department for environment, food and rural affairs

Department for Environment, Food and Rural Affairs

Research and Support for Developing a UK Strategy for Managing Contaminated Marine Sediments

Task 5: Establishing Best Practice for Current Disposal and Treatment Options for Contaminated Dredged Marine Sediments

April 2010

Entec UK Limited

Report for

Defra

Nobel House

17 Smith Square

London SW1P 3JR

Main Contributors

Shaun Nicholson

John Pomfret

Ed Gilligan

Rohit Mistry

Approved by

John Pomfret

Entec UK Limited

Northumbria House Regent Centre Gosforth Newcastle upon Tyne NE3 3PX England Tel: +44 (0) 191 272 6100 Fax: +44 (0) 191 272 6592 Doc Reg No. 19993\cr041 e:\data\19993 defra marine seds\latest versions of reports\19993 defra cms

task 5 final report.doc

Department for Environment, Food and Rural Affairs

Research and Support for Developing a UK Strategy for Managing Contaminated Marine Sediments

Task 5: Establishing Best Practice for Current Disposal and Treatment Options for Contaminated Dredged Marine Sediments

April 2010

Entec UK Limited

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Glossary and Abbreviations

Action Level Action levels, in terms of contaminant concentrations, are used for assessing the suitability of dredged sediments for sea disposal in England and Wales. Action Levels contribute, together with a range of other assessment methods (e.g. bioassays, historical data, physical properties and disposal site characteristics), to a weight of evidence approach used to make management decisions on the fate of dredged material. In general, contaminant levels in dredged material below Action Level 1 are of no concern and are unlikely to influence the licensing decision. Dredged material with contaminant levels above Action Level 2 is, however, generally considered to pose an unacceptable toxicological risk to aquatic organisms and unsuitable for sea disposal. Dredged material with contaminant levels between Action Levels 1 and 2 requires further consideration and testing before a decision on disposal option can be made.

Bioassay A test using exposure of living animals to water or sediment that is used to ascertain the toxic properties of the material. Bioassays are conducted in controlled laboratory conditions to prevent environmental factors confounding the test result.

Bioturbation The process whereby organisms influence sediment properties through the movement of sediment particles (such as through the creation of burrows, particle ingestion) and sediment porewater.

Borrow Pit An area/ depression in the seabed that has been used by the extractive industry to ‘win’ deposits such as aggregate (e.g. sand, gravel) for the construction industry. Such pits can be much deeper than the natural seabed level (e.g. in Hong Kong the pits may be up to ~30m below the level of the seabed and dredged material is used to backfill the pit and restore to levels extant prior to aggregate dredging).

CAD Contained aquatic disposal facility. CADs are seabed depressions or excavations (such as in exhausted borrow pits) used for the placement of CDMS followed by capping with uncontaminated material (sand, mud) to prevent mobilisation of CDMS and also isolate the CDMS from the overlying water and aquatic life. CADs are typically engineered structures although naturally deep seabed basins such as in the Norwegian fjords have also been used as CADs.

Note: some authors use the term ‘Confined disposal facility (CDF)’ for CAD and in this report CDF has only been used to describe disposal facilities that use engineered structures such as a dyke to retain CDMS, whereas CAD has been used to describe those facilities that use depressions or excavations in the seabed (including borrow pits) for CDMS disposal.

Capping The placement of uncontaminated material such as sand or mud over contaminated sediment to isolate the latter from the environment.

CDF Confined (or contained) disposal facility. These disposal facilities use engineered dike structures to isolate CDMS (or in some cases clean dredged sediment) from the adjacent aquatic environment. Several types of CDF can be used for CDMS disposal all of which are filled with CDMS then capped when the CDF reaches capacity. CDF options include Island CDF (an engineered structure in open water that is filled with CDMS then capped when filled), Near Shore CDF (engineered structure built partly in the coastal environment) and Upland CDF (a CDF constructed on land).

CEA Cost Effectiveness Analysis (CEA) is widely used (but not restricted) to determine the least cost means of achieving pre-set targets or goals. CEA can be aimed to identify the least cost option among a set of alternative options that all achieve the targets. In more complicated cases, CEA is used to identify combinations of measures that will achieve the specified target.

Cefas The Centre for Environment, Fisheries and Aquaculture Science. Cefas is an executive agency of Defra and advises on the suitability for disposal of CDMS at sea.

CDMS Contaminated marine dredged sediment. This report only deals with contaminated material that has been dredged.

Contamination The amount of a substance in the environment in excess of the quantity that is present naturally.

Equilibrium Partitioning Approach (EqP)

This approach assumes that the most critical factor for predicting the toxicity of sediments is the contaminant concentrations present in the sediment pore water. Such an approach is often useful because environmental quality standards that have been developed for water can be applied to sediment pore water. The approach is most useful for hydrophobic organic chemicals as they tend to partition between the solid phase and the sediment pore water.

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FEPA Food and Environment Protection Act 1985. For any works requiring the disposal of CDMS, developers are, amongst others, required to apply for a Food and Environment Protection Act 1985 (FEPA) licence and may also require consent under the Coast Protection Act 1949 (CPA) with respect to the deposit of materials below mean high water spring tides (MHWS). These arrangements will soon be updated in England and Wales by the Marine and Coastal Access Act 2009, which replaces FEPA licences by a new Marine Licence

Hydrophobic Tending not to combine or mix with water

Infauna Those animals living either within or on the sediment matrix.

Macrofauna Animals that are normally >1mm long. The term is usually applied to benthic organisms.

MCEU Former Marine Consents and Environment Unit. MFA is now responsible for the role previously undertaken by MCEU.

MFA Marine and Fisheries Agency is an executive agency of Defra with responsibility for marine consents and licensing; duties include supporting the fishing industry and marine environment around England and Wales. The MFA was responsible for administration of FEPA licence applications until 2010, a role formerly undertaken by the MCEU.

NIMBY Not In My Back Yard (NIMBY) is a term typically used to describe opposition by residents who are in close proximity to a proposed development. The proposed development may have benefits to society (in this context the treatment/disposal of CDMS) but may have negative impacts on the local landscape and nearby residents. As a consequence, local residents may consider the development undesirable and may prefer that the development be built elsewhere.

PAH Polycyclic Aromatic Hydrocarbons (PAHs) are produced as by-products of burning fuels such as oil and coal or by natural pyrogenic processes. PAHs are hydrophobic (do not mix with water) and in the marine environment tend to accumulate in sediments. PAHs are of concern because some are toxic.

PCB Polychlorinated bi-phenyl. Class of man-made (synthetic) compounds that were used for various purposes including as coolants for transformers. Many PCBs are highly toxic, persistent in the environment and difficult to metabolise (break-down) when accumulated in the body and their manufacture was banned in the 1970s.

Probable Effects Level (PEL)

The contaminant concentration above which adverse effects in biota may be expected.

PIANC International Navigation Association (formerly known as the Permanent International Association of Navigation Congresses)

Pollution In this report, the term pollution has been used to describe only the presence of contaminants at levels that result in harm or injury to animals and plants (compare contamination).

Polychaete A type of marine worm including ragworms and lugworms.

Receptor Receptors are defined features including water quality, ecological and/ or commercial entities such as fish and shellfish and humans, that are sensitive or have the potential to be impacted through activity such as dredging. Typically, a Source-Pathway-Receptor model is used to predict those receptors that can be impacted (i.e. where no pathway exists from a contaminant source to the receptor then there is unlikely to be an impact).

Ripening The process whereby CDMS is placed on land that facilitates dewatering and consolidation. Such treatment techniques can help to oxidise organic material including certain contaminants such as PAHs within the CDMS and the overall process results in material that is easier to handle and suitable for engineering and beneficial use purposes.

SQG Sediment quality guideline. Other than the Cefas Action Levels that cover a limited number of contaminants, there are at present no formal environmental quality standards adopted in the UK to assess contaminated sediments. Guidelines have, however, been developed elsewhere for coastal sediments (e.g. Environment Canada’s Sediment Quality Guidelines [SQG]). In the absence of any UK standard, these SQGs can be used to predict the likelihood that substances in sediments will induce toxicity in aquatic organisms. SQGs have been derived from toxicity data available for a number of contaminants; the Threshold Effects Level (TEL) of a contaminant is the contaminant concentration which is not considered to pose a significant hazard to most aquatic organisms and the Probable Effects Level (PEL) is a contaminant concentration that is usually associated with adverse effects in biota.

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SPI Sediment Profile Imagery (SPI) uses a portable camera that is lowered to the seafloor and takes in-situ photographs of relatively undisturbed sediment (and any fauna present) as it penetrates the upper sediment layers. The camera is mounted on a prism that cuts through the upper sediment layers and penetrates the sediment with minimal disturbance so sediment properties can be recorded photographically without the disturbance that occurs during grab sampling. SPI has been used for various applications including checking cap thickness, differentiation of sediment layers present (such as CDMS, sand and mud) and the evaluation of the presence of infauna such as worms and any bioturbation of sediment layers.

Stabilisation Treatment technique that immobilises contaminants within the CDMS but does not remove them. Such a process may involve mixing CDMS with cement and/or various chemicals that bind contaminants strongly thereby reducing the bioavailability.

TBT Tributyltin is the active biocidal (toxic to life) component of certain antifouling paints. The compound is highly toxic to non-target organisms and has now been banned from the application to ship hulls. TBT is hydrophobic and partitions between water and sediment and it can be highly persistent (>20 years) in sediments (notably anoxic sediment) and can induce toxicity over extended periods.

Threshold Effects Level (TEL)

The contaminant concentration below which the contaminant is not considered to pose a significant hazard to most aquatic organisms

Treatment Term used to refer to a process (e.g. physical, thermal, chemical techniques) used to change the character of the CDMS to produce a product that is less hazardous or toxic

US ACE United States Army Corps of Engineers (US ACE) is responsible for maintaining navigation in the US.

US EPA United States Environmental Protection Agency (US EPA) shares with US ACE the regulatory responsibility for the management and disposal of CDMS in US coastal waters. The EPA is responsible for establishing (in conjunction with the USACE), relevant guidelines pertaining to the evaluation of CDMS disposal activities and performing oversight actions.

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Acknowledgements

During the preparation of this report, there was extensive consultation with a large number of

individuals and organisations that have kindly provided information and input to the study. We

also thank the contractors and bodies overseas involved in disposal of CDMS, who contributed

to our understanding to current practice in the treatment and disposal options for CDMS who

have provided information that has been invaluable to the preparation of the Best Practice

Guidelines.

In particular we would like to thank the below for the kind help provided:

Graeme Batley CSIRO, Australia

Todd S Bridges, US Army Engineer Research and Development Center, USA

Mark Gillingham, Envirotreat, UK

Marjan Euser, SedNet Secretariat, The Netherlands

Joe Germano, Germano & Associates Inc., Bellevue, WA, USA

Pol Hakstege, Rijkswaterstaat, Centre for Public Works, Utrecht, The Netherlands

Joe Jersak, Technical Manager, Biologge AS, Norway

Torild Jorgensen, Oslo Port Authority, Norway

John Leckie, A&P Tyne

Steven Lewis, A&P Ports

Mike Reynolds, Port Operations Director, A&P Falmouth

Phil Lynch, Port of Tyne

Brian Reeve, Port of Tyne

Brendan O’Rourke, DEME, Belgium

Guy Pomphrey, DEME, Belgium

Chris Vivian, Coastal Development Topic Leader, Cefas, Burnham on Crouch

Amanda Ginn, Magnox North Ltd., Berkeley

George Ding, Civil Engineering and Development Department, Hong Kong SAR Government

Edwin Zengerink, Tencate Geosynthetics Netherlands

Axel Netzband, HPA Hamburg Port Authority

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Contents

1. Background 1

1.1 Contaminated sediment and dredged material 1

1.2 Background to this project 2

1.3 Project definition 3

1.3.1 Scope of the project 3

1.3.2 Project objectives 3

1.3.3 Project tasks 4

1.4 This document 4

1.5 Defining the problem and risk 5

1.5.1 What is CDMS? 5

1.5.2 Risks 5

1.5.3 Sediment standards 6

1.5.4 Environmental Quality Standards for sediments in the EU 7

1.6 Legislative barriers to disposal 8

1.6.1 Overview 8

1.6.2 Disposal at sea 8

1.6.3 Status of dredged material 10

1.6.4 Regulation of waste disposal 10

1.6.5 Falmouth Cruise Terminal project case study 10

1.7 Scope of Task 5: Best Practice Guidelines for the Treatment and Disposal of CDMS 11

1.8 Information sources 12

1.9 The content of this document 12

2. Treatment Options 13

2.1 Introduction 13

2.2 Dewatering 14

2.2.1 Natural dewatering and lagooning 15

2.2.2 Mechanical dewatering 17

2.2.3 Geobags 18

2.3 Separation of particle size fractions 18

2.4 Contaminant separation 20

2.4.1 Chemical extraction 20

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2.4.2 Thermal desorption (volatilisation) 24

2.4.3 Electrokinetic separation 25

2.5 Contaminant destruction 27

2.5.1 Biological treatment 27

2.5.2 Thermal oxidation 31

2.6 Contaminant immobilisation 32

2.6.1 Thermal immobilisation 32

2.6.2 Chemical immobilisation 34

2.7 Treatment methods for TBT Contaminated Sediments 37

2.8 Radioactive contamination 41

2.9 Summary 43

3. Beneficial Use of CDMS 45

3.1 Introduction 45

3.2 Summary 47

4. Disposal Options 49

4.1 Disposal options 49

4.2 Purpose-built confined disposal facilities (CDF) 51

4.3 Other landfill sites 56

4.4 Silt lagoons 57

4.5 Contained aquatic disposal (CAD) 57

4.5.1 Method 57

4.5.2 Integrity of caps 62

4.6 Level bottom disposal at sea with capping/isolation techniques 64

4.6.1 Effectiveness 64

4.6.2 Constraints on capping CDMS at sea 67

4.7 Geotextile bags for disposal at sea 68

4.8 Disposal options summary 70

5. Socio-Economics 73

5.1 Introduction 73

5.2 Treatment 74

5.3 Disposal 83

5.4 Summary 91

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6. Best practice 95

6.1 Overview 95

6.2 Framework for CDMS Disposal After Treatment 96

6.3 Conclusion 98

Table 1.1 Summary of the six commissioned work packages within the project. 4 Table 1.2 Action Levels used by Cefas for Dredged Material Assessment 6 Table 2.1 Summary of Natural and Mechanical Dewatering. 14 Table 2.2 Summary of Separation Techniques 20 Table 2.3 Summary of Sediment Washing 21 Table 2.4 Summary of Thermal Desorption 24 Table 2.5 Summary of Ex-situ Electrokinetic Treatment Technology 27 Table 2.6 Summary of Bioreactor Treatments 28 Table 2.7 Summary of Landfarming / Ripening 30 Table 2.8 Summary of Phytoremediation 31 Table 2.9 Summary of Thermal Immobilisation – Brick Production 33 Table 2.10 Summary of Thermal Immobilisation and the Production of Lightweight Aggregate 34 Table 2.11 Summary of Stabilisation/Immobilisation 35 Table 2.12 CDMS Treatment Option Applicability for a Range of Contaminants 43 Table 2.13 Technical Criteria for Treatment Option selection 44 Table 4.1 Comparative Summary of CDMS Disposal Options 70 Table 5.1 Natural Dewatering - Economic, environmental and social implications of CDMS

treatment 75 Table 5.2 Mechanical Dewatering - Economic, environmental and social implications of CDMS

treatment 76 Table 5.3 Separation - Economic, environmental and social implications of CDMS treatment 77 Table 5.4 Thermal Desorption - Economic, environmental and social implications of CDMS

treatment 78 Table 5.5 Bioreactors - Economic, environmental and social implications of CDMS treatment 79 Table 5.6 Phytoremediation / Phyto-extraction - Economic, environmental and social implications of

CDMS treatment 79 Table 5.7 Landfarming - Economic, environmental and social implications of CDMS treatment 80 Table 5.8 Thermal immobilisation - Economic, environmental and social implications of CDMS

treatment 81 Table 5.9 Stabilisation / chemical immobilisation - Economic, environmental and social implications

of CDMS treatment 82 Table 5.12 Confined Disposal Facility - Economic, environmental and social implications of CDMS

disposal 83 Table 5.13 Land disposal - Economic, environmental and social implications of CDMS disposal 85 Table 5.14 Contained Aquatic Disposal Facilities - Economic, environmental and social implications

of CDMS disposal 86 Table 5.15 Open Sea Disposal with Capping - Economic, environmental and social implications of

CDMS disposal 88 Table 5.16 Geotextile bags - Economic, environmental and social implications of CDMS disposal 90 Table 5.17 Summary of the estimated costs and benefits of treatment and disposal options (2008 £

prices) 93 Table A1 Treatment and Disposal and Beneficial Use Options and Data Source A1 Table A2 Treatment Consultations A5 Table A3 Disposal and Beneficial Use Consultations A6 Table B1 SQG Criteria for Marine Sediment Quality Classification in Hong Kong B2 Table B2 Whole-Sediment Bioassay Test Species and Endpoints B2

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Figure 2.1 Dewatering/lagooning field (aerial photography courtesy of DEME) 15 Figure 2.2 Mobile Dewatering Plant. (Photo courtesy of DEME) 17 Figure 2.3 Ex-SituStabilisation / Immobilisation (Photo courtesy of DEME) 35 Figure 2.4 Main Plume Remediation Area, Dounreay (Courtesy of http://www.dounreay.com/particle-

cleanup/). 41 Figure 2.5 A Remotely Operated Vehicle ROV, Dounreay (Courtesy of

http://www.dounreay.com/particle-cleanup/). 42 Figure 4.1 Schematic of the various disposal options for CDMC 50 Figure 6.1 Framework for assessing dredged material for possible beneficial use 96 Figure 6.2 Possible decision tree for CDMS disposal (without treatment) 98

Figure B1 Management Framework for Dredged Sediments in Hong Kong (ETWB, 2002) B2

Appendix A Information sources and contact details Appendix B Management of CDMS in Hong Kong

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1. Background

1.1 Contaminated sediment and dredged material

It is Government policy to promote the use of the sea for the transport of goods around the UK

and also to encourage a modal shift from road to water1. As the global and UK economy

continue to grow there is an increasing demand for port capacity and the UK’s success in the

global market place depends, in part, upon the ability of ports to adapt and operate efficiently as

gateways to international trade2. There is, therefore, a fundamental need to ensure ports

continue to operate and their activities are sustainable and not detrimental to the environment.

The dredging and subsequent use or disposal of marine sediment is an essential activity to

maintain ports, harbours and shipping channels open for navigation. Where contaminated

dredged material3 is encountered, a management decision is required on the best means of

disposing of it. The disposal of CDMS must, therefore, be compliant with and have regard to,

amongst other things, legislation, environmentally sound practices and socio-economics, so that

sustainable, cost-effective treatment and disposal options are adopted.

Whilst the majority of sediments in many inshore areas around the UK coast are relatively clean

or only mildly contaminated, in many industrialised estuaries of the UK the seabed sediments

are contaminated with a variety of chemical species due to port and industrial activity. These

include heavy metals, organo-metal complexes (e.g. tributyl tin - TBT), and various organic

congeners e.g. polychlorinated biphenyls - PCBs. Whilst such inputs are now highly regulated

through restrictions on discharges imposed by the Environment Agency (through discharge

consents and environmental permits), contaminants in sediment are often persistent (i.e. do not

readily break down) and levels of contaminants measured in the estuarine sediments often

reflect historic industrial activity and waste discharges of coastal communities. This historic

legacy means that contaminants are often still at levels that could pose a risk to both aquatic life

and human health. Contamination of sediments is occasionally so high that disturbance to the

contamination which arises during dredging and during subsequent management of the dredged

material may result in unacceptable contaminant loss, dispersal and pollution within the

biosphere. The key issue is to develop a management approach to dredging and disposal of

contaminated seabed sediments in port regions in which the economic objectives are achieved

but in which there is minimal risk to the environment.

Contaminated dredged material has typically been viewed as a waste in the UK and there has

been limited use of CDMS for beneficial purposes. Contaminants in CDMS can also be

stabilised using various treatment options and the resultant material can be a valuable resource

1 The Department for Transport has estimated that about 95% of goods either consumed or produced in the UK are either imported

or exported by sea (http://www.dft.gov.uk/pgr/shippingports/).

2 Department for Transport, 2007. Ports Policy Review.

http://www.dft.gov.uk/pgr/shippingports/ports/portspolicyreview/portspolicyreviewinterimreport

3 These sediments are referred throughout this report in abbreviated form as CDMS (‘contaminated dredged marine sediment’).

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and used for numerous engineering and environmental use4. Whilst efforts to find sustainable

and beneficial use of CDMS have largely been driven in locations such as Hong Kong and The

Netherlands by a lack of disposal space, increasing environmental awareness and progressively

stricter legislation, options for CDMS treatment and disposal have received little strategic

attention in the UK, although they have been addressed on a case-by-case basis. Beneficial use

has been addressed more widely over the last 10-15 years and we are beginning to see the

benefits of that in slowly increasing numbers of proposed schemes and at an increasing scale.

There is currently little in the way of a management framework for dredged marine sediment in

the UK, and contaminated dredged material is usually either disposed of at sea (in open sea

disposal sites subject to chemical and biological screening criteria) or when contaminant levels

are too high transported and dumped in a land fill. To date, there has been limited investigation

into the treatment, disposal or beneficial use of contaminated dredged sediment in the UK, and

other than the OSPAR and London Protocol Guidelines there is no formal sediment

management framework that details the procedures for the treatment, disposal or beneficial use

of contaminated marine sediment.

1.2 Background to this project

In 2006, Defra’s Marine Consents and Environment Unit (MCEU) completed an internal review

of the current situation regarding CDMS. It identified that there is a lack of information on the

extent of contaminated marine sediments in UK waters, and clarity was required regarding the

current industry options for dredging and disposal and associated factors such as liability and

legislation. Currently there is no consistent guidance in place to help industry address the issue

and furthermore, the information known is largely unavailable through a common portal.

In May 2006, as part of a Defra initiative, a Contaminated Marine Sediment Steering Group was

set up, comprising representatives from the Centre for Environment, Fisheries and Aquaculture

Science (Cefas), Natural England, the Welsh Assembly Government, the Scottish Executive,

The Crown Estate and representatives from industry including port authorities (Associated

British Ports, Bristish Ports Association, Port of London Authority), other conservation agencies

and green NGOs. The aim of this group was to discuss and formulate a programme of research

work to underpin provide a comprehensive guidance document (‘strategy’) for managing

contaminated marine sediments in the UK. The specific Terms of Reference (TOR) for this

group were:

‘to assist and facilitate the development of the UK strategy for handling and

managing contaminated material to be dredged from UK marine waters, and to

support and advise on the practical implementation of the strategy’.

Extensive consideration of the principal issues gave rise to a research framework comprising six

work packages (Tasks) that commenced on 1st April, 2007. Whilst it was initially decided to

address all seabed areas which were known to be contaminated, the Steering Group

subsequently focussed the research on contaminated areas where dredging was likely to occur or

be necessary in the future.

4Murray, L.A. 2008. Dredged material as a resource. Terra et Aqua 112: 3-10.

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1.3 Project definition

1.3.1 Scope of the project

This project was conceived within several highly specific terms-of-reference. These are:

• that the sediment is contaminated (although the severity of contamination is not

known); and

• that a decision to dredge has already been made;

• that minor revisions to existing legislative statutes can be entertained;

• that all UK waters and thus potentially out to 200 miles (and inland as far as tidal

limits) are considered.

These terms are important because they dictate at what point within a decision hierarchy the

results of this research will be invoked. For example, this project is not intended to underpin or

provide a framework for evaluating, permitting and controlling the dredging of contaminated

marine sediments.

1.3.2 Project objectives

The aims of the project were set out under ‘Background and Overview’ in the original project

specification and are that it will underpin development of a UK strategy for managing

contaminated marine sediments that need to be dredged which will:

• aid in the transparent and objective assessment of all dredged material disposal

options through the Best Practicable Environmental Option (BPEO) assessment

process, taking into consideration the principles of sustainable development

(including the polluter pays principle and the precautionary principle) on a case by

case basis;

• take into account the scale, extent, implications and impacts of DCDMS on the

marine environment;

• act as a focus for existing work, draw together best practice and ensure work is not

duplicated elsewhere e.g. by The London and Oslo Paris (OSPAR) Conventions,

the International Navigation Association (PIANC), Central Dredging Association

(CEDA) etc;

• produce a simultaneous and inclusive consultation process to replace the current

methodology of approaching one regulator at a time in order to make disposal

decisions;

• identify where regulations are preventing the BPEO from being used and reflect

the associated risks to the marine environment these represent with examples

where flexibility in regulations allows the common sense approach to prevail being

highlighted;

• define the nationwide scale of the problem and disposal solutions at sea as well as

beneficial use, identify current sea disposal sites receiving contaminated, but

acceptable, material and suggest guidance for their use, identify remedial

4


measures for material deemed acceptable for sea disposal and establish guidelines

for preventing further contamination.

1.3.3 Project tasks

The research was delivered through a series of six commissioned work packages, or Tasks.

These were let through a competitive tendering process. The project co-ordination was also let

via the same process and was awarded to Partrac Ltd (Glasgow). Table 1.1 summarises the six

tasks.

Table 1.1 Summary of the six commissioned work packages within the project.

Task Title Consultant

Task 1 Characterising the Issue and Delivering a National Database of Contaminated Marine Sediments in UK Waters

ABPMer

Task 2 Exploring Liability and the Polluter Pays Principle ABPMer/York University Law School/IEEP

Task 3 Identifying Existing Relevant Legislative and Regulatory Barriers, and Guidelines and Protocols, with Respect to CDMS

ABPMer

Task 4 Establishing Best Practice for the Prevention of Pollution arising from CDMS

ABPMer

Task 5 Establishing Best Practice for Current Disposal and Treatment Options for CDMS

Entec

Task 6 Identify relevant marine sediment research and development relevant to the management of CDMS

National Oceanography Centre, Southampton

1.4 This document

This document is the Best Practice Guidelines for the Treatment and Disposal of contaminated dredged marine sediment (CDMS) required under Task 5 of the study. It sets out the findings of a review of practices used both within the UK and overseas for the treatment, disposal and beneficial use of CDMS.

The document has been prepared by the Entec UK Ltd. project team.

5


1.5 Defining the problem and risk

1.5.1 What is CDMS?

There is no universally accepted definition of CDMS5,6,7, but a useful operational definition for

the purpose of this study is:

“sediment (soils, sand, organic matter or minerals) present on the bottom of the

sea (or estuary) that contains one or more chemicals at concentrations that are

above background, exceed an environmental quality standard or guideline and

may pose an adverse threat to the environment or to human health.”

1.5.2 Risks

Sediment is a major sink (and source) for contaminants in coastal waters – a sink in that

particulate matter, including suspended solids, typically scavenges contaminants in the water

column and such matter is ultimately deposited as bottom sediment. Sediment can also act as a

source of contaminants through bioturbation by burrowing organisms or physical disturbance

through meteorological events or human intervention, such as dredging. Contaminants

mobilised in this way can result in impacts/toxicity to aquatic receptors, including shellfish and

fish. Sediments integrate contaminant inputs to aquatic systems over time and in many

industrialised areas of the world serious sediment contamination has resulted in impacts on

aquatic benthic fauna including fish8 that has resulted in restrictions on commercial fishing and

health advice restricting consumption of seafood.

Contaminants in sediment are transported to the water column by two major mechanisms,

namely diffusion from porewater and resuspension of sediment, sometimes aided by biological

activity within the sediment. Disturbance of sedimentary material into the water column can

form plumes and this is mainly influenced by the nature of the sediment and the prevailing

hydrodynamic conditions9. Sediment granulometry influences plume formation, as sediment

that is predominantly composed of finer (silt/clays) material form plumes more readily than

coarser sands, however, their cohesive nature and surface biological growth in shallow waters

may restrict their erosion potential so the relationship between sediment granulometry and risk

of contaminant mobilisation is a complex one. Contaminants such as metals are often mobilised

from sediments during disturbances (such as dredging operations), as the oxidation of reduced

metal species in sediments can result in the release of these sediment-bound contaminants to the

water column. The solubility or partitioning behaviour (i.e. the potential for contaminants to

5 USEPA, 1998. EPA’s Contaminated Sediment Management Strategy. United States Environmental Protection

Agency, Office of Water. EPA-823-R-98-001.

6 Keillor, P. 2007. Deciding About Sediment Remediation. A step-by step guide to making the decisions. Wisconsin

Sea Grant.

7 Simpson, S.L., Batley, G.E., Chariton, A.A., Stauber, J.L., King, C.K., Chapman, J.C., Hyne, R.C. Gale, S.A.,

Roach, A.C., and Maher, W.A. 2005. Handbook for Sediment Quality Assessment. CSIRO, Bangor, New South

Wales.

8Flatfish living on contaminated harbour sediments have been found with various skin lesions including ulcers and

tumours that have been directly correlated with the contaminant (e.g. PAH) content of sediments.

9 CIRIA (2000) Scoping the assessment of sediment plumes from dredging

6


desorb/release from the solid phase [sediment particles] to water) and hence the risk of

mobilisation and likelihood of impacting water quality and aquatic-dependent ecological

receptors is affected by the physical and chemical properties of chemical contaminants

themselves, as well as the properties of the organic and inorganic sediment constituents and the

characteristics (e.g. salinity, dissolved oxygen concentration) of the overlying water.

The risk of mobilisation of contaminants can be assessed based on their sediment-water

partitioning coefficients. For example, it is well known that contaminants such as zinc and

arsenic are less tightly bound to sediments and are more readily mobilised during dredging

operations than are other heavy metals. Many contaminants mobilised from sediments are

rapidly scavenged from the water column as they readily re-bind to suspended particles (rather

than become dissolved) thereby limiting their potential impact to biota in the water column.

1.5.3 Sediment standards

To predict the probability of risk from CDMS to marine life, numerical values known as

screening values or Sediment Quality Guidelines (SQG) are often used. In the UK, Cefas

Action Levels10 are used as sediment chemical screening values for dredged material to

determine the disposal option11 for sediment and are applied to a suite of contaminants routinely

tested in dredged sediment as part of consideration of marine disposal licence applications12.

Where chemical substances in CDMS are below certain thresholds they are generally considered

to be of low risk to aquatic organisms and toxicity on exposure is considered unlikely. Where

chemicals are above certain thresholds, then toxicity is considered likely in most aquatic

organisms. In general, contaminant levels in dredged material below Cefas Action Level 1 are

of no concern and are unlikely to influence the licensing decision as toxicity is unlikely. CDMS

with contaminant levels above Action Level 2 is, however, generally considered unsuitable for

sea disposal because there is a high likelihood of toxicity to marine life. Between these two

Action Levels is an area of uncertainty where cases-by-case investigation is required.

The Cefas Action Levels currently used for dredged sediment assessment in the UK are

presented in Table 1.2.

SQGs have either been adopted or proposed in other countries in Europe, Hong Kong, Australia

and North America and have been reviewed in various publications and are not repeated here.

Table 1.2 Action Levels used by Cefas for Dredged Material Assessment

Contaminant Action Level 1 (mg/kg dry wt.) Action Level 2 (mg/kg dry wt.)

Arsenic 20 100

Mercury 0.3 3

Cadmium 0.4 5

10 The Cefas Action Levels are used for a range of inorganic and organic contaminant parameters.

11 Depending on the level of contamination, dredged sediment may be disposed at sea or on land (disposal to landfill

is under Environment Agency licensing).

12Cefas has set action levels for As, Hg, Cd, Cr, Cu, Ni, Pb, Zn, organotin compounds (TBT, DBT, MBT), PCBs,

DDT and Dieldrin.

7


Contaminant Action Level 1 (mg/kg dry wt.) Action Level 2 (mg/kg dry wt.)

