of Broxbourne and Welwyn Hatfield Borough Council) and Essex (Epping Forest District Council).

4.4.2 The London Plan (July 2011) anticipates that there will be significant growth and regeneration in north London, including through planned redevelopment and enhancement within identified opportunity areas. London Plan policies promote sustainable growth, with the provision of necessary infrastructure and services to support existing and new sustainable communities, including access to employment and services, exemplary approaches to water, energy and water use, management and provision. The London Plan also recognises that adequate sewage treatment provision is a key component to achieving sustainable communities.

4.4.3 The main focus of growth and regeneration within the Deephams catchment will be within that part of the catchment that lies within the Upper Lee Valley Area of Opportunity. The 2011 London Plan anticipates that across the 3,884 hectares that make up the Opportunity Area, 15,000 jobs will be created and a minimum of 9,000 new homes will be constructed in the plan period up to 2031. In addition to this, each Borough has other development proposals set out within their respective Local Plans or Local Development Frameworks.

4.4.4 The existing sewage infrastructure at Deephams Sewage Works does not have capacity to accommodate the forecast increase that is required. Therefore the infrastructure needs to be upgraded in order to deliver the planned capacity increase. The constrained nature of the site limits the potential for expansion of the existing works.

4.4.5 In providing this capacity, regard must also be had to the challenges of climate change. With the increased probability of wetter winters, more intense rainfall events and greater climate variability, there will be greater pressure on public sewer systems and sewage treatment works. The heavier the rain, the greater the flow the sewers have to carry and the larger the storm capacity necessary at the sewage works. Sewage works will need to accommodate the risk that they may become overwhelmed in storm events, leading to water quality and flooding impacts. The constraints of the existing infrastructure at the Deephams Sewage Works are expected to increase as a result of these challenges.

4.4.6 The approach to accommodating growth in the upgrade project is consistent with London Plan Policy 5.14. This states “The Mayor will work in partnership with the Boroughs, appropriate agencies within London and adjoining local planning authorities to: … ensure that London has adequate and appropriate wastewater infrastructure to meet the requirements placed upon it by population growth and climate change…..”.

4.4.7 The Deephams Sewage Works currently serves a Population Equivalent (PE) of 885,000 (2010). To meet the level of growth proposed within the catchment and to fulfill its statutory duty to treat and dispose of sewage, we forecast that the capacity of the sewage works needs to be increased to serve 941,000 PE. This would accommodate growth within the catchment to at least 2031.




4.5 Upgrading and improving assets

4.5.1 The majority of the three sewage treatment streams at Deephams (streams A, B and C) date back to the 1950s and 1960s. Over the years, the sewage works has been upgraded and expanded to meet more stringent discharge standards and to treat greater quantities of sewage generated by growth within the catchment. However, a significant proportion of the infrastructure at Deephams is now over 50 years old.

4.5.2 The need to maintain high environmental standards of sewage treatment and effluent discharge is becoming more challenging to achieve with the existing infrastructure at Deephams Sewage Works. This is particularly the case for treatment streams ‘A’ and ‘B’, which date back to the 1950s. The combination of age and the condition of the assets means that it is increasingly difficult for us to effectively and efficiently treat the sewage arising within the catchment both now and in terms of meeting future needs.

4.5.3 Our main concerns relating to the current infrastructure are summarised below:

• The geometry of the existing tanks is not in keeping with modern design standards. Streams A and B’s Primary Settlement Tanks (PSTs) have no slopes on the floors and the aeration tanks are much shallower than modern designs require. The latter has a significant impact on the volumes of air required to promote biological degradation of the sewage, significantly increasing the operating costs of the blowers

• Poor hydraulic design of, pipes and channels within the site. These have low flow velocities which allows suspended material to be deposited. This can inhibit the flow of sewage through the treatment processes and result in an uneven distribution between treatment streams. This has a significant impact on the efficiency of the processes, a particular concern once the new consent is applied

• Poor structural condition of the PSTs and aeration lanes

• Excessive amounts of screenings (rag) passing the fine screens within the sewage treatment process affecting the efficiency of flow distribution and hence the amount of flow that can be treated

• Excessive fat, oil and grease within the sewage treatment process affecting the efficiency of flow distribution and hence the amount of flow that can be treated

• Reduced capacity of the sludge treatment works due to the poor performance of the inlet works – rag and floating debris is passing into the treatment processes

• Poor mechanical operation of the PSTs and ineffective sludge removal

• Flooding during storm events raises the water level of the Salmon’s Brook which backs up the level of effluent in the final effluent culvert and results in surcharging of the Final Settlement Tank (FST) weirs. This causes a process failure leading to solids from the FSTs entering the final effluent that is subsequently discharged

• Fluvial (river and stream) flooding (approximately 4-6 times a year) of the storm tanks arising from the backing up of the stormwater discharge channels

4.5.4 We are continuing to invest in improvements to Deephams Sewage Works, including works under construction, but these improvements will not meet the need identified above. The upgrade will provide sewage treatment for north London that is sustainable in the long term, including the improvement or upgrade of infrastructure much of which is over 50 years old.




5 Approach to site selection


5.1.1 This section explains how we have established a robust methodology to assess potential strategies for meeting the need for the upgrade. It details the implementation of the methodology, assessing high level strategies for solving the problem, outlining the identification and assessment of sites where the solution could be implemented, and the consideration of potential treatment technologies. The section concludes with the identification of a preferred site.

5.2 The Assessment Methodology

5.2.1 We have developed an assessment methodology for selecting the most appropriate option for delivering the upgrade, including the assessment of site and treatment technology options.

5.2.2 The methodology allows for the assessment of a range of strategies for meeting the need for the upgrade. It allows for the consideration of potential locations for new sewage treatment infrastructure. The construction of the upgrade could be on the existing site, although the site would have to remain operational throughout the upgrade, or it could be on a new site nearby. A suitable treatment technology option has to be selected, to meet the treatment requirements of the new discharge consent within funding, programme, treatment technology and physical site constraints.

5.2.3 The proposed methodology is a multi-stage process, with stages undertaken both in parallel and sequentially. Figure 5 below illustrates this process diagrammatically.

Figure 5: The assessment methodology




5.2.4 The assessment process firstly allows an initial review of strategic options for meeting the need for the upgrade to be undertaken. From this, an assessment of treatment options is undertaken, alongside an assessment of potential development sites. A more detailed assessment of a shortlist of sites is then undertaken to enable a preferred site to be identified. The preferred site is then published for a first phase of public consultation and stakeholder engagement.

5.2.5 Alongside, and following on from, the first phase of consultation we will undertake scheme design development work and further work on treatment options. This will confirm the feasibility of our preferred site and treatment technology options for delivering the upgrade. A second phase of public consultation will then be held on our proposals and designs for the upgrade and information on the potential environmental impacts of the upgrade and our plans for mitigating them. An application for permission to build the upgrade will then be submitted.

5.2.6 An essential part of the methodology is an ongoing process of review and checking the validity of previous assessments through feedback loops at all stages of the process. This enables the assessment of sites and options to remain valid as information changes and new information is obtained. Thus, where necessary, stages in the process may be repeated (or ‘back-checked’) in order to take account of new information or other changes of circumstance.

5.2.7 The Assessment Methodology is explained in more detail in the separate “Assessment Methodology report”.

5.3 Strategic options review

5.3.1 A list of strategies which might have the potential to meet the need for the upgrade was established through discussion within the project team and with stakeholders.

5.3.2 A review of the following strategic options was undertaken:

• Strategy 1: reduce or remove enough of the incoming flow to Deephams Sewage Works to permit each of the existing three treatment streams to be turned off and upgraded in turn.

• Strategy 2: combining various options from Strategy 1 to reduce or remove incoming flow to Deephams Sewage Works.

• Strategy 3: build part of a new sewage works on another site and transfer flow between the new plant and the existing Deephams Sewage Works for treatment.