Chromium 40 400

Copper 40 400

Nickel 20 200

Lead 50 500

Zinc 130 800

Organotin compounds (TBT, DBT, MBT) 0.1 1

PCBs (sum of ICES 7) 0.01 None

PCBs (sum of 25 congeners) 0.02 0.2

DDT 0.001

Dieldrin 0.005

Notes: CDMS with contaminant concentrations below Action Level 1 are regarded as of ‘no concern’ and unlikely to influence the

licensing decision. CDMS with contaminant concentrations above Action Level 2 is generally not considered suitable for disposal at sea

(disposal is typically by other route such as landfill). CDMS with contaminant concentrations between Action Level 1 and 2 requires

further consideration and testing before a decision on disposal can be made.

TBT= tributyltin; DBT=dibutyltin; MBT= monobutyltin; PCB=polychlorinated biphenyl; DDT= dichlorodiphenyltrichloroethane.

1.5.4 Environmental Quality Standards for sediments in the EU

In the European Union, Article 16 of the EC Water Framework Directive (2000/60/EC) (WFD)

required definition, as Annex X of the WFD, of a list of priority substances requiring control

through reduction or cessation of discharges, emissions and losses of the substance. The list

was formally established in December 2008 by the EC Directive on environmental quality

standards in the field of water policy (2008/105/EC), which sets EQS for these substances in

water and populates Annex X of the WFD. Article 3(2) of this Directive allows member states

to apply EQS for sediment and/or biota instead of for water and defines EQS for mercury,

hexachlorobenzene and hexachlorobutadiene in biota. EQS for sediments and other EQS for

biota are to be determined by the Member State but must offer at least the same level of

protection as the EQS for water.

Article 4 of the WFD requires action to prevent deterioration in the status of water bodies. If

contaminant levels are being used as an indicator of pollution that might affect the status of the

water body, this implies a presumption against any significant increase in existing levels of

contamination in water, biota and sediments.

8


1.6 Legislative barriers to disposal

1.6.1 Overview

This section summarises briefly some of the legislative barriers that currently limit disposal of

CDMS. Fuller discussion is contained within the Task 3 report for the project13.

1.6.2 Disposal at sea

In the 1970s, the London Convention 1972 was adopted by certain countries for the control of

the disposal of material at sea with a particular focus on the disposal of harmful substances. It

has since been amended in minor ways or through policy agreements but in 1996, the "London

Protocol" was agreed to modernise the Convention and, eventually, replace it. Under the

Protocol all dumping is prohibited, except for certain listed exceptions, which include dredged

material. The Protocol entered into force on 24 March 2006.

The Oslo and Paris Convention (OSPAR Convention), which entered into force in 1998,

combined and strengthened controls set out in the former Oslo Convention and the Paris

Convention but also specifically excluded dredged material from the definition of wastes or

other matter whose dumping at sea is prohibited.

Recognising that only a small proportion of dredged material is contaminated, its disposal is

now considered as a special case and at the heart of the two conventions are two basic

principles, namely the ‘precautionary principle’ (by virtue of which preventative measures are

to be taken when there are reasonable grounds for concern that substances introduced into the

marine environment may result in harm) and the ‘polluter pays’ principle (by virtue of which

the costs for pollution prevention, control and reduction measures are borne by the polluter).

Controls on deposits of materials in the sea are imposed in the UK through licensing

requirements established by the Food and Environment Protection Act 1985, which thus

effectively transposes the Conventions into UK law. These arrangements will soon be updated

in England and Wales by the Marine and Coastal Access Act 2009, which replaces FEPA

licences by a new Marine Licence.

The London Convention, which has been adopted by many countries worldwide, developed

guidelines for assessment of dredged material proposed for disposal at sea. The OSPAR

Guidelines for the Management of Dredged Material are more detailed than those of the London

Convention, (although entirely consistent with the London Convention guidelines), and the

OSPAR guidelines are followed in the UK. These provide generic guidance on determining the

conditions under which dredged material may (or may not) be deposited at sea.

The guidelines evaluate and provide guidance on the disposal option and cover aspects such as:

• physical characterisation;

• chemical characterisation;

13 ABPmer, 2009. Research and Support for Developing a UK Strategy for Managing Contaminated Marine

Sediments. Task 3: Identifying Existing Relevant Legislative and Regulatory Barriers, and Guidelines and Protocols,

with Respect to CDMS.

9


• biological testing;

• Action Levels adopted on a national and regional basis to set environmental quality

standards;

• evaluation of the potential for beneficial use;

• selection of an appropriate site for sea disposal taking account of the likely fate and

effects of the disposed material;

• impact assessments detailing the likely consequences of the disposal option (e.g.

differences in dredged material flux between retentive and dispersive sites);

• licensing of sea disposal; and

• field monitoring and assessment.

Further details can be found in the primary convention or a more recent publication Bray

(2008)14 provides a useful review.

In the UK, a risk-based approach is used to assess the quality of in situ sediment prior to

operations such as maintenance and capital dredging. Cefas advises Defra on the suitability for

disposal of dredged material at sea. In England and Wales, the body carrying out the dredging

is responsible for the chemical testing of dredged material, while Cefas checks and assesses the

results. As described earlier, Cefas compares contaminant concentrations against numerical

values (Action Levels), as part of a ‘weight of evidence’ approach, in line with best practice, to

assess whether dredged material is suitable for disposal at sea.

In England and Wales, for any works requiring the disposal of dredged material, developers are,

required to apply for a Food and Environment Protection Act 1985 (FEPA) licence and may

also require consent under the Coast Protection Act 1949 (CPA) with respect to the deposit of

materials below mean high water springs (MHWS)15. These arrangements will shortly be

updated in England and Wales by the Marine and Coastal Access Act 2009, which replaces

FEPA licences by a new Marine Licence. The Marine and Fisheries Agency (MFA) administers

these consents on behalf of the Department for Environment and Rural Affairs (Defra). Marine

Scotland has a similar role in Scotland and the Northern Ireland Environment Agency in

Northern Ireland. Marine projects consented under marine licences or consents are generally

covered by the Marine Works (Environmental Impact Assessment) Regulations (SI 2007:1518)

but these regulations do not specifically include disposal of dredgings.

The application process for consent for marine dredgings disposal is relatively straightforward,

provided that all the information necessary is readily available. The two consent applications

are submitted together on a single form, together with all supporting information and

documentation (forms can be submitted to the MFA electronically and this both streamlines the

process and is cheaper). Provided that no major hurdles are encountered, consents are typically

granted within four months of the submission of applications.

14 Bray, R.N., 2008. Environmental Aspects of Dredging. Taylor & Francis/Balkema, The Netherlands.

15 CPA may apply to both the excavation/ dredging and placement of dredged material in the marine environment

whereas FEPA only applies to placement.

10


1.6.3 Status of dredged material

Whilst the licensing of direct disposal of dredged material to sea is relatively simple, the

complications arise in relation to the status of the material if it is to be re-used beneficially on

land or treated for purposes such as modifying it so that it is less toxic. On land, different

regulatory regimes come into play and if it is treated, it will no longer be regarded as ‘dredged

material’, which means that disposal at sea is removed as an option, as this would be contrary to

the Conventions.

1.6.4 Regulation of waste disposal

Dredged material is treated as waste by the Conventions and current marine disposal licences

are compliant with the Waste Framework Directive i.e. as waste disposal licences. The

alternative of disposal on land is regulated in England and Wales under the Environmental

Permitting (England and Wales) Regulations 2007 (as amended).

1.6.5 Falmouth Cruise Terminal project case study

A convenient example illustrating the way in which the regulatory regimes are applied is

provided by the project to provide a new cruise ship terminal at Falmouth. Various options

were investigated for the disposal of about 100,000m3 of dredged material that would be

generated by dredging to deepen a navigation channel at the port. A number of options for

CDMS disposal, including treatment prior to disposal, were investigated. There were, however,

numerous barriers encountered and a summary is provided below. Further details can be

obtained from the Environmental Statement prepared for the proposed project 16.

Disposal of Untreated CDMS at Sea

This option was to dispose CDMS at the Falmouth Bay offshore disposal ground. Due to the

elevated contaminant concentrations present, following consultation with Cefas, it was

determined that this option would not be permitted under s.5 of the the Food and Environment

Protection Act 1985, then in force.

Disposal at Sea of Treated CDMS

Any dredged material that is treated on land is currently considered an industrial waste and

cannot, under the present Food and Environment Protection Act 1985 licensing regime, then be

disposed of at sea. This is based on the argument once treated it can no longer be regarded as

‘dredged material’ and would therefore not fall within the exemptions listed in the Conventions.

This option was therefore ruled out for the Falmouth Cruise Project. However, this does not

rule out the use of such materials beneficially on land within for example a construction project,

subject to compliance with EC Directives and national legislation.

Disposal at Sea of Untreated CDMS followed by Capping with Clean Material

Whilst there is experience of this technique in countries such as the USA there is limited

experience in the UK aside from the trial conducted at the Port of Tyne (see Box 4.5 below).

This disposal technique places CDMS on the seabed followed by deposition of a clean cap to

16 Royal Haskoning, 2008. Falmouth Cruise Project. Environmental Statement: Non-Technical Summary. Report

prepared for Falmouth Harbour Commissioners and Falmouth Docks and Engineering Company. Final Report No.

9S4181.

11


isolate the contaminated material. There were concerns over the environmental and technical

performance of the capping technique and this option was ruled out for the Falmouth Cruise

Terminal Project.

Disposal at Sea of Treated CDMS as a Fill Material for Wharf Improvements

During the study various locations were identified where treated CDMS could potentially be

used as fill material, such as improvement to wharves and other structures in the docks. The

permitting route is, however, complicated as both the Environmental Permitting Regulations

2007 and Food and Environment Protection Act 1985 would apply, as the treated CDMS would

have been placed both above and below mean high water springs (MHWS). It was considered

that the treated CDMS could have been used as an infill material for the wharf structures below

MHWS under Food and Environment Protection Act 1985 (placement of construction material)

and the wharf structure above MHWS may have been covered by Schedule 3 of the

Environmental Permitting Regulations (i.e. for ground improvement works). This option was

also ruled out for the Falmouth Cruise Project because the option was not considered feasible

and only part of the CDMS could be re-used. Despite the various disposal options appraised,

due to legal barriers, opportunities for treatment and re-use of CDMS proved difficult. About

20,000m3 of CDMS could be treated/ stabilised and used above MHWS for ground

improvement and coastal erosion protection, subject to these uses being exempted from

permitting requirements. The only option for the further 60,000m3 of CDMS was to send it to

landfill.

1.7 Scope of Task 5: Best Practice Guidelines for the Treatment and Disposal of CDMS

The major aim of this task is to review the best practice methods used both in the UK and

overseas in the treatment and disposal options for CDMS. Specific guidance has been

formulated from the review, together with an assessment of costs, and case studies are presented

that review current practices.

The Key Tasks were as follows.

• The study required close liaison with industry and contractors to identify best practice within the UK and the international arena with respect to current disposal and treatment options for CDMS. This took account of the environmental, economic, social and legislative aspects, strategic and public acceptability and provided case studies of both good and bad management in order to understand the reasons behind successes and failures.

• An assessment of currently available techniques is provided that identifies and comments on the newly emerging technologies, such as biological conversion techniques and existing novel ideas currently employed at quarries, coal, gas and oil production facilities, steel industry and in soil treatment.

• Best Practice Guidelines have been produced for the treatment and disposal of CDMS, incorporating current disposal protocols.

The focus of the present study is on dredged CDMS; the in-situ treatment of contaminated material, as may be used for remedial purposes, is outside the scope of the study. The reader

12


may refer to other sources of information on the various in-situ treatment techniques, which include source control measures to stop or reduce contaminant inputs (i.e. where the contaminant source, such as waste water discharges, is strictly controlled or terminated) and natural processes whereby CDMS is capped by uncontaminated material (i.e. ‘natural attenuation’). Other methods include the sequestration (binding in an inactive state) of contaminants in CDMS by mixing in-situ sediment with activated carbon17, and stimulation of bioremediation through in-situ aeration (biosparging)18. These aspects fall within Task 4 of the overall study.

1.8 Information sources

Sources of information and details of contacts are given in Appendix A.

1.9 The content of this document

The remainder of this Report is detailed below.

• Section 2 reviews treatment options for CDMS.

• Section 3 summarises the potential beneficial uses of CDMS after it has been suitably

treated.

• Section 4 reviews various options for CDMS disposal used both in the UK and

overseas.

• Section 5 provides a review of the socio-economic aspects of CDMS treatment and

disposal.

• Section 6 presents pointers towards best practice.

17 Zimmerman, J.R., Bricker, J.D., Jones, C., Dacunto, P.J., Street, R.L. and Luthy, R.G. 2008. The stability of marine

sediments at a tidal basin in San Francisco Bay amended with activated carbon for sequestration of organic

contaminants. Water Research 42: 4133-4145.

18Thomas, J., Beitinger, E., Grosskinsky, H., Koch, T. and Preuss, V. 2008. Innovative in situ treatment options for

contaminated sediments. In: Urban Sediment Management and Port Redevelopment & Sediment in River Basin

Management Plans pp. 73-74. Abstracts of presentations and posters presented at the 5th International SedNet

Conference, Oslo, Norway.

13


2. Treatment Options

2.1 Introduction

Various treatment technologies have been developed for CDMS and these use a wide range of

different processes to reduce the overall amount of dredged CDMS for disposal. In many cases

the aim is not simply to remove the contaminants but to put the treated sediment to beneficial

use.

One of the overriding factors to consider when treating CDMS is the variability of the types and

concentrations of contaminants and also the variability in sediment characteristics, including

grain size, organic content and mineralogy. CDMS is rarely contaminated by a single class of

contaminant and a spectrum of grain sizes is encountered in dredged material. Different

contaminants require the application of different treatment techniques. In some cases, CDMS

can be effectively treated using relatively simple techniques; others are more complex, requiring

several stages of treatment using different infrastructure or equipment.

Treatments are used to separate, destroy or immobilise contaminants. Extraction of

contaminants results in waste streams that may require further treatment or in some cases

provide a beneficial product. Destruction of contaminants involves changing the contaminant

chemistry or state and often involves high energy consumption. Immobilisation reduces the

availability of contaminants to biological receptors. Technological advances now enable

immobilisation in-situ (although this is beyond the scope of this report). Each method has

advantages and disadvantages. The choice of treatment option is likely to warrant evaluation on

a case by case basis.

Ex-situ treatment options are utilised in situations where the sediment must be moved for

example to maintain harbours and ports and other waterways. Depending on the level of

contamination treated and the regulatory regime, CDMS can be put to beneficial use, disposed

of at landfill/CDF or re-deposited in the same waterway.

The technologies described below are at various stages of development from laboratory (or

bench) scale tests to full scale operations capable of dealing with many thousands of cubic

meters of sediment. The treatment options for CDMS presented below are grouped by

mechanism (i.e. thermal, biological), with the exception of TBT and radioactive contamination

for which the contaminant class is used, and available treatments are discussed. At the end of

each section a treatment summary is given.

Some techniques, such as dewatering, may be used for pre-treatment of CDMS before

application of other methods. Where techniques are typically used in combination, this is

identified in the various subsections below.

The various treatment techniques described below have been grouped into a number of overall

types:

• dewatering techniques;

• separation of size fractions;

14


• separation of contaminants (including chemical, thermal and electrochemical

methods);

• destruction of contaminants (including biological and thermal methods);

• immobilisation of contaminants (including thermal and chemical methods);

• treatment of sediments contaminated with TBT;

• treatment of radiologically contaminated sediments.

2.2 Dewatering

Three methods of dewatering are used; these are natural (gravity), mechanical and geobags.

These are summarised in Table 2.1 and described subsequently.

Table 2.1 Summary of Natural and Mechanical Dewatering.

Natural (gravity)dewatering

Mechanical Dewatering Geobags

Advantages Low energy costs

Large volumes and high production rates

Marketable end product for construction

Higher dry matter content than with natural dewatering giving higher volume reduction and increased beneficial use

Mobility of plant

Lower space requirement

Low energy costs

Large volumes and high production rates

Marketable end product for construction

Disadvantages Space and time required, some NIMBY

Higher energy costs Some space and time required, some NIMBY, space can be minimised as geobags can be stacked.

Application Generally better for sandy sediments

Fine grained sediments Suitable for a wide range of sediments

Location Belgium Belgium, Germany, Netherlands

Sweden

Developmental phase Industrial scale Industrial scale Industrial scale

Costs 10-25 €/m3 10-30 €/m3 (fixed installation) 12-35 €/m3 (mobile installation)

20-30 €/m3

Information Sources: Bortone G., Palumbo, L. 2007.SedNet. Sustainable Management of Sediment Resources, Vol 2. Sediment and Dredged Material Treatment. Published by Elsevier, Holland.

DEC, 2002. Overview of practical experience obtained in Flanders in the treatment of contaminated sediments. DEME.

DEC, 2004. Treatment and Beneficial Re-Use of Contaminated Sediments. DEME.

DEC. 2004. Experience with Mobile Dewatering of Dredged Sediments. DEME.

DEC. 2005. Treatment of Contaminated Sediments in Harbour Areas. Review on 10 Years Experience. DEME

Dewatering Contaminated sediments (using Geobags) - http://www.tencate.com/smartsite.dws?id=8530

15


2.2.1 Natural dewatering and lagooning

Gravity dewatering of dredged material takes place to some extent on board suction dredgers

where the material pumped from the seabed is discharges to the hold of the ship and excess

water overspills back into the sea. More controlled dewatering is sometimes undertaken on

board barges used as receptors for material dredged using backhoe or grab methods.

For large volumes of sediments natural dewatering is the most efficient treatment method for

CDMS material. Dredged material is laid out in windrows on dewatering fields (Fig. 2.1) to

enable drying and consolidation. The sediments are mechanically turned (usually using a

mechanical excavator) during this period. Subsequent layers are added at intervals when the

previous layer reaches the desired level of dewatering. Whilst much of the water content

evaporates, the base of the lagunation fields has a complex series of drains which consist of

filters and plastic pipes. The drainage system is designed so that, once drainage has started, a

capillary action is created which promotes suction of water from the sediment pores. Water that

drains from the lagunation fields is pumped to a wastewater treatment plant. 19 20.

Oxidation of minerals and organic matter often improves the engineering properties of the

sediment and a limited amount of biological degradation can occur. However, oxidation of

sulphate in marine sediments leads to production of sulphuric acid which can then increase

leaching of metals and other contaminants and cause a serious problem. Although natural

dewatering is a low energy process, a large area is required, therefore stimulating agents and

nutrients can be added to decrease the overall time needed. The time required for drying and

hence addition of new layers depends on sediment type; the best results are shown with sandy

sediments and in terms of costs natural dewatering is the best dewatering method for silts.

Costs are estimated to be 10–25 €/m3 21.

Figure 2.1 Dewatering/lagooning field (aerial photography courtesy of DEME)

19 DEC, 2002. Overview of Practical Experience Obtained in Flanders in the Treatment of Contaminated Sediments.

DEME.

20 DEC. 2005. Treatment of Contaminated Sediments in Harbour Areas. Review on 10 Years Experience. DEME

21 Bortone G., Palumbo, L. 2007. SedNet. Sustainable Management of Sediment Resources, Vol 2. Sediment and

Dredged Material Treatment. Published by Elsevier, Holland.

16


An example of use of natural dewatering at the Port of Hamburg is described in Box 2.1.

Box 2.1 Port of Hamburg: The Application of Sand Flushing Fields, Dewatering and METHA technologies

Background: In 1980 the Hamburg Dredged Material Management Concept (HDMMC) was developed to address the growing need to incorporate environmental best practice into dredging operations and maintain operational capability of the Port of Hamburg. The concept involved development and assessment technologies for treatment, beneficial use and disposal while considering wider regional environmental impacts. The assessment has considered a number of treatment options and with respect to chains of beneficial use. Key technologies of the HDMMC are the mechanical treatment and dewatering of dredged material from maintenance dredging.

The primary objective of the concept is the beneficial use of the sand fraction and where possible the silty sediment or treated silt fractions. Where silt cannot be put to beneficial use they are disposed of; for this purpose two landfill sites with a total capacity of 18 Million m3 were constructed. The landfills were constructed on old flushing fields, which enable the waste water to be captured; this is then treated in a treatment plant. In total the HDMMC treats 1.25 – 1.45 Million m3 of contaminated dredged marine sediments annually.

Treatment Technologies

Sand Flushing Field Ellerholtz:

The Ellerholtz facility has two active flushing fields with a total active area of 4ha and 2.5ha with a storage capacity of 90,000m3 and 60,000m3, respectively. The principle of operation relies upon the settling behaviour of sand, silt and organic matter. The sand fraction settles out under gravity following flushing at the end of the channel whilst the finer, lighter grains and the organic matter are flushed out at the end of the channel. The channel base is sloped and also acts as a drainage layer. Annual throughput of sediments from three runs is capable of producing up to 450,000m3 of sand.

Overflow water from the site must first reach the appropriate environmental standards before it is discharged to the River Elbe. This has proved to be possible only through sedimentation prior to discharge i.e. drop-out of the finer particles. The facility at Ellerholtz therefore has a 1.1ha sedimentation basin for this purpose. Final effluent is discharged to the Elbe and quality and volumes are monitored.

Dewatering Facility Moorburg:

Moorburg is a 100 ha site consisting of 31 dewatering fields of about 2-4ha each. The site has the capability to treat approximately 200,000 m3 annually. The sediments are laid out on the fields to an average height of 1.3m. The site is situated on a conventionally flushed field and groundwater is additionally protected using a silt layer with a drainage layer at its surface. Following a settling period for the silt of a few weeks, the supernatant water is removed. Following this the actual drying period begins. Drying fissures’ are the first indication that the silt surface is dried and this is then removed and stacked for further drying. Depending on the required technical specification for the end-product the stacking process continues. Generally the required solid content is 55-60% by weight which, in Hamburg takes a period of approximately 9 months to 1 year.

Mechanical Treatment of Harbour Sediments (METHA) Plant:

Dredged sediment is input via a pump intermittently into a basin with an approximate total volume of 250,000 m3. From here the material is discharged into a bar-screen close to the basin and coarse (>60µm) fractions are separated. Following this a further screening (to 10µm) takes place in a rotary screen. The ‘suspension’ is subsequently pumped into two parallel operating lines of 100t/h (dry substance) capacity each where they are sorted into sands and silts in a hydrocyclone. Debris such as wood, coal and fine silts that remain in the sand are subsequently removed in an upstream current classifier.

To achieve a construction material grade sand, the coarse product leaving the upstream current classifier is dewatered on a vibrating screen to achieve a moisture content of 10-15% by weight. Of the silt suspension fraction from the hydrocyclone, 50% is used as input material into a second separation process (20µm) and as previously the suspension is pumped into two parallel lines with a capacity of 25t/h (dry substance). A hydrocyclone is used to separate this suspension into fine a fine fraction (<20µm) and a fine sand fraction (20-150µm). Additional cleaning is required however in order to remove the debris. The final step in the process is dewatering to a moisture content of 15-17% by weight using a vacuum belt filter.

The annual throughput of the METHA plant is about 550,000t (dry sediment) or 1 Million m3, in a ratio of 50%:50% by weight silt and clay. Following treatment of the CDMS the residue can either be further treated or put to beneficial use.

Windrows

17


Wastewater Treatment

A key consideration for the treatment of dredged material is management of waste water. Effluent from sediment treatment processes is often contaminated with metals, inorganic and organic particles as well as ammonium from farming, industry and households which are bound to suspended particles. The SARA plant is situated adjacent to the Francop Landfill and has the capability to treat 600m3/h (2.3 Million m3/year). The principle of this treatment is based on removal of the suspended particles by sedimentation aided by flocculation. After sedimentation, nitrification and oxidation takes place. The nitrification process relies upon bacteria and so control of the temperature is necessary for maximum efficiency. The nitrification and sedimentation processes reduce the organic contamination in the waste water. The Chemical Oxygen Demand is reduced to an average value of 55 milligrams per litre, a reduction of 32%. Dead bacteria from this process forms a sludge which is removed using a series of drum belt filters and disposed of at the silt mound.

Information Source: Detzner, H. D. and Knies, R. 2003. Treatment and beneficial use of dredged sediments from Port Hamburg, 2004. cited in: Proceedings of the XVIIth WODCON (paper B2-2), Hamburg, Germany, 2003.

2.2.2 Mechanical dewatering

When compared to natural methods, mechanical dewatering is more energy dependent but

requires less space and time. In addition higher dry matter contents and hence volume reduction

can be achieved. Other benefits resulting from the higher shear stress of the dry material

include increased acceptability for beneficial use or landfill disposal. Conditioning materials,

such as lime or cement, may be added to absorb water and improve the physical characteristics

of the treated material. Mechanical dewatering is conducted using filter presses or belt presses

of which there are various types, each including storage and conditioning tanks for the dredged

material. Estimated costs are estimated to be 10-30 €/m3 for fixed plants and 12-35 €/m3 mobile

plants (Fig. 2.2)21 22 23 24. An example of the application of mechanical dewatering is presented

below in Box 2.2.

Figure 2.2 Mobile Dewatering Plant. (Photo courtesy of DEME)

22 Netzband, A., Hakstege, A.L., Hamer, K. 2002. Treatment and Confined Disposal of Dredged Material. Dutch-

German Exchange on Dredged Material.

23 DEC. 2004. Experience with Mobile Dewatering of Dredged Sediments. DEME

24 DEC, 2004. Treatment and Beneficial Re-Use of Contaminated Sediments. DEME.

18


Box 2.2 Mechanical Dewatering: Miami River 15ft Deepening Project by Dredging.

Between 2004 and 2008 a 5.5 mile stretch of the Miami River was dredge deepened. The project included dredging of 720,000 cubic yards of sediments that were mainly contaminated with metals which were then treated prior to being put to beneficial re-use and / or disposal.

A fully automated mobile processing plant was employed to separate the sediments into different re-usable fractions and the fine fractions were mechanically dewatered. The average production rate was in excess of 1500 m3 per day.

Post treatment sediments were loaded directly onto trucks and taken off-site, thus no temporary stockpile was necessary and the overall operational footprint kept to a minimum.

Information Source: http://www.boskalisdolman.nl/. (Accessed 16/02/2009).

The AMORAS project currently being implemented in Antwerp involves a combination of

sieving to remove sand and lagooning to consolidate the remaining CDMS, followed by

dredging the consolidated material and dewatering using a filter press, before final disposal to

landfill25.

2.2.3 Geobags

Dredged material can be deposited or pumped into bags or tubes made of woven geotextile

materials that allow water to drain out while retaining the solids26. After the final containment

and dewatering cycle, the retained fine grained materials in the filtration tubes continue to

consolidate by desiccation, while residual water vapour escapes through the small pores of the

fabric. Bags or tubes can be stacked several layers high in a dewatering basin and the dried

material can then be disposed of to landfill in the bags or removed and handled in bulk. Further

detail is given in section 4.7.

2.3 Separation of particle size fractions

Separation relies upon the density differences between particles and bulk liquids and gravity;

Separation is achieved using floatation techniques, separation ponds, hydrocyclones or geobags.

In some cases, e.g. for low level contamination, pre-treatment techniques alone may reduce

contamination to comply with environmental standards.

Contaminants in sediment generally have a propensity for binding or sorption to the finer grain-

sized particles. Additionally higher contamination concentrations are often associated with

sediments with higher organic matter content. Thus separation of finer particle size or less

dense organic fractions of CDMS can be an effective method of separating out contaminants,

where this is the case. However, it is necessary to establish the distribution of contaminants

across sediment particle size and density fractions in each case, as some contaminants, for

example TBT, may be associated with much larger particles, sometimes of anthropogenic

origin, for example paint flakes from shipyard operations. Further discussion on TBT is set out

in section 2.7.

25 See http://www.deme.be/Press/press_item.asp?iID=106

26 See www.tencate.com/smartsite.dws?id=8530

19


Two main methods are widely used, mechanical and longitudinal classification.

Longitudinal classification is carried out using sloped flushing fields (or sedimentation basins)

which are elongated basins with a gently sloping base through which sediment slurry transits

slowly from one end to the other. As the sediment moves across the slope it settles out of the

water. Longitudinal separation relies upon the differential settling behaviour of fine and coarse

particles which enables the separation of slightly from highly contaminated sediments.

Mechanical classification, can be achieved using sieves and/or hydro-cyclones27 or separation of

fine from coarse particles using sediment basins or a variety of apparatus such as upstream-

current-classifiers, spirals, jigs, flotation-cells28 (which are mechanical devises for separating

sediments by size). Examples in which separation and classification is employed are shown in

Box 2.1 and Box 2.3. Separation techniques alone are generally not sufficient to enable

beneficial use or disposal of the sediments and have to be combined with other techniques such

as dewatering21. A summary of these techniques is provided in Table 2.2.

Box 2.3 De Slufter Rotterdam, Sand Separation Demonstration Project.

De Slufter is a Confined Disposal Facility (CDF) in the Port of Rotterdam with a sand/silt separation basin. The site is 260-hectares in size and has a capacity of 1million cubic metres. Approximately 3 million m3 per year of sediments derived from maintenance dredging of the Rotterdam harbours are transported to the site via barge or dredger. Much of the silt fraction of dredged material is contaminated.

Sand is separated from contaminated silts using a sand separation basin in order to retain as much capacity as possible. The sand separation basin expands in the direction of flow to allow the sand to settle naturally while fine grained (contaminated) sediment remains in suspension and flows into De Slufter.

The Sand Separation Demonstration Project is an initiative of the Dutch Ministry of Transport, Public Works and Water Management. The original intention of the project was to process approximately 70,000m3 of fine sand (100,000 tons of dry product) over two years, however, a project extension meant that 150,000m3 was treated out over 3.5 years

Information Sources: http://www.boskalisdolman.nl/ (Accessed February 2009). http://el.erdc.usace.army.mil/elpubs/pdf/doert1.pdf - Innovations in Dredging Technology: Equipment, Operations, and Management, DOER, 2000. ERDC TN-DOER-T1. Accessed 27/03/2009

27 A hydrocyclone is a mechanical device in which sediments are rotated or ‘spun’ which allows differential settling

of the various grain sizes which enables them to be separated.

28 M Vanthuyne, A Maes, Cauwenberg, P. (2003) The use of flotation techniques in the remediation of heavy metal

contaminated sediments and soils: an overview of controlling factors. Minerals Engineering 16, 1131-1141

20


Table 2.2 Summary of Separation Techniques

Longitudinal Separation Mechanical Separation

Advantages Low energy costs Lower space requirement

Disadvantages Large area required Higher energy costs (but still relatively low)

Application Generally better for sandier sediments.

Must be combined with other technique for beneficial use.

Fine grained sediments. Must be combined with other technique for beneficial use.

Location Netherlands and Germany Hamburg, Rotterdam

Developmental phase Industrial scale Operational and demonstrated

Costs 3-11 €/m3 3-11 €/m3

Information Source: Bortone G. & Palumbo, L. 2007. Ibid

2.4 Contaminant separation

2.4.1 Chemical extraction

Also known as sediment or soil washing, this technology utilises a combination of mechanical

actions, water and chemical additives to remove contaminants from sediments. It relies upon

transferring the contaminants from the sediment to the washing solution. Depending on source

material, each scenario may warrant a different approach to treatment. Soil washing technology

can be very simple; using water and mechanical action or more complex, involving extraction

agents such as acid, bases, chelatants, surfactants, biosurfactants and reducers.

This technology is suitable for all types of inorganic and organic contaminants and hence those

contaminants typically found in urban harbours. Both fine and coarse grained sediments can be

treated although higher contaminant removal efficiencies are reported with coarse fractions.

Washing is most suitable for more weakly bound metals in the form of hydroxides, oxides and

carbonates. Mercury, lead, cadmium, copper, nickel, zinc and chromium can be removed by

electro-chemical processes, although precipitation is not suitable for metal sulphides29.

Biodegradable biosurfactants have been demonstrated to remove low exchangeable fraction

metals with high efficiencies (up to 100% zinc & 70% copper) from contaminated soils and are

also be capable of remediating sediments. The biodegradability of the surfactants can have the

additional benefit of enhancing hydrocarbon degradation29. General removal efficiencies for a

range of contaminants in fine sediments are in the range 60 to 80%. Higher efficiency is

possible with coarse grained material. Contributing factors to the removal efficiencies are initial

29 Mulligan, C. N., Yong, N., Bibbs, B. F. 2001. An evaluation of technologies for the heavy metal remediation of

dredged sediments. Journal of Hazardous Materials. 85: 145-163.

21


contaminant concentration and type(s), sediment matrix, total organic carbon content (TOC) and

extent of treatment21. A summary of soil washing is presented below in Table 2.3.

Table 2.3 Summary of Sediment Washing

Contaminant extraction by sediment washing

Advantages Effective on a wide range of contaminants and sediment sizes, low environmental impact, socially acceptable, low emissions, beneficial products

Disadvantages Wastewater treatment requirements, not all contaminants extractable

Application All sediment types. More effective for coarse sediments. All contaminant types.

Pre treatment None

Location USA, Canada, Hong Kong, Italy, UK

Developmental phase Commercial

Costs Scaled: 55-80 €/m3 (300,000m3) / 90-130€/m3 (50,000m3)

Information Source: Bortone, G., Palumbo, L. 2007. Ibid

http://www.biogenesis.com/ssebbs.html (Accessed February 2009)

A schematic diagram of the Biogenesis Technology is presented in Box 2.4. An example of the

use of soil washing is presented in Box 2.5.

22


Box 2.4 Soil Washing: Biogenesis Technology

One of the most well known trade names for soil washing is BioGenesis which was developed in the USA and awarded a U.S. patent in 2001. A schematic is shown below. This technology has been globally tested and has been scrutinised by multiple agencies. The company has worked with Environment Canada, the US Environmental Protection Agency (USEPA), the State of New Jersey Department of Transportation and the Port Authority of Venice26. Biogenesis has demonstrated full scale implementation in the Port of New Jersey with a decontamination program for 24,000m3 of sediments. Prior to large scale implementation it is first important to understand the nature of the sediment and contamination on a case by case basis, so remediation should begin with a feasibility study in order to assess the optimal treatment parameters.

Information source: Bortone, G., Palumbo, L. 2007. Ibid

http://www.biogenesis.com/ssebbs.html (Accessed February 2009)

BioGenesis Schematic. Courtesy of BioGenesis Inc 2009

23


Box 2.5 Examination of potential use of Using BioGenesisSM

Washing Technology for Ex-Situ Remediation of Hong Kong Marine Sediments

Introduction

The Biogenesis sediment washing technology was one of several treatment methods under consideration by the Government of the Hong Kong Special Administrative Region for the potential ex-situ treatment of heavily contaminated marine sediments within the Kai Tak Airport Approach Channel (KTAC), as part of the South East Kowloon Development project. Appraisal of the treatability on approximately 800 litres of homogenized marine sediment from seven sampling locations in the KTAC was undertaken using BioGenesis Technology.

South East Kowloon Development Area and KTAC

Methodology

The testing program proceeded in three stages: initial chemical/physical examination, optimisation, and treatment validation. The KTAC sediment consisted of essentially clayey silt with fine sand, with high concentrations of a wide variety of organic and inorganic contaminants. Four mixtures of chemicals and reagents were tested.

Results

Analytical results for treated sediment showed that the BioGenesis technology produced significant reductions in inorganic and organic compounds; inorganics were reduced from 30% to greater than 99%, while organics were reduced from 23% to greater than 92%. The total organic carbon (TOC) content was reduced by 79% in the treated sediment. Based on the results of the treatability testing, the KTAC CDMS is treatable in terms of sediment physical characteristics, and contaminant removal using the soil washing technology.

Validation

Results indicate that ex-situ sediment washing using the BioGenesisSM technology is applicable to the remediation of contaminated KTAC sediment; the proprietary chemical mixture can easily be optimized to achieve cleanup goals for the project.

Information Source: DeDen, M., K. Johnson, C. Stevens, M. Amiran, and C. Wilde. 2003. Ex-Situ Remediation of Hong Kong Marine Sediments Using BioGenesis

SM Washing Technology. Proceedings of the Second International Conference

on Remediation of Contaminated Sediments, Venice, Italy, September 30 – October 3, 2003. Available at http://www.biogenesis.com/venicedocs/10TechPap03.pdf Accessed January 2009

24


2.4.2 Thermal desorption (volatilisation)

Thermal Desorption

For dredged material that contains contaminants that can be volatilised at temperatures less than

650 ˚C, thermal desorption can be used as a treatment for removal of contaminants from CDMS.

Typical contaminants include PAHs, PCBs, mineral oil, mono aromates, cyanides, chlorinated

solvents and TBT. Thermal desorption involves several stages. For marine or estuarine

sediments it may be necessary to desalinate by washing with fresh water. For sediments with

high water content, pre-treatment by mechanical or passive dewatering is necessary to reduce

the moisture to below 30%. After pre-treatment CDMS is heated in a rotary kiln where

contaminants are volatilised, the gases are then removed and treated. Five-part gas treatment

systems have been developed which remove particles, destroy organic contaminants (with a

99.99% efficiency) and optionally can reduce sulphates (should they be present)21 30 31.

Thermal desorption is capable of removing organic contaminants with concentrations up to

many thousands of mg/kg. Inorganic compounds cannot generally be removed using this

technique, except for volatile metals such as mercury. Should the input CDMS be free of

inorganic contamination the output material from this technique can be considered suitable for

re-use. A summary of thermal desorption is presented below in Table 2.4 and Box 2.6 presents

two examples of desorption technology.

Table 2.4 Summary of Thermal Desorption

Thermal Desorption

Advantages 99.99% efficiency for organic removal Applicable for TBT Beneficial secondary products

Disadvantages Energy intensive / complex

Application Organic contaminants and some high volatility metals.

Pre-treatment Dewatering/separation of sand fraction and/or washing to desalinate in some cases

Location Netherlands, Belgium and Germany

Developmental phase Industrial scale

Costs 50-70 €/ton excluding pre-treatment

Information Source: Bortone G., Palumbo, L. 2007. Ibid

Hamer, K., Karius, V., 2002. Producing bricks from dredged harbour sediments – an industrial scale experiment. Waste Management 22: 521-530

30 Detzner, H. D. and Knies, R. 2003. Treatment and beneficial use of dredged sediments from Port Hamburg, 2004.

cited in: Proceedings of the XVIIth WODCON (paper B2-2), Hamburg, Germany, 2003

31 Hamer, K., Karius, V. 2002. Producing bricks from dredged harbour sediments – an industrial scale experiment.

Waste Management 22: 521-530.

25


Box 2.6 Thermal Desorption of Mercury:

Examples

A commercial process to remediate mercury contaminated sediments has been operational since 1994. The principle of operation is a two step process involving a drying stage and desorption of the mercury and relies upon the mixing of proprietary material and the contaminated sediment at temperatures between 150-650˚C. The vapour is condensed to form a 99% pure metallic form of the vapour phase; the air emissions do not contain mercury vapour. Unit capacities range from 0.5 to 10t/hour and treatment plants can be mobile or fixed, batch, continuous or semi-continuous. The technology is capable of accepting mercury as oxides, chlorides and sulphides. Treated material contains less than 1mg/kg of mercury.

Another process to treat mercury in CDMS utilises a rotary dryer operated at 400-650˚C for desorption and reduction of mercury oxides and sulphides. Gas treatment systems receive the vapours via a carrying medium (nitrogen). About 0-30% of the gas is removed by a dust scrubber, the liquid from which is treated to separate water, organics, mercury and sludge. A two stage condenser is then employed to condense mercury to a purity sufficient for removal from the system. The gas from the condenser is then passed through a mist eliminator to remove droplets then 5-10% of this is passed through particulate filter and carbon absorption system before discharging to the atmosphere. The remainder is recycled through the rotary dryer. This technology has been shown to reduce the concentration of mercury in soils and sediments from 130-34,000mg/kg to 1.3-228mg/kg.

Information Source: Mulligan, C. N., Yong, R. N., Bibbs, B. F. 2001. Ibid.

2.4.3 Electrokinetic separation

The general principle involves the use of low voltage DC currents which are applied through

electrodes directly to the contaminated sediment which acts as a conductive medium. Soluble

contaminants move via electro-osmosis and electromigration because of the applied currents

and due to hydraulic, chemical and electrical gradients. Migration of charged ions occurs in the

pore water of the sediments, which allows the collection of cations (e.g. metal ions) at the

cathode and anions at the anode. So far, many of the case studies have involved in-situ

remediation, however it is also feasible that ex-situ treatment could be carried out with adapted

infrastructure. Although electrokinetic remediation technologies are not a new concept, the

technologies are not widely used and large scale application has not been widely adopted by

industry. To date most work with electrokinetic treatment is confined to bench, pilot or small

scale field and few attempts have previously been made to investigate metal removal from

saline reduced sediments32, 33.

Electrokinetic treatment technologies have been shown to have variable results for both fresh

and marine water sediment sources and have the potential to remove metals, radionuclides and

some organic compounds33. Much of the available data have been collected from experiments

with soils rather than sediments but theoretically many of the principles are transferable to

marine sediments.

The efficiency of electrokinetic treatment of soils depends upon sediment type, mineral

composition and pore fluid conditions. More recent studies specifically investigated the

feasibility of electrokinetics to remove copper from saline reduced sediments with mixed

success32. Experiments using sediments collected from Pagham Harbour in West Sussex in an

32 Altaee, A., Smith, R., Mikhalovsky, S. 2008. The feasibility of decontamination of reduced saline sediments from

copper using the electrokinetic process. Journal of Environmental Management. 88: 1611-1618.

33 Alshawabkeh, A. N. 2001. Basics and Applications of Electrokinetic Remediation, Short Course Northeastern

University.

26


electrokinetic cell were prepared and spiked with copper. The results of these experiments

showed that copper removal from saline reduced sediments cannot be achieved using

conventional unenhanced electrokinetic experiments. In order to achieve a higher copper

removal, the addition of an agent is necessary which depolarises the cathode reactions. In a

field demonstration project conducted by ERDC at Point Magu, USA in 199434 it was apparent

that brackish water caused significant problems with conductivity. To overcome the problem a

relatively high current density was used and overall energy expenditure was found to be

reasonable and a good efficiency of chromium and cadmium removal was achieved35.

The removal of some organic contaminants from soils has been proven in a number of studies

using electrokinetic techniques. Contaminants such as phenol, gasoline hydrocarbons and

trichloroethylene have all been removed from soils at varying levels of success but some studies

indicate that free phase non-polar organic compound removal is more questionable. Of the

treatment options available for CDMS, few are considered as solutions for actively removing

radionuclides. Bench studies by indicate that electrokinetics have the potential to remove

radionuclides from soils, which may also indicate its potential for treatment of radiologically

effected marine sediments; the study demonstrated that uranium can be efficiently removed

from kaolinite by formation of uranium hydroxide precipitate at the cathode although the

technique appears to have limited potential to remove thorium and radium 36 37. In the case of

thorium, the limiting factor was thought to be related to the precipitation of a hydroxide gel

which prevented transport to the cathode; for radium, the low removal efficiency was thought to

be related to either the propensity for binding to clay minerals or the precipitation of radium

sulphate.

In general the potential of electrokinetic treatment has only been examined at the experimental

or pilot scale and large scale industrial applications capable of treatment of thousands of cubic

meters of sediments have not been realised. By and large the technology is designed to be

operated in-situ using electrode wells sunk at intervals across the site; however experiments

with the electrokinetic cells indicate the potential for shore based ex-situ treatment units.

Electrokinetic technologies are complex, expensive and potentially would require a lot of

energy to run on a large scale. In addition different sites are likely to require different

approaches and it is not yet known if the techniques will be applicable to sediments

contaminated with a wide range of substances. A summary of electrokinetic treatment is given

in Table 2.5.

34 ESEC 2000. In-Situ Electrokinetic Remediation of Metal Contaminated Soils Technology Status Report. US Army Environmental Center. Report Number: SFIM-AEC-ET-CR-99022. July 2000

35 Alshawabkeh, A. N. et al. 1997. Effect of solubility on enhanced electrokinetic extraction of metals. In Situ

Remediation of the Geoenvironment, Minneapolis, Minnesota, October 5-8, 1997.

36 Ugaz, A., Puppala, S., Gale, R.J., and Acar, Y. B. 1994. Electrokinetic soil processing: complicating features of

electrokinetic remediation of soils and slurries: saturation effects and the role of the cathode electrolysis. Chemical

Engineering Communications, 129:183-200.

37 Acar, Y. B., Li, H., and Gale, R. J. 1992. Phenol removal from kaolinite by electrokinetics. Journal of

Geotechnical Engineering, 118:1837-1852.

27


Table 2.5 Summary of Ex-situ Electrokinetic Treatment Technology

Ex-situ Electrokinetic Treatment

Advantages Potential for radiological remediation

Can be effective with clay soils

As an ex-situ method, advantages are limited considering the high costs, high energy and complex apparatus and there is little available experience of such treatment.

Disadvantages Doubts over sustainability and risk management i.e. groundwater pollution

Application Polar organic and inorganic compounds, metals and radionuclides (uranium and strontium)

Mostly sandy sediments but also clays if low water table

Pre-treatment N/A

Location USA

Developmental phase Emerging concept / demonstration scale

Costs Not established

Alshawabkeh, A. N. 2001.Ibid

Ugaz, A., Puppala, S., Gale, R.J., and Acar, Y. B. 1994. Electrokinetic soil processing: complicating features of electrokinetic remediation of soils and slurries: saturation effects and the role of the cathode electrolysis. Chemical Engineering Communications, 129:183-200.

Acar, Y. B., Li, H., and Gale, R. J. 1992. Phenol removal from kaolinite by electrokinetics. Journal of Geotechnical Engineering, 118:1837-1852.

2.5 Contaminant destruction

2.5.1 Biological treatment

Bioreactors

Biological degradation enhances the breakdown of organic contaminants into harmless

compounds through the action of micro-organisms. Such techniques are generally not suitable

for recalcitrant contaminants such as metals (except organometallic compounds) and also tend

to be slower than some other treatment technologies.

The purpose of a bioreactor is to provide optimal conditions for the desired biodegradation

process under controlled conditions, usually either by introducing oxygen to sediment in order

to break down organic contaminants under aerobic bacterial/microbial action or in an enclosed

reactor, to achieve anaerobic digestion. Aerobic treatment is carried out using large tanks or

basins which have a form of mechanical mixing device to enhance the reactions. The technique

is most suitable for contaminants that rapidly degrade such as low ring PAHs and mineral oils,

which can be treated within a relatively short time-scales involved in this process21, however,

more recalcitrant compounds can also be treated using a cascade of different conditions to

enhance a phased degradation.

28


Degradation within bioreactors can reduce PAHs and mineral oils within sediments and are

capable of varying efficiencies which range from 10 – 90%38. Costs and efficiency of treatment

are directly proportional to time. Costs of >100€/m3 have been experienced in practice which is

prohibitively expensive compared to other methods of treatment. A bioreactor is intended as an

industrial treatment method which should aim to reduce organic contaminants to a degree where

the output sediment is suitable to make a reusable product. For production of a usable soil

product a further dewatering step may be necessary22.

Bioleaching

Bioleaching is possible in bioslurry reactors (or by heap leaching39) for the extraction of metals

from sediments. One proposed method involves bacteria which reduce sulphur compounds

under aerobic and acidic conditions at temperatures between 5-55˚C. Leaching can be direct or

indirect either by the production of sulphuric acid resulting from acidification of sulphur

compounds leading to metal desorption or solubilising metal sulphides as a result of oxidation

to metal sulphates. Laboratory tests have demonstrated a 70-75% removal rate of metals from

sediments with the exception of lead and arsenic. Bacteria and sulphur compounds are added to

both heap leaching and bioslurry reactors. The sediments are then mixed and pH is controlled.

Copper, gold and uranium have been reported to have been removed using this technology29. A

summary of bioreactors is presented in Table 2.6 below.

Table 2.6 Summary of Bioreactor Treatments

Bioreactors

Advantages Wide range of contaminants

Disadvantages Variable results Expensive

Application Generally organic contaminants but some success demonstrated with metals and applicable to all types of sediments

Pre-treatment None

Location Netherlands

Developmental phase Pilot scale but intended to be industrial

Costs >100 €/m3 up-scaling will reduce costs


38 H. Seidel, J. Ondruschka, P. Morgenstern, U. Stottmeister, Water Sci. Technol. 37 (1998) 387. In: Mulligan, C, N.,

Yong. R, N., Bibbs. B, F., 2001. Ibid.

39 ‘Heap leaching’ is a method of removing metals whereby sediments are heaped on an impermeable membrane and

an appropriate leaching solution is added to the heap. The solution percolates through the sediment, removing metals

by binding to them and then flushing out at the bottom of the heap.

29


Field experiments38 have demonstrated that percolation leaching is more efficient when sulphur

is used as a substrate rather than sulphuric acid and in addition oxic sediments showed a higher

metal removal efficiency (62%) than anoxic sediments (9%) indicating that anoxic sediments

should be ripened (see below) prior to treatment.

Passive Landfarming/Ripening

In common with aerobic bioreactors, the use of land farming relies upon oxygenating sediments

over time to achieve the reduction of organic contaminants to acceptable residual concentrations

and likewise effectiveness is directly proportional to time. This technique is most suitable for

easily aerateable sediments i.e. low in clay content and well drained, however the technique can

be applied to clay soils if organic matter is present but sediments with high clay contents must

be deposited in thinner layers21.

Passive landfarming, whereby no or limited activity is employed in the maturation process is

much slower than intensive landfarming in which optimisation techniques are used to reduce the

time involved to that comparable with a bioreactor, however the passive method is capable of

treating both rapid and slow degrading compounds.

Ripening is the name often given to dewatering the sediments and developing of the correct soil

structure and is often necessary prior to landfarming although can be conducted concurrently in

the passive process. The expected timescales for dewatering are between several months and

several years. Due to the long time-scales (decades) and large areas involved in passive

landfarming, another beneficial use must be made of the site in order to make it economically

viable.

As an example, in the Netherlands, biomass crops of willow are grown and used as a fuel when

fully matured. During the growth period, the willow trees aid dewatering of the contaminated

medium whilst biodegradation of organic contaminants occurs. In Belgium willows are also

cultivated as a biomass but have been put to use on land given over to the deposition of CDMS

that would otherwise have been put to no use. Instead the land can be reclaimed due to

dewatering, landscaping and the introduction of wildlife. Willow species with high metals

tolerances have been chosen for cultivation in Belgium. Some experimental results show that

the willow failed to grow on substrates with sand contents of 60%21 22 29 30. A summary of this

treatment is given in Table 2.7.

Active Landfarming

Active landfarming involves additional activity to promote the speed of treatment compared to

passive landfarming. Two main factors that affect the speed of the treatment are the availability

of the contaminants and the time taken for diffusion of the organisms to the contaminated parts

of the sediment. Promoting vegetation growth substitutes active dewatering which is rapid once

good coverage is achieved. Readily available PAHs are reported to be biodegraded between

one and three years but less available PAHs can take between three and six years to be degraded

by 50%22.

Metals will not be reduced ‘naturally’ using landfarming techniques and stabilising agents (such

as lime or gypsum) must be added to the soil to decrease their availability such as those

techniques already utilised for this purpose in agricultural soils. Methods of landfarming have

been developed using chelating organic acids, with soil conditioners and nutrients which has the

effect of both chelating metals and initiating biodegradation29.

30


Table 2.7 Summary of Landfarming / Ripening

Landfarming / Ripening

Advantages Beneficial secondary products including biomass

Disadvantages Large space required

Long time required: 1-2 years (intensive) decades (passive)

Application Organic contaminants. Metals only in conjunction with stabilisation


Pre-treatment Sorting

Location Netherlands, Belgium, US

Developmental phase Industrial scale

Costs 15-20 €/m3. Beneficial use profits are deductible

Information Sources: Bortone, G., Palumbo, L. 2007. Ibid; Netzband, A., Hakstege, A. L., Hamer, K. 2002. Ibid.

Confined Disposal Facilities

Use of confined disposal facilities as a disposal method is discussed in detail in section 4.2.

Their use as sites for undertaking bioremediation is being undertaken in the USA in a joint

project between Dredging Operations and Environmental Research (DOER), the US Army

Corps of Engineers (USACE) and the Environmental Protection Agency (EPA)40. This aims to

develop low-cost bioremediation technologies aimed at transforming CDFs from disposal to

treatment facilities, allowing material to be removed for beneficial use, thus recreating storage

capacity for further dredged material. Technologies reviewed include composting, landfarming,

and land treatment. The research is showing promise for practical application to recalcitrant

organic contaminants in dredged material.

Phytoremediation / Phyto-extraction

Plants can be used to remediate contaminated soils and sediments in a number of ways. In

general they are used for remediating metals due to the potential for uptake but can also be used

for the remediation of organic compounds including TBT.

Phytoextraction, a summary of which is given in Table 2.8, is the uptake and concentration of a

substance from the soil into the plant biomass.

Passive uptake species can be used where crops are easily grown or rapid uptake is not desired.

Active uptake by certain species of plant quickens the process and these plants are known as

‘hyper-accumulators’. Induced or assisted hyper-accumulation is the addition of a chelator fluid

or other agent which increases the mobilisation or solubility of metals, thus increasing the

potential for uptake. Where chelators (such as a weak acid) are used there is the potential for

leaching of metals through the soil into groundwater in non-controlled environments. A

concrete basin can be used to collect leachate, which can then also be treated either using plants

or with more conventional treatment technologies 21 22 30.

40 Myers T.E. & Williford C.W. (2000) Concepts and Technologies for Bioremediation in Confined Disposal

Facilities. DOER Technical Notes Collection (ERDC TN-DOER-C11), US Armey Engineer R&D Center,

Vicksburg, MS

31


Table 2.8 Summary of Phytoremediation

Phytoremediation

Advantages Simplicity of technique

Beneficial secondary products including biomass

Disadvantages Large space likely to be required

Doubts over sustainability and risk management i.e. Risk of groundwater pollution

Application Organic compounds and metals


Pre-treatment N/A

Location Netherlands

Developmental phase Emerging concept / demonstration scale

Costs Not established


2.5.2 Thermal oxidation

Thermal treatment can be used to remove, breakdown and immobilise many contaminants.

Incineration is a thermal treatment process that can be used to destroy PAHs, PCBs and dioxins

at high temperature but does not destroy metals. Incineration is, however, a technology that has

high running costs.

Plasma Furnace Treatment

Plasma arc (PA) treatment is a high energy technology able to treat a range of scheduled wastes

including CDMS. In plasma arc treatment a thermal plasma field is created by directing an

electric current through a low pressure gas stream. Plasma arc fields can reach 5000 to

15000°C. The intense high temperature can be used to dissociate the waste into its atomic

elements41. In a PA system, organic constituents are volatized, pyrolyzed, or combusted, while

inorganic material and non-volatized metals are bound in the molten pool. Off-gas from the PA

furnace typically contains products of incomplete combustion, volatized metal, particulates,

hazardous oxides, and acid gases that require further treatment in a pollution abatement system.

Plasma arc technology has many applications for metallurgical manufacturing and purifying

processes that have been in use for many years. However its relatively high costs compared to

incineration with respect to processing of wastes means that it is considered as an emerging

technology that has limited established commercial applications. PA technology is considered

viable however for ‘special’ wastes. This includes nuclear wastes where treatment options are

less developed. To date, two facilities based around the PA technology are operational and

considered viable for nuclear waste. The Zwilag Radwaste Vitrification facility in Switzerland

was the first fully licensed PA facility in the world able to process low/medium level nuclear

41 http://www.environment.gov.au/settlements/publications/chemicals/scheduled-waste/swtt/plasma.html (Accessed

February 2009)

32


waste, using a plasma arc centrifugal treatment (PACT-8™) furnace, receiving its licence in

March 2000. Also in Switzerland, the MGC/Retech Waste Vitrification Facility in Muttenz

demonstrated its capability to process low/medium nuclear waste in addition to organic and

inorganic waste streams. The vitrified output material from non-radioactive wastes are

transported to a landfill or used as aggregate in the construction industry as the glassy material

is non-leaching and considered safe. Vitrified nuclear wastes however are transported to a long

term storage facility.

Currently the use of plasma arc technology is emerging and has seen limited wide scale

application due to its high costs when compared to conventional incineration for many standard

waste types. However it is likely that the legacy of nuclear power, weapons research, testing

and use will become a more pressing issue with respect to treatment of contaminated sediments.

Despite its high costs, plasma arc technology appears to be a one of only a few viable treatment

options for radioactive waste.

2.6 Contaminant immobilisation

2.6.1 Thermal immobilisation

Thermal Immobilisation

Bricks, cement, artificial basalt and light weight aggregates are four examples of products that

can be made using CDMS that has had contaminants immobilised through thermal techniques

(see examples in Box 2.7 and Box 2.8). High temperatures are employed in the production of

building materials like the above examples. Dredged material of appropriate grain size can be

utilised after pre-treatment as a substitute raw material (e.g. fine grain-sized material is a

substitute for clay in brick production). Partial contaminant removal can occur during pre-

treatment or as a result of gas removal and remaining contaminants are bound within the

product. Organic contaminants are removed/destroyed by thermal desorption as described

above and inorganic contaminants are fixed within the matrix of the final product. The final

products have undergone rigorous testing in Europe where they have been found to satisfy

environmental standards for contaminant leaching. Gas emissions from production plants are

lower than that from conventional plants due to more stringent controls for production from

secondary raw materials,21 30 31. Table 2.9 and Table 2.10 give summaries of thermal treatment

technologies that incorporate the production of a secondary product.

Box 2.7 Thermal Immobilisation: Brick production with dredged harbour sediments.

A volume of 600.000m3 harbour sediments is annually dredged out of the Bremen Harbour basin to maintain water depth. Because of its perpetual availability, homogeneity and mineralogical, petrologic and chemical composition, the sediment is regarded as a suitable raw material for brick production. During production, the environmental standards concerning waste-water treatment and the quality of exhausted gas are sufficiently fulfilled. Bricks specified as “building bricks” are produced according to German industrial standards. The parameters pH-value and grain size were varied in leaching tests performed on the bricks as both parameters are likely to change in the course of the brick’s life cycle. The leaching data showed that arsenic was stabilised and metals were immobilised in such a way that the bricks are not hazardous to soil or groundwater neither by their use, for example, in masonry, nor afterwards, when they will be deposited as mineral demolition mass.

Information Source: Bortone G., Palumbo, L. 2007. Ibid.; Hamer, K., Karius, V. 2002. Ibid

33


Box 2.8 Thermal Immobilisation: Sediments for Beneficial Use: Light Weight Aggregates.

Background

The growth rate of The Port of New York/New Jersey was lagging behind its major international and U.S. competitors and they were faced with serious challenges with respect to the maintenance of the existing waterways. The problem in the port arose due to lack of disposal sites for the millions of tonnes of sediment that was dredged annually to maintain the existing waterway depths.

In 1992 revisions were made to the criteria for ocean disposal of sediments due to concern over long term impacts to ecology. The port incurred CDMS treatment and disposal costs of $118/cubic yard (0.76 m3) that was required to stabilise, ship by barge (to Texas) and then transport by train to Utah to be used as landfill cover.

The new criteria for sea disposal of dredged sediments included increased sensitivity of the detection limits and a more rigorous assessment of chronic effects. This resulted in an estimated 75% less dredged sediment meeting the criteria for sea disposal and other means of use were investigated.

Technology Overview

An initial assessment of the mineralogical characteristics of the dredged material from The Port showed that the characteristics were analogous to that of traditional raw materials for end products used in the construction industry. A range of technologies are employed in the thermal treatment of CDMS for production of lightweight aggregates. Initially pre-treatment is necessary; this involves initial sizing and debris removal, dewatering and pelletising. Following the initial stages the dewatered pellets are transported to a plant for final processing in refractory lined rotary kilns. Here, temperatures of between 1150 – 1200˚C achieve thermal desorption of the organic contaminants and addition of flux materials cause reactions with the SiO2 to form a complex matrix which binds and immobilises the metal constituents. Air bubbles within the matrix enhance the thermal insulation properties of the final product.

Following further optimisation of the technique the process is able to use existing infrastructure to process 380 000m3 of dredged material annually.

Information Source: Bortone G., Palumbo, L. 2007. Ibid.

Table 2.9 Summary of Thermal Immobilisation – Brick Production


Advantages Beneficial secondary products off-sets process costs

Preservation of primary natural resources (clay)

Waste gas and water of higher environmental standard due to stricter emissions requirements


Application Organic, inorganic and metals. Fine grained sediments

Pre-treatment Dewatering / separation depending on source

Location Netherlands, Germany

Developmental phase Industrial scale in Germany / Pilot in the Netherlands

Costs 15-30 €/m3 excluding pre-treatment


34


Table 2.10 Summary of Thermal Immobilisation and the Production of Lightweight Aggregate


Advantages Beneficial secondary products off-sets process costs

Preservation of primary natural resources (clay)

Waste gas and water of higher environmental standard due to stricter emissions requirements


Application Organic, inorganic and metals. Fine grained sediments and some sand grade

Pre-treatment Dewatering / separation depending on source

Location Netherlands, USA

Developmental phase Industrial scale since 2004

Costs 15-32 €/m3 excluding pre-treatment

Information Source: Bortone G., Palumbo, L. 2007. Ibid

2.6.2 Chemical immobilisation

Stabilisation/Chemical Immobilisation

Physico-chemical treatment processes use treatment to remove, change or stabilise contaminants

in CDMS. Following dewatering of contaminated sediments, they can be treated with a range

of additives in order to stabilise them for beneficial use and/or immobilise contaminants. This

technique aims to reduce leaching, erosion, dispersion and bioavailability of contaminants by

altering the physical and chemical properties of the sediment21.

Stabilisation/immobilisation techniques are closely linked to the pH of the sediment and its

natural buffering capacity. Marine sediments usually have higher buffering capacities than

soils. A high buffering capacity reduces the need for additives because the pH of the sediment

is less likely to be reduced as a result of chemical reactions post dewatering. The addition of an

immobilising agent will reduce the mobility of contaminants but not remove them from the

sediment, however the environmental risk is reduced. Some stabilisation additives are

considered ‘industry secrets’ however a range of additives are known; these include cement,

calcium aluminates, fly-ash, bentonite, various clays, phosphates, lime, oil residue, silicate fume

and activated carbon42.

Immobilisation techniques can be applied in-situ (not considered herein) or ex-situ. Ex-situ

treatments can be conducted using mobile mixing plant on land (Fig 2.3) or on a floating

pontoon. Sediments treated with these techniques have a range of tried and tested beneficial

uses/applications including liners for disposal facilities (e.g. Hamburg CDFs) and road and dyke

construction. Some methods of immobilisation can be effective for treatment of organic

contaminants including TBT (see Box 2.9 and Box 2.10). A summary of

stabilisation/immobilisation is presented below in Table 2.11 22.

42 Environment Agency (2004) Guidance on the use of Stabilisation/Solidification for the Treatment of

Contaminated Soil. Science Report: SC980003/SR1

35


Whilst there are clear beneficial uses of stabilised sediments, the Dutch authorities place

restrictions on the allowable initial organic matter content due to limited leaching tests.

Additionally there is some concern over the potential long term effects with respect to

construction stability and pore water chemistry due to the generally higher organic contents of

dredged material in comparison to clays and silts.

Figure 2.3 Ex-Situ Stabilisation / Immobilisation (Photo courtesy of DEME)

Table 2.11 Summary of Stabilisation/Immobilisation

Stabilisation/Chemical Immobilisation

Advantages Re-use of sediments possible Preserves natural resources Beneficial secondary material production

Disadvantages Long term stability untested

Application Some organic compounds including TBT, although organic matter may influence stability, inorganic compounds, metals. Mostly silts and clays for re-use but can stabilise sands.