• Strategy 4: construct on Deephams Sewage Works.

5.3.3 These involve either removing enough of the sewage flows into Deephams Sewage Works (165Ml/d) to allow one of the three existing treatment streams to be turned off and upgraded at a time; building new treatment plant on another site; or reconfiguring the existing site and building a new plant to treat part of the flow before upgrading the other treatment streams in turn. Each of the strategies has to ensure that treatment of the sewage flows from the Deephams catchment is maintained throughout.

5.3.4 The strategic options were subjected to a high level review to identify those potentially viable options that met the need for the upgrade. This assessed the opportunities, risks and uncertainties associated with each strategic option, concluding whether each option should go forward for further consideration in the appraisal process. Clear conclusions were reached for each option, leading to a reduced list of potential options being taken forward for more detailed assessment.




5.3.5 The conclusions of this work were that only the following options were considered to have sufficient potential for delivering the upgrade to justify further more detailed assessment:

• Strategy 3: build part of a new sewage works on another site and transfer flows between the new plant and the existing Deephams Sewage Works for treatment.

• Strategy 4: construct on Deephams Sewage Works.

5.3.6 Details of the strategic options and their assessment can be found in the separate “Strategic options review stage 1 report”.

5.4 Site assessment

Context

5.4.1 The existing Deephams Sewage Works must continue to operate throughout the construction of the upgrade. This places significant engineering constraints on our ability to implement the upgrade within the boundaries of the existing sewage works site.

5.4.2 Having considered strategic options, an assessment of sites that could potentially deliver these viable strategies (strategies 3 and 4) has been undertaken. Details of the sites and the assessment outcomes can be found in the Site Assessment Report. The upgrade will either have to be implemented alongside the existing sewage works, on a phased basis, or a new site will need to be found for the upgrade.

5.4.3 We identified a longlist of potential sites within the area for assessment to determine whether they could be developed for sewage treatment use. This process is described below.

Site search area

5.4.4 There will be a need for a significant amount of energy to pump preliminary treated sewage from the existing Deephams Sewage Works Inlet works to any new site for treatment, and for the return of treated effluent and sludge to the existing Deephams Sewage Works. This energy is additional to the energy necessary for the treatment processes themselves.

5.4.5 This additional energy is wasted – in that aside from transferring the raw sewage, treated effluent and sludge, it has no beneficial treatment purpose. In addition to the financial cost to our customers, this energy use involves significant CO2 emissions. The further the new site is from the existing sewage works, the greater the need to pump and the greater the energy use. Pumping increases both with distance from the Deephams Sewage Works, and with increases in elevation from Deephams.

5.4.6 In line with Government policy on sustainable development, we defined the search area having regard to the additional energy requirements associated with pumping to and from the new site. The greater the distance from Deephams, the greater the ongoing energy use and financial cost of pumping. In addition, there will be additional construction costs and likely environmental effects from construction and operation of the transfer tunnel / pipeline.




5.4.7 We adopted a threshold point to define the boundary of the site search area, where the required energy for the transfer of flows is greater than 20 per cent of the existing mains energy requirement for the current Deephams Sewage Works treatment processes. Adopting a 20 per cent threshold limits the search area radius to an approximate maximum of 3km from the existing Deephams Sewage Works site. The resulting site search area is shown in Figure 6.

Figure 6: Plan of site search area

5.4.8 The search area is focused within the London Borough of Enfield, but extending into London Borough of Waltham Forest to the east and London Borough of Haringey to the south. The search area extends along the floor of the Lee Valley, to its maximum distance of 3km to the north and south of the existing sewage works. To the east and west the search area is constrained by the increased elevation up the sides of the valley, particularly to the east.

5.4.9 We presented this approach to identifying a site search area to the Deephams Stakeholder Forum (see our Consultation Strategy document for more information on the Forum). Comments on the general approach and any alternatives that stakeholders wished to suggest were invited. No negative comments were received.




Identifying potential sites

5.4.10 A longlist of sites was created using a desk based search for sites of a minimum of 8 hectares within the above search area. A site of 8 hectares is considered to be about the minimum area needed for the more compact potential treatment technologies to treat 100 per cent of the future flows.

5.4.11 Existing business and employment sites, allocated strategic industrial sites, allocated place shaping priority areas and area action plans identified in the relevant adopted Unitary Development Plans, Core Strategies and other Development Plan Documents were all examined to identify potential sites.

5.4.12 The search also included areas of formal and informal open space, and areas of land that we already own.

5.4.13 Identified sites were assessed against a number of different property, legal, planning and environmental, and engineering criteria (see separate “Assessment Methodology report”). The criteria ensured that a sufficiently broad base of information on the potential sites was collected and assessed to enable a shortlist of sites to be formed robustly. Key property, legal, planning and environmental and engineering issues that could constrain or affect the development of treatment processes on the sites were identified through this process. This includes relevant planning and environmental factors that are referred to in the London Plan and Development Plan Documents. These criteria were discussed with stakeholders through the Deephams Stakeholder Forum.

5.4.14 We applied only limited exclusionary criteria when seeking to identify potential sites, so as to ensure that an appropriately wide list of potential sites were available for assessment. These criteria were:

• Sites in a residential use

• Sites in use as a school or hospital

• Cemeteries

Site assessment outcomes

5.4.15 From the site assessment work that has been undertaken, we reached the following conclusions:

• There are few identified sites within the Upper Lee Valley that are considered suitable to accommodate the upgrade.

• Within the identified site search area, a total of 22 potential sites above 8ha were identified, including the Deephams Sewage Works site.

5.4.16 These 22 sites are listed in Table 2 below and shown in Figure 7.




Table 2: Longlist of sites

Site name Area (ha)

Summary of identified current use

1 Deephams Sewage Works 35 Existing sewage works site (inclusive of preliminary treatment and sludge treatment areas)

2 Durants Park, Hertford Road 16 Public open space – park, including pitches, tennis courts and grassed areas

3 Brimsdown Industrial Estate (Jeffreys Road)

23 Mixed industrial and employment uses

4 Brimsdown Industrial Estate (East Duck Lees Lane)


5 Woodall Road industrial area 9 Industrial estate and former gasholders site (Academy school proposed on part of the site)

6 Meridian Business Park (Morson Road)


7 Elliots Field Allotments, Aspen Way

8 Allotments

8 Jubilee Park 20 Public open space – park including sports pitches, ball courts, playgrounds and pitch and putt

9 Lee Valley Leisure Complex Golf Course (southern part)

23 Southern part of larger golf course, and campsite

10 Chingford RFC and golf driving range

15 Sports pitches, rugby football club, driving range and adjoining car showroom and retail uses

11 Ardra Road North Industrial Area

21 Mixed employment uses

12 Pymmes Park 16 Public open space – sports pitches, woodland, open water and related uses

13 Montagu Industrial Estate 12 Mixed industrial and employment uses

14 Edmonton Eco Park 15 Edmonton Incinerator and adjoining industrial estate – mixed industrial and employment uses

15 Lower Hall 16 Thames Water operational land, partially used for construction & demolition waste management site

16 Eley Industrial Estate 27 Mixed industrial and employment uses

17 Meridian Water East 19 Mixed industrial and employment uses. Part of Meridian Water Allocation

18 Meridian Water Central 30 Retail and retail warehousing (Tesco, Ikea and others) and employment uses

19 Brantwood Road Industrial Area 18 Mixed industrial and employment uses

20 Watermead Way Industrial Area 21 Mixed industrial and employment uses

21 Land at Walthamstow Avenue and Chingford Road

25 Retail (Sainsbury & Morrisons), open space (sports pitches), allotments, hotel and employment units

22 Tottenham Marshes (northern part)

18 Open space / common land and nature reserve in the Lee Valley Regional Park




Figure 7: Plan of longlist sites

Identification of shortlist site options

5.4.17 From the assessment of the longlist sites against the criteria, the following shortlist of site options were identified and taken forward for further evaluation.