Pre-treatment Dewatering

Location France, Germany, Netherlands, UK

Developmental phase Industrial / Pilot

Costs 10-15 €/m3 (Capping material) / 23-41€/m3


36


Box 2.9 Ex situ stabilisation of TBT contaminated sediment from the Tyne

In addition to using cement to bind CDMS and use in reclamations, stabilised CDMS has also been disposed in landfill as, the stabilisation technique reduces water content and the material is more suitable for transport by road. TBT contaminated CDMS on the tidal reaches of the River Tyne adjacent to the approaches to the dry dock of a ship repair yard were deemed unsuitable for open sea disposal as the organotin concentrations exceeded the Cefas Action levels. The dockyard explored various options to dredge and dispose the CDMS. Initially it was intended to immobilise the TBT contaminated CDMS in cement based stabilisers available commercially then use the material for landscaping purposes at the dock. Following concerns over the potential for TBT leaching from the cement, however, an alternative option to treat and dispose the CDMS was adopted.

The TBT contaminated CDMS at the dock entrance was dredged using an environmental grab installed onto a backhoe dredger and a silt curtain to minimise suspended solids losses encircled the whole operation. The dredged CDMS was placed in barges, transported to lined lagoons in a separate dock area and mixed with cement and allowed to settle for about one day. Mixing with cement effectively reduced the water/moisture content of the CDMS and the resultant material was, therefore, more easily transported to landfill. Whilst such an operation proved highly effective in CDMS treatment and disposal it ultimately resulted in larger volumes of material disposed in landfill and the costs compared to standard sea disposal were approximately 15 times higher (i.e. disposal to sea typically costs the yard about £5/m3 whereas disposal following mobilisation in cement about £80/m3). This clearly highlights the constraints many operators are faced with when trying to deal with CDMS that has limited options for beneficial use.

Box 2.10 Ex-Situ Immobilisation: St Sampsons Harbour Redevelopment - Treatment of TBT Contamination.

Background

TBT has long since been established as a major concern in waterways after its extensive use as a biocide on ships and subsequent accumulation in sediments has been proven to disrupt the endocrine systems and impair the immune systems of marine organisms.

There is no generally accepted international concentration limit for the disposal of CDMS contaminated with TBT to sea, however Germany has issued guidelines of a reduction from 300µg/kg in 2005 to 60µg/kg in 2010 and Belgium has set its own limit of 7µg/kg. It is generally accepted that even at very low concentrations TBT can have a detrimental effect on marine ecosystems. Studies conducted in the UK use a benchmark of 1µg/g as an indicator of potential harm.

Treatment of Contaminated Sediments

In order to redevelop the small harbour of St Sampsons in Guernsey (Channel Islands) into a Marina, 25,000 m3 of TBT contaminated sediments had to be dredged and treated. The concentrations of TBT in the sediments ranged from 642µg/kg to 1770µg/kg. Disposal of the sediments was not a viable option due to the limited availability of disposal sites. The Guernsey authorities requested that the sediments were put to beneficial use in the reclamation of part of the harbour however due to the underlying groundwater in this area there was concern over the potential for leaching of TBT. In order to re-use the sediments the TBT had to be immobilised.

Remediation of St Sampsons Marina

A crucial part of the project was the selection of a suitable additive to prevent leaching of TBT and other metals. The first stage therefore, consisted of laboratory trials. It was found that ordinary Portland cement was not suitable as it didn’t prevent leaching of TBT; as a result it was necessary to dewater the sediments by lagooning prior to further treatment. It was found that clays had no negative effects on leaching of TBT and the use of an in-house additive was sufficient to treat all of the sediments and prevent leachate from exceeding Guernsey standards.

Information Source: DEC, 2004. Guernsey Channel Island, UK. Dredging and Immobilisation of Sediments Contaminated with TBT. DEC.

37


2.7 Treatment methods for TBT Contaminated Sediments

Arguably one of the most pressing contamination issues with respect to marine sediments is that

of tri-butyl tin (TBT) because of its global prevalence, environmental persistence and negative

effects to the marine flora and fauna at very low concentrations. Although most if not all

countries have outlawed its use, many ships around the world are still and continue to be coated

with TBT antifouling paints.

Past studies suggested that there were few feasible options for treatment of TBT-containing

marine sediments and only mechanical separation and sorting was practiced43. More recent

studies, however, have shown that a greater number of treatment options are now available and

emerging technologies are becoming more feasible and economically viable. Some of the

treatment options are discussed elsewhere in this chapter and as such are described briefly

below with respect to their applicability to TBT treatment. Box 2.11 presents a case study of the

applicability of ex-situ immobilisation for treatment of TBT contaminated sediments. In this

case, solidification/stabilisation facilitated re-use on land by solidifying the CDMS to make it a

sufficient construction material (e.g. load bearing) and by stabilising it to avoid it leaching

contaminants into the surrounding environment. Unfortunately, the Environmental Permitting

Regulations can restrict the opportunity for use of such techniques in the UK because

stabilisation does not destroy or remove contaminants, so this option is not reliably available.

Separation techniques using hydrocyclones and floatation have often (but not always) proved to

be successful at removing particulate TBT. Grain size and density are important factors in

separating TBT. It is likely however that a range of grain sizes of TBT paint will be encountered

as it occurs in the environment as paint flakes and as diffuse contamination adsorbed to

sediments. Experiments with separation and floatation have proved an association of TBT with

lighter (low density) particles and a proportional relationship between paint flake size and

sediment grain size. Studies have shown showed that a 60 – 70% reduction in contaminated

sediment volume was possible and that a reduction of approximately 80% of grossly

contaminated sediment volume was possible but this is highly dependent upon site-specific

sediment and paint particle characteristics. Other experiments using pilot scale density

separation and laboratory scale floatation for the treatment of sand fractions resulted in a 60-

65% cleaning efficiencies of the TBT in contaminated sediments 44 45 46.

Bioremediation, the underlying principle of which is the breakdown of organotins, can be used

to treat TBT contaminated sediments. TBT has a breakdown chain leading first to di-butyl tin

43 DEFRA, 2002. The Fate of TBT in Spoil and Feasibility of Remediation to Eliminate Environmental Impact. CSG

Report 15.

44 Reed, J., Waldock, M.J., Jones, B., Blake, S., Roberts, P., Jones, G., Elverson, C., Hall, S., 2001. Remediation

techniques applied to reduce the environmental impact of paint derived TBT in dredged material: a pilot study. In:

Champ, M. (Ed.), Proceedings Pollution Prevention from Ships and Shipyards. Oceanology 2001 Symposium. Vol. 1.

US Office of Naval Research, Arlington VA, USA, pp. 93–97 In: DEFRA, 2002. Ibid.

45 Goethals, L., Pieters, A., 2005. Remediation of sediments, treatment of the solid phase, in: Anonymous (Ed.),

Development of an Integrated Approach for the Removal of Tributyltin (TBT) from Waterways and Harbours:

Prevention, Treatment and Reuse of TBT Contaminated Sediments, LIFE02 ENV/B/000341, available at

http://www.portofantwerp.be/tbtclean/, (accessed January 20, 2008).

46 Kotrikla. A. 2009. Environmental management aspects for TBT antifouling wastes from shipyards. Journal of

Environmental Management. Vol. 90 (S1): S77-S85.

38


(DBT), then mono-butyl tin (MBT), which is the least environmentally harmful organo-tin

species, and finally to inorganic tin. The UK currently applies the same action levels for sea

disposal to all three organo-tin species. A number of promising results have been demonstrated

using various bioremediation techniques. Aerobic conditions promote the rate of breakdown of

TBT and so too does increasing temperature. In comparative experiments it was found that

complete degradation (no measurable concentration at detection limits of 1µg/kg dry weight) of

TBT was possible in sediments at temperatures of 55˚C. In contrast untreated sediments

showed a degradation of 10% per year and sediments that were regularly restacked and aerated

showed degradation of TBT of approximately 30% in 10 months, whilst similar experiments by

other groups have yielded 60-70% removal of TBT47. It is important to note that there was no

significant increase in TBT concentrations in groundwater or the surrounding land.

TBT removal from harbour sediments using phytoremediation has been shown to reduce

contamination with no uptake in the harvest however the technique did not meet the disposal

criteria of some countries. Lagooning in addition to plant growth can be used to degrade

organotin compounds. The plant growth is thought to accelerate the breakdown by dewatering

the soil, thus aerating it. Various species can be used but in experiments by Novak and Trapp

(2005), barley was found to work best and grew despite the saline conditions48.

Separation and sorting of TBT contaminated sediments allow a proportion of the overall volume

be disposed of but do not degrade TBT. Thermal treatment offers a solution for the treatment of

the remainder of the contaminated fraction. Sediments containing 14mg/kg TBT, collected from

the River Tyne were remediated to 0.097mg/kg TBT using a combination of techniques43.

Firstly the sediments were sieved into <500µm and 180µm fractions. The 180µm fraction

which contains the highest concentrations of TBT was then treated using a steam stripping

technique which was reported to remediate contaminated material by up to >98%46.

Pilot studies by Mostofizadeh (2001) in most cases showed the degradation of all organotin

compounds, including MBT, using thermal treatment under high pressure49. This process is

known as pressure thermolysis. Over a period of 3 hours at a pressure of 35 bar and a

temperature of 230˚C, TBT levels were reduced to <1µg/kg of dry weight. The use of high

pressure mitigated the need to evaporate the water prior to treatment, thus reducing energy

expenditure. Following these tests two full scale units were designed (but not built) with

capacities of 85,000 and 170,000 tons per year in order to calculate consumption values and

costs. In simple terms the apparatus consists of a reaction tank and heating jacket into which

stream and compressed air are pumped. Steam and ‘Strip’ air are drawn off and via pipe system

and the condensed effluent is treated separately. Finally the treated sludge is released. In order

to save energy, heat is transferred to the next batch of untreated sludge via a heat exchanger.

47 Pensaert, S., De Becker, G., De Clercq, B., De Puydt, S., Van de Velde, K., Trapp, S., Novak, J., 2005. Treatment

of sediment, in: Anonymous (Ed.), Development of an Integrated Approach for the Removal of Tributyltin (TBT)

from Waterways and Harbours: Prevention, Treatment and Reuse of TBT Contaminated Sediments, LIFE02

ENV/B/000341, available at http://www.portofantwerp.be/tbtclean/ (accessed July 1, 2007 January 20, 2007). In

Kotrikla. A. 2009. Ibid

48 Novak, J., Trapp, S., 2005. Growth of plants on TBT-contaminated harbour sludge and effect on TBT removal.

Environmental Science Pollution Research. 12:332–341.

49 Mostofizadeh, Ch., 2001. Elimination of TBT Compounds from Dredged Material by Means of Pressure

Thermolysis. Institute of Energy and Process Technology (IEV), Bremerhaven. In: Kotrikla. A. 2009. Ibid.

39


Estimates of running cost for the large scale plants are 11.5 to 15 €/ton and energy consumption

is estimated to be 140kWh/tonne for thermal energy and 16kWh/tonne for electrical energy46.

Another potential method for the thermal treatment of TBT is by evaporation. Due to the low

boiling point and high volatility of TBT, it may be possible to adapt a technique that is already

extensively tested for treatment of other volatile compounds. Evaporation of TBT from the

solid phase to the gas phase followed by oxidation into harmless carbon-compounds and tin

may be an area of development for the future.

Electrochemical oxidation has been demonstrated to be an efficient method of TBT degradation

using direct treatment of the suspended sediment (or slurry electrolysis)50 51. In contrast much

lower efficiencies were achieved when using a leaching step as part of the process. In scaled up

versions of initial experiments the University of Bremen achieved high efficiencies for TBT

degradation, however MBT was less efficiently degraded. An additional benefit of this process

is that PAHs were found to be degraded by approximately 90%. No changes in PCB,

chlorobenzenes or pesticides occurred. Larger-scale trials were conducted with this technology

in 2003 which utilised modular plant with a capacity of 160m3 which is reported to be suitable

for use even in large harbours and has an estimated flow rate of 6 to 30m3/h. Treatment costs

were found to be 15 €/ton46.

Whilst some electrochemical treatments appear promising in terms of efficacy for the removal

of TBT, the production of toxic wastes and inherent safety risks of the technologies meant that

they were deemed unviable on a commercial scale following pilot experiments at the Port of

Antwerp52. Similar problems were encountered in experiments with chemical oxidation

techniques using potassium permanganate.

The treatment of marine sediments will inevitably involve waste water which may also be

contaminated with various chemical compounds. Further information on wastewater treatment

options for the remediation of effluent resulting from treatment of TBT contaminated sediments

can be found at http://www.portofantwerp.be/tbtclean/).

Other treatment methods have been tested including investigations into moisture content,

wetting and drying cycles and the contribution of exposure to UV light. Of these only the

wetting/drying cycling reduced TBT. This was thought to be related to microbial stimulation.

Box 2.11 Ex-situ Stabilisation, Mylor Harbour, Cornwall.

Background

Mylor Harbour is situated on the Fal Estuary in Cornwall. As a consequence of historical use of TBT, both the harbour and surrounding estuary sediments were identified as being contaminated.

50 Stichnothe, H., Keller, A., Tho¨ming, J., Lohmann, N., Calmano, W., 2002. Reduction of tributyltin and other

organic pollutants of concern in contaminated sediments by means of an electrochemical oxidation. Acta Hydrochim.

Hydrobiol. 30 (2-3), 87–93.

51 Arevalo, E., Keller, A., Stichnothe, H. and Calmano, W. (2004) Optimisation of the operation of an electro-

chemical process to treat TBT-contaminated sediments on a pilot scale. Acta Hydrochim. Hydrobiol. 32, 401-410.

52 http://www.portofantwerp.be/tbtclean/ (accessed February 2009).

40


Objective

Contractors were initially commissioned with the view to stabilise the contaminants prior to transport to landfill. Earlier desk studies indicated that the contaminated material could not be disposed at sea and disposal cost to land was prohibitive, in fact few landfill sites in the UK could accept it, and the nearest was 200 miles away at Swindon. It was, therefore decided that the heavily contaminated sediment would treated and be re-used in the construction of a dry-dock at the harbour

Area of harbour dredged for treatment.

An initial appraisal was conducted in order to establish which materials were appropriate for the stabilisation of the TBT within the sediments. A modified organoclay (E-Clay) and OPC (Ordinary Portland Cement) were found to achieve both effective remediation and overall geotechnical properties. The proposed treatment was agreed with both the EA and English Nature.

Silts were dredged from the harbour area using land based and marine plant; the contaminated silts were then transported to the treatment area, whereby they were combined with the E-Clay and OPC at the prescribed levels to achieve the necessary remediation requirements and geotechnical properties. The treatment technology was applied ex-situ for the treatment of 2,300m3 of CDMS.

The treatment slurry comprised OPC and modified E-clay applied at an addition rate of 5% and 0.5% weight-by-volume, respectively. Contaminated CDMS were treated in 10m3 batches; this allowed a known volume of contaminated material to be mixed with a known volume of treatment slurry, thus enabling a homogenous mix to be produced. The treated CDMS was then re-used on site in place of clean infill, thus eliminating the need for offsite disposal and import of clean infill material. Within 28 days of the treatment the area had sufficiently cured to allow cars to use the parking area.

Placement of treated material within Admiralty Quay

Validation and results

Following completion of the project, it was demonstrated that the treatment approach achieved the stated objectives of addressing TBT contamination on-site. Samples were taken for every 10m3 of material treated and allowed to cure for a nominal period of 28 days. The samples were leached following the standard NRA methodology, the leachates were analysed for TBT. All of the leachate results for TBT were well below the remediation target of 50µg/l indicating that successful chemical immobilisation of TBT had taken place (i.e. a reduction of over 99% following treatment).

Envirotreat. Case Study: Ex-situ Immobilisation. Mylor Harbour, Cornwall.

41


2.8 Radioactive contamination

Treatment technologies for radiologically contaminated marine sediments, is a subject little

addressed or discussed within the literature and yet there is a global legacy of contamination

from anthropogenic sources including power stations, medicine and research. In the UK, a

number of marine sites have been shown to have been contaminated with radionuclides. One

example of particular interest is Dounreay in northern Scotland. Following a public

consultation, Dounreay has decided to proceed with the remediation of offshore marine

sediments contaminated with radionuclides.

It was found that discrete contaminated hotspots were present in coastal waters off Dounreay

(illustrated in Fig 2.4). As a result targeted dredging of hotspots is required for remediation

purposes. Due to the inherent dangers of diving activities and the oceanic swells, divers were

not allowed to undertake the surveys for radioactive particles. Instead, remotely operated

vehicles (ROVs) on tracks (crawler) fitted with gamma probes specifically adapted to operate

underwater were used (Fig 2.5). These crawlers can detect the activity of the individual fuel

fragments and can also acquire the gamma spectrum for assessment of radionuclide types.

Additionally they are capable of recovering the radioactive particles and small volumes of

associated sediment for detailed laboratory analysis.

Figure 2.4 Main Plume Remediation Area, Dounreay (Courtesy of http://www.dounreay.com/particle-cleanup/).

The capability of remote recovery is essential when the deployment of the ROVs is for

remediation purposes rather than for investigation. However, their slow speed and the

complexity of operations make an ROV rate of area coverage/remediation very low.

42


Because the ROV survey was complex and slow, a different method that dredged CDMS and

scanned it with probes was developed to remove radioactive particles. This method lifts CDMS

from the sea bed, loads it onto a conveyor belt and scans it with a bank of detectors mounted on

the conveyor belts. This fully automated system detects the radioactive fragments and diverts

the volume of sediment identified as contaminated by the probe into a separate container.

Uncontaminated sediment is sent placed back to sea. This system is characterised by high

capital costs and is very intrusive in terms of associated impacts on the marine environment.

However its throughput can be significantly higher than an ROV’s.

Figure 2.5 A Remotely Operated Vehicle ROV, Dounreay (Courtesy of http://www.dounreay.com/particle-cleanup/).

Diffuse Contamination

An alternative approach, suitable when the contamination is of a diffuse nature, is the recovery

of the sediment and its bulk treatment. The radioactive sediments are initially separated from

areas of low contamination and are further concentrated by washing.

The washing process concentrates the contamination adsorbed on the surface of sediment grains

by transferring the contamination to the liquid phase. Following washing the liquid might be

treated on ion exchange columns; the soluble contaminants are extracted from the liquid phase

and the activity is concentrated on the resin columns. The choice of the washing liquid depends

on the chemical-physical characteristics of the radioactive contaminant.

This process is a good example of the general philosophy of ’Concentrate and Segregate’ the

activity (i.e. is shifting the radioactive contamination from a mobile phase into a solid and

concentrated form). Following the concentration, the radio-activity is disposed of into a

dedicated authorised facility and the de-contaminated sediments can be re-deposited in the sea.

This treatment process is generally suitable for caesium contamination however may not be

suitable for other elements due to the different behaviour and adsorption characteristics to

sediment grains.

43


A note on in situ Treatment

Pilot scale studies have also been conducted using vitrification treatments of sediments that are

radiologically contaminated. The process relies upon passing current through the sediments

which are super heated as a result and melt. The solidification of the sediments when the

current is switched off binds the radiological elements into a glassy matrix. This principle of

this treatment technology is that the mobilisation potential and thus the availability of the

radiological elements is reduced and risk to receptors is reduced. This technology is state of the

art, complex and expensive and as a result wide scale application may not be viable.

2.9 Summary

The choice of treatment of CDMS depends upon the class of contaminant, the degree of

contamination, the desired end use for clean sediment and the availability of space and

infrastructure as well as economic factors. It is often the case that CDMS will contain more

than one class of contaminant and its treatment can involve one or more of the technologies

described above. A wide range of success has been reported during pilot and laboratory scale

experiments for emerging technologies. Some of the treatments described are only viable for

one contaminant class and certain technologies are only applicable to sediments with certain

grain size and water content. Feasibility studies may need to be conducted in order to assess the

treatability and whether any of the options is appropriate, on a case by case basis. Table 2.12

shows the theoretical applicability of each treatment type to contaminant class and Table 2.13

gives a general indication of the technical criteria for each treatment option.

Table 2.12 CDMS Treatment Option Applicability for a Range of Contaminants

Technique Metals PAH PCB TBT Radioisotopes

Evaporation X �� X

Mechanical Dewatering �� X

Classification ��

Sorting ��

Soil Washing ��

Thermal Desorption ��* �� X

Electrokinetic �� * ��* ��

Biological Oxidation ��* �� X

Phytoremediation �� X

Landfarming ��* �� X

Chemical Oxidation / Stabilisation �� X X ��* X

Thermal Immobilisation ��

* indicates partial applicability.

44


Table 2.13 Technical Criteria for Treatment Option selection

Type of Sediment (granulometry, composition)

Level of Contamination Level of Contamination Technique

Silty Sandy/silty Sandy Low High Organic Inorganic

Classification + + + + + + +

Sorting + + + + + + +

Evaporation + + + + + + +

Mechanical Dewatering + + + + + - +

Chemical Extraction + + + - + - +

Thermal Desorption + + + - + + -

Biological Reduction - + + + + + -

Chemical Oxidation + + + - + + -

Thermal Oxidation + + + + + + -

Chemical Immobilisation + + + + + + +

Thermal Immobilisation + + + + + + +

Notes: + process complies with environmental standards. - process does not comply with environmental standards.

Information source: Detzner, H. D., Hakstege,A. L., Hamer, K., Pallemans, I. 2007. Overview of Treatment and Disposal Options In: Sustainable Management of Sediment Resources, Vol 2. Sediment and Dredged Material Treatment. (Ed. G. Bortone and L. Palumbo), pp59-66. Elsevier. Holland.

45


3. Beneficial Use of CDMS

3.1 Introduction

Under the present marine licensing regimes in the UK, the applicant must consider alternative

means of disposal of marine dredged material that may include the beneficial use of CDMS

following treatment (‘clean-up’)53. Whilst the existing UK licensing regime for disposal of

dredged materials would appear to be a commendable attempt to re-use dredged materials, in

practice, it appears that few ports and harbour authorities actually implement sediment re-use

due to legislative barriers, lack of disposal space, excessive costs and environmental concerns.

Most dredged material in the UK is also composed of mud, which because of its geotechnical

properties is generally unsuitable for use in land reclamations and only really suitable for

projects such as wetland and mudflat restoration.

Where the characteristics of the dredged material are such that contaminant levels exceed

criteria for disposal at sea (e.g. the material fails to meet OSPAR Convention requirements or

exceeds Cefas Action Levels) treatment options may be considered54. Such options can be used

to reduce or control impacts and can use several treatment techniques (see Section 2) such as

separating contaminated fractions that make the material suitable for beneficial use and can be

considered as alternatives to disposal at sea.

There are few options for the beneficial use of highly contaminated CDMS and re-use is

generally restricted to material that has been treated (such as thermal treatment to volatise

contaminants) followed by using the sediment in the manufacture of construction material such

as bricks. There is, however, a level of consumer resistance to products such as bricks made

from CDMS even where contaminants have been removed or immobilised or reduced to safe

levels and do not pose a risk. It is, however, possible to use a risk based approach to the use of

CDMS that has received no treatment before disposal. For example, in Hong Kong, CDMS

with medium levels of contamination is used to backfill deep (~30m below the seabed surface)

seabed pits that were created during extraction of aggregate (sand) from the seabed (see

Box 4.2). The CDMS is disposed to within a few metres of the top of the pit and then capped

with uncontaminated material and hence restores the seabed to its former state and provides

habitat for benthic marine fauna.

CDMS that is not too contaminated is also used in The Netherlands and Belgium for various

purposes including road embankments and raising the level of the land.

The beneficial re-use of CDMS is challenging because of the physical and chemical nature of

such sediments and the requirement that contaminated material is separated from the non-toxic

sediment fraction, so that on re-use the risk to the environment and /or human health is

53 MCEU, 2006. The Control of Marine Works Dredging / Disposal at Sea and Approval of Oil Dispersants. Marine

Consents and Environment Unit, Defra. Annex 5 presents a flow chart that identifies alternatives for the use of

dredged materials.

54 OSPAR, 2004. Guidelines for the Management of Dredged Material (Reference number: 2004-08); OSPAR

Convention for the Protection of the Marine Environment of the North-east Atlantic. OSPAR Commission.

46


minimal6. Owing to increasing concerns over the placement of CDMS and lack of disposal

space, and increased scope for separation and stabilisation treatment to reduce the

contamination risk, there has been growing interest in the beneficial use of dredged material and

increasingly such material is considered far from a waste but an important environmental and

economic resource55. Aside from sediment that is contaminated with radioactive substances,

many contaminants in CDMS can, following suitable treatment, be used as a resource and put to

beneficial use, although the additional step in the treatment chain does result in higher costs for

the handling of contaminated dredged material.

Treatment of CDMS may be undertaken to reduce and /or remove contaminants from CDMS so

that the resultant material is suitable for beneficial use. There are numerous potential uses of

treated CDMS (including CDMS that has been mixed with cement to immobilise contaminants)

for both engineering and environmental purposes. There is, however, a need to balance the

benefits of reuse of treated CDMS with the risks in gaining consent in the UK for such

operations (e.g. there have been instances when dockyards could not use cement treated CDMS

for landscaping) although there are examples of using treated CDMS to create high value

intertidal habitat and use in flood defences elsewhere in the world. Additionally, treatment of

highly contaminated CDMS may be necessary to render the material suitable for disposal (e.g.

after treatment, contaminant concentrations may be reduced to acceptable levels for disposal of

the material). Examples of re-use include Newlyn, where CDMS mixed with cement and an

additive was used it to raise land levels in an area for future development56 and the Mylor

Harbour example in Box 2.11.

The beneficial use of treated CDMS in the UK is largely driven by the need to do something

with the dredged sediment rather than due to a demand for the material. It has been estimated

that about 60-70% of dredging costs are attributable to disposal and there is considerable scope

for diverting finances associated with obtaining licenses, monitoring and cost of disposal into

the beneficial use of such dredged material.

Reviews of the beneficial use of CDMS are provided by The Department for Transport57,

USACE58, Keillor (2007)6, Murray (2008)4 and PIANC (2009)55 and are only summarised

below.

The beneficial use of treated CDMS has been reviewed in greater detail in The Department for

Transport’s Guidelines for Beneficial Use of Dredged Material together with guidance on the

type of material that can be used in various projects and the recent PIANC publication. There

are numerous potential beneficial uses of treated CDMS and the USACE website also provides

guidance on the material type that can be used in each project and also lists case studies. These

may include the following57,59,60, subject to treatment where appropriate to remove or reduce

55 PIANC (2009) Dredged material as a resource. Options and constraints. PIANC Report 104 by

EnvicomWorking Group 14.

56 Dredging and Port Construction July 2006 p.24-25

57 Department for Transport, 1996. Guidelines for the Beneficial Use of Dredged Material. Report prepared by HR

Wallingford Limited.

58 http://el.erdc.usace.army.mil/dots/budm/budm.cfm (accessed 22 October 2008).


47


contaminant levels and subject to compliance with the appropriate marine or land-based

regulatory regime:

Engineering

• beach nourishment;

• shoreline protection, coastal defence structures;

• reclamation and land formation (mud is unsuitable for later construction of buildings

but the reclaimed land may be suitable for other purposes such as landscaping,

recreation and car parks);

• construction materials including bricks;

Environmental Enhancement

• wildlife habitat creation, such as islands for birds, mudflat and saltmarsh creation;

• fishery habitats (e.g. provision of refuge habitats for fish);

• wetland restoration;

• maintaining sediment supply to estuarine systems through transport in the water column

or direct recharge of the foreshore;

Agricultural Use

• agriculture (e.g. for non-food crops), forestry, horticulture.

3.2 Summary

There are numerous criteria that must be evaluated when considering the potential beneficial use

options for treated CDMS, as detailed by the US ACE, including the contaminated status of the

material, site selection, technical feasibility, environmental acceptability, cost/benefit analysis

and legal considerations61. There is a wealth of information on the beneficial use of dredged

material from the USA (see the US ACE website for further details) and PIANC62 55. Such use

reduces the quantity of material for disposal and hence reduces pressures on valuable disposal

space.

In the UK, to date there has been little incentive for the beneficial use of dredged material

despite the need to consider such alternatives as part of the marine licensing regime. However,

with regard to uncontaminated dredged material, progress has been made, albeit slowly, and

over the past 10-15 years quite large volumes of dredged material considered not to pose a risk

60 Brandon, D.L. and Price, R.A. 2007. Summary of Available Guidance and Best Practices for Determining

Suitability of Dredged Material for Beneficial Uses. US Army Corps of Engineers, Engineer Research Development

Center. ERDC\EL TR-07-27.


62 PIANC. 1992. Beneficial Uses of Dredged Material. A Practical Guide. Report of PIANC working group 19.

Brussels: Permanent International Association of Navigation Congresses (PIANC).

48


to the environment have been used for various coastal engineering schemes, such as flood

defence, raising land levels63 and wetland creation (e.g. Wallasea Island in the Crouch Estuary).

There has been a recent example of using CDMS beneficially in the UK at the Port of Falmouth

but the port operator encountered numerous environmental and legislative constraints that

resulted in only a small proportion64 of the dredged material being put to a useful purpose; the

majority was disposed of in landfill.

The EU Waste Framework Directive regards dredged material as waste. However, the recently

agreed exception in Article 2(3) exempts the relocation of sediments for specified purposes

from the provisions of the Directive, although it is still waste:

“Without prejudice to obligations under other relevant Community legislation, sediments

relocated inside surface waters for the purpose of managing waters and waterways or of

preventing floods or mitigating the effects of floods and droughts or land reclamation shall be

excluded from the scope of this Directive if it is proved that the sediments are non-hazardous.”

A project-specific risk-based approach to the beneficial use of treated and untreated (where not

highly contaminated) CDMS would, therefore, go a long way in reducing the costs often

encountered for disposal, reduce pressure at disposal sites and help stimulate options for the

beneficial use of material that can be a valuable resource. With advances in equipment,

treatment and handling technologies the options for beneficial use are increasing60. If such

techniques could be accepted by the public and regulators, then there could be the potential to

improve sediment management in the UK through a wider acceptance of beneficial use.

63 Clay, N. Bray, N. and Hesk, P. 2008. Maximising beneficial reuse through the use of a novel dredging contract.

Terra et Aqua 111: 13-20.

64 Approximately 20,000m3 of CDMS was treated and used for sea protection purposes (above MHWS) whilst about

60,000m3 went to landfill.

49


4. Disposal Options

4.1 Disposal options

There are various options for the disposal of dredged material but the most cost effective is

usually to dispose of it in open water at sea near the source. However,, in the UK under the

present marine licensing arrangements, dredged material that is considered too contaminated

and. therefore, unsuitable for open sea disposal will need to be transported to a disposal site on

land, such as a landfill. The costs associated with dewatering, double handling and transporting

the dredged material to a landfill site, in addition to any charges made by the landfill operator,

may be significantly greater than for sea disposal.

Options as alternatives to open sea disposal or placement in landfill have been developed

overseas using various disposal facilities including disused borrow pits and purpose built

depots. There are, however, inherent risks (human health, ecological) and cost implications

associated with each of the various disposal options. The risk associated with CDMS disposal

is beyond the scope of the present study but a review of the human health and ecological risk

together with cost estimates for CDMS disposal using various disposal options (e.g. confined

disposal facilities [CDF], contained aquatic disposal [CAD], landfill, immobilisation in cement)

has recently been conducted for CDMS in the US65. Further discussion is provided by USEPA66 67.

Whilst a number of disposal options are available for CDMS, a key consideration is whether

such alternatives are within the present UK licensing regime for disposal. CDMS is presently

considered a waste and there are many barriers to its disposal even after the material has been

treated on land. There is, therefore, presently limited scope to re-use treated CDMS in the UK

and such material is invariably sent to landfill, often at considerable additional cost compared

with sea disposal.

A schematic showing the various disposal options is presented in Figure 4.168.

65Kiker, G., Bridges, T.S. and Kim, J. 2008. Integrating comparative risk assessment with multi-criteria decision

analysis to manage contaminated sediments: An example for the New York/ New Jersey Harbor. Human and

Ecological Risk Assessment 14: 495-511.