• Site 1: Deephams Sewage Works

• Site 6: Meridian Business Park

• Site 9: Lee Valley Leisure Complex Golf Course (southern part)

• Site 11: Ardra Road North Industrial Area

• Site 15: Lower Hall




5.4.18 Site 1 – Deephams Sewage Works: The Deephams Sewage Works site performed well against the range of assessment criteria. It has less property / legal, planning and environmental constraints than the other longlist sites. There are, however residential areas close to the site boundary, particularly to the north and west, and a Site of Metropolitan Importance for Nature Conservation within the site boundary, both of which represent a constraint. The delivery of the upgrade on this site represents a major challenge in engineering terms due to the need to retain a fully operational sewage works throughout the upgrade.

5.4.19 If those engineering constraints can be overcome and an upgrade scheme designed that is acceptable in planning and environmental terms, within the environmental and financial constraints imposed by the regulators and at an acceptable level of risk to us, then the Deephams Sewage Works site would be an appropriate location for the upgrade.

5.4.20 Our assessment of the Deephams Sewage Works site (Site 1) was that it should become a shortlist site option. Further assessment is needed of the engineering constraints and phasing requirements for this site, together with the potential for impacts on sensitive land uses and off site planning and environmental designations, including any mitigation that may be required.

5.4.21 Site 6 – Meridian Business Park: This is a designated employment site, protected under planning policies as Strategic Industrial Land, a regionally important designation. The London Borough of Enfield has identified the potential loss of employment opportunities as a result of the upgrade as a significant constraint.

5.4.22 Whilst London Plan policy would not require replacement or compensation employment land to be provided, London Borough of Enfield has indicated that it would seek its provision in order to mitigate or compensate for the loss of the site. The site also lies within the Ponders End Place Shaping Priority Area within which London Borough of Enfield policies also seek to retain employment opportunities.

5.4.23 The site has flood risk, and other planning and environmental constraints that would need to be overcome for it to be developed for the upgrade and the site is located in close proximity to residential and other sensitive receptors. The site is in multiple landownerships and there are considerable risks and costs relating to land acquisition.

5.4.24 The site lies in relative close proximity to the existing Deephams Sewage Works site, which lies beyond the Lee Valley Leisure Complex to the south. The required transfer pipeline to and from these sites, when compared to other sites more remote or beyond significant engineering constraints, would be less complex in engineering terms, be expected to have less environmental impacts during construction, and involve less financial and carbon costs both during construction and operation.

5.4.25 Our assessment of the Meridian Business Park site was that it has potential to accommodate the upgrade. We concluded that the site should become a shortlist site option, enabling its development potential for the upgrade to be assessed in more detail. This more detailed assessment would need to include further consideration of issues relating to land acquisition, flood risk, and issues relating to the loss of employment opportunities as a priority.




5.4.26 Site 9 – Lee Valley Leisure Complex Golf Course (Southern part): This site lies almost immediately north of the Deephams Sewage Works site, beyond Picketts Lock Lane. The site is owned by the Lee Valley Regional Park Authority. It is in active use as part of the Lee Valley Leisure complex, partly as a campsite but mostly as part of the wider publicly accessible golf course. Any redevelopment involving the loss of the golf course and campsite would be expected to have to re-provide these existing uses elsewhere. The site lies immediately adjacent to the nationally significant Lee Valley Athletics Centre and has residential properties immediately south of the site. These sensitive receptors place constraints on the development of the site.

5.4.27 The whole of the site lies within the Green Belt and Lee Valley Regional Park. It is also designated as open space by London Borough of Enfield planning policies. There are flood risk constraints affecting parts of the site and the site is crossed by high voltage overhead power lines.

5.4.28 The site is considered to represent a good opportunity to accommodate the upgrade in engineering terms, given its close proximity to the Deephams Sewage Works site. However, the site is assessed as having significant and complex land acquisition risks – given the fact that it is in public ownership by the Lee Valley Regional Park Authority and the Authority has indicated that the site would not be made available for the upgrade.

5.4.29 It is considered that a compulsory purchase order would be necessary to secure the land, with the potential for special parliamentary procedures to also be required to be followed as the site is open space. The Lee Valley Regional Park Act also adds further potential complexities to land acquisition as there are legal provisions relating to the acquisition or disposal of land within the Park.

5.4.30 The required transfer pipeline to and from this site would be less complex in engineering terms, be expected to have less environmental impacts during construction, and involve less financial and carbon costs both during construction and operation, than other more remote sites.

5.4.31 Despite the land acquisition risks and complexities, given the close proximity of the site and the relative lack of engineering constraints, our assessment was that this site should become a shortlist site option. Further assessment of the land acquisition issues would need to be undertaken, and formal (written) confirmation of the Lee Valley Regional Park Authority’s position on the future of the site secured and assessed, with appropriate legal advice as necessary.

5.4.32 Site 11 – Ardra Road North Industrial Area: This site is a designated employment site, protected under planning policies as Strategic Industrial Land, a regionally important designation. The London Borough of Enfield has identified the potential loss of employment opportunities as a result of the upgrade as a significant constraint. Whilst London Plan policy would not require replacement or compensation employment land to be provided, London Borough of Enfield has indicated that it would seek its provision in order to mitigate or compensate for the loss of the site in accordance with policies in the Local Plan.

5.4.33 The site has flood risk, and other planning and environmental constraints that would need to be overcome for it to be developed for the upgrade. The site is located in close proximity to residential and other sensitive receptors. The site is also in multiple land ownerships and there are considerable risks and costs relating to land acquisition.




5.4.34 The site lies in close proximity to the existing Deephams Sewage Works site, it being immediately adjoining it to the south. The required transfer pipeline to and from this site would be less complex in engineering terms, be expected to have less environmental impacts during construction, and involve less financial and carbon costs both during construction and operation, than other more remote sites.

5.4.35 We assessed the Ardra Road site as having potential for accommodating the upgrade. We concluded that the site should become a shortlist site option, enabling its development potential for the upgrade to be assessed in more detail. This more detailed assessment would need to include further consideration of issues relating to land acquisition, flood risk, and issues relating to the loss of employment opportunities as a priority.

5.4.36 Site 15 – Lower Hall: We own the Lower Hall site and it is located in relatively close proximity to the Deephams Sewage Works site. The site is subject to planning and environmental designations including, Green Belt, Lee Valley Regional Park and Site of Metropolitan Importance for Nature Conservation. It is also subject to flood risk constraints and close to residential properties in Chingford.

5.4.37 The development of this site would need to overcome these constraints, and other engineering challenges including the need to build a transfer pipeline between this site and Deephams Sewage Works – either under or over the Lee Navigation. The development of the site would also be in relatively close proximity to the toe of the existing William Girling Reservoir embankment, and close to high voltage overhead power lines and major underground water mains.

5.4.38 Notwithstanding the planning, environmental and engineering constraints to the development of the site, Lower Hall represents the single largest off site opportunity within our ownership for delivering the upgrade. The lack of existing development and land acquisition risks and costs means that the upgrade could potentially be delivered faster on Lower Hall than on any of the other longlist sites.

5.4.39 Our assessment was that the Lower Hall site should become a shortlist site option. This would enable the Green Belt, regional park and other significant constraints to be explored in more detail, alongside the assessment of engineering and other issues relating to its potential future development.

Rejected sites

5.4.40 Our assessment of the other longlist sites concluded that none had any significant property/legal, planning and environmental, or engineering opportunities that made them more appropriate as shortlist sites, when compared to the five shortlist sites identified above. The sites had significant additional constraints to the five shortlist sites, including being located a greater distance away from the Deephams Sewage Works site, necessitating a longer, more complex transfer pipeline.

5.4.41 A sensitivity test of a wider area did not identify any other potential sites that had preferable characteristics and / or improved opportunities or fewer constraints when compared to the 22 identified sites. Further discussions through the Stakeholder Forum have also not identified any further sites either inside or outside the site search area that would be suitable for assessment for the upgrade.

5.4.42 Details of the site assessment can be found in the separate “Site assessment stage 2a report”.




5.4.43 The conclusions of the site assessment process is summarised in Figure 8 below which identifies the shortlist site options.

Figure 8: Plan of shortlist sites

5.5 Treatment options assessment

5.5.1 An engineering assessment of potentially viable treatment options has been undertaken. The purpose of the assessment was to consider potentially viable sewage treatment technology options against factors that affect their suitability for the upgrade. No final decisions have been taken on treatment options at this stage. Those decisions will be informed by feedback from the phase 1 public consultation, and from our further consideration of cost, sustainability, resilience, flexibility and speed of construction of the treatment options.

5.5.2 The preliminary conclusions reached at this stage of the assessment are set out below.




Primary treatment options

5.5.3 In the primary treatment stage, sewage flows through large tanks, commonly called primary settlement (or sedimentation) tanks (PSTs). The tanks are used to settle out organic solids/sludge whilst fats, oils and grease (floatables) rise to the surface and are skimmed off. PSTs are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank, from where it is pumped to sludge treatment facilities.

5.5.4 Primary treatment can take place in conventional radial or horizontal flow PSTs or PSTs fitted with lamella plates. Both conventional and lamella primary tanks were considered to have potential for use as part of the upgrade. Conventional tanks are currently in use at Deephams. Lamella primary settlement systems use inclined plates to reduce the footprint area needed to settle solids from the sewage. Lamella based treatment is considered less reliable as Lamella plates can become partially blocked by accumulations of fat, oil and grease. However, their more compact nature warrants inclusion. Lamella based primary tanks will need an upstream Fats, Oil and Grease (FOG) plant to be included to remove material which may stick to the plates. Tanks will also need to be able to be removed from service for regular pressure washing of the lamella plates to remove any accumulations of fat, oil and grease.

Secondary treatment options

5.5.5 Secondary treatment is designed to substantially remove the biological pollution content of the sewage. The majority of municipal sewage works undertake secondary treatment by treating the settled sewage which exits from the primary treatment stage using aerobic biological processes. The secondary treatment stage has the greatest number of potential alternative process technologies. The conclusions of the assessment of all of the secondary treatment options considered are reported below.

5.5.6 Following completion of the assessment it was concluded that the following secondary treatment technology options would not be taken forward for more detailed evaluation, as they were not considered appropriate for the upgrade:

• Reed beds/wetlands

• Trickling filter technology (or biotowers)

• Sequencing Batch Reactors (SBRs)

• Conventional activated sludge followed by N-SAF

• Rotating Biological Contactors (RBCs)

• Oxidation ditches

5.5.7 Further details of the assessment of these treatment options can be found in the Treatment Options Assessment Report.

5.5.8 The following secondary treatment options were identified as meriting further assessment:




5.5.9 Conventional Activated Sludge (CAS): CAS is a suspended-growth system where a biomass (called ‘activated sludge’) is mixed with the settled sewage (sewage following treatment in the PSTs) to accelerate natural degradation of the organic pollution load. CAS uses dissolved oxygen to promote the selective growth of particular types of bacteria in the aeration tanks that substantially removes the organic material. Upon exiting the aeration tanks, these bacteria are settled out of the flow in final settlement tanks (FSTs) and collected for return to the head of the aeration tanks as the ‘activated sludge’. Periodically the concentration of biomass rises above the level required for efficient operation of the process and the excess biomass is drawn off as ‘surplus activated sludge’ which is thickened and treated along with the sludge from the Primary Treatment stage. The liquid fraction of flows entering the FSTs exits these tanks over weirs constructed around the tank perimeters.

5.5.10 This technology offers space savings when compared to the traditional fixed film growth systems such as trickling filters. CAS provides a well understood process, having a high confidence in its capability of achieving the effluent quality required for the Deephams upgrade. Most of the largest sewage works in the world are CAS plants and we have significant experience with operating large CAS plants. The conventional activated sludge process is currently used at Deephams Sewage Works, however this existing plant does not have capacity to meet the new consent.

5.5.11 Integrated Fixed-film Activated Sludge (IFAS): IFAS includes a fixed-film or attached growth system suspended within the activated sludge process. The objective is to increase the concentration of biomass activity within the aeration tank by maximising the correct type of biomass growth. This greatly enhances the tank performance and increases the purification capabilities and capacity.

5.5.12 IFAS technology allows for a smaller activated sludge plant, otherwise it is similar to CAS. This technology provides some confidence in meeting the Deephams upgrade consent, although it is not widely used on scales comparable to Deephams Sewage Works. There are only a limited number of suppliers who provide systems of a suitably reliable nature to guarantee meeting the future consent, which may restrict any competitive tender process.

5.5.13 Membrane Bio Reactor (MBR): MBR combines activated sludge treatment with a membrane liquid-solid separation process. The membrane component uses low pressure microfilter or ultrafiltration membranes and eliminates the need for subsequent clarification and tertiary filtration stages. The membranes are typically immersed in the aeration tank, however, some applications utilise a separate membrane tank. One of the key benefits of an MBR system is that it effectively overcomes the limitations associated with poor settling of sludge in conventional activated sludge processes.

5.5.14 The technology permits bioreactor operation with considerably higher biomass concentrations. These elevated concentrations allow for very effective removal of both soluble and particulate biodegradable materials and increased sludge retention times. Over time, membrane filters can become clogged with grease or abraded by suspended grit. MBRs typically require more electrical energy than conventional systems and have a limited membrane filter life span (7-10 years). This increases the cost of MBR plants.




5.5.15 The technology has become increasingly popular for reliably pre-treated waste streams. The small area of land needed of MBR systems and the high quality effluent produced, make them particularly useful for wastewater recycling applications or meeting tight effluent standards such as those needed for the Deephams upgrade. The MBR process needs fine screens to protect the membranes, but offers a very compact treatment process requiring a small area of land. This technology provides good confidence of meeting the Deephams upgrade consent. However, the technology is not widely used on scales comparable to Deephams Sewage Works.

5.5.16 Biological Aerated Flooded Filter (BAFF): BAFF plants comprise a reactor filled with an inert granular material (filter media). The media is either buoyant and retained in place by a suitable grid at the top of the filter or submerged and supported by a gravel layer at the bottom of the filter. The dual purpose of this media is to provide a surface upon which a highly active biomass can adhere and grow, and to physically filter suspended solids from the flow. BAFF systems are very compact when compared to conventional activated sludge.

5.5.17 To meet the Deephams upgrade consent a two-stage BAFF process is envisaged as a minimum. The first stage is for Biological Oxygen Demand removal, with the second stage being required to achieve the ammonia standard. A two stage BAFF system provides only some confidence of meeting the Deephams upgrade consent. It is likely that a three stage system would be required. The technology is not widely used on scales comparable to Deephams Sewage Works in the configuration required.

Comparative assessment of the considered secondary treatment technologies

5.5.18 Following the review summarised above, a further more detailed comparison of these secondary treatment technologies identified the following outcomes.

Table 3: Comparison of secondary treatment technologies

Evaluation Criteria CAS IFAS BAFF MBR

Confidence in technology meeting new discharge consent?

Good confidence Some confidence Some confidence Good confidence

Experience of technology on Deephams scale?

Good experience No or very limited experience

Limited experience

No or very limited experience

Speed of construction on a cleared site

Medium

3 – 4 years

Medium

3 – 4 years

Medium

3 – 4 years

Medium

3 – 4 years

Comparison of Whole Life Costs

Medium

Medium

Medium

High

Comparison of power consumption

Medium

Medium

High

Medium




Evaluation Criteria CAS IFAS BAFF MBR

Competition in Construction Tenders

Good competition Acceptable competition Poor competition Good competition

Comparison of odour generation

Medium odour generation, mitigation possible




Assessment of tertiary treatment options

5.5.19 Where very high quality final effluent is required, additional filtration processes are used to reduce the amount of fine suspended solid material in the effluent. This also reduces the BOD levels. These filtration stages include sand or gravel filters and natural systems such as ponds or wetlands. The MBR processes described above already incorporate effective solids removal processes and therefore do not require a separate tertiary treatment stage for enhanced solids capture.