66 USEPA. 2005. Contaminated Sediment Remediation Guidance for Hazardous Waste Sites. EPA-540-R-05-012.

Washington, DC: United States Environmental Protection Agency, Office of Solid Waste and Emergency Response

(OSWER) 9355.0-85.

67 USEPA/USACE. 2004. Evaluating Environmental Effects of Dredged Material Management Alternatives - A

Technical Framework. Washington, DC: United States Environmental Protection Agency, Office of Water (4504F);

Department of The Army, U.S. Army Corps of Engineers. Report nr EPA842-B-92-008 Revised. 95 p

68For further details of disposal facility options see Kiker, G., Bridges, T.S. and Kim, J. 2008. Ibid 65.

50


51


The following disposal options are considered in this chapter:

• disposal on land:

� purpose-built confined disposal facilities (CDF);

� other landfill sites;

� silt lagoons;

• disposal in open water at sea:

� contained aquatic disposal;

� level bottom disposal at sea with capping;

� disposal in geotextile bags.

The land-based options are all effectively different forms of landfill and in the UK at present are

regulated under the Environmental Permitting Regulations 2007 in England and Wales and the

waste management licensing regime in Scotland. The marine options are all regulated under the

relevant marine licensing regimes. These regimes potentially overlap in some cases, for

example where a CDF is constructed that is situated partly or wholly below mean high water

mark but the existing regulations for land disposal explicitly exclude from their remit operations

that are licensed under the relevant marine licensing regime.

4.2 Purpose-built confined disposal facilities (CDF)

Purpose built engineered structures that act as confined disposal facilities (CDF) have been

developed to isolate CDMS from the environment. CDFs are engineered ‘depots’ used for the

containment of CDMS as they reduce contaminant migration and differ from other measures

used to manage CDMS, such as treatment options that aim to destroy or immobilise

contaminants present in CDMS. The use of CDFs is widespread in locations where there are

constraints on space to receive CDMS and such facilities are now well developed and tested in

locations overseas69 including The Netherlands, Norway70 and are used in the USA71, where

they are one of the most widely used options to dispose of CDMS and non-contaminated

sediment. However, at present there are no examples of CDFs licenced in the UK under the

marine licensing regimes and the option of building a CDF in the marine environment may be

considered unacceptable under the existing legislatory framework.

69 PIANC 2002 ‘Environmental guidelines for aquatic, nearshore and upland confined disposal facilities for

contaminated dredged material’

70 NGI, 2008. Stability of Contaminated Sediments. NGI’s Strategic Institute Program 2003-2008. Final Report, 30

September 2008. Norwegian Geotechnical Institute.

71 Palermo, M. and Averett, D. E. 2000. Confined disposal facility (CDF) containment measures: A summary of field

experience. DOER Technical Notes Collection (ERDC TN-DOER-C18), US Army Enineer Research and

Development Center, Vicksburg, MS, USA.

52


There are three major type of CDF and they can be constructed in ‘upland’ sites (similar to a

traditional landfill), in near shore areas where one or more of the engineered walls (dykes) are in

water or as island depots surrounded by water72 (see Figure 4.1). CDMS is normally at least

partially stored under water in island and nearshore CDFs and such storage especially if the

overlying water is anoxic (no ambient oxygen) helps prevent the mobilisation of metals73.

Despite being engineered from relatively impermeable material such as clay and sand, there are

several potential contaminant loss pathways from CDFs. Pathways include spills of effluent

during filling operations, surface run-off after heavy rain, migration of leachate to ground water

and loss of certain contaminants such as volatile organics to air72. Nevertheless, it is possible to

use operational controls on site to limit contaminant pathways from CDFs and these may

include the selective placing of CDMS followed by uncontaminated sediment layers to

‘sandwich’ contaminated layers; placement of a final capping layer comprising uncontaminated

sediment; placement below water to keep the CDMS anaerobic (low dissolved oxygen) that

reduces the potential for mobilisation of contaminants such as metals to the dissolved phase72.

In The Netherlands, The Dutch Ministry of Transport, Public Works and Water Management oversee CDMS disposal and there are numerous case studies in the use of purpose built CDFs facilities (CDMS storage depots). A case study of the use of the largest nearshore CDF, De Slufter in Rotterdam is provided below in Box 4.1

74. Another CDF, The Hollandsch Diep was recently (October 2008) officially opened to receive CDMS. The Hollandsch Diep was constructed on a section of river linked to the Rhine and Meuse and will be used to store CDMS dredged from the lower reaches of both rivers. The Hollandsch Diep CDF is 1,300m long by 500m wide and 32m deep and has a capacity to hold 10.2Mm3 of CDMS75. When the CDF is filled (estimated to take about 20 years based on present forecasts) it will be covered with uncontaminated material and turned into a nature reserve.

CDF facilities have also been constructed in The Netherlands to receive contaminated dredged

material from freshwater systems. An island CDF (the Ijsseloog CDF) has been constructed in

Lake Ketelmeer and used as a storage depot for material dredged from within the lake. The

Ijsseloog CDF is approximately 1km in diameter and about 45m deep and enclosed by a 10m

high dyke and covers an area of 250ha. The Ijsseloog CDF has a >1m thick clay liner at its base

and is designed to hold 23Mm3 of contaminated material dredged from the lake. The CDF is a

unique construction project designed to store contaminated material dredged from Lake

Ketelmeer and is not considered further here although the design, operation and management of

72 Palermo, M. and Averett, D. E. 1999. Design features of confined disposal facilities (CDFs) for contaminated

sediments. 31st Texas A&M Dredging Seminar/ WEDA XIX. 17-20 May 1999, Louisville, Kentucky.

73 Hakstege, P. 2004. Sub-aquatic confined disposal. Draft SedNet Report on Treatment and Disposal of Dredged

Material. August 2004. 74 Hong Kong previously had plans for island CDFs that would be filled and then capped and used for various

recreational purposes but to date none has been constructed.

75 Dredging News Online, 3 October 2008. Hollandsch Diep disposal site formally opened.

http://www.sandandgravel.com/

53


the CDF has parallels with CDFs located in the coastal zone to receive CDMS and further

details are available elsewhere76.

CDFs are also constructed to receive CDMS in terrestrial locations away from the coast and are

termed ‘upland CDFs’. Upland CDFs store CDMS above the groundwater level and share

many similarities with a traditional landfill in their design and operation and an example of an

upland CDF ‘The Francop’ used in Hamburg, Germany is presented below in Box 4.2.

Box 4.1 Near-shore Confined Disposal Facility, De Slufter, The Netherlands

In the Netherlands, the Rijkswaterstaat (Ministry of Transport, Public Works and Water Management) is responsible for dredging operations and management of dredged material in ‘state waters’ that includes access channels to ports. Port authorities including the Port of Rotterdam are responsible for dredging in ports. Water boards are responsible for the maintenance of inland waterways such as drainage channels.

It is national policy to consider dredged material as a resource and consider options for its use. Uncontaminated dredged material is placed into aquatic systems and also used for flood defence purposes on the banks of waterways. Opportunities are also sought for engineering and environmental uses including backfilling of borrow pits for environmental beneficial use. Highly contaminated CDMS is stored in sub-aquatic engineered disposal facilities (CDFs). Prior to disposal in CDFs fractions of the CDMS that may have a beneficial use such as sand are removed by separation techniques in sedimentation basins and clay is removed in clay ripening lagoons.

The largest CDF used is De Slufter located at Maasvlakte, Port of Rotterdam, which is owned and operated jointly by the Rijkswaterstaat and the Port of Rotterdam. The port is one of the largest in the world and routinely dredges large volumes of CDMS. Other CDFs owned by the Rijkswaterstaat have also been built (Ijsseloog on Lake Ketelmeer, Hollandsch Diep). The Holandsch Diep CDF was recently constructed at a cost of approximately € 50 million and has a capacity of 10 million m3. Regional authorities including the water boards can make use of the Rijkswaterstaat CDFs for a tariff of about € 10 / m3 of CDMS.

De Slufter, Rotterdam, NL

76 PIANC Working Group ENVICOM 5 Guidelines for Marine, Nearshore and Inland Confined Disposal Facilities

(Ijsseloog CDF case study).

54


There are four classes of CDMS in The Netherlands, with class 4 material being the most highly contaminated. The Slufter CDF mainly holds class 2/3 CDMS with some class 4. The Port of Rotterdam is one of the largest ports in the world and requires frequent maintenance dredging for navigation. Following an Environmental Impact Assessment carried out in the 1980s it was decided that the best option for disposal of categories 1-3 CDMS would be in a purpose built CDF. The Slufter CDF was built on the nearshore area of the industrial Maasvlakte with an overall storage capacity of 150Mm3 and was operational by 1987.

Design and Operation of the CDF

The Slufter CDF is a nearshore engineered (bunded) disposal facility that covers an area of approximately 260ha (200ha of which is the pit). The dykes bunding the pit were constructed by dredging by dredging sand from within the pit down to 22m below sea level. On completion the dykes were planted with grass to stabilise the walls and prevent erosion, In addition, on the seaward side of the Slufter a recreational area was created and an inspection road built on top of the dykes with public viewing areas to observe inside of the pit. A nature reserve was also created to attract waterbirds including gulls, ducks and waders.

The Slufter is typically filled with CDMS hydraulically whereby trailer suction hopper dredges discharge CDMS by pumping material through pipelines into the pit. Inside the Slufter a moored disposal pontoon that can be moved into position within the pit is used to discharge the CDMS in layers via a diffuser.

To maximise pit capacity and reduce the volumes of material deposited within the CDF, settlement basins were constructed to separate the CDMS from sand. The useful sand deposits can be separated from the CDMS in the settlement basins by mechanical sieving. The sand is then used either for bund construction or sold commercially. Clay is also now produced in clay ripening fields. The category 2 (lower contamination levels present) CDMS is allowed to settle in the ripening fields for about one year and can then be used for engineering purposes such as in the dykes.

Effluent Control

The water depth within the Slufter is kept at a desired level through discharging excess volumes into the adjacent coastal waters. In order to prevent the unacceptable discharge of water with high levels of suspended material the water is first passed through a pump pit and return pipeline and the effluent quality is continuously monitored. If the suspended matter exceeds desired concentrations then the silt-laden water is routed into a specially designed settlement basin that reduces sediment levels prior to discharge.

Monitoring

A borehole system located both within the pit and in the dyke is in place to monitor any migration of contaminants from the placed CDMS. Water samples are collected from within the boreholes and analysed for contaminants. The CDF functions as a freshwater bubble that ‘floats’ on the salt groundwater below with little interaction between the two systems . Should contaminant migration become evident then a drainage system will become operational to pump contaminated water to a treatment facility.

Constraints: The capital outlay for the CDF construction is substantial and operational / monitoring costs are also high (see below). For nearshore CDFs, a large area of foreshore is also required and this may conflict with other users such as nature conservation, navigation.

Opportunities: Much is now known from the construction of the CDFs in the 1980s and the influence on the environment including contaminant migration are less than originally thought. CDFs can by their enclosed nature accommodate highly contaminated CDMS that may not be suitable for other disposal routes. Separation techniques for sand have proven to be economically viable at De Slufter at least locally. The created habitat can be used for both recreational use and by wildlife including nesting seabirds.

Cost: The Slufter was constructed for about €68 million and annual operating costs are about €9 million. These costs do not include costs associated with disposal barges and dredgers or costs for maintenance of the pit equipment.

Information Sources: Pol Hakstege (Rijkswaterstaat); PIANC Working Group ENVICOM 5 Guidelines for Marine, Nearshore and Inland Confined Disposal Facilities (Slufter CDF case study)

55


Box 4.2 Upland Confined Disposal Facility, The Francop CDF, Hamburg, Germany

Various treatment technologies for the decontamination and re-use of CDMS have been investigated by the Port Authority of Hamburg. Based on technical feasibility, economics and environmental impact, the option selected is to dewater CDMS and store the contaminated silt material in safe storage sites (CDFs) on land. The main purpose of the Hamburg CDF facility is to separate fine sand from contaminated silt and dewater the CDMS (mostly silt fraction) and use the sand for either beneficial use or disposal.

An upland CDF is an engineered structure used to store CDMS above groundwater levels and is bounded by a dyke. In many respects, upland CDFs are not too dissimilar to a standard landfill in their construction and operation. Only de-watered sediment is disposed in the CDF. An upland CDF was constructed in the Francop district of Hamburg to receive highly contaminated CDMS arising from the Elbe at the Port of Hamburg. There are two upland CDFs in Hamburg (the Feldhofe CDF was commissioned in 2001 and has the capacity to store 9Mm3 of dewatered CDMS) but the Francop CDF is the oldest and permission to construct the facility was granted in 1991. The Francop CDF has the capacity to hold 7Mm3 of dewatered CDMS and covers an area of 120 ha. The CDF is located in an agricultural area adjacent to a river and with a height of about 38m has the potential to be visually intrusive in a flat landscape. Landscape planners have, however, designed the CDF so it appears natural as possible and cultivation of the site has taken place with grass and planted trees.

Operation and Monitoring

Prior to disposal in the CDF, CDMS is sorted and dewatered in a treatment installation using hydrocyclones and a series of screen and belt presses to reduce water content in a facility termed the METHA (Mechanical Separation of Harbour Sediments) that separates the contaminated silt fraction from uncontaminated sand and dewaters the silt slurry. The CDMS is placed in the CDF in layers with alternate sand drainage layers. Silt layers (1.5.m thick) alternate with 0.3m wide sand drainage layers. To reduce contaminant leakage to groundwater the CDF base is sealed. Because of the natural self-sealing properties of silt, dewatered silt with a low permeability is used as a sealing material at the CDF base and a 2.5mm thick double high density polyethylene liner.

The CDF is surface sealed with a 1.5m thick layer comprised of dewatered silt, sand and soil to reduce contaminant emissions to air. The final cover system comprises a loam soil (barrier to plant roots) and an arable soil layer that is cultivated for landscaping purposes. The seepage water arising from the CDF is treated in a wastewater treatment installation that removes contaminants (flocculation of suspended material and biological process [trickling filters] to remove ammonia) before water is discharged to the River Elbe.

Cross-section of the Francop upland CDF (based on Detznar, 2004)

56


There is the potential for both air emissions and leachate seeping to groundwater with these CDFs. There is, therefore, a need to monitor contaminant emissions around the CDF after CDMS disposal operations have ceased and the disposal facility has been put to other uses. Once completed, upland CDFs can be landscaped and used for recreational purposes and the Francop CDF will form a park facility for use by the general public.

Opportunities: These disposal facilities are highly regulated, isolated from most pollution pathways and can be used for highly contaminated CDMS. There is also experience in their use in countries including the USA and Germany. When the capacity of the CDF has been exhausted and the seal cultivated with vegetation and any emissions have ceased, the site may be converted for recreational use (e.g. park facilities) by the general public.

Costs: The CDF cost about €70M (in 1993) and costs for disposal of CDMS including long-term monitoring at the CDF site range between €10-75/m3. High overall costs associated with upland CDFs as disposal is at the end of the treatment chain involving dredging – separation of sand from contaminated silt – dewatering – transport – disposal.

Information Sources: Detzner, H-D. 2004. Confined upland disposal. Draft SedNet Report on Treatment and Disposal of Dredged Material. August 2004.

Detzner, H. D., Bode, W., Weiss, T. 1994. The treatment of dredged Elbe sediments from the Port of Hamburg. Paper presented at Environmental and Mineral Processing, June 30 –July 1 1994, Ostrava, Tschechien.

Dealing with Dredged Material in Hamburg. http://www.dredging-in-germany.de/Hamburg/eng%20sites/schlick/schlick_m.html Accessed January 2009.

Netzband, A., Hakstege, A.L., Hamer, K. 2002. Treatment and Confined Disposal of Dredged Material. Dutch- German Exchange on Dredged Material.

Tresselt, K. et al. 1998. Harbour sludge as a barrier material in landfill cover systems. Water Science and Technology 37: 307-313.

4.3 Other landfill sites

The CDF facilities used in Hamburg, Germany that operate like landfills are discussed above in

Box 4.2. There are, however, other Ports that use land based disposal at Bremen and Rostock

and these have been reviewed by Netzband et al. 2002.

The Port of Bremen dredges about 300,000m3 of sediment annually that is contaminated with a

range of metals, organic contaminants and TBT. Dredged sediment is dewatered and stabilised

in (ripening) fields within one year. Following the dewatering process, CDMS is deposited in

sealed landfill (silt mounds; upland CDF). Excess water is collected via drainage layers and

diverted to a treatment plant; cost for the dredging and disposal of the CDMS are about €10-

13M per year22.

At the Port of Rostock on the Baltic coast, elevated nutrient concentrations in the CDMS do not

allow the material to be disposed at sea. CDMS is dewatered in ripening fields and used

beneficially for earthworks or agricultural use77.

The Port of Antwerp is about to start a 25 year scheme called the AMORAS Project to landfill

sieved, dewatered contaminated sediments. The cost estimate for this project was €28-31 per

tonne dry matter. This is approximately €10-15 per cubic metre in situ.

77 Netzband, A., Hakstege, A.L., Hamer, K. 2002. Ibid.

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4.4 Silt lagoons

The land disposal of CDMS can be an efficient management tool for disposal of contaminated

dredged material where storage space is available within the immediate vicinity of the area to be

dredged and environmental conditions are favourable. Whilst many port operations have

limited capacity for the storage of CDMS on-site there are UK examples of this type of

operation, for example silt lagoons at Rainham used by the Port of London Authority and

lagoons at Woolston used by the Manchester Ship Canal Company. There are also examples of

CDMS dredging and storage from other industries.

The power station located on the tidal stretch of the River Severn at Oldbury requires a

continuous supply of water for cooling purposes and a tidal reservoir with cooling water intake

was constructed on mudflats adjacent to the site in order to provide water even at low tide. The

tidal reservoir constructed at Oldbury power station is a large structure (approximately 2.2km

long at its longest wall) and holds approximately 2,273,000m3 of water. Owing to the retentive

capacity of the tidal wall however the reservoir accumulates sediment and since 1993, the

reservoir has been dredged twice a year to maintain its capacity. On average, approximately

60,000m3 of sediment material is removed annually and the station uses a series of silt lagoons

for the on-site storage of CDMS.

The estuarine dredged material is deposited in a series of clay-lined and bunded silt lagoons that

were constructed next to the power station on the farmland just behind the sea wall. There are

three silt lagoons (two have been backfilled) and Lagoon 1 operated from 1975-1983; Lagoon 2

from 1983-1993 and Lagoon 3 from 1994 to present. Sediment is removed from the tidal

reservoir by a suction dredger and transferred to the silt lagoons along wide bore pipes.

Lagoon 1 was backfilled by 1983 and after dredged material had settled, was capped with

topsoil. The lagoon has been colonised with grass and is used as pasture land.

When backfilling operations ceased at Lagoon 2, the backfilled material was colonised naturally

by saltmarsh vegetation and salt tolerant species and provides an important habitat for wildlife

including birds. Further details on the environmental value of the vegetated silt lagoons to the

Severn’s wetland birds are provided in Merrit 199478.

4.5 Contained aquatic disposal (CAD)

4.5.1 Method

These facilities involve open water disposal and typically comprise excavated pits or

depressions in the seabed that receive CDMS from plant such as split-bottom barges or, in

deeper water (~70m) such as found in the Norwegian fjords, via a vertical pipe from an

anchored vessel. CAD disposal cells can reduce the risk from CDMS disposal by confining the

sediments to a small footprint, increasing contaminant diffusion times and isolating CDMS from

waters where the hydrodynamics could result in sediment transport. All of these factors can

eliminate and/or reduce exposure pathways thereby reducing risk to both natural environmental

78 Merritt, A. 1994. Wetlands, Industry & Wildlife: A Manual of Principles and Practices. The Wildfowl and

Wetlands Trust, Gloucester.

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The CAD facility used for CDMS disposal in Hong Kong

receptors and human health79. CAD has been used around the world for CDMS disposal79

including in Hong Kong80,81, Belgium (Antwerp)82, The Netherlands83, Norway84, and in a

number of navigation projects throughout the USA including Boston, Rhode Island, Los

Angeles and the Puget Sound Naval shipyard in Washington79.

As CAD disposal areas are ‘open facilities’ during backfilling operations and until they are

capped with uncontaminated material, the CDMS material is not fully isolated from the

receiving environment (see level bottom disposal above in Section 4.6 and CDF in Section 4.2).

When the CDMS is disposed within the CAD area to the desired depth it may, however, be

capped with inert material such as sand and mud to replace the seabed to its former state and

isolate the CDMS layers below from aquatic receptors. Purpose built CAD facilities have been

used in Hong Kong for about 16 years (since 1992) and a case study of the use of a CAD facility

and management of CDMS from Hong Kong is provided below in Box 4.3. The placement of

CDMS in CAD facilities is normally undertaken in lower energy environments where the near

bed shear stress does not result in waves and currents mobilising CDMS from the disposal

facility.

Box 4.3 Contained Aquatic Disposal Facility in Hong Kong

Hong Kong experiences substantial deposition of sediments within its inshore waters. Following many decades of rapid growth and uncontrolled industrial and municipal waste discharges into the coastal environment, the surficial sediments are often highly contaminated (notably in urban areas). Owing to a relatively small area of land space available for urban growth, much of the natural coastal area has been reclaimed and consequently large volumes of contaminated sediment have been removed through dredging associated infrastructural projects.

79Fredette, T.J. 2006. Why confined aquatic disposal cells often make sense. Integrated Environmental Assessment

and Management 2: 35-38.

80 Whiteside, P., Ng, K.C., Lee, W.P. 1996. Management of contaminated mud in Hong Kong. Terra et Aqua 65: 10-

17.

81 Nicholson, S., Hui, Y.H., Rodger, J.G. and Ding, W.W. 2004. The management and monitoring of contaminated

dredged material disposal in Hong Kong. In: International Conference on Coastal Infrastructure Development-

Challenges in the 21st Century, pp. 70. HKSAR Government.

82 http://www.portofantwerp.com/annualreport/2006/en/fut_amoras.html

83 Hakstege, P. and Heineke, D. 2008. Re-use of borrow pits: The Dutch approach to storing (contaminated) dredged

material. Terra et Aqua 112: 11-14.

84 The Oslo Harbour Remediation Project, undated. Oslo Fjord Clean Up.

http://www.renoslofjord.no/cgi-bin/ohv/imaker?id=42754&visdybde=1&aktiv=42754 (accessed 30 October 2008.)

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Hong Kong’s success as a major trading entity is also due to its strategic position as a major entry port to China and has world-class port facilities and is one of the busiest international shipping ports. Whilst it is the policy of the Hong Kong Government to leave CDMS in-situ and minimise dredging where practicable, owing to the substantial deposition of sedimentary material from the Pearl River, dredging is often necessary to keep the port and navigation channels open for shipping.

In order to prevent significant deleterious environmental impacts, the dredging and subsequent disposal of contaminated material in Hong Kong is subjected to rigorous legislative and environmental controls. Since, December 1992 contaminated dredged material has been disposed in a series of purpose-built seabed pits and exhausted borrow pits located to the East of Sha Chau (ESC; an island lying off the present international airport at Chek Lap Kok). The land area of Hong Kong is relatively small (comprising approximately 1,064 km2 of which 40% is classed as Country Park where development is highly regulated) and land-based disposal of CDMS is highly impractical. Recognising the constraints on disposal of CDMS and need for environmental protection, trials were undertaken to ascertain the efficacy of disposal of CDMS in seabed pits (CAD facilities). Extensive disposal trials were conducted using uncontaminated sediment and acoustic Doppler current profilers (ADCP) used to track suspended material and the ESC area was selected in 1992 as the preferred CDMS disposal site in Hong Kong.

The ESC site has several hydrological attributes that provide a suitable location for CDMS disposal. The water is comparatively shallow (~6-8m) and the tidal currents are relatively slow (typical velocity is approximately 0.3m s-1). During filling operations, CDMS placed in the CAD is still exposed to the overlying water and when the pit is backfilled to a certain level (to about 3-6m from the seabed level) the CDMS is capped with inert material to isolate the contaminated material from the adjacent environment and also return the seabed to its former state. The ESC disposal facility is probably the largest CAD site used for disposal of CDMS in the world and several studies have concluded that it represents the most appropriate environmentally acceptable option for placement of CDMS in Hong Kong.

Design and Operation of the CAD

The coastal waters around the CAD facility hold aquatic ecological receptors and are important nursery grounds for spawning fish and also the core habitat for dolphins. The CAD facility was, therefore, designed to minimise the potential for the mobilisation of CDMS from the uncapped pit during storm-induced bed shear stresses because the pits are only capped when they have been backfilled to within approximately 3-6m below the seabed and CDMS disposal operations may last several years. The CAD design also minimises the potential for erosion of the uncontaminated material used to cap the backfilled pit and the cap is also sufficiently thick to prevent soft-bottom invertebrate fauna from burrowing into the contaminated material. When the CAD is backfilled and CDMS disposal operations cease, an inert layer of uncontaminated marine sand and mud (terrestrially sourced mud has also been used in recent times) at least 3m thick is deposited to cap the CDMS and this returns the pit to the same level as the adjacent seafloor which allows the seabed to restore to its previous pre-dredged state.

Once the inert cap is in place it is important to ensure that it isolates the deeper CDMS layers within the pit and the cap integrity has been investigated using vibrocores and geophysical surveys. Samples collected through the cap (by viborcore) have been tested for metals and also used to determine the capping material thickness. Such sampling has revealed the capping to be is effective in isolating the CDMS from the ambient environment and when cap layers consolidate and sink over time, maintenance-capping operations are deployed to bring the layer up to the seabed level.

To ensure that the receiving environment is not impacted due to CDMS disposal an environmental monitoring and audit programme is in place that monitors various environmental media for any evidence of contamination (water, sediment, biota; see Appendix B). A 24-hour on-site management barge is in operation year-round at the CAD facility and the on-site management team register all incoming barges and allocate a disposal area in cells within the pits depending on the tidal current at the time of disposal. For example, during the ebb tide the disposal cells in the northern area of the pit are most commonly used as any sediment in suspension will be carried to the south and likely settle out of suspension within the pit. Further precautions also include ensuring that the barges used to dispose the CDMS are allowed to drift over the target cells to minimise the movement of vessels under power over the pit as this may cause some propeller wash of the placed sediments. Disposal of CDMS material within the CAD facility is also evenly distributed and regular bathymetric surveys are conducted to determine the levels of contaminated mud within the pit.

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Constraints: Disposal of CDMS into CADs located in deep water (>20m) and high tidal currents can result in unacceptable losses during placement if plant such as split bottom barges used (unacceptable CDMS losses can, however, be overcome by use of deep submerged pipelines for CDMS placement). Presence of CDMS may result in a ‘no go zone’ on the seabed resulting in constraints to future seabed infrastructure (e.g. pipes, cables). Uncontaminated cap material is required to cap the CDMS in place at the end of the operational life of the CAD. Monitoring of cap with possible re-capping may be required to ensure cap integrity. Other seabed users (e.g. bottom trawlers) may impact cap integrity.

Opportunities: The backfilling of deep seabed pits with CDMS followed by capping returns the seabed to its former state and is a useful beneficial use of CDMS. CADs can be constructed close to the dredging location (e.g. within the same tidal basin) and the shorter transport distances and use of barges for disposal are likely more cost-effective than longer transport routes to landfill and consequent greater use of petrol and inherent air quality issues. Compared to open sea disposal with capping, CADs are often also more easily accepted by the general public as CADs provide more ‘comfort’ to people as they appear to provide a high level of protection from natural events including storms and waves (Fredette, 2006). Compared to other disposal options including open sea and CDFs, use of CADs for disposal of CDMS results in less handling of contaminated material and fewer contaminant transfer pathways (Fredette, 2006). For example disposal of CDMS in upland CDF facilities may result in greater level of handling by site workers and contaminants transfer via pathways to groundwater resources.

Cost: In many cases, the seabed pit will have been used for the purpose of aggregate extraction and hence costs will be covered during the winning of marine aggregate resources. Where CADs are purpose-built and dredging of the seabed is required, the cost is usually about 2-3 times higher (in some cases this may be considerably higher) than open sea disposal but it is often found that the additional costs are acceptable when balanced with public acceptance, agreement with regulators and expediency (Fredette, 2006). Capping and any environmental monitoring to ensure no impacts in the receiving environment may entail major cost implications.

Information sources: Nicholson, S., Hui, Y.H., Rodger, J.G. and Ding, W.W. 2004. The management and monitoring of contaminated dredged material disposal in Hong Kong. In: International Conference on Coastal Infrastructure Development- Challenges in the 21st Century, pp. 70. HKSAR Government.

Fredette, T.J. 2006. Why confined aquatic disposal cells often make sense. Integrated Environmental Assessment and Management 2: 35-38.

The hydraulic regime (i.e. waves, currents) is a key consideration in the use of CADs for

placement of CDMS but they can be designed with bunds to prevent the risk of mobilisation of

CDMS from the seabed pit. For example, in tidal rivers that experience unilateral flow a bund

could be built at the upper boundary of the pit to prevent CDMS losses (a CAD facility ‘the

Cromstrijen’ that incorporates a dike structure on the upper boundary of a former borrow pit has

been built in the lower reaches of Rhine-Meuse delta and has a capacity to receive about 10Mm3

of CDMS)83. For those CAD facilities constructed in coastal and estuarine locations a

submerged ring dike may be necessary to provide sufficient protection of CDMS placed in the

pit83.

Whilst shallow water engineered CAD facilities are used in Hong Kong and The Netherlands,

there is also experience using a natural basin as a deep water CAD facility in the Norwegian

Oslo fjord (Box 4.4).

Whilst many CAD facilities are capped at their end of their operational life (i.e. once they have

been filled with CDMS), there are options to leave the pit un-capped, as the CDMS will still be

relatively isolated from the adjacent environment. Dredging can result in the ‘bulking up’ of

sediment as water is entrained during the dredging process resulting in overall greater volumes

of CDMS than present in situ. Such bulked material may have different properties to bedded

sediment and may be more easily mobilised from within the pit. However, there are dredging

techniques such as backhoes and special grabs that can excavate sediment with minimal water

entrainment, so that they largely maintain their in situ characteristics. Following CDMS

placement, the finer sediments will, however, consolidate as increasingly greater volumes of

material are placed on top.

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Box 4.4 Contained Aquatic Disposal in Oslofjord

The Norwegian fjords are generally deep and do not usually require maintenance dredging but serious contamination of sediments is often evident in coastal areas. The contaminants reflect the historic industrialisation around the harbours and sediment layers show distinct contaminant gradients from deeper layers that have elevated PAH from a society that relied on coal burning, PCB concentrations then increase from their past use in transformers and electronic equipment and the upper layers are contaminated with TBT following extensive use of the antifoulant in paint applied to ship hulls70. The Norwegian Ministry of the Environment has identified CDMS as a top priority for remediation70. The marine sediment contamination in Norway is so serious that in 120 areas restrictions have been placed on the consumption of fish and fish products from fjords and harbours over an area covering 1200km2 70.

In the Oslo fjord, there are naturally occurring deep water basins (50-150m deep) and the current velocity is about 3cm/s. A naturally formed deep sea basin with a capacity to hold about 700,000m3 of CDMS was identified85. Dredged CDMS (salt is added to the CDMS to increase the specific gravity and improve sedimentation) is transported to the CAD area by barge and pumped down to 70m into the CAD facility using a vertical pipe85. To ensure that water quality is not impacted during CDMS placement an on-line monitoring system is operated using buoys that monitor turbidity and water currents. Additionally, water samples are periodically taken and passive samplers (polymer strips) and sediment traps deployed to measure dissolved contaminants and suspended solids85. When backfilling operations are completed, a cap will be placed over the CDMS.

Technique used in Oslo Fjord for CDMS disposal into a deep (CAD) basin (Source: The Oslo Harbour Remediation Project – Oslo Fjord Clean-up)

Because the upper layers of the CDMS in the CAD facility will be below the seabed surface,

physical forces will affect the surrounding seabed before mobilising sediment within the CAD

facility and in practice it is likely that when the adjacent seabed is being re-suspended, the CAD

area will act as a sediment receptor and substantial erosion of CDMS is unlikely (Fredette,

2006). The risk of CDMS mobilisation even in uncapped CAD areas is, therefore, much

reduced and probably a better alternative to disposal in open sea sites with capping which may

be readily affected by natural events such as storms and anthropogenic impacts including

bottom trawling, anchor damage or disturbance by the downwash from ship’s propellers.