5.5.20 Where needed following the secondary treatment stage, tertiary treatment is assumed to require additional filtration in deep bed sand filters. These are called Rapid Gravity Filters (RGFs) and are normally arranged as a series of filters which operate together to treat the flow. RGFs operate by trapping fine suspended solid particles within the depth of the sand filter bed.

Phosphorus removal options

5.5.21 Phosphorus from the sewage can be removed either biologically in a process called enhanced biological phosphorus removal or through chemical precipitation, usually with salts of iron or aluminium. The chemical phosphorus removal process requires a significantly smaller equipment footprint, is easier to operate and often more reliable than biological phosphorous removal. In addition, from our experience, biological phosphorous removal processes generally need chemical dosing support.

5.5.22 Two chemical options are available, either dosing chemicals into the primary tanks (termed pre-dosing) or dosing different chemicals into the aeration plant (termed simultaneous dosing). Chemical dosing into primary treatment was chosen as the common basis for evaluating the different secondary treatment options that passed the screening exercise.

Conclusions on treatment options assessment

5.5.23 The preliminary assessment of treatment options has identified a number of potential treatment technologies that could be used as part of the upgrade. The secondary treatment stage was identified as the area where the highest variation in the available treatment process technologies are found. There were nine secondary treatment option solutions assessed. Upon completion of this assessment only the following three were identified to be taken forward to support the preferred site identification process:

• Conventional Activated Sludge (CAS) for the treatment of the entire flow

• Integrated Fixed-film Activated Sludge (IFAS) for the treatment of the entire flow, or parts of the flow (IFAS could reduce the required footprint when compared to CAS treatment)




• Membrane Bio Reactor (MBR) for the treatment of parts of the flow (there are concerns over MBR experience on a Deephams Sewage Works scale, and on energy and cost grounds, however the use of MBR could reduce the area of land required when compared to CAS treatment)

5.5.24 No final decisions have been taken on the treatment options at this stage. We will use feedback from the Phase 1 public consultation, and from discussions with potential contractors, to inform our decisions on the final treatment options for the upgrade. The final treatment option could be one of those identified as suitable above, or an alternative option if our further assessment demonstrates it to be the most appropriate treatment process for the upgrade.

5.6 Identification of preferred site

Approach to the assessment

5.6.1 The outcome of the site assessment (longlist) stage was the identification of five shortlist sites. Key issues that could constrain or affect the implementation of the upgrade on these five shortlist sites were reviewed in more detail. The conclusions of the earlier longlist assessment were also tested and verified as part of this process.

5.6.2 Preliminary design work was undertaken on each of the shortlist sites to enable the assessment work to be undertaken. This included the preparation of preliminary plant layouts, based on the treatment technologies identified, which showed the extent of the built development. Due to their size or shape, some of the shortlisted sites could only accommodate a single treatment technology option, whereas others could accommodate a number of treatment options. Potential construction programmes were also developed for each site. The outcome of the shortlist site assessment process was the identification of a preferred site for the upgrade.

5.6.3 The decision on the number of preliminary layouts that were prepared for each site took account of the extent to which different treatment options were considered likely to give rise to significantly different assessment results on the shortlisted sites, that may affect the selection of a preferred site. The layouts were chosen so that a reasonably comprehensive assessment of the results on the shortlist sites was given.

5.6.4 For example, on the Deephams Sewage Works site, construction programmes could vary as a result of the need to phase development alongside the existing operational works. Different treatment technology options could also result in different development areas being required on the Deephams Sewage Works site.

5.6.5 On sites where there would not be a phasing requirement, the differences between treatment technology layouts and programmes, and the impacts potentially arising from them may be more limited. For these sites, we developed a layout for the treatment technology with the largest footprint compatible with the site. Sites were not eliminated at this stage on the basis of the treatment technology that was assessed on the site.

Results of the shortlist assessment

5.6.6 Summary results of the options assessed and the outcomes of the assessment is set out below. Further details can be found in the separate “Site assessment stage 2a report”.




Deephams Sewage Works site

5.6.7 Three potential treatment technology options were assessed on this site at this stage of the assessment process:

• Conventional activated sludge (CAS) to treat all of the sewage

• Integrated Fixed-film Activated Sludge (IFAS) to treat a third of the sewage and conventional activated sludge to treat the remaining two thirds

• A Membrane Bio-Reactor (MBR) to treat a third of the sewage and conventional activated sludge to treat the remaining two thirds.

5.6.8 We assessed this range of treatment technologies to give us confidence that the conclusions of our assessment of the site would be robust for whichever treatment technology is selected for the upgrade. These and other options will be assessed again in the light of feedback from the public consultation and from potential contractors before final decisions are taken on the treatment technology for the upgrade.

5.6.9 Illustrative layouts for these options are shown on the following pages in Figures 9, 10 and 11.

5.6.10 The Deephams Sewage Works site is an established site that has been in use for sewage treatment for decades. The continuation of this use is an appropriate use for the site, and the principle of developing the upgrade on the existing site is consistent with national, regional and local planning policies. All the existing sewers within the catchment drain to the existing Deephams Sewage Works site and the consented discharge point for treated effluent is at the Salmon’s Brook. Upgrading the existing site will take advantage of this existing infrastructure and avoid the need for costly and complex pipeline transfers of sewage, effluent and sludge to and from Deephams.

5.6.11 Redeveloping and upgrading the existing site is technically challenging, as the existing works must continue to function throughout construction works. There is only limited space available on the site currently, meaning that an upgrade would have to be built in a series of phases over a number of years. There are resulting risks to our ability to meet the current discharge consent requirements during this period. A multi-phased redevelopment is also more costly to build than a single phased build on an undeveloped site.

5.6.12 The development would bring sewage treatment operations closer to some existing residential properties and other sensitive receptors. This will require further detailed assessment and the preparation of appropriate design solutions and the inclusion of any necessary noise, odour or other mitigation.

5.6.13 Our assessment of the three treatment technologies gives us confidence that the conclusions of our assessment of the site would be robust for whichever treatment technology is selected for the upgrade. Our assessment showed that whilst the options resulted in variations to the extent of the built development, construction programme and costs, there were not significant differences between the assessment outcomes for each option. We therefore have confidence that an appropriate treatment technology can be identified for development on the Deephams Sewage Works site.




Figure 9: Illustrative Deephams Conventional Activated Sludge plant layout




Figure 10: Illustrative Deephams IFAS / Conventional Activated Sludge layout




Figure 11: Illustrative Deephams MBR / Conventional Activated Sludge layout

Meridian Business Park

5.6.14 The assessment of the Meridian Business Park site reviewed an Integrated Fixed-film Activated Sludge (IFAS) treatment technology. This treatment technology has the benefit of requiring a slightly smaller area of land than for Conventional Activated Sludge, which could not be accommodated due to constraints on available space at the site.




5.6.15 An illustrative layout for the infrastructure that would need to be built at the Meridian Business Park site as part of this option is shown in Figure 12 on the following page. Figure 13 below identifies the infrastructure that would need to be built at Deephams Sewage Works as part of this option.

Figure 12: Illustrative Meridian Business Park IFAS layout




Figure 13: Illustrative layout for infrastructure required at Deephams Sewage Works site in association with any of the off site development options

5.6.16 The Meridian Business Park site is in multiple ownerships and we would need to acquire the land and relocate any existing uses before construction of the upgrade could commence. It was assessed that there was a risk that a compulsory purchase order might be required to achieve this. Land acquisition costs and risks were assessed as being complex, with the potential to delay commencement of construction at best, or at worst to frustrate the delivery of the upgrade on this site. Given the existing use of the site, the costs of land acquisition and relocation of uses was assessed as being high, increasing the overall costs of the upgrade.

5.6.17 The site is designated as SIL and is important for the job opportunities that it provides. The site also lies within one of LB Enfield’s place shaping priority areas, for which it is an important area of job opportunities. The development of an upgrade on this site was assessed as conflicting with the LB Enfield’s planning policies relating to employment and regeneration.

5.6.18 The development of the upgrade on this site is assessed as leading to potential impacts on the environment and local community. It would introduce a new odour source into the Ponders End area, which would need to be controlled and mitigated to avoid odour impacts. The development of the upgrade would also be within close proximity of residential properties and a new Academy school and close to the Green Belt, LVRP and the Lee Valley Leisure Complex Golf Course.




5.6.19 The layout that has been prepared for this site indicates that there would be limited space available for any on site landscaping or other environmental mitigation. The site would not be capable of being screened from views, and flooding, nature conservation and other mitigation may have to be provided off site.

5.6.20 There would be a need to build transfer pipelines to and from this site to the existing Deephams Sewage Works for sewage, treated effluent and sludge, incurring additional financial costs and environmental impacts during construction. There would then be ongoing financial and carbon costs associated with pumping during the operation of the upgraded works.

Lee Valley Leisure Complex Golf Course (Southern Part)

5.6.21 The assessment of the Lee Valley Leisure Complex Golf Course (southern part) site reviewed a modern version of the sewage treatment processes on the existing Deephams Sewage Works site – a Conventional Activated Sludge treatment process. This process is robust and resilient, has an established track record and is proven on the scale required at Deephams.

5.6.22 An illustrative layout for the infrastructure that would need to be built at the Lee Valley Leisure Complex Golf Course (southern part) site as part of this option is shown in Figure 14 below. Figure 13 above identifies the infrastructure that would need to be built at Deephams Sewage Works as part of this option.

Figure 14: Illustrative Lee Valley Leisure Complex (Southern part) Conventional Activated Sludge layout




5.6.23 This site is wholly owned by the Lee Valley Regional Park Authority and it is in active use as a golf course and camp site. It forms part of the wider Lee Valley Leisure Complex, including the regionally important athletics centre. The Regional Park Authority has confirmed that it would not make the land available for development and would strongly resist any attempts to acquire it. It considers that there would be significant local, regional and national support for its opposition to the use of the site for the upgrade.

5.6.24 Our assessment is that a compulsory purchase order would be required to acquire the site. We have assessed the land acquisition risks as complex with the potential to delay or frustrate the development of the upgrade. Given the landowner’s strongly stated position, it is not possible to conclude that the site could be successfully acquired, and therefore the site should not be considered to be readily available for the upgrade.

5.6.25 The site is also within the Green Belt and LVRP and it is in active open space/recreational use. The development of the upgrade on this site would conflict with national, regional and local planning policy that seeks to prevent inappropriate development within the Green Belt, to protect the Regional Park and to prevent the loss of designated Open Space. The development of the upgrade on this site is also assessed as leading to potential impacts on the environment and local community.

Ardra Road North Industrial Area

5.6.26 The assessment of the Ardra Road North site reviewed a modern version of the sewage treatment processes on the existing Deephams Sewage Works site – a Conventional Activated Sludge treatment process. This process is robust and resilient, has an established track record and is proven on the scale required at Deephams.

5.6.27 An illustrative layout for the infrastructure that would need to be built at the Ardra Road North site as part of this option is shown in Figure 15 below. Figure 13 above identifies the infrastructure that would need to be built at Deephams Sewage Works as part of this option.

5.6.28 This site is in multiple ownerships and we would need to acquire the land and relocate any existing uses before construction of the upgrade could commence. It was assessed that there was a risk that a compulsory purchase order might be required to achieve this. Land acquisition costs and risks were assessed as being complex, with the potential to delay commencement of construction at best, or at worst to frustrate the delivery of the upgrade on this site. Given the existing use of the site, the costs of land acquisition and relocation of uses was assessed as being high, increasing the overall costs of the upgrade.

5.6.29 The site is designated as SIL and is important for the job opportunities that it provides. The development of an upgrade on this site is assessed as conflicting with the LB Enfield’s planning policies relating to protection of these employment opportunities.




Figure 15: Illustrative Ardra Road Conventional Activated Sludge layout

5.6.30 The development of the upgrade on this site is assessed as leading to potential impacts on the environment and local community. The development of the upgrade would be within close proximity of residential properties and close to the Green Belt and LVRP. Parts of the site are at risk of flooding and flood mitigation would be required to be provided either on or off site.

5.6.31 As the site lies immediately adjacent the Deephams Sewage Works site, the transfer pipelines for sewage, treated effluent and sludge would only need to be short, incurring less additional financial costs and environmental impacts than the other shortlist sites.

Lower Hall

5.6.32 The treatment technology option that we assessed at the site was a more modern version of the sewage treatment processes on the existing Deephams Sewage Works site – a Conventional Activated Sludge treatment process. This process is robust and resilient, has an established track record and is proven on the scale required at Deephams.




5.6.33 An illustrative layout for the infrastructure that would need to be built at the Lower Hall site as part of this option is shown in Figure 16 below. Figure 13 above identifies the infrastructure that would need to be built at Deephams Sewage Works as part of this option.

Figure 16: Illustrative Lower Hall Conventional Activated Sludge layout

5.6.34 We already own the Lower Hall site. There would be a need to end the current waste processing operations on the land (Camden Plant) in order to build the upgrade on this site. However, apart from this there are no property or legal costs or risks associated with the development of the upgrade at this site. As the site would not need to be acquired, and the upgrade could be built in a single phase, Lower Hall is the option that could deliver the upgrade in the shortest timescale, subject to securing consents for the scheme.

5.6.35 The site is within the Green Belt and LVRP. The development of the upgrade on this site would conflict with national, regional and local planning policy that seeks to prevent inappropriate development within the Green Belt and to protect the Regional Park. In relation to the Green Belt policies, we would need to justify why an exception to normal policies should be made in order to secure permission on this site. This would have to include consideration of the availability of alternative options including the options on the existing Deephams Sewage Works site.




5.6.36 The development of the upgrade on this site is also assessed as leading to potential impacts on the environment and local community. Development would introduce a new odour source into the local area, which would need to be controlled and mitigated to avoid odour impacts. The development of the upgrade would also be within close proximity of residential properties and within the flat valley floor for which mitigation of visual impacts would be limited. Parts of the site are subject to nature conservation and flood risk constraints, and on or off site mitigation would be required.

5.6.37 The layout that has been prepared for this site shows that there would be very limited space available for any on site landscaping or other environmental mitigation. The site would not be capable of being screened from views, and flooding, nature conservation and other mitigation may have to be provided off site.

Identification of the preferred site

5.6.38 The Deephams Sewage Works site is an existing operational site where sewage treatment processes have been undertaken for many years. The development of the upgrade on this site would be in compliance with planning policy. It would not involve the potential compulsory purchase of land, land acquisition costs and related risks to the timetable and delivery of the upgrade. This means that the use of the Deephams Sewage Works site for the upgrade has significant advantages over other shortlist sites.

5.6.39 Provided that the challenges of upgrading the existing site alongside the continued operation of the existing works can be met, the conclusion of our assessment is that our preferred site for delivering the upgrade is the existing Deephams Sewage Works site.

5.6.40 We will undertake further work to ensure these challenges can be met and that an upgrade can be delivered on the Deephams Sewage Works site to a timetable acceptable to the Environment Agency (the environmental regulator), at a financial cost to our customers that is acceptable to Ofwat (our financial regulator) and to a design with potential environmental impacts and associated mitigation proposals that is acceptable to London Borough of Enfield (the local planning authority), other stakeholders and affected communities.