85 Jorgensen, T., Jensen, K.L and Halvorsen, P.O. 2008. The Oslo harbour remediation project In: Urban Sediment

Management and Port Redevelopment & Sediment in River Basin Management Plans pp. 14. Abstracts of

presentations and posters presented at the 5th International SedNet Conference, Oslo, Norway.

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4.5.2 Integrity of caps

The geotechnical performance of the caps used to isolate CDMS is often assessed using

vibrocores and geophysical survey. There are concerns that contaminants may migrate through

the cap over time and chemical analyses through the cores drilled through the cap can provide

information on the spatial location of contaminants. To assess the ecological performance of

the cap, benthic grab samples can be collected to determine recolonisation of the capping layers

by benthic fauna (see example from Hong Kong Appendix B). The ecological integrity of the

capping layers has also been assessed with sediment profile imaging (SPI) that deploys a frame-

mounted camera that uses a prism to penetrate the upper sediment layers (~22cm) with minimal

disturbance and takes images of the sediment layers together with the benthic fauna present86.

Various materials including sand, silty-sediment and geotextile liners have been used to

effectively cap CDMS in various projects worldwide. The SedWeb website provides a

summary of CDMS capping projects worldwide and links to information and graphics on the in-

situ capping of CDMS87. A key constraint on the use of ‘traditional’ thicker caps in inshore

waters can be the potential to impeded navigable depth and innovative thinner capping

technologies have been proposed. Whilst thinner caps may potentially form a suitable barrier to

migration of contaminants from CDMS, they may not prevent burrowing organisms breaching

the cap and hence careful consideration is required in cap design. In addition, it is likely to be

difficult to put such thin caps in place in deeper waters or where there are strong currents.

Where there is sufficient risk of organisms burrowing through the cap and there is a need to

maintain navigable depth that precludes a thicker capping layer, it may be feasible to

incorporate a geotextile into the cap that is impenetrable to infauna.

A fundamental consideration in the use of CADs is the design of the cap used to isolate CDMS

and act as a barrier for CDMS dispersion and potential contaminant pathways and also to isolate

CDMS from burrowing in-fauna some of which may be in food webs linked to human food

resources. The design and thickness of the cap used to isolate CDMS is, therefore, a key design

and cost consideration to prevent the breach of the cap by benthic organisms and a review of the

design criteria to isolate CDMS from marine fauna is presented below in Box 4.5.

Box 4.5 Capping to Prevent Penetration of Burrowing Organisms

Background

It is necessary to consider bioturbation (i.e. the movement or alteration of sediment particles and porewater mediated by benthic fauna) in the design of capping layers (‘caps’ comprising uncontaminated material such as sand and marine mud) used to isolate CDMS deposited in CADs. The caps used in CADs are designed to serve three primary functions:

• physical isolation of the CDMS from the benthic biological environment;

• sequestration of CDMS so that is not re-suspended or transported;

• reduction in the flux of contaminants leaking from CDMS into the water column.

86 Nilsson, H.C. 2008. Sediment quality assessment by sediment profile imaging (SPI) in contaminated Norwegian

harbour and fjords. In: Urban Sediment Management and Port Redevelopment & Sediment in River Basin

Management Plans pp. 23. Abstracts of presentations and posters presented at the 5th International SedNet

Conference, Oslo, Norway.

87 See http://www.sediments.org.

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An important consideration in cap design is the selection of material used to cap the CAD. Whilst it is important to select material with suitable geotechnical properties so as to ensure long-term cap stability, it is also important to consider the grain size characteristics, as these will have implications for the fauna that will colonise the cap and ultimately through bioturbation, potential implications for cap integrity. The cap thickness must, therefore, consider the potential for bioturbation within the cap in addition to the properties that are required to ensure the cap is operational and effective at CDMS isolation.

Bioturbation and Cap Design

Animals including polychaetes (worms), crustaceans and burrowing bivalves are key bioturbator organisms in marine systems. The use of uncontaminated material to cap CDMS has been long established but a key concern is the effects of bioturbation on cap integrity and efficacy of the cap to isolate the CDMS from biota. To address such concerns, controlled laboratory studies were conducted that demonstrated the need to incorporate a safety margin in cap thickness as it was found that the burrowing polychaete (Nereis virens) was capable of penetrating 50cm thick caps composed of sand, silt or clay. Other field based studies revealed clear successional changes in the faunal assemblages that colonised caps. Initially, small tube-dwelling worms (that pumped water into and out of their tubes to depths of about 3cm deep) and crustaceans were present on the cap. Following ensuing stages of cap colonisation, (2-5 years) tube-dwellers were replaced by deeper (~10cm) burrowing organisms (such as burrowing worms) were present. Later stages of colonisation were dominated by less dense assemblages of larger organisms (e.g. deposit feeding polychaetes) many of which penetrated sediments to about 30cm. Whilst biological activity tends to be greater in the upper well oxidised sediment layers and activity tends to decrease with depth (down to 10-40cm), other studies have revealed that many marine organisms can burrow considerably deeper. For example, echiuran worms build large burrows up to 80cm in muddy sediment, mud shrimp and mantis shrimp (family Squillidae) can construct burrows to depths below 1m. In addition to invertebrates burrowing through caps there has also been interest in the role of megafauna such as fish including skates and rays that forage in large pits excavated in bottom sediments.

Best Practice Cap Design

Various site-specific considerations have been used to estimate cap thickness required isolate CDMS from most burrowing organisms. In the New York Bight, based on a review of the regional macrofauna present, a cap thickness of 30cm was considered sufficient to isolate CDMS from present (the authors considered that the mean biological mixing zone would only be 10cm deep but a further 20cm of cap was required to provide a long-term barrier to porewater migration). Other studies have estimated that when fauna are present with a potential for deep bioturbation then a cap thickness of 45cm has been suggested to isolate CDMS.

Whilst it is recognised that thicker caps will be more effective barriers to burrowing fauna, there is a fine balance between conservative cap designs and construction costs. Usually caps will be designed with other considerations in mind such as increased thickness to offset the consolidation of the cap as lower sediment layers settle and cap erosion and in many cases the cap will be of sufficient thickness to reduce the risk of deep bioturbators penetrating the cap and affecting integrity. There may, however, be instances where local ecological conditions dictate that a thicker cap must be used for example in locations where fauna such as deep-burrowing shrimps are present and in these cases caps may need to be over 1m thick. In Hong Kong, where both burrowing mantis shrimp are common and high shipping density is the norm, the cap on the CAD is about 3m thick and this design was used to prevent both fauna and anchor damage from smaller vessels breaching the cap. Some cap designs have incorporated a gravel layer (20cm thick) placed below the sand cap to prevent penetration by shrimp burrows but it is unknown whether such designs have been used in practice and there may be concerns over higher density gravel sinking through into unconsolidated CDMS.

In summary, there are three distinct zones of biological activity through sediment profiles. The upper well oxidised cap layers are intensively reworded by organisms, the mid-depth zone has less activity, the deeper zone is less well understood but the density of bioturbators is much reduced. Whilst local biological fauna must be considered in cap design, in most coastal locations, bioturbation can be predicted to extend to about 20-60cm and caps should be designed accordingly.

Information source: US Army Engineer Research and Development Center, 2001. Subaqueous Cap Design: Selection of Bioturbation Profiles, Depths, and Process Rates. Technical Note ERDC TN-DOER-C21, August 2001.

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With only limited space for disposal of CDMS, Hong Kong has adopted innovative measures to

the management and disposal options for CDMS and can serve as a model for other countries.

A risk based approach is used for the disposal of dredged material.

• Uncontaminated material is disposed in the open sea at licensed disposal grounds.

• Slightly contaminated material (i.e. CDMS that fails SQGs but proves non-toxic in

bioassays) is disposed in Dedicated Open Sea Disposal Sites (i.e. in sea bed pits

located in an area with higher tidal currents).

• Contaminated CDMS is disposed in specially designed CADs and managed by an

extensive Environmental Monitoring and Audit programme to ensure that there are

no impacts in receiving waters (more details in Box 4.3 and Appendix B).

• Highly CDMS must be subject to ‘special treatment’ (e.g. treatment may be

required before disposal in the CADs).

Further details on the management of CDMS in Hong Kong are presented in Appendix B.

4.6 Level bottom disposal at sea with capping/isolation techniques

4.6.1 Effectiveness

A cost effective disposal option for CDMS disposal is placement of the dredged material

directly on the seabed. There are, however, obvious concerns over such placement of CDMS in

the open sea as there are potential pathways for contaminants to affect receptors, including fish.

The disposal at sea of highly contaminated CDMS directly onto the seafloor that is then capped

has been used historically in countries overseas, including the USA, but there has been limited

use of such a disposal technique aside from a trial involving predominantly TBT contaminated

CDMS at the Port of Tyne.

Whilst the capping of CDMS placed at sea is a useful technique there are concerns / perceptions

that such a disposal option is not environmentally acceptable. The capping of CDMS disposed

at sea is often perceived as ‘harder to sell’ as the remedy of choice for regulators and the general

public as contaminants are left in place88. Whilst capping can effectively ‘seal’ underlying

CDMS it also accentuates consolidation of the CDMS, which may result in release of

contaminants from the CDMS porewater to the water column. The significance of such

consolidation will, however, depend on the water content of the CDMS that is affected by the

method of dredging, together with the stress placed on the underlying contaminated sediments70.

Following CDMS placement the material must be capped with uncontaminated layers to prevent

migration of contaminants. An open sea disposal trial was conducted off the Tyne with

sediments contaminated with TBT and various metals. The concentrations of TBT and some

metals in the CDMS exceeded the Action levels for open sea disposal but in agreement with

88 Forster, U. and Apitz, S.E. 2007. Sediment remediation: U.S. focus on capping and monitored natural recovery.

Journal of Soils and Sediments 7: 351-358.

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Defra the sediment was dredged and placed out at sea and then capped with mud and sand. An

extensive monitoring programme was used to ascertain the effectiveness of the trial and details

of this study are provided below in Box 4.6. Potential liabilities arising from this project are

discussed in the Task 2 report89.

Box 4.5 Port of Tyne Open Sea Disposal Trial and Capping

Background

The Port of Tyne needed to dredge approximately 60,000m3 of sediment that was contaminated with TBT. The CDMS was contaminated with TBT at concentrations that exceeded levels normally deemed suitable for sea disposal. The CDMS was disposed at sea depths of about 45 m. The port agreed an innovative trial with Defra whereby the CDMS would be placed at sea then capped with layers of silt and then sand to isolate the contaminated material from the marine environment. The cap was designed to be composed of a 1m silt layer overlaid with a 0.5m sand layer to protect from erosion. The volume of material used in the original cap was about 135,000m3.

Before disposal, the environmental, social, legal and economic risks were evaluated and risks were considered acceptable. The sea disposal of the CDMS followed by capping was deemed the best practicable environmental option but there was no experience of such a technique used previously either in the UK or the North Sea (depth of waters are about 50m) and a detailed stakeholder consultation exercise was held with stakeholders including Environment Agency, Natural England and Cefas (Defra).

CDMS Placement at the Licensed Disposal Ground and Monitoring Programme

CDMS was dredged using a backhoe dredger with an enclosed bucket from the Tyne Estuary (between December 2004-March 2005) and carefully placed in split hopper barges to ensure that the sediment maintained its cohesive properties. The CDMS was then placed by split hopper barge at the licensed sea disposal site (Souter Point Outer Disposal Ground) and formed a mound within an ellipse of ~500m diameter and a maximum height of about 1.5m. The barges disposed the CDMS at a central site that ensured limited spread of the bulk of the placed material. However, due to these operations taking place during the winter, bad weather affected the dredging/disposal operations leading to an extended period over which the contaminated dredged material (CDM) was deposited. This long period of exposure of the CDM before being capped (about 3 ½ months), almost certainly greatly increased the spread of thin layers of CDM around the deposition site. This then required a greater area to be capped than had been planned.

89 York Law School, 2009. Research and Support for Developing a UK Strategy for Managing Contaminated Marine

Sediments. Task 2: Exploring liability and the polluter pays principle.

Schematic cross-section of open sea disposal trial, Port of Tyne

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In March-April 2005, the silt and sand cap was deposited over the CDMS. During initial cap placement using silt pumped down the dredge pipe of a trailer suction dredger it was found that the silt dispersed widely. Thus, placement of silt was stopped and the cap design altered by placing greater quantities of sand than had been planned to compensate for the greatly reduced silt cap. The key objectives of the trial were to ensure that there was minimal loss of CDMS, the placement of an adequate cap to isolate the CDMS and investigate the long-term integrity of the cap.

Various techniques were used to assess the cap thickness and cap integrity. Such surveys included:

• Bathymetry (provides depth contour plots and data on the CDMS and cap thickness)

• Side scan sonar (provides data on any transport of cap material due to tidal and wave action)

• Sub-bottom profiling (geophysical technique that provides images of sub-surface sediment layers and can provide information on sediment thickness)

• Sediment Profiling Imagery (photographic technique used to take images of relatively undisturbed sediment and provide information on cap thickness and integrity)

• Grab samples (to assess both surficial sediment contamination and presence of infauna)

• Beam trawls (to assess epifauna present such as fish and tissue analyses for contaminants)

• Sediment vibrocores (to ascertain the relative thickness and distribution of each capping layer)

Results of the Capping Trial

During the period after the capping had taken place, the various tools used to ascertain cap thickness revealed that the cap was thickest at the centre of the disposal site at up to 1m thick and graduated to thinner capping layers at the edge of the capping site. Median cap thickness were between 0.2 m and 0.25 m– significantly less than the designed thickness of 1.5 m. The coverage of the cap was very patchy with significant variations in the thickness of the cap. . Following surveys in 2006, additional capping works was undertaken between July and December 2006 whereby a further 119,678 m3 of predominantly sand was used to top-up the cap. Concentrations of TBT in the cap layer were below Action level 1 suggesting that TBT had not migrated through the cap. There was also evidence of colonisation of the cap by macroinvertebrates.

Surveys conducted two years post-capping indicated that whilst the cap has shown some changes in morphology and some sediment movement and a few areas that had not been fully capped, the trial was largely successful as the majority of placed CDMS was capped by a 0.6m thick layer over the central part of the disposal ground. TBT concentrations in the cap layers were also below Action level 1. Further replenishment capping would be undertaken if future survey results indicate changes in cap thickness. There was also evidence that epifauna were not affected in the longer term by capping operations and tissue analysis for TBT also showed that concentrations were not elevated in fauna following placement and capping of CDMS with high TBT levels.

Constraints: Disposal of CDMS in deep water may be constrained by high tidal currents result in unacceptable losses during placement, however, this was not a significant problem in this case. Presence of CDMS may result in a ‘no go zone’ on the seabed resulting in constraints to future seabed infrastructure (e.g. pipes, cables). Uncontaminated cap material is required to cap the CDMS in place at the end of the operational life. Monitoring of cap with possible topping-up may be required to ensure cap integrity. Other seabed users (e.g. bottom trawlers) may impact cap integrity (the enforcement of an exclusion zone around the disposal location, whilst likely to be controversial and to result in compensation claims from fishermen, would help protect the cap integrity. However, there are legal difficulties in imposing such exclusions.).

The Port of Tyne capping trial was a qualified success in the disposal of highly contaminated CDMS. While the objectives of the trial were met, the trial did encounter a number of problems. Lessons were learnt from those problems that would enable such an operation to be carried more effectively in the future. Over 3 years and a number of storms the CDM and cap has remained in place with environmental risks minimised and the surface sediments returning towards pre-disposal conditions. However, the cap has had to be topped up over that time to maintain the cap to a suitable thickness. With continued bathymetric surveys in the years to come, the integrity of the cap can be monitored and the cap topped up when required. However, until the technique is accepted by the regulators, that disposal technique is unlikely to be used in the UK in the shorter term.

Opportunities: Relatively simple disposal technique that could be located within close proximity of the port thereby reducing disposal costs. However, it can only be done where the physical circumstances are appropriate e.g. where the seabed is not exposed to significant currents from tides or wave action nor impacted by human activities such as fishing and anchoring. Thus, it will never be a generally applicable option. Ongoing maintenance dredging of uncontaminated material such as sand could be used to replenish the cap. Disposal near to source could be used to keep the CDMS within the same system / basin.

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Cost: The overall cost of the trial was about £3M, about 53/m3 or £33/m3 if the costs of monitoring are excluded. Capping and any environmental monitoring to ensure no impacts in the receiving environment may entail major cost implications. Routine monitoring is also required to ensure cap integrity in perpetuity or until such time as the cap can be demonstrated to be stable over the long-term. This is a significant commitment.

Information sources (main data source):

Envirocentre, 2008. Port of Tyne Sea Disposal Trials of Contaminated Tyne Estuary Sediment: Post Placement Monitoring – Tier 1 Second Annual Monitoring. Report No. 3308, June 2008.

BPA conference, 2007. Sea Disposal Trials of Contaminated Estuary Sediment. Presentation by George Fleming (Envirocentre) and Brian Reeve (Port of Tyne). Gateshead, UK.

4.6.2 Constraints on capping CDMS at sea

Conventional sand caps act as a barrier to the migration of CDMS contaminants to overlying

environmental compartments because they slow the travel times for contaminants. Whilst caps

are useful when used in seabed pits (such as CADs) because they both act as an impermeable

barrier and also return the seabed to its former level, there are constraints with such caps placed

over CDMS disposed on the open or flat-bottom seabed. A major constraint on using capping

material in some locations is the potential for conventional thick (typically >1m thick) sand caps

to intrude into the overlying water column and constrain navigable depths where the initial

depth is limited. For example, caps can limit the water depth in shallow inner harbours and

ports where shipping traffic is concentrated90, although interference with shipping would not be

permitted under UK legislation. Traditional caps (used for both in-situ capping of CDMS and

CADs) have tended to be thick to prevent contaminant migration but thinner caps can be used

where sorptive material is incorporated into the cap design. For example, modelling studies

using various capping media such as carbon sand mixtures in caps 5-10cm thick have shown

that even thin caps (5cm) have the capability to reduce the bioavailability of contaminants to

ecological receptors91. Pilot trials (for sediment remediation purposes rather than capping

placed CDMS) have also shown that thin layer capping in-situ CDMS with layers of material

containing activated carbon was a cost effective (1-2€ /m2) way to bind contaminants strongly

and reduces uptake in fauna and transport to environmental compartments92 and has obvious

potential for the capping of CDMS disposed in the open sea.. However, these thin caps may be

easily eroded or disturbed by natural events or human activities.

90 Jersak, J., Aasen, F.B., Hull, J. and Collins, J. 2008. “Non-conventional” capping remedies for risk based

management of contaminated sediments in shallower inner-harbor areas. In: Urban Sediment Management and Port

Redevelopment & Sediment in River Basin Management Plans pp. 33. Abstracts of presentations and posters

presented at the 5th International SedNet Conference, Oslo, Norway.

91 Ruiz, C.E. and Schroeder, P.R. 2008. Capping for management of contaminated sediments and dredging residuals.

In: Urban Sediment Management and Port Redevelopment & Sediment in River Basin Management Plans pp. 29.

Abstracts of presentations and posters presented at the 5th International SedNet Conference, Oslo, Norway.

92Cornelissen, G., Oen, A.M.P., Brandli, R., Eek, E. and Breedveld, G.D. 2008. Opticap, thin layer capping of

sediments using reactive and non-reactive materials, field studies. In: Urban Sediment Management and Port

Redevelopment & Sediment in River Basin Management Plans pp. 30. Abstracts of presentations and posters

presented at the 5th International SedNet Conference, Oslo, Norway.

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Numerous engineering products and impermeable or reactive materials can be used to cap

CDMS. Engineered reactive and /or low permeability products are available commercially93 and

include AquaBlok94, Reactive Core Mats, Bauxsol and organoclays90. Natural materials that can

be used as capping layers that are relatively reactive or have low permeability include activated

carbon, bentonite, apatite, zeolite and bauxite90.

4.7 Geotextile bags for disposal at sea

Placement of CDMS in geotextile bags or liners to isolate it from the ambient environment can

be adopted followed by either disposal in the open sea (where conditions permit) or more

sheltered waters such as estuaries and such disposal has been trialled overseas. The use of

geotextile containers or bags, has focussed on filling the containers with uncontaminated

dredged material for various coastal engineering applications such as breakwaters and

groynes95,57. A UK example is use filled with uncontaminated material to provide an artificial

surfing reef at Boscombe. In recent years, however, there has been growing interest, notably in

the USA and Hong Kong,96.in the feasibility of using geotextile fabrics to cap in-situ sediment

and also as bags or containers to isolate CDMS disposed in the marine environment57

Trials with geo-bags and geo-containers have been undertaken in the USA. Over 40,000 bags

and 700 containers were deposited in waters 18m deep with current strengths of 1.2m/s

apparently without failure of any of the units and with a high accuracy of placement57.

Field trials were undertaken in Hong Kong in to asses the feasibility of using geotextile

containers for the disposal of CDMS that was highly contaminated and not suitable for disposal

at sea. Geosynthetic fabric was placed into a split bottom barge, filled with sediment, the liner

sealed/sewn and the container disposed through the bottom of the barge. The trials were

undertaken at the existing CAD facility (see Box 5.3 above) and used uncontaminated material

and polystyrene balls as indictors of container integrity. A number of controlled disposal trials

were carried out using different container designs and sizes. The initial trials highlighted

problems in the container design and deployment as bags ruptured and/or tore on the barge

doors during placement. Following amendments to the disposal technique the field trials were,

however, considered a success and the technique could be used with confidence in meeting

requirements for contained disposal of contaminated sediments in Hong Kong96. In practice,

geobags are likely to need to be capped to ensure their integrity in the face of tidal and wave

action as well as to prevent damage by human actions such as fishing and anchoring, although at

93 Entec does not endorse, nor has any information on the efficacy of any of these products and the list is shown for

information purposes only. The product manufacturers should be contacted for further information.

94 This capping material is composed of clay-based particles that expand when hydrated and forms a relatively soft

capping material. When placed over CDMS it forms a physical barrier limiting contaminant transport to the overlying

water (see http://www.aquablokinfo.com/)

95 Van Zijl, M., Vlasblom, W.J., De Gut, J.G., Broos, E., De Boer, J. 2006. Feasibility study of the continuous

Geotube®. Terra et Aqua 102: 19-24.

96 Chan, B.B.P., Cheek, P.R. 2004. Disposal of dredged contaminated marine sediments using geosynthetic

containers In: International Conference on Coastal Infrastructure Development- Challenges in the 21st Century.

ETWB, Hong Kong SAR Government.

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Boscombe the geobags were purposefully positioned where they would cause waves to break

and they did not require capping. The latest geotextiles are stronger and double-skinned to

prevent damage..

A summary of the use of geotextile liners for disposal of CDMS is presented below in Box 4.7.

Box 4.7 Geotextile containers for CDMS Isolation and Disposal

Background

There is growing interest in the use of geotextile sealed containers for the safe disposal of CDMS that is unsuitable for other disposal routes. The containers effectively seal the dredged CDMS and can be used as to isolate CDMS.

Filling and Placement

CDMS is placed into the container either in-situ, or from a split bottom barge or by pumping. The geotextile is then sealed using portable industrial stitching equipment.

Fabric Design

Various fabrics are available commercially that have different properties such as porosity and strength. Design parameters must consider:

• sufficient permeability to relieve excess pressure from the accumulation of gas during decomposition of material in the CDMS;

• sufficient retention of CDMS (non-woven liners retain almost 100% of fine grained material);

• resistance to pressures from loadings during filling without failure of the fabric or seam;

• resistance to mechanical abrasion during filling and tearing or puncturing during placement at disposal site;

• resistance to UV light and other pressures at the disposal location.

Constraints: Geo-bags and containers filled with CDMS can only be placed in locations where the risk of rupture from prevailing environmental conditions (e.g. storm surges) and human activity (e.g. dredging, fishing, anchoring) is sufficiently low. In practice, they will probably need to be capped to ensure their integrity.

Opportunities: Flexible geotextile fabrics filled with dredged material have been used for a number of applications including the placement of tube units in estuaries for reclamation projects and to contain CDMS. Geotextile containers have also been used along coastlines where because of their flexibility they can be used to help beach nourishment as they can be laid to trap drift sediments and they can also be used to form offshore breakwaters to protect sand dunes.

Cost: The costs for geotextile containers are not easily calculated but a manufacturer of such a disposal system has advised that, not including transport costs, a geocontainer suitable for holding CDMS would cost about 45€ /m3 and installation into a barge for disposal costs a further ~30€ /m3. Also, an additional cost needs to be taken into account for capping.

Information sources:

Department of Transport, 1996. Guidelines for the Beneficial Use of Dredged Material. Report prepared by HR Wallingford Limited.

Chan, B.B.P., Cheek, P.R. 2004. Disposal of dredged contaminated marine sediments using geosynthetic containers In: International Conference on Coastal Infrastructure Development- Challenges in the 21st Century. ETWB, Hong Kong SAR Government.

Ten Cate See http://www.tencate.com/smartsite.dws?id=82 under the Industrial Fabrics menu item and Dewatering Contaminated sediments at http://www.tencate.com/smartsite.dws?id=8530.

Van Zijl, M., Vlasblom, W.J., De Gut, J.G., Broos, E., De Boer, J. 2006. Feasibility study of the continuous Geotube®. Terra et Aqua 102: 19-24.

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4.8 Disposal options summary

A summary of the various CDMS disposal techniques together with the various constraints and

opportunities is presented below in Table 4.1.

Table 4.1 Comparative Summary of CDMS Disposal Options

Disposal Technique Risks / Constraint Benefits / Opportunities Cost

Confined Disposal Facility (CDF)

Potential for both air emissions and leachate leaking to groundwater but this can be reduced with well designed barriers, leachate collection and treatment, and adequate caps.

CDMS may not be disposed of in same system ‘basin’ thus may affect sediment balance. Transport of CDMS from source may have negative environmental effects and result in higher disposal cost.

The existing marine licensing regime in the UK limits the opportunities for use of CDFs in the marine environment.

Can be used for highly contaminated CDMS that poses too high a risk for other disposal routes.

CDF can be used to create habitat that can be used for both recreational use and by wildlife including nesting seabirds.

£££

Landfill Landfills approved for hazardous materials are recommended for these sediments. All hazardous waste that is sent to landfill will need to be treated (typically dewatering) prior to disposal, but there is the risk of leakage to groundwater and issues associated with exposure to air.

CDMS is not disposed in same system ‘basin’ thus may affect sediment balance. Transport of CDMS from source may have negative environmental effects and result in higher disposal cost

Can be used for highly contaminated CDMS that poses too high a risk for other disposal routes.

£££

Contained Aquatic Disposal (CAD)

CDMS is exposed until capping is completed and contaminants remain in the sediment. Long-term migration of persistent substances through sediments and cap can occur into the water column.

There is also a risk of cap erosion and hence an environmental monitoring programme is required to check cap integrity.

The existing marine licensing regime in the UK limits the opportunities for use of CAD in the marine environment.

CDMS can potentially be disposed in same system ‘basin’ thus reducing transport costs.

Disposal into seabed pit reduces risk of CDMS losses such as during wave exposure, wake from boats. Backfilling followed by capping of seabed pits used for aggregate extraction can return seabed to former state.

££

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Disposal Technique Risks / Constraint Benefits / Opportunities Cost

Level Bottom with Capping

CDMS is exposed until capping is completed and contaminants remain in the sediment. Migration of persistent substances through sediments and cap materials may occur into the water column. Thin layers may not be capped.

There is also a risk of cap erosion and hence an environmental monitoring programme is required to check cap integrity. Topping up of the cap is required if the cap is eroded.

Use of this technique in the UK has been limited to trial implementation and disposal of CDMS to sea is usually not permitted under the current marine licensing regime.

CDMS can potentially be disposed in same system ‘basin’ thus maintaining sediment balance and reducing transport costs.

£

Geotextile Bags Risk of bag rupture during placement and when in situ and hence monitoring programme is required to check integrity.

The existing marine licensing regime in the UK limits the opportunities for use of geotextile bags for disposal of CDMS in the marine environment.

CDMS can potentially be disposed in same system ‘basin’ thus reducing transport costs.

Isolates CDMS from the environment and can be used for medium-highly contaminated CDMS that poses too high a risk for other disposal routes.

£

Note: The above comparison of techniques does not consider existing legal barriers in the UK.

£-low cost; £££= highest cost.

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5. Socio-Economics

5.1 Introduction

This section provides a review of the costs and benefits of treatment and disposal techniques so

that informed decisions can be made on the economic suitability of options in relation to the site

specific management of CDMS. It is important to note that the cost relate in isolation only to

the techniques described. In practice, dredging contractors will often give an ‘all-in’ price for

dredging, treatment and disposal, taking account of costs of mobilisation/demobilisation,

equipment wear and tear, handling and transport of dredgings and sometimes risk factors such

as downtime due to bad weather.

In addition to the economic feasibility, other factors to consider include the technical and

practical feasibility of CDMS disposal or treatment. This will be dependent on multiple factors

such as the level of contamination in the sediment and the specific characteristics of where the

CDMS is located (e.g. is it possible to dispose CDMS at sea or will this result in unacceptable

restriction of ship navigation?). The technical and practical consideration of these options has,

where possible, been considered in earlier sections.

The socio-economic impacts of each treatment and disposal options has been broadly

categorised into three main groups; economic, environmental and social impacts. The types of

aspects considered within these three main groups are briefly described below.

• Economic aspects – These consider the costs to those responsible for undertaking the

treatment/disposal of CDMS and any subsequent monitoring that may be required. They

also consider benefits in terms of any possible economic efficiencies in the overall process

(e.g. making use of resources that may already readily available) and producing valuable

by-products that have commercial value (e.g. sand and aggregates).

• Environmental aspects – These consider the adverse environmental impacts (such as

leakage to groundwater and emissions to air such as during transportation) and the extent of

environmental benefits (e.g. reductions in ground/surface water contamination, air

emissions and energy consumption) of treatment/disposal.

• Social aspects – These broadly include impacts that affect parties who do not bear the costs

of implementation and monitoring (if relevant) but are indirectly affected by the

treatment/disposal of CDMS. Social costs include for example; visual impacts, possible

constraints to other seabed users (e.g. cable operators) from disposal and “not in my back

yard” (NIMBY) reactions by local stakeholders. Social benefits include for example, the

creation of amenity areas from a CDF facility.

Although it is not possible to undertake a formal cost effectiveness analysis (CEA) because

there is a general lack of quantifiable environmental benefits in existing literature (e.g. what

improvements have meant in terms of fish stocks, recreational uses, water quality, etc), where

possible, it has been noted, if a particular option is, in general, more cost-effective than another

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in terms of the costs of CDMS treatment/disposal per m3 (although there may be site specific

conditions which need to be considered).

Based on the review of existing cost and benefit information concerning CDMS treatment and

disposal options, the only quantifiable (and therefore monetised) impacts found were

implementation and monitoring costs (either represented as an overall cost for a particular

project or a cost per m3 treatment/disposed). The remainder of impacts (i.e. other economic

costs, benefits, environmental and social impacts) were typically only qualitatively described. It

should be noted the severity of impacts will vary in relation to the quantity of CDMS and site

specific conditions.

The costs presented in this section97, have been converted into British sterling (£) and presented

as 2008 prices. Estimates have been converted using historical exchange rates98 and updated to

present terms using the Retail Price Index (RPI). This approach is consistent with guidelines set

out in the HM Treasury Green book99.

5.2 Treatment

The main costs and benefits of selected treatment options that were presented in Section 2 are

summarised in Tables 5.1 to 5.11.

97 Source dates shown in this section have been based on publication date as most literature sources do not specify

the dates for prices quoted.