5.6.41 However, should the options for delivering the upgrade on the existing Deephams Sewage Works site prove not to be deliverable following further assessment, we believe that the Lower Hall site would be the next most appropriate site for the delivering the upgrade. This site lies within our ownership, in close proximity to the existing Deephams Sewage Works site. Whilst it is designated Green Belt and within the Lee Valley Regional Park, we consider it represents the next most appropriate site to accommodate the upgrade.

5.6.42 The other shortlist sites, Meridian Business Park, Ardra Road, and the Lee Valley Leisure Complex Golf Course (southern part) sites, have existing use and acquisition costs and risks such that they are not preferred sites for delivery of the upgrade.




6 Development of the preferred site


6.1.1 This section provides a brief explanation of the development of the upgrade on the existing Deephams Sewage Works site as we currently foresee it, representing work undertaken to date. Further detailed work will be carried out in consultation with LB Enfield and other stakeholders on the proposals for the site, with further consultation planned, before any application for planning permission will be made.

6.2 Phasing of development

6.2.1 We plan to build the Deephams Sewage Works Upgrade in phases, allowing us to keep the existing works running while we build the new sewage treatment process on the same site. Throughout the construction works, we have to continue to treat the sewage that arrives at the works each day to meet the current standard set by the Environment Agency.

6.2.2 It may be possible for us to make a number of improvements to the existing sewage treatment facilities without having to completely rebuild the sewage treatment plant. However, we expect that we will have to replace the existing primary and secondary treatment facilities. If this is the case, we plan to build the upgrade in two or three main phases, allowing us to keep the existing works running while we build the new sewage treatment plant on the same site.

6.2.3 To allow us to keep the sewage works running at full capacity and to meet the required treatment standards, we plan to build and start using a new section of the upgrade first, before decommissioning and rebuilding a section of the existing treatment process.

6.2.4 As we are still at the early stages of planning and design for the upgrade, we are unable to say how long each phase of construction will take, but it is likely to take us between four and six years to build the upgrade and put the new treatment process into full use.

6.2.5 Figures 9, 10 and 11 above show examples of potential phasing for a two or three phase construction of the upgrade on our preferred site.

6.3 Illustrative layouts

6.3.1 Figures 9, 10 and 11 above show illustrative layouts for how the upgrade could be developed at the Deephams Sewage Works site.

6.3.2 These illustrative layouts will change depending on the treatment technology option that is eventually selected for the upgrade, following further design development work, and in response to feedback received during the public consultation. The broad principles illustrated in those figures do however provide an indication of how the proposals would look.

6.3.3 Illustrative aerial views of the Deephams Sewage Works site have been prepared, looking from the north, using the illustrative treatment options layout as its base, as shown in Figures 17 to 19 below.




Figure 17: Illustrative aerial view of Deephams Conventional Activated Sludge layout

Figure 18: Illustrative aerial view of Deephams IFAS / Conventional Activated Sludge layout




Figure 19: Illustrative aerial view of Deephams MBR / Conventional Activated Sludge layout

6.3.4 Assessing the amount of available space on the existing Deephams Sewage Works site, it may not be possible to provide the necessary area of land on the site for the temporary contractor’s construction compound. If this is the case, the compound may have to be located on a separate site close by. One possible location being considered is part of the land that we own at Lower Hall.

6.4 Sludge strategy

6.4.1 Sludge is a product of the sewage treatment processes. Currently the sludge treatment facilities on the site operate using conventional digestion followed by thickening, with an open storage area for the final product before it is taken away from the site to be recycled as fertiliser on farmland. As a consequence of the upgrade project, a small increase in sludge production is expected. We anticipate the capacity of the existing plant will be capable of treating this increase.

6.4.2 The sludge treatment process does not directly contribute to the quality of the effluent discharged form the site to the Salmon’s Brook, but the upgraded treatment works must be designed to treat liquids separated from the sludge during treatment and thickening, which are returned to the treatment works. It is proposed that the existing sludge treatment infrastructure shall remain on the Deephams Sewage Works site in all site options considered for the upgrade. The sludge treatment facilities have therefore not been a contributing factor to the site selection.

6.4.3 The development works within the upgrade project intended for the area in which the sludge treatment facility resides will include covering the existing secondary digesters (subject to funding from Ofwat), constructing a tertiary treatment plant and installing combined heat and power engines.




6.4.4 Through undertaking these works around the existing sludge treatment infrastructure, it is foreseen that this could result in a configuration of the sludge treatment facilities that is not optimum in the long term.

6.4.5 We are currently undertaking a review of the sludge treatment facilities at the existing sewage works as part of our wider sludge strategy. Possible options for this sludge strategy include a range of on site options from extending the existing technology of conventional digestion through to the development of a new ‘Thermal Hydrolysis Plant’ (THP) to enhance the current arrangements for sludge treatment and increase the volume of ‘biogas’ generated that can be used to make electricity to be used on site.

6.4.6 The illustrative layout in Figure 20 below shows the potential configuration of the sludge area if a THP was installed. The illustrative aerial view in Figure 21 shows the same configuration, but looking from the north of the Deephams Sewage Works site.

Figure 20: Illustrative potential Deephams THP Sludge Treatment layout

Figure 21: Illustrative aerial view of potential Deephams THP Sludge treatment layout




6.5 Odour management

6.5.1 We recognise that there are a large number of people who live or work close to the Deephams Sewage Works who at times may be affected by smells from the works and other waste and industrial operations in the area. Reducing odour from sewage treatment is one of our top priorities for the upgrade project.

6.5.2 Sewage can give off odour when it is treated, or moved around during the treatment process. Although it is mainly water, sewage contains other biological and chemical materials that produce gases when they are treated. If the treatment process takes place in treatment tanks that are open to the air, these gases can be released into the air and cause odour. The amount of odour from a sewage treatment works depends on a range of issues including what is in the sewage, how it is treated, the direction and strength of the wind and how warm the weather is on a particular day (sewage can smell more on hot days).

6.5.3 The main sources of odour at the Deephams Sewage Works are the screening, primary treatment and sludge treatment stages of the sewage treatment process. In summer 2012 we will complete a construction project to improve the part of the treatment process where the sewage first arrives at the site, which will reduce the levels of odour produced by that part of the existing sewage works.

6.5.4 Figure 22 below illustrates how odour from the sewage works currently leaves the existing Deephams Sewage Works site and affects the surrounding area. The red area of the map is where the odour is strongest and the yellow area is where it is weakest.

Figure 22: Illustration of existing Deephams Sewage Works odour generation




6.5.5 We are carrying out modelling to predict odour levels likely to be produced from the different treatment options we are considering as part of the upgrade, and to assess the effectiveness of different types of odour control. We will take into account the amount of odour produced by each treatment option when we decide upon a preferred sewage treatment process and site layout for the upgrade. We have also reviewed the odour complaints that we and the London Boroughs of Enfield, Waltham Forest and Haringey have received from customers and local residents since 2006.

6.5.6 We will design the new sewage works to reduce the current odour levels. This is likely to include introducing measures to control odour at the most odorous parts of the sewage works. As part of the upgrade, we plan to include measures to manage and control the odour produced by the primary treatment stage of the sewage treatment process. This may involve covering the large settlement tanks used at this stage, or putting this part of the process inside a large building and treating the gases produced before releasing them higher above the sewage works.

6.5.7 We also plan to cover part of the sludge treatment stage of the sewage treatment process, which will further reduce the amount of odour produced by the sewage works. We believe that these measures will provide a significant reduction in odour and we will develop our proposals in more detail as the design of the upgrade progresses. This work to manage and control odour produced by the upgraded sewage works will be subject to approval of the necessary funding by Ofwat, our financial regulator.

6.5.8 Figures 23 and 24 below give an illustration for how odour generation from the Deephams Sewage Works site could be reduced through the current construction work, and as a result of further potential improvements through the upgrade project.

Figure 23: Illustration of potential future Deephams Sewage Works odour generation




Figure 24: Illustration of potential odour reductions at Deephams Sewage Works

6.6 Protection of the environment and amenity

6.6.1 Deephams Sewage Works is located in a built up part of north London, close to housing, areas of Green Belt, the Lee Valley Regional Park, nature conservation sites, recreational sites and several rights of way. We will design and build the upgrade to reduce its potential impacts on the environment and local communities, both during construction and as part of the day-to-day running of the upgraded site. We will work closely with the London Borough of Enfield, other stakeholders and the local community to prepare our detailed plans for the upgrade.

6.6.2 To help us identify and manage the positive and negative effects of the proposed upgrade, we will carry out an Environmental Impact Assessment (EIA). The EIA will allow us to understand the environment around any work sites and identify the likely effects the upgrade may have on a wide range of environmental areas, including air quality, transport, archaeology, ecology, noise and odour.

6.6.3 Understanding the possible environmental impacts will allow us to change the design of the upgrade, wherever possible, to avoid negative effects. If there are any negative effects that cannot be avoided, we will work out where and when we need to do things differently to reduce these effects. The findings of the EIA will be set out within a report called an Environmental Statement. This will be submitted with the application for permission to build the upgrade.

6.6.4 We realise that the possibility of noise, dust and traffic during construction of the upgrade will be concerns to the large number of people who live or work close to the Deephams Sewage Works. Reducing noise, dust and other potential impacts from the construction work are key priorities for the upgrade. Similarly, noise, traffic and odour from the operation of the upgraded works will be a further concern that will be assessed.




6.6.5 Depending on the outcomes of our Environmental Impact Assessment, there are a number of ways we can try to reduce noise while we are building the upgrade. These include limiting the noise coming from the site by putting up specially designed hoarding to reduce noise levels and by using noise suppressed equipment, wherever possible. The upgraded parts of the sewage works will be designed to work within acceptable noise levels. Dust could be limited by damping down during demolition works, and by keeping roads and vehicles clean, as these normally produce the most dust on a site. Construction traffic will be routed away from residential areas as much as practicable.

6.6.6 We will develop our proposals in more detail as the design of the upgrade progresses. We will set out our draft proposals and consult on these before any planning application is made.

7 Funding the upgrade 7.1.1 The costs of designing, building and running the upgrade will be paid for by our 14

million wastewater customers through their sewerage bills. Similar upgrades to our other sewage works in London have also been funded in this way.

7.1.2 Customer bills and how much they change each year are set by our economic regulator Ofwat (The Water Services Regulation Authority). Ofwat’s role is to make sure that water and sewerage companies provide customers with good standards of service and value for money.

7.1.3 Every five years, we have to submit our plans to Ofwat for how we will meet the needs of our customers and maintain and improve the levels of service we provide to them. Ofwat looks in detail at these plans to make sure that customers only pay for the work and improvements that are necessary. Ofwat then decides how customers’ bills will change over the following five years. This process is known as the Price Review.

7.1.4 At the last Price Review in 2009, Ofwat gave us the funding to begin the planning and design work for the Deephams Sewage Works Upgrade. Following the two phases of public consultation and after we have completed the design work and submitted a planning application for the upgrade, we will ask Ofwat to allow the remainder of the necessary funding at the next Price Review which is determined at the end of 2014. This determination will also cover any funding for sludge treatment or odour control measures.

7.1.5 At this early stage of development, it is difficult to say exactly how much the upgrade will cost, but we estimate it will be between £300-400 million. This estimate will be subject to ongoing refinement as detailed planning of the upgrade progresses.

8 Next steps 8.1.1 Details of our preferred site will be published as part of the phase 1 public

consultation. We will invite the submission of comments both on our conclusions and the assessment work we have undertaken to date.

8.1.2 Alongside and following the consultation we will undertake further work on the design of the upgrade, including the treatment technology options that could be adopted on our preferred site. This work will be informed by feedback from the consultation process, our own design work, and input from potential contractors.




8.1.3 We will then continue to develop and assess the upgrade in accordance with our Assessment Methodology. This will involve a further phase of consultation, further design development work and environmental impact assessment (EIA). At the conclusion of this process, we will select the scheme we intend to take forward for application.

8.1.4 The EIA will identify potential impacts arising from the construction and operation of the upgrade, utilising environmental baseline and appraisal information gathered during the earlier stages of the project. The EIA will also enable detailed proposals to be developed, including modifications to the detailed design to mitigate impacts that are predicted to arise. The EIA will be undertaken in close consultation with the relevant regulators and consultees, including the local authorities, LVRPA, Environment Agency, Greater London Authority, Department for Transport, Natural England, English Heritage, Transport for London and others.

8.1.5 Applications for the planning and other consents will be prepared, with a further formal stage of public consultation held as part of this pre-application process. That consultation will enable comments and feedback on the proposed design of the upgrade, and the potential impacts and mitigation proposals to be made. We will amend our plans to incorporate comments into the design of our proposals where feasible.

8.1.6 There are two possible routes that our planning application could take. If following the first phase of public consultation, our preferred site is confirmed as the selected site for the upgrade and our planning application is for permission to build the new sewage works on the existing Deephams Sewage Works site, then we anticipate that an application for planning permission will be submitted to the London Borough of Enfield.

8.1.7 The Borough’s Planning Department will be responsible for deciding our planning application, and will take into account the documents we submit as part of the application and the views of local people and consultees. The Greater London Authority will also provide its comments on the proposals during this process. It is likely to take around nine months for the London Borough of Enfield to make its decision.

8.1.8 However, if following the first phase of public consultation, a different site is confirmed as the selected site for the upgrade and our planning application is for permission to build new sewage treatment plant on a new site, then the upgrade is likely to be a Nationally Significant Infrastructure Project (NSIP) and an application for a Development Consent Order will be submitted to the Planning Inspectorate under the Planning Act 2008.

8.1.9 In this case, the application would be considered by a panel of planning inspectors who would give their recommendations to the Secretary of State. It would then be for the Secretary of State to make the final decision (and not London Borough of Enfield). This procedural route to consent might take approximately six months longer than shown in the timetable below.

8.1.10 Whichever route our application takes, there will be an opportunity for the public and others to make representations on the application to the body considering the application.




8.1.11 Subject to planning and other consents being approved and conditions discharged, construction of the upgrade would then commence. There will be regular and ongoing communication with the local communities throughout the construction process as set out in our Consultation Strategy.

8.1.12 Our indicative timetable for these stages of the project is set out below.

Table 4: Indicative timetable

July – October 2012

Phase 1 consultation Consultation on the preferred site, potential treatment options and our proposals for building the upgrade. This is the current phase of consultation.

October 2012 – December 2012

We assess the consultation responses, issue a consultation feedback report, make changes to our proposals and confirm the preferred option for the Deephams Sewage Works Upgrade project.

December 2012 – summer 2013

We prepare more detailed designs of our proposals for the upgrade and carry out an Environmental Impact Assessment.

Summer/autumn 2013

Phase 2 consultation Consultation on the more detailed designs of our proposals for the upgrade, including details of our assessment of environmental and other impacts and our proposals for avoiding and mitigating any potential negative environmental and community effects of the upgrade.

Winter 2013 – spring 2014

We assess the consultation responses, issue a consultation feedback report and make changes to and finalise the documents needed for the application for permission to build the upgrade.

May 2014 We submit our application for permission to build the upgrade.

Summer 2014 Opportunity for members of the public and others to make representations to the planning authority on the application.

April 2015 Earliest date for approval of our application for permission to build the upgrade.

August 2015 Earliest date for starting the main construction work, which will be completed in phases over a number of years.

2020 - 2021 Anticipated date for completion of the Deephams Sewage Works Upgrade.

Project Overview

Documents

picketts lock

illustrative

freshwater

public consultation

thermal hydrolysis

sequencing

project overview

biochemical