98 Historical euro to sterling exchange rates were obtained using the European Central Bank ECB) Statistical Data

Warehouse: http://sdw.ecb.europa.eu/quickview.do?SERIES_KEY=120.EXR.A.GBP.EUR.SP00.A

99 HM Treasury The Green Book, Appraisal and Evaluation in Central Government http://www.hm-

treasury.gov.uk/data_greenbook_index.htm

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Table 5.1 Natural Dewatering - Economic, environmental and social implications of CDMS treatment

Natural Dewatering

Economic costs Economic benefits

Van Dessel & Cnuddle (2006)100 estimate the economic costs of natural dewatering to be in the region of £7-19 /m3. This is dependent on factors such as the:

• dry matter content (i.e. different dredging techniques influence the water content of sediments);

• area available for installation of dewatering fields;

• time needed for dewatering (i.e. weather);

• marketability of end product;

• economies of scale.

Depending on national (and local) regulations, the outputs (dried sediments) can be used as:

1. soil/clay;

2. building material.

These products can be applied as construction material e.g. as a general fill, foundation material, sealing material in disposal sites, covering of disposal sites, construction of dikes and noise barriers etc.

The marketability and revenues of these products will depend on the market price for alternatives (e.g. primary aggregates such as gravel and soil) and regulations for use of sediments as building materials.

Environmental risks Environmental benefits

Natural dewatering requires ample space (see social costs) and time. The less time required for natural dewatering, the cheaper the costs will be. However the process can take months to years.

The time required will depend on the weather conditions (i.e. wind, rain and sunshine) and the required level of dewatering given the composition of the CDMS. For example, the time increases with the amount of fine particles and organic matter present in the CDMS.

Risks are also related to treatment of waste streams.

The overall energy consumption of the natural dewatering process is low. The energy is consumed during the use of cranes or special window turners to handle and move sediments.

The emissions in air are negligible although drainage water may need to be treated prior to discharge to meet certain environmental criteria.

Social costs Social benefits

It is necessary to spread the CDMS to a maximum height of 1-2m. The necessary surface area required is large and often quite expensive. A contingency/buffer area is generally also required especially for larger scale projects.

Some NIMBY opposition could occur if there is a lack of communication concerning CDMS and the benefits of natural dewatering.

If these products can be re-used (e.g. in construction), then less primary aggregates may be required. There are multiple benefits of using less primary aggregates such as; the preservation of rural landscapes and the reduction in emissions associated with the extraction, mining and transportation of primary aggregates.

Experiences and uncertainties

Van Dessel & Cnuddle (2006) states this technique is operational on a full scale and is often used to dewater sediments. It is suitable for large volumes with relatively high production rates (e.g. construction material).

100 SEDNET (2006) Sustainable Management of Sediment Resources: Sediment and Dredged Material Treatment.

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Table 5.2 Mechanical Dewatering - Economic, environmental and social implications of CDMS treatment

Mechanical Dewatering


De Weirdt & Detzner (2006)100 estimate the costs of mechanical dewatering, inclusive of water treatment, to be:

• fixed installations: £7-22 /m3 in situ (or £14-44/ton dry solids);

• mobile installation: £9-26 /m3 in situ (or £18-52/ton dry solids);

• Geobag dewatering: €20-30 per cubic metre plus handling costs.

The use of filter presses which are semi-mobile installations means that a lot of transport handling can be avoided (e.g. can be installed on a pontoon or mobilised and demobilised closer to the dredged CDMS).

The higher shear strength (in comparison to natural dewatering) makes the treated CDMS more acceptable for beneficial use or landfill disposal. Because of the higher dry matter context, the material can also be handled easier, reducing the costs of transportation, loading and handling.


Risks are associated with treatment of waste streams.

The use of filter presses means that limited surface space is required in comparison to natural dewatering and it is also not affected by weather conditions.

The total volume of CDMS is reduced using filter presses, as it is possible to achieve higher dry matter content compared to natural dewatering (40-45% up to 65-80%). The volume reduction will depend on the content of fine grained particles in the dredged CDMS.


Negligible. If these products can be re-used (e.g. sand in construction), then less primary aggregates may be required. There are multiple benefits of using less primary aggregates such as; the preservation of rural landscapes and the reduction in emissions associated with the extraction, mining and transportation of primary aggregates.


De Weirdt & Detzner (2006) states that large scale dewatering facilities are in operation in different European countries, e.g. Belgium, Germany and the Netherlands.

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Table 5.3 Separation - Economic, environmental and social implications of CDMS treatment

Separation


Separation is usually not used as a stand alone technique but rather combined with other treatment options. In all cases a combination with other technologies e.g. dewatering (and the associated cost of this), is required to ensure a further beneficial use or disposal of the CDMS.

Depending on the type of technique, scale and local conditions, Detzner (2006)100 estimates the economic costs of separation are in the region of £2-8/ m3 in situ. The disposal costs of residues are not included.

There are two main economic benefits of separation.

1. Valuable material such as sand can be removed from the CDMS which can have a beneficial re-use value e.g. sealing material in construction and secondary raw material in the ceramic industry.

2. The amount of CDMS requiring disposal and the associated costs associated with disposal can be reduced.


The main disadvantage of separation is the energy consumed during the process, where mechanical separation is used.

Risks of leakage also exist.

Separation of CDMS is sustainable and sensible if the:

• volume of the contaminated sediment is reduced;

• capacity of available disposal sites is saved;

• sediment fractions can be used as earthwork material or secondary raw materials in the building and other industries.


A NIMBY position can arise if the location of the site is not well chosen or communicated (e.g. public consultation).

Detzner (2006) argues that separation is generally accepted by the public because the environmental effects (space and energy consumption) are quite low and/or because the process results in beneficial re-use products.

If these products can be re-used (e.g. sand in construction), then less primary aggregates may be required. There are multiple benefits of using less primary aggregates such as; the preservation of rural landscapes and the reduction in emissions associated with the extraction, mining and transportation.


Detzner (2006) states that this technique is applied on an industrial scale: longitudinal separation is in operation in comparable variations e.g. in the Netherlands and Germany. Mechanical separation plants on an industrial scale are in operation in Hamburg and have been demonstrated in Rotterdam from 1996-1998.

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Table 5.4 Thermal Desorption - Economic, environmental and social implications of CDMS treatment

Thermal Desorption


In order to use thermal desorption, the CDMS must be pre-treated either using mechanical or natural dewatering. The maximum moisture content of CDMS will need to be 30% prior to entering the installation. De Weirdt (2006) 100 estimates the cost, excluding any pre-treatment operations, to be £37-52 / ton.

Similar to separation, there are two main economic benefits of thermal desorption:

1. beneficial use of secondary material;

2. reduction in the amount of CDMS requiring disposal, and the associated costs associated with disposal.


The process is energy intensive and emissions need to be controlled to meet national and local environmental standards.

Thermal desorption can be used to treat CDMS containing contaminants that can be volatised at temperatures below 650°C. These include: mineral oil, PAH’s, PCB’s, cyanides, chlorinated solvents, mercury and TBT. Many contaminants, even with very high concentrations (several thousand of ppm), can be removed with very high efficiency. This technique however is only suitable for organics and does not work for metals except mercury.


Potential for NIMBY objections. If these products can be re-used (e.g. sand in construction), then less primary aggregates may be required. There are multiple benefits of using less primary aggregates such as; the preservation of rural landscapes and the reduction in emissions associated with the extraction, mining and transportation of primary aggregates.


De Weirdt (2006) states that thermal desorption plants are in operation at full scale in a number of countries (e.g. Belgium, the Netherlands and Germany).

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Table 5.5 Bioreactors - Economic, environmental and social implications of CDMS treatment

Bioreactor


The estimated cost of treatment in a bioreactor are high (>£74/m3), but can be reduced by up-scaling. Long treatment times make the bioreactor an expensive treatment option but faster and less space-intensive than landfarming.

There are two main economic benefits of using a bioreactor:

1. soil as a beneficial use of secondary material –However additional dewatering will be necessary.

2. reduction in the amount of CDMS requiring disposal, and the associated costs associated with disposal.


Bioreactors use energy for mixing and aerobic reactors require an addition of oxygen. As a result they emit CO2. Emissions of hydrogen sulphide may also occur when using sediments with a low pH but emissions from reactors are much more easy to control than those in landfarms, The process is only applicable to degradable contaminants such as organics and can not be used to breakdown metals etc in CDMS.

The technology can be used to remove degradable compounds such as low ring PAHs and mineral oil.


Potential for NIMBY objections. The space required is quite limited, and there should be less political and social objections to a bioreactor.


Harmsen (2006) 100 argues because of the high costs and limited results possible, no full scale applications have been developed to date. This will also be because the treatment process using a bioreactor requires a lot of time.

Table 5.6 Phytoremediation / Phyto-extraction - Economic, environmental and social implications of CDMS treatment

Phytoremediation / Phyto-extraction


There is insufficient information at present to do a qualitative assessment of the costs and benefits of using this technique. Harmsen (2006)100 states that no successful application has been published on this technique as yet. It is still an emerging concept as far as technical implementation is concerned, and there are concerns on sustainability and risk management grounds as well as practicability. More recently, Kotrikla (2008)46 indicates based on a feasibility study of contaminated harbour sludge that barley could be a suitable plant as it can grow well despite the salinity of the dredged sediments. It also had a significant positive benefit effect on TBT removal and showed no measurable uptake of TBT or the other contaminants into the harvest product.

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Table 5.7 Landfarming - Economic, environmental and social implications of CDMS treatment

Landfarming (intensive and passive)


It is necessary to dewater the CDMS and to develop a proper soil structure (“ripening”). The time necessary varies between a few months and several years. This means that the site must have another beneficial use to be economically viable.

Harmsen (2006)100 estimates the costs of landfarming both intensive and passive are about £15 / m3, and DGE (2002) estimate costs in the region of £8-27 / m3 (excluding the costs of dredging and transportation).

In the Netherlands (on an experimental farm in Oostwaardhoeve) and in Belgium, the land has been used to grow willow as a biomass for energy production. The profits can be used to offset some of the costs of landfarming, although in the Netherlands profits from willow biomass have reported to be low.

Additional products can include reusable CDMS for building material as well as clean sediments. Landfarming also reduces the amount of CDMS requiring disposal, and the associated costs associated with disposal.


The energy consumption ranges from the energy used in normal agricultural practice (passive landfarming) to the high energy consumed in intensive landfarming. Emissions from landfarming are comparable to a bioreactor but less readily controlled.

Results of intensive landfarming can be comparable with a bioreactor. High degradation percentages can be obtained but over a long time.


The amount of space required is high, and a period of 1-2 years is necessary for intensive landfarming and passive landfarming may last several decades. The time required can be a problem in densely populated areas (DGE 2002)101.

Biomass production has multiple social benefits such as reduction in use of natural resources (e.g. oil and gas) for energy purposes, biomass is also considered a renewable energy source, and trees planted can improve the visual landscape.


Harmsen (2006) states that intensive landfarming for soil is already in use for a long period, but is unclear whether this technique has been used for CDMS.

101 DGE (2002) – Part 2: Treatment and Confined Disposal of Dredged material – Dutch-German Exchange on dredged material

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Table 5.8 Thermal immobilisation - Economic, environmental and social implications of CDMS treatment

Thermal Immobilisation (producing bricks, light-weight aggregates [LWA] and artificial basalt)


Regardless of the product being made after CDMS treatment (e.g. brick, LWA or artificial basalt), pre-treatment is required. The costs after pre-treatment are estimated below for each product.

• Brick - Hamer & Ulbricht (2006) 100 estimate that the costsof thermal immobilisation to produce bricks was in the region of £11-22 /m3 in situ. This was based on the assumption of; treating ~200,000m3, de-watered sediment, an investment cost of £26m, 20 years of depreciation and a price for the bricks sold roughly 10% below the usual market price.

• LWA - Hamer (2006) estimate the costs for producing LWA vary between £11-24 / m3 in situ. This price includes consulting and planning, investment and operational costs and depreciation.

• The treatment for CDMS to produce LWA in Dusagrind is estimated at between £18-26 / m3. The cost depends on the amount of sand and the composition of the dredged material. DGE (2002) estimates the cost of producing LWA using CDMS at £12-25 / m3.

• Artificial basalt - Hakstege (2006) estimates the costs for large-scale production at about £52 / m3 in situ (or £72/ton dry matter input) in granulate form, whilst the production to make blocks is more costly.

There are two main economic benefits of thermal immobilisation:

1. beneficial use of secondary material:

a. bricks – HZG in Germany developed a process using dredged sediments as clay replacements in high-value building materials – e.g. façade bricks. In Japan and in the Netherlands a pilot has been carried out to make bricks out of CDMS;

b. LWA – This can be used as a substitute for natural gravel as geotechnical fill material or additives to light concrete;

c. artificial basalt - The columnar blocks and granulates were used for dike revetment in a small harbour in Woudrichem.

2. no costs of disposal of any CDMS and the associated costs associated with disposal.

The extent to which treated CDMS can have re-use value in construction will depend on the actual properties of the product and legislative barriers for building materials.


The energy consumed using CDMS during the thermal immobilisation process is generally higher in comparison to using natural clay due to the higher water content in CDMS.

The thermal immobilisation process is compliant with emissions standards relative to conventional plants. The technique results in the immobilisation of organic and inorganic contaminants in the product.


NIMBY effect can be met by informing people that the CDMS does not need to be sent to a disposal site, and possibly could replace the production of products that previously used natural resources.

The space consumption of the plant (regardless of the end product) is lower than for a landfill disposal site. The use of the CDMS would mean a reduction in the use of natural resources and no space would be consumed at a disposal site.


Bricks – According to Hamer & Ulbricht (2006) the use of dredged sediments has been used in Japan, HZG in Germany and a pilot scheme in Netherlands.

LWA- The production of LWA using CDMS is a technique that has been applied for decades. Industrial scale application started in the Netherlands in 2004.

Artificial basalt – Experience with producing artificial basalt using CDMS has had limited success - Hakstege (2006) states that based on a pilot project in the Netherlands, a small part of the basalt was formed as hexagonal blocks, the rest as granulate. It turned out that the making of hexagonal blocks was a difficult process, where many blocks cracked during the cooling process. Because of these negative experiences and the high costs, the technique is not considered feasible for large scale application.

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Table 5.9 Stabilisation / chemical immobilisation - Economic, environmental and social implications of CDMS treatment

Stabilisation / Chemical Immobilisation


Depending on the use of the treated CDMS, the costs of stabilisation excluding the costs of transport and dewatering are estimated by Hamer, Van Dessel and Hakstege (2006) 100 to be £17-30/m3 in order to produce construction material.

The process of stabilisation involves a chemical addition, such as cement, lime, calcium aluminates and fly ashes.

There are two main economic benefits of using stabilisation chemical/ immobilisation are:

1. beneficial use of secondary material:

a. use as land covers – By covering up land, there is the avoided cost associated with the removal of rainfall in abandoned landfills. In addition, Douglas, Maher and Jafari (2005) explain that treated sediments were used in the Elizabeth Landfill in Elizabeth, New Jersey, USA so that the land could be re-used. The remediated site was sold for commercial development that includes a 2-story shopping mall and a parking lot, 3 hotels, and a movie theatre. Future plans include office towers and a marina;

b. use in construction material – The treated CDMS can be used for the production of aggregates (mainly with the addition of cement). These aggregates are usually used for road material;

c. capping material e.g. for abandoned mines and CDFs.

2. no costs of disposal of any CDMS or costs associated with disposal.

The extent to which treated CDMS can have re-use value in construction will depend on the actual properties of the product and legislative barriers for building materials.


There may be risks of emissions of nutrients and contaminants. However according to Douglas, Maher and Jafari (2005)102 reduction in potential contaminant release can be achieved by ensuring that best management practices be employed with regard to storm-water management, fugitive dust and erosion control.

Although this technique does not remove contaminants from the CDMS, the contaminants are bound to additives less mobile and therefore less bioavailable. This technique is applicable to TBT, which can effectively be immobilised resulting in less leachability below detection values. Based on the Elizabeth landfill, there were no indications of short- or long-term environmental harm caused by the using the sediments.


Potential for NIMBY objections. When the treated CDMS is used to cover landfills, this is likely to be perceived as a positive social benefit due to the environmental benefits of covering the landfill, but also the beneficial use of that land (where applicable).


Hamer, Van Dessel and Hakstege (2006) state that this technique has been applied on an industrial scale – It is used for liners in CADs in Hamburg and a disposal site for industrial waste in Bremen. A pilot scheme was also conducted in Hamburg and Bremen for using the treated CDMS for constructing part of the backward dike line at the coastline.

Douglas, Maher and Jafari (2005) state the New Jersey Department of Transportation/Office of Maritime Resources (NJDOT/OMR) initiated a pilot study to evaluate the feasibility of sediment dredged material (SDM) as a fill material for road embankments.

102 Douglas, Maher and Jafari (2005) - Analysis of Environmental Effects of the Use of Stabilized Dredged Material from New York/New Jersey Harbor, USA, for Construction of Roadway Embankments - SETAC (2005) - Integrated Environmental Assessment and Management — Volume 1, Number 4—pp. 355–364

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5.3 Disposal

Tables 5.12 to 5.16 in this sub-section outlines the main costs and benefits of selected disposal

options for CDMS that were presented in Section 4. Note that all the costs shown have been

converted and updated to 2008 prices (£). Costs of direct disposal to open sea or to the marine

environment for habitat creation or beach nourishment purposes are not included here, as these

options are not available for CDMS under the existing marine licensing regime in the UK.

Table 5.12 Confined Disposal Facility - Economic, environmental and social implications of CDMS disposal

Confined Disposal Facility (CDFs)


The capital outlay for the CDF construction is substantial and operational / monitoring costs are also high.

CDF is often the last step of a total treatment chain e.g. dredging → (separation) → dewatering → disposal Detzner (2004)100. There is potential for higher disposal costs as CDMS must be transported to the CDF. In order to monitor the functional efficiency of the disposal site safety systems are also necessary.

The costs for disposal of CDMS including long-term monitoring at the CDF site range between £8-59/m3 Detzner (2004).

The Holandsch Diep CDF was recently constructed at a cost of approximately £39 million and has a capacity of 10 million m3. The Slufter CDF, with a capacity of 150Mm3 was constructed for about £53 million and annual operating costs are about £7 million. These costs do not include costs associated with disposal barges and dredgers or costs for maintenance of the disposal facility equipment (See Box 1.4 for further details)

For moderately contaminated dredged material in the Great Lakes Basin, CDF was considered more economical than landfilling for more than 6,000m3 of mechanically dredged sediment and for more than 16,000m3 of hydraulically dredged sediment.

Separation techniques for sand have proven to be economically viable at De Slufter at least locally. The sand can be used either for bund construction or sold commercially.

Additionally, the technique follows the polluter pays principle, where practicable. For example, regional authorities including the Water boards can make use of the Rijkswaterstaat CDFs for a tariff of about £ 8/m3 of CDMS (see Box 1.4 for further details).


There is the potential for both air emissions and leachate leaking to surface or groundwater with these CDFs. These can be reduced with well designed landfills with conservatively safe barriers, leachate collection and treatment, and adequate caps.

Disposal of CDMS in CDF facilities may result in a greater level of handling by site workers and contaminants transfer via pathways to groundwater resources.

Detzner (2004) states that salts can accumulate at the surface of the dredged material, especially on the edge of cracks, created when the dredged material dries out. Rainfall events tend to dissolve and remove these salt accumulations. Certain metal contaminants may also become dissolved and transported out of the CDF by surface runoff.

Complex contaminants during the drying process can also be released to surface runoff, soil pore water, and leachate. There is then a risk that subsequent plants and animals that colonise the upland site may bioaccumulate these related contaminants.

On-land confinement structures are not exposed to the destructive hydrodynamic forces from vessels and storm surges and may not be subject to floods or land movements.

Similar to a landfill site, the base and the top are isolated to reduce emissions of contaminants.

Based on experiences with CDFs built during the 1980s the risks to the environment, including contaminant migration are less than originally thought. CDFs can by their enclosed nature accommodate highly contaminated CDMS that may not be suitable for other disposal routes.

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Confined Disposal Facility (CDFs)


Space consumption can be a disadvantage of CDF especially if navigational routes are compromised or space is at a premium. For nearshore CDFs, a large area of foreshore is required and this may conflict with other users such as nature conservation, navigation.

Detzner (2004) argues that a NIMBY-position of the public can occur if the site is not well chosen or not well communicated. Public consultation with key stakeholders is advisable and this would incur a cost.

CDFs can be landscaped in several ways to be used e.g. for recreation uses and nature reserves such as at De Slufter in Rotterdam, where the created habitat can be used for both recreational use and by wildlife including nesting seabirds.


CDFs are one means of disposal for large amounts of moderately to highly contaminated dredged material. There are decades of experience with design, operation, and maintenance of CDFs in the United States and this technique has been applied on an industrial scale in Bremen and Hamburg (See Box 1.5 for further details)

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Table 5.13 Land disposal - Economic, environmental and social implications of CDMS disposal

Land Disposal (e.g. landfill)


Land application is only economical in small- to medium-sized applications. The Port of Bremen dredges about 300,000m3 of sediment annually that is contaminated with a range of metals, organic contaminants and TBT. Cost for the dredging and disposal of the CDMS are about £7-10m per year.

There will be a costs associated with the transportation of CDMS to landfill. Elskens and Harmsen (2007)103 estimate that the costs of transport of dredged sediments (excluding the cost of loading and unloading) are:

• truck: £0.07-0.11/tonne-km;

• ship: £0.02-0.09/tonne-km;

• pumping: £0.06-0.11/tonne-km.

Additional costs may be associated with dewatering and possibly stablisation, as liquid wastes are no longer acceptable at landfills (see Tables 5.1, 5.2 and 5.9).

At the time of this report, dredged material sent to landfill is currently exempt from landfill tax104. However all hazardous waste that is sent to landfill will need to be treated prior to disposal. It is difficult to give a definitive cost of treatment for disposal at landfill as this will vary given the type and level of contamination, but assuming this is low, the costs can range from £1-205/t. Experiences with CDMS at a dry dock on the Tyne showed that mixing dredged CDMS with cement and then sending the material to landfill cost approximately £80/m3. This includes the stabilisation and landfill cost.

Landfill costs for the AMORAS project in Antwerp are estimated as €10-15 per cubic metre

For highly contaminated dredged sediment or lower volumes, the costs of landfill are generally less than the cost for disposal in a CDF.


Landfills (approved for hazardous materials if required) are recommended for these sediments. Waste that is sent to landfill may need to be treated (typically dewatering) prior to disposal, but there is the risk of leakage to groundwater and issues associated with exposure to air.

Risks of air pollution from transport may also be high if hazardous waste is involved, as there are few sites licensed to receive hazardous waste.

Negligible.


Landfill space is already at a premium and further use will only exacerbate the scale of the problem. There are negative impacts associated with landfills such as visual and odour. There have also been reports of health risks associated with landfill, although this is not conclusive105.

Negligible.


There is generally a lot of experience with the landfill of CDMS, although as with all the disposal options, there are uncertainties over the long term effects to the environment.

103 Frank Elskens and Joop Harmsen - Chapter 8 – Costs of Treatment Chains

104http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=page

Excise_ShowContent&id=HMCE_CL_000509&propertyType=document#P159_12557

105 http://www.environment-agency.gov.uk/research/library/data/34423.aspx

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Table 5.14 Contained Aquatic Disposal Facilities - Economic, environmental and social implications of CDMS disposal

Contained Aquatic Disposal Facilities (CADs)


In general CADs are very cost-effective if compared to other treatment chains. Large variations in costs can occur depending on the circumstance (e.g. scale, disposal costs). For example making use of existing pits is less expensive than engineered CDFs since pits generally only need small alterations.

In the Netherlands it was estimated the costs of CADs including investment, operation and long term monitoring be in the region of £4-26/m3 Hakstege (2006)100.

In the CAD described earlier in Box 1.2, to ensure that the receiving environment is not impacted due to CDMS disposal an environmental monitoring and audit programme is in place that monitors various environmental media for any evidence of contamination. This entails major cost implications (~£1.2m/year) and there will be the costs of monitoring the integrity of the cap.

Where CADs are purpose-built and dredging of the seabed is required, the cost is usually about 2-3 times higher than open sea with capping. However it is often found that the additional costs are acceptable when balanced with other issues such as public acceptance and agreement with regulators (Fredette, 2006)79.

CADs can be constructed close to the dredging location (e.g. within the same tidal basin) and the shorter transport distances and use of barges for disposal are likely more cost-effective than longer transport routes to landfill, which also incur the cost of fuel and inherent air quality issues.

When a seabed pit has been used for the purpose of aggregate extraction, then the costs of CADs can be covered or at least offset by the commercial value of the aggregate resources.

The technique can follow the polluter pays principle, where monitoring costs can be passed on proportionality (e.g. project proponents pay for each m3 of CDMS disposed) to the users of the CAD facility. The problems with this approach arise where, for example, a port is having to deal with historic pollution and the polluter no longer exists.


The main environmental impacts to consider are potential emissions of contaminants to surface water. Contaminant reduction is very limited although biological degradation of organic contaminants takes place but at a very slow rate.

Similarly to capping, there is a long-term risk of environmental exposure from toxic chemicals in sediments at the disposal site. Management of this long-term risk requires an open-ended commitment to monitoring by the party who accepts responsibility for receipt and storage of the materials. Disposal may therefore involve a transfer of liability for any future harm.

In deep waters (>20m) and during high currents, there is a risk of losing CDMS during placement. However, if the sediment is cohesive, dredged such that it maintains its character e.g. backhoe and dumped using a split-hopper barge then these risks can be minimised.

CADs can be an environmentally sound solution if properly designed, constructed and monitored.

Compared to other disposal options including open sea capping and CDFs, the use of CADs can result in less handling of contaminated material and fewer contaminant transfer pathways (Fredette, 2006).

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Contained Aquatic Disposal Facilities (CADs)


Space should be available and public acceptance is a very important factor. A NIMBY-position of the public can occur if the site is not well chosen or not well communicated. There is the disadvantage for the community (especially fishermen) due to possible hindrances for shipping. Adequate time should be considered for planning, consultation, designing and implementing a CAD facility. Typically an environmental impact assessment (EIA) and an ongoing environmental management programme are also required.

When large volumes of CDMS have been disposed in the CAD, this may contribute to the area becoming a ‘no go zone’ or put constraints to future seabed infrastructure.

Compared to open sea disposal with capping, CADs are often also more easily accepted by the general public as CADs provide more ‘comfort’ to people as they provide a higher level of protection from natural events including storms and waves (Fredette, 2006).


CADs have been used around the world for CDMS disposal including in Hong Kong, The Netherlands, Norway, and in a number of navigation projects throughout the USA including Boston, Rhode Island and Los Angeles.

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Table 5.15 Open Sea Disposal with Capping - Economic, environmental and social implications of CDMS disposal

Open Sea Disposal with Capping


The Port of Tyne needed to dredge approximately 60,000m3 of sediment that was contaminated with TBT. The CDMS cap was designed to be composed of a 1m silt layer overlaid with a 0.5m sand layer to protect from erosion. The volume of material used in the original cap was about 135,000m3. The overall cost of the trial was about £3.2M.

The advantage of open sea disposal is that there are lower transportation costs compared to treatment where CDMS is transported to land. The costs of material used for capping will vary depending on whether there are any excess sand or soil from construction work. Hauge & Hamer (2006)100 estimate that if soil sand of reasonable quality is used this will be £2-3/m2 but if there is excess sand or soil, which can be easily sourced from maintenance dredging at ports, the costs of capping material can go to zero. In Hong Kong, a surplus of soil excavated on land was used beneficially as capping material.

Hauge & Hamer (2006) also estimate that when using thin capping the material cost will be around 20% of the normal cap. Pilot trials (for sediment remediation purposes rather than capping placed CDMS) have shown that thin layer capping in-situ CDMS with layers of material containing activated carbon was cost effective at around £1-2/m2.

Some form of dredging plant will also be required to collect, transport and dispose the material at a licensed sea disposal site – There will also be associated resource costs and equipment costs to consider, although these are likely to be minimal.

However associated cost for monitoring can easily be underestimated or overlooked. Routine monitoring will be required to ensure the integrity of the cap. Keillor (2007)6 states that risk management of a capping project requires an open-ended commitment of funds and other resources (time and expertise) on a very long time scale that is not common to local governments who often become the “local sponsors” or parties responsible for maintenance of such facilities. Governments may not be willing, or able, to sustain such a commitment to risk management.

Where permitted, open sea disposal with capping is a relatively simple disposal technique that could be located within close proximity of the port thereby reducing disposal costs.

It also eliminates the need for transport, treatment and disposal at a new storage site. Consequently energy consumption, space and time requirements and production of by-products are negligible.

It is considered a cost efficient alternative to ex situ treatment especially for lightly contaminated dredged sediment (e.g. used in the Great Lakes Basin).

Ongoing maintenance dredging of uncontaminated material such as sand could be used to replenish the cap.


Emissions into the ambient environment can be controlled but the CDMS is still exposed until the capping is completed and contaminants still remain in the sediment. Long-term migration of persistent toxic substances through sediments and cap materials can occur into the water column.

Indefinite monitoring is also required as contaminants may migrate through the cap into surface water. There is also a risk of cap erosion and contaminated sediment resuspension by boat wake, propeller wash, flood currents, storm surge, and wind waves.

There might be a reduction in sediment and contaminant losses to the water column if toxic sediments remain capped.

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Open Sea Disposal with Capping


Hauge & Hamer (2006) argue that local opinions are likely to be in favour of removal and treatment of the CDMS to restore the seabed. Capping is likely to be viewed as “wiping the problem under the carpet”.

An exclusion zone may be required (particular if the cap >1m) to ensure the integrity of the cap. An exclusion zone or a particularly thick cap can cause constraints to navigable depths and could be an issue with shallow water depths near harbours and ports where shipping traffic is concentrated. In some cases, it may be necessary to compensate fishermen for the loss of fishing grounds to ensure the integrity of the cap.

Possible recreational benefits from improved sediment / environmental quality such as fishing and water-sport related activities.


Whilst several pilot schemes and full scale capping projects have been carried out, the long-term effectiveness (>30 years) and integrity of a cap is unknown. Significant short-term experience (several decades) with capped sediment deposits at multiple sites in estuarine and ocean waters have shown that capping has kept concentration levels below necessary action levels. However there is limited long-term experience with cap integrity including during extreme storm and flood events.

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Table 5.16 Geotextile bags - Economic, environmental and social implications of CDMS disposal

Geotextile bags


It is estimated that the total cost for CDMS disposal in geotextile bags would be around £80/m3 (personal communications with TenCate106).

The Geocontainer® system would cost around £36/m3. The hopper (or another type of barge) may have to be modified to carry geosynthetic containers or other geotextile methods of containing CDMS. The installation cost of a Geocontainer into a barge, inclusive the barge is estimated at around £24/m3 inclusive filling and closing of the Geocontainer®. The transportation will depend on the transport distance of course and for indicative purposes the costs could be around £12/m3. In practice, capping would probably be required which would add significantly to the cost.

Where permitted, a Geotextile bag is a disposal technique that could be located within close proximity of the port thereby reducing disposal costs.

It also eliminates the need for transport, treatment and disposal at a new storage site. Consequently energy consumption, space and time requirements and production of by-products are negligible or not existing.


There are risks of leakage during and after filling.

Cheek and Yee (2006)107 show that based on pilot schemes run in Hong Kong, if the fabric tensile strength is insufficient relative to the volume of CDMS disposed, then the bag will rupture on exit or in contact with the seabed.

Capping would likely be required to ensure integrity of the bags in the face of tidal and wave action as well as to prevent damage by human actions such as fishing and anchoring.

Bags can be used to create underwater structures e.g. groynes.


The use of geotextile bags could be seen as “wiping the problem under the carpet”. Similarly to capping, it may have implications on navigation if an exclusion zone is required to ensure the integrity of the containers/bags.

Bags have been used for infill in construction


Geosynthetic containers have been successfully used for coastal engineering (containment dykes, river groynes, breakwater core construction, etc) and sediment disposal applications in many parts of the world. The use of such a technique for CDMS disposal is less tried and tested.

106 Email (dated 02/02/09) communication with Tencate Geosynthetics Europe

107 Cheek and Yee (2006) - The use of geosynthetic containers for disposal of dredged sediments – a case history -

2006 Millpress, Rotterdam, ISBN 90 5966 044 7

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5.4 Summary

Table 5.17 summarises the treatment and disposal options discussed in this section and where

possible includes monetised costs along with a simple qualitative scale for the scale of non-

monetised impacts. It should be noted these are a subjective generalised analysis, whilst the

severity of impacts in reality will vary depending on site specific characteristics (e.g. space

available, weather conditions), the type and level of CDMS, market conditions (e.g. the price of

aggregates, labour costs) and the resources available at hand (e.g. is there already a pit nearby).

In general, looking purely at the economic costs, disposal is cheaper than treatment. However

in the absence of any weighting assigned to a particular type of impact, the environmental and

social costs are generally higher with disposal options relative to treatment options and from

society’s perspective, the benefits are generally higher with treatment options rather than

disposal options.

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Draft - See Disclaimer

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Table 5.17 Summary of the estimated costs and benefits of treatment and disposal options (2008 £ prices)

Option Costs Benefits Uncertainties

Economic Environmental Social Economic Environmental Social

Natural Dewatering £7-18/m3 -- --- ++ +++ +++ Low

Mechanical Dewatering Fixed - £7-22/m3

Mobile - £9-26/m3 - - ++ +++ +++ Low

Separation £2-8/m3 -- -- ++ ++ +++ Low

Thermal Desorption £37-52/tonne

+ Pre-treatment --- - ++ +++ +++ Low

Bioreactor >£74/m3 -- - ++ ++ ++ High as no full scale application

Landfarming £8-27/m3

+ dewatering --- --- ++ ++ ++ Low

Phytoremediation / Phyto-extraction Unknown Unknown Unknown Unknown Unknown Unknown High


Brick - £11-22/m3

LWA - £11-26/m3

Artificial basalt - £52/m3

-- - +++ +++ +++ Limited experience except with LWA

Stabilisation / Chemical Immobilisation Covering land £7-11/m3

Construction £17-30/m3 - - +++ +++ +++ Low

In situ chemical £44-74/m3 - - +++ +++ +++ Low

In situ biological £11-22/m3 -- - ++ ++ + Limited knowledge of the long term effect

Draft - See Disclaimer

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Option Costs Benefits Uncertainties

CDFs

£8-59/m3

+ dewatering + monitoring costs

--- --- +++ ++ ++ Low

Landfill disposal £1-205/tonne --- --- + + + Limited knowledge of the long term effect

CADs £4-26/m3

+ monitoring costs --- --- ++ ++ ++ Low

Open Sea with Capping £33/m3

+ monitoring costs --- --- ++ + + Limited knowledge of the long term effect

Geotextile bags for disposal at sea £80/m3 -- -- + ++ + Low

Subjective scale: Economic (impacts in additional to those monetised): + (-) small, ++ (--) medium, +++ (---) high

Environmental: + (-) no further deterioration/improvement, ++ (--) small change, +++ (---) major change

Social: + (-) small, ++ (--) medium, +++ (---) high

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6. Best practice

6.1 Overview

While re-use and disposal options at sea will be available if contamination levels within the

CDMS are acceptable and if other the characteristics of the CDMS (e.g. physical properties such

as volume, consolidation, etc) are acceptable given the sensitivities of the specific receiving

environment, in the UK, current legislation significantly limits the options for disposal of

CDMS at sea (whether treated or not) for the following reasons.

• Re-use and disposal options at sea will be unavailable or, at best, very unlikely to

be available if contamination levels within the CDMS are unacceptable (in other

words, it is not best practice to assume that CAD or CDF options will be available).

• Where contamination levels within the CDMS are unacceptable for direct disposal

at sea, re-use and disposal options at sea cannot be made available by pre-

treatment/treatment techniques because treated CDMS will cease to be ‘dredged

material’ under existing UK legislation and thus will cease to qualify for the

specific exclusion under the OSPAR Convention of dredged material from the

definition of wastes or other matter whose dumping at sea is prohibited.

• Re-use and disposal options on land depend on contamination levels, treatment

applied, availability of exemptions or options for beneficial re-use under the

Environmental Permitting/Waste Management Licensing regimes and, in terms of

landfill availability, on whether contamination levels within the CDMS are non-

hazardous or hazardous.

Regarding treatment techniques:

• pre-treatment and treatment techniques should only be used where they facilitate a

chosen re-use/disposal option (in other words, it is not best practice to assume pre-

treatment or treatment will make re-use and disposal options available);

• pre-treatment and treatment techniques may not have the same production rates as

the dredger producing the CDMS (i.e. the dredging process will become less cost-

effective if it is slowed down by a subsequent pre-treatment / treatment technique);

• pre-treatment may be required to facilitate treatment (e.g. dewatering prior to

thermal desorption) or disposal (e.g. dewatering prior to landfill disposal);

• some pre-treatment techniques (e.g. de-watering, solidification) and treatment

techniques (e.g. stabilisation, immobilisation) can increase the volume of CDMS

that needs to be re-used or disposed of;

• not all treatment techniques are proven and therefore may not be available (e.g.

proven to reduce contaminants by sufficient amounts, proven to work beyond pilot

96


studies, proven to be cost / time effective for large volumes of CDMS; proven to

work in ex situ conditions);

• not all treatment techniques are currently available in the UK or via transfrontier

shipment of waste;

• different treatment techniques may only be effective for certain contaminants (e.g.

organic contaminants, inorganic contaminants, organo-metal contaminants);

• immobilisation techniques do not reduce or destroy the contaminants and,

therefore, they do not necessarily make available re-use or disposal on land options

associated with non-hazardous waste where the original CDMS is classified as

hazardous waste, the options will need to be assessed on a case-by-case basis.

6.2 Framework for CDMS Disposal After Treatment

There are a number of options for the disposal of CDMS following treatment that renders the

material suitable for beneficial use. An example of a flow chart adapted from USACE60 is

presented below that provides a possible framework for the testing, evaluation and

environmental suitability of CDMS for beneficial use (without or with treatment) that could

(subject to legal compliance) potentially be adopted in the UK.

Evaluate Environmental suitability

Evaluate Physical and Engineering Suitability for Proposed Uses

Processing to EnhancedSuitability

Reason to Believe Contaminated?

Biological Evaluation

Chemical Evaluation

Biological Evaluation

Chemical Evaluation

Treatment

Stop: Unsuitable for Beneficial Uses

Suitable for Beneficial Use

Adverse Impacts

No Adverse Impacts

No Adverse Impacts

Adverse Impacts

No Adverse Impacts

No Adverse Impacts

Adverse Impacts

Adverse Impacts

Yes

Yes

No

97


Figure 6.1 Framework for assessing dredged material for possible beneficial use

The above simplistic framework could be easily adopted in the UK for risk-based, project

specific evaluation of CDMS for beneficial use. Where chemical and/or biological screening of

CDMS suggests that the material may have adverse impacts at the disposal site, the CDMS is

treated (where applicable using a contaminant specific technology and assumed to be on land)

and re-tested to ascertain the likelihood of adverse impacts of beneficial re-use, which under

existing regulatory regimes in the UK will normally only be possible on land.

If, following re-analysis, CDMS contaminants have been removed or immobilised and adverse

impacts are no longer likely, then a beneficial use option may be sought. Whilst wider

application of the treatment of CDMS and subsequent beneficial use where possible would

result in an overall improvement in CDMS management in the UK, there are several constraints.

Notably, there is currently a lack of proven treatment technologies for CDMS, most such

techniques having only been tested at the pilot scale. The lack of commercial application of

treatment technologies thus poses questions about their effectiveness and cost60 and hence

further hinders the potential for beneficial use of CDMS.

While the principle of use of suitable untreated dredged material for beneficial use at sea is

established in the UK (a recent example is dredging of the Princes Channel by the Port of

London Authority), use of treated CDMS is not. Thus only land-based beneficial uses are likely

to be available for CDMS (whether treated or not) under existing legislative regimes. While

beneficial use within the marine environment of treated material is theoretically possible,

provided that the material meets environmental criteria, past experience suggests that obtaining

agreement of a majority of OSPAR parties to use of waste materials for beneficial use in the sea

is likely to be very difficult. These aspects are discussed further in the Task 3 report13.

In cases where adverse impacts are likely after treatment, the dredged material is not suitable for

beneficial use and disposal options need to be considered. While disposal of CDMS without

treatment can be considered according the hierarchy in Figure 6.2, as described earlier, many of

the options are considered not to fall within what is permitted under the current UK legislative

regime (this is explored further in the Task 3 report). Once CDMS is treated, it is unlikely to

fall within the exemption for ‘dredged material’ in the OSPAR Convention, thus its disposal at

sea will not be permitted under the Convention, effectively ruling out most of these options

under the current interpretation of the Convention.

98


Lightly ContaminatedCMS

Moderately ContaminatedCMS

Highly Contaminated CMS

•Level Bottomwith Capping

•Level Bottom With Capping

•CAD

•Geotextile Liner

•CDF

•Landfill

•Geotextile Liner placed in CAD

Cost

Risk(expressed as losses to environment)

Figure 6.2 Possible decision tree for CDMS disposal (without treatment)

6.3 Conclusion

Thus there are a number of reasons why treated CDMS that cannot be beneficially used cannot

currently be disposed of at sea in the UK, including:

• any treatment is likely to change the characteristics of the CDMS so that it will no

longer be considered ‘dredged material’, irrespective of its contaminant and

sediment properties, therefore it will no longer fall within the exemption allowing

sea disposal for ‘dredged material’ in Annex II, Article 3 (2a) of the OSPAR

Convention;

• any treated material is likely to be regarded as waste that does not fall within any of

the other exemptions in Annex II, Article 3 (2a) of the OSPAR Convention;

• any chemical treatments would be bound to leave at least traces of the chemicals

involved and these may render the material unacceptable for sea disposal;

• if the treatment involved adding anything to the sediment to stabilise or bind the

contaminants, then it would no longer be ‘dredged material’.

Thus many of the treatment and disposal options discussed in this Task 5 report cannot, under

the existing international legislative regime, be used in combination with one another.

Appendix A1


Appendix A Information sources and contact details 7 PagesIntroduction

Baseline information and data were collated by desk study using published data including

various publications, reports, and methods available from contractors involved in the treatment

of CDMS, and the various UK and overseas agencies involved in CDMS disposal (e.g. USACE,

USEPA, The Netherlands, The Dutch Ministry of Transport, Public Works and Water

Management, Hong Kong Civil Engineering Department, Dutch-German Exchange on dredged

material). In addition, there was extensive consultation with the dredging industry and ports and

information was sought from various bodies including SedNet, British Ports Association108,

IADC and CEDA.

The major information and their sources are listed below in Table A1.

Table A1 Treatment and Disposal and Beneficial Use Options and Data Source

Baseline Topic Data Source

Treatment

Baseline information / General literature / Consultation

Bortone G., Palumbo, L. 2007.SedNet. Sustainable Management of Sediment Resources, Vol 2. Sediment and Dredged Material Treatment. Published by Elsevier, Holland.

Netzband, A., Hakstege, A.L., Hamer, K. 2002. Treatment and Confined Disposal of Dredged Material. Dutch- German Exchange on Dredged Material.

Detzner, H-D. 2004. Confined upland disposal. Draft SedNet Report on Treatment and Disposal of Dredged Material. August 2004.

Detzner, H. D., Bode, W., Weiss, T. 1994. The treatment of dredged Elbe sediments from the Port of Hamburg. Paper presented at Environmental and Mineral Processing, June 30 –July 1 1994, Ostrava, Tschechien.

http://www.boskalisdolman.nl/

http://www.pianc.org/

Bray, R.N., 2008. Environmental Aspects of Dredging. Taylor & Francis/Balkema, The Netherlands.

Detzner, H. D. and Knies, R. 2003. Treatment and beneficial use of dredged sediments from Port Hamburg, 2004. In: Proceedings of the XVIIth WODCON (paper B2-2), Hamburg, Germany, 2003.

US EPA Contaminated sediments in water at http://www.epa.gov/waterscience/cs/

US EPA Contaminated sediments – Publications at http://www.epa.gov/waterscience/cs/pubs.htm#management

US EPA (2003) Evaluating environmental effects of dredged material management alternatives – A technical framework. EPA-A842-B-92-008 at http://www.epa.gov/glnpo/sediment/gltem/tech-frame-rev04.pdf

108Nicholson, S. 2008. Contaminated dredged marine sediments: Developing a UK management framework – join the

debate and have your say. British Ports Association Newsletter, October.

Appendix A2



US EPA (2005) Contaminated sediment remediation guidance for hazardous waste sites. EPA-540-R-05-012 at http://www.epa.gov/superfund/health/conmedia/sediment/guidance.htm

US EPA (1997) National conference on management and treatment of contaminated sediments, Cincinnati, Ohio, 13-14 May 1997, at http://www.epa.gov/tio/download/remed/sedconf.pdf

US EPA (1993) Selecting remediation techniques for contaminated sediments. EPA-823-B93-001 at

US EPA (1998) EPA’s Contaminated sediments management strategy. EPA-823-R-98-001 at http://www.epa.gov/waterscience/cs/library/strategy.pdf

Urban Sediment Management and Port Redevelopment & Sediment in River Basin Management Plans. Presentations and posters presented at the 5th International SedNet Conference, Oslo, Norway available at http://www.sednet.org/library/library-sednetconference5.htm

DEC, 2002. Overview of Practical Experience Obtained in Flanders in the Treatment of Contaminated Sediments. DEME.

DEC, 2004. Treatment and Beneficial Re-Use of Contaminated Sediments. DEME.

DEC. 2004. Experience with Mobile Dewatering of Dredged Sediments. DEME.

DEC. 2005. Treatment of Contaminated Sediments in Harbour Areas. Review on 10 Years Experience. DEME

Federal Remediation Technology Round Table (2002) Remediation technologies screening matrix and reference guide, 4th Ed. http://www.frtr.gov/matrix2/

Soil Washing Mulligan, C, N., Yong. R, N., Bibbs. B, F., 2001. An evaluation of technologies for the heavy metal remediation of dredged sediments. Journal of Hazardous Materials. 85: 145-163. http://www.biogenesis.com/ssebbs.html

Thermal Desorption / Immobilisation

Hamer, K., Karius, V., 2002. Producing bricks from dredged harbour sediments – an industrial scale experiment. Waste Management 22: 521-530.

Herman, J. D., Shlieper, H. A., 1998. Decontamination and Beneficial Reuse of Dredged Material Using Existing Infrastructure for the Manufacture of Lightweight Aggregate. Proceedings of the 19th Western Dredging Association (WEDA XIC) Annual Meeting and Conference, Louisville, Kentucky.

Electrokinetic Herman, J. D., Shlieper, H. A., 1998. Ibid

Netzband, A., Hakstege, A.L., Hamer, K. 2002. Ibid

Altaee, A., Smith, R., Mikhalovsky. S, 2008. The feasibility of decontamination of reduced saline sediments from copper using the electrokinetic process. Journal of Environmental Management. 88: 1611-1618.

Alshawabkeh, A. N. 2001. Basics and Applications of Electrokinetic Remediation, Short Course Northeastern University.

Reddy et al, 1997. Effect of soil composition on removal of Chromium by electrokinetics, Electrochemical Decontamination of Soil and Water, Journal of Hazardous Materials 55:135-158.

Alshawabkeh, A. N. et al. 1997. Effect of solubility on enhanced electrokinetic extraction of metals. In Situ Remediation of the Geoenvironment, Minneapolis, Minnesota, October 5-8, 1997.

Lageman, R. 1993 “Electro-Reclamation”, Journal of Environmental Science and Technology, 27:2648-2650.

USACE, 2000. In-situ Electrokinetic Remediation of Metal Contaminated Soils Technology Status Report. Published by US Army Environmental Center.

Ugaz, A., Puppala, S., Gale, R.J., and Acar, Y. B., 1994. Electrokinetic soil processing: complicating features of electrokinetic remediation of soils and slurries: saturation effects and the role of the cathode electrolysis," Chem. Engineering Communications, 129:183-200.

Acar, Y. B., Li, H., and Gale, R. J., 1992. Phenol removal from kaolinite by electrokinetics." Journal of .Geotechnical Engineering., ASCE, 118:1837-1852.

Appendix A3



TBT DEFRA, 2002. The fate of TBT in spoil and feasibility of remediation to eliminate environmental impact. CSG Report 15.

Reed, J., Waldock, M.J., Jones, B., Blake, S., Roberts, P., Jones, G., Elverson, C., Hall, S., 2001. Remediation techniques applied to reduce the environmental impact of paint derived TBT in dredged material: a pilot study. In: Champ, M. (Ed.), Proceedings: Pollution Prevention from Ships and Shipyards. Oceanology. 2001 Symposium. Vol. 1. US Office of Naval Research, Arlington VA, USA, pp. 93–97.

Kotrikla, A. 2009. Environmental management aspects for TBT antifouling wastes from shipyards. Journal of Environmental Management. 90:S77-S85.

Goethals, L., Pieters, A., 2005. Remediation of sediments, treatment of the solid phase, In: Anonymous (Ed.), Development of an Integrated Approach for the Removal of Tributyltin (TBT) from Waterways and Harbours: Prevention, Treatment and Reuse of TBT Contaminated Sediments, LIFE02 ENV/B/000341, available at http://www.portofantwerp.be/tbtclean/, (accessed January 2009).

Pensaert, S., De Becker, G., De Clercq, B., De Puydt, S., Van de Velde, K., Trapp, S., Novak, J., 2005. Treatment of sediment, in: Anonymous (Ed.), Development of an Integrated Approach for the Removal of Tributyltin (TBT) from Waterways and Harbours: Prevention, Treatment and Reuse of TBT Contaminated Sediments, LIFE02 ENV/B/000341, available at http://www.portofantwerp.be/tbtclean/ (accessed January 2009)

Novak, J., Trapp, S., 2005. Growth of plants on TBT-contaminated harbour sludge and effect on TBT removal. Environmental Science Pollution Research. 12:332–341.

Mostofizadeh, Ch., 2001. Elimination of TBT Compounds from Dredged Material by Means of Pressure Thermolysis. Institute of Energy and Process Technology (IEV), Bremerhaven. In: Kotrikla, A. 2009. Environmental management aspects for TBT antifouling wastes from shipyards. Journal of Environmental Management. 90:S77-S85.

Stichnothe, H., Keller, A.., Thoming, J., Lohmann, N., Calmano, W., 2002. Reduction of tributyltin and other organic pollutants of concern in contaminated sediments by means of an electrochemical oxidation. Acta Hydrochimica et Hydrobiologica. 30:87–93.

Disposal Options

General USACE, 1983. Engineer Manual, Dredging and dredged material disposal. USACE, EM 1110-2-5025.

EPA/USACE, 1998. Evaluation of Dredged Material Proposed for Discharge in Waters of the U.S. - Testing Manual. EPA-823-B-98-004, Washington, D.C.

EPA/USACE, 1991. Evaluation of dredged material proposed for ocean disposal – Testing manual. EPA-503/8-91/001. http://www.epa.gov/waterscience/itm/ITM/toctxt.htm

Capping of placed CDMS and isolation methods such as covers and/or liners

USACE, 1998. Guidance for Subaqueous Dredged Material Capping. USACE Technical Report DOER-1. . [New version is in preparation.]

Envirocentre, 2008. Port of Tyne Sea Disposal Trials of Contaminated Tyne Estuary Sediment: Post Placement Monitoring – Tier 1 Second Annual Monitoring. Report No. 3308, June 2008.

Urban Sediment Management and Port Redevelopment & Sediment in River Basin Management Plans pp. 54-55. Abstracts of presentations and posters presented at the 5th International SedNet Conference, Oslo, Norway.

Sea disposal in exhausted borrow pits, CADs, CDFs

In: Urban Sediment Management and Port Redevelopment & Sediment in River Basin Management Plans pp. 54-55. Abstracts of presentations and posters presented at the 5th International SedNet Conference, Oslo, Norway.

Bray, R.N., 2008. Environmental Aspects of Dredging. Taylor & Francis/Balkema, The Netherlands.

Hakstege, P and Heineke, D. 2008. Re-use of borrow pits: The Dutch approach to storing (contaminated) dredged material. Terra et Aqua 112: 11-14.

Nicholson, S., Hui, Y.H., Rodger, J.G. and Ding, W.W. 2004. The management and monitoring of contaminated dredged material disposal in Hong Kong. In: International Conference on Coastal

Appendix A4



Infrastructure Development- Challenges in the 21st Century, pp. 70. HKSAR Government.

Whiteside, P., Ng, K.C., Lee, W.P. 1996. Management of contaminated mud in Hong Kong. Terra et Aqua 65: 10-17.

Nicholson, S. 2001. Biological-based screening in the management of dredged or excavated sediment in Hong Kong. SETAC Globe 2: 38-40.

CDFs Netzband, A.., Hakstege, A.L., Hamer, K. 2002. Treatment and Confined Disposal of Dredged Material. Dutch- German Exchange on Dredged Material.

PIANC(2002) ‘Environmental guidelines for aquatic, nearshore and upland confined disposal facilities for contaminated dredged material’.

USACE, 1987. Engineer Manual, Confined disposal of dredged material. USACE 1102-2-5027.

USACE, 2003. Evaluation of dredged material proposed for disposal at island, nearshore or upland confined disposal facilities – Testing manual. USACE, ERDC-EL-TR-03-1.

Beneficial Use

General USEPA and USACE 2007. Identifying, Planning, and Financing Beneficial Use Projects Using Dredged Material. Beneficial Use Planning Manual, EPA Report EPA-842-B-07-001, 114 pp. Downloadable from: http://www.epa.gov/owow/oceans/ndt/publications/pdf/2007_beneficial_use_manual.pdf.

See http://el.erdc.usace.army.mil/dots/budm/budm.cfm

PIANC (2009) Dredged material as a resource.

Re-use of CDMS, blended with cement, bricks, aggregates for construction industry

Keillor, P. 2007. Deciding About Sediment Remediation. A step-by step guide to making the decisions. Wisconsin Sea Grant.

Engineering uses of CDMS, re-use

Murray, L.A. 2008. Dredged material as a resource. Terra et Aqua 112: 3-10.

Engineering uses, environmental use, construction material

Department of Transport, 1996. Guidelines for the Beneficial Use of Dredged Material. Report prepared by HR Wallingford Limited.

Brandon, D.L. and Price, R.A. 2007. Summary of Available Guidance and Best Practices for Determining Suitability of Dredged Material for Beneficial Uses. US Army Corps of Engineers, Engineer Research Development Center. ERDC\EL TR-07-27.

Socio-Economics

Disposal and treatment costs

Keillor, P. 2007. Deciding About Sediment Remediation. A step-by step guide to making the decisions. Wisconsin Sea Grant.

Netzband, A.., Hakstege, A.L., Hamer, K. 2002. Treatment and Confined Disposal of Dredged Material. Dutch- German Exchange on Dredged Material.

Treatment Options

A desk based literature review was conducted in order to collate information on existing and

emerging treatment options and their applicability to various contaminants. A search was

conducted for contractors involved in the remediation of CDMS in order that consultations

could be made. In addition in-house specialists provided information in relation to radionuclide

contamination.

Appendix A5


A search of the Brownfield Briefing Database was made for contractors involved in the

treatment of contaminated land. A questionnaire was prepared and circulated by email on the

6th October 2008 to 59 contractors to ascertain their capability together with costs involved for

treating CDMS. The full list of the contractors approached is given in Table A2. Consultees

who responded and expressed a capacity to treat CDMS are marked with an asterisk. In

addition BioGenesis were consulted (in February 2009) and provided information on soil

washing technology.

Table A2 Treatment Consultations

Consultee Consultee

Alpha Environmental Systems Group Land & Water Remediation Ltd

Ardabus Ltd Land Clean Ltd

BAE Systems Environmental Lloyds Environmental Waste Management Ltd

Beach Group Macaulay Enterprises Ltd (MEL)

Biogenesis ,USA * May Gurney

Biogenie Site Remediation Ltd MB Envirotech Ltd

Buckingham Group Contracting Ltd MEL Ltd

CA Blackwell (Contracts) Ltd Morrison Construction Services Ltd

Cognition Land & Water Ltd O'Keefe Soil Remediation

Cornelsen Ltd Pryor Mourik Ltd

Cowens QDS Environmental Ltd

CSC Land Restoration Regenesis Ltd

DEME Environmental Contractors (DEC NV) * REC Remediation Ltd

Ecologia Environmental Solution Ltd Remedios Ltd

Edmund Nuttall Remedx Remediation Services

EDS (Euro Decommissioning Services Ltd) Rockbourne Environmental Ltd

Encia Group Ltd Shanks Waste Management

Environmental Land Solutions Ltd Slater (UK) Ltd

Environmental Reclamation Services Ltd Soilfix

Envirotreat Ltd * Tamdown Regeneration

ENVISAN N.V Telluric Remediation Management Ltd

Erith Contractors Ltd Terra Vac (UK) Ltd

GeoFirma Terreco UK

GeoSierra LLC Terrsula Ltd

Ground Remediation Systems Ltd United Retek (UK) Ltd

Hazrem Environmental Vertase FLI Ltd

Hitech Environment VHE Construction Plc

Appendix A6


Consultee Consultee

I&H Brown Ltd Virotec Europe Ltd

IEG Technologies UK Ltd Westminster Dredging Ltd/Boskalis Dolman *

J McArdle Contracts Ltd

Keller Ground Engineering

Disposal Options and Beneficial Use - Consultations

A screening exercise, comprising literature review, was undertaken to identify UK and overseas port authorities, regulators and stakeholders involved in the dredging and disposal of dredged material. Following the initial screening exercise, relevant organisations, regulators (e.g. contracting parties to the London Convention etc) and industry associations and organisations were consulted to determine their present practice for the disposal and beneficial use of dredged material. A summary of the consultation is provided below in Table A3. An article was also published in the BPA Newsletter108 of October 2008 that highlighted the study and requested feedback from UK port authorities.

Table A3 Disposal and Beneficial Use Consultations

Consultee Reason for Consultation

UK

A&P Falmouth Disposal and treatment of CDMS

A&P Tyne Disposal and treatment of TBT contaminated CDMS

British Ports Association Disposal, treatment and beneficial use of CDMS

CEFAS Beneficial use of CDMS; disposal and treatment of CDMS

Magnox Oldbury silt lagoons

Port of Tyne TBT capping study

Five Waters International Disposal in geotextile bags

Appendix A7


Consultee Reason for Consultation

Overseas

CSIRO, Australia Disposal and treatment of CDMS; sediment management

Civil Engineering and Development Dept., Hong Kong Disposal in CAD facilities, including geotextile bags

Environmental Protection Department, Hong Kong Disposal of CDMS in CAD facilities

Rijkswaterstaat, Centre for Public Works, The Netherlands Disposal in CDFs, De Slufter

Tencate, The Netherlands Geotextile bags

Hamburg Port Authority, Germany Upland CDF, Francop

Port of Oslo Authority, Norway Deep sea CAD disposal (Oslofjord CAD)

Biologge AS, Norway Capping material

University of Aegean (Dept. of Shipping), Greece Disposal and treatment of CDMS; sediment management

Germano and Associates Inc. USA Capping of CDMS, SPI of capping layers

Environment Canada Disposal and treatment of CDMS; beneficial use

Army Engineer Research and Development Center, USA Disposal and treatment of CDMS; beneficial use

Environmental Protection Agency, USA Disposal and treatment of CDMS; beneficial use

Note: Only key consultees are included above.

Appendix A8


Appendix B1


Appendix B Management of CDMS in Hong Kong 6 Pages

Introduction

A risk-based tiered management framework for the disposal of dredged/excavated has been

developed in Hong Kong109. The framework uses a risk based approach in the management of

CDMS. Uncontaminated dredged material poses a low risk of adverse impacts to the receiving

environment and is placed in Open Sea Disposal grounds. CDMS that is considered to have

lower potential for adverse impacts in the receiving environment because it has passed a suite of

bioassays is disposed in Dedicated Open Sea Disposal facilities (these are exhausted borrow pits

present in marine waters exposed to stronger tidal currents). The dredged material that is highly

contaminated is carefully disposed in a dedicated Contained Marine Disposal facility (exhausted

borrow pits termed ‘mud pits’ at East of Sha Chau). CDMS that is extremely contaminated must

undergo Special Treatment and may include treatment to render the material suitable for

disposal in the CADs (mud pits) located at East of Sha Chau.

Hong Kong’s CDMS framework is based on best practice and requires the chemical testing of a

suite of inorganic and organic contaminants and when the concentrations of these contaminants

exceed sediment quality values and hence may induce toxicity, the dredged CDMS must also

undergo biological testing in order to determine the ultimate disposal option (Figure B1).

Sediment Quality Guidelines (SQGs) and Chemical Screening

Hong Kong’s sediment management framework incorporates a suite of contaminants and

includes arsenic, metals, organics including polycyclic aromatic hydrocarbons (PAHs),

polychlorinated biphenyls (PCBs) and tributyltin (TBT). The framework uses a tiered system

for dredged CDMS assessment based on both chemical and biological screening to determine

the disposal option (i.e. open sea disposal for uncontaminated material, contained disposal for

contaminated material and the option for special treatment prior to disposal for highly CDMS;

see Figure B1). The contaminants (determined through analytical chemistry) present in

dredged sediments are compared to both lower and upper chemical screening criteria (sediment

quality guidelines) that provide an indication as to the likelihood for the sediment to induce

toxicity in marine organisms when disposed at sea110. The lower chemical screening criterion is

referred to as the Lower Chemical Exceedance Level (LCEL) and represents a value at which

the contaminant may have a toxicological impact in marine biota whereas the Upper Chemical

Exceedance Level (UCEL) is the contaminant concentration that frequently induces toxicity

(Table B1).

109 ETWB, 2002. Management of Dredged/Excavated Sediment. Environment, Transport and Works Bureau Technical Circular

(Works) No. 34/2002.

110 Nicholson, S. 2001. Biological-based screening in the management of dredged or excavated sediment in Hong Kong. SETAC

Globe 2: 38-40.

Appendix B2


Sediment to be Disposed of

Tier I

Desk Top Studyof Available Data

Tier IIChemical Screening

Category L Material<Lower ChemicalExceedance Level

Category M Material>Lower & <Upper Chemical

Exceedance Level

Category H Material>Upper Chemical

Exceedance Level

Type 1 –Open Sea Disposal

(see Note 1)

Type 3 –Special Treatment/

Disposal(see Note 3 & 4)

Type 2 –Confined Marine

Disposal(see Note 3)

>10 x Lower ChemicalExceedance Level

Insufficient Data or Data Indicates Potential Contamination

Data Indicates Little or No Contamination

Pass Pass FailFail

Type 1 –Open Sea Disposal(Dedicated Sites)(see Note 1 & 2)

Tier IIIBiological Screening

Tier IIIBiological Screening

(Dilution Test)

NO

Yes

Management Framework for Dredged/Excavated Sediment

Figure B1 Management Framework for Dredged Sediments in Hong Kong (ETWB, 2002)

Table B1 SQG Criteria for Marine Sediment Quality Classification in Hong Kong

Contaminant Lower Chemical Exceedance Level (LCEL)

Upper Chemical Exceedance Level (UCEL)

Metals (mg kg-1 dry wt.)

Cadmium 1.5 4

Chromium 80 160

Copper 65 110

Mercury 0.5 1

Nickel* 40 40

Lead 75 110

Silver 1 2

Zinc 200 270

Appendix B3


Contaminant Lower Chemical Exceedance Level (LCEL)

Upper Chemical Exceedance Level (UCEL)

Metalloid (mg kg-1 dry wt.)

Arsenic 12 42

Organics-PAHs (µµµµg kg-1 dry wt.)

Low Molecular Weight PAHs 550 3160

High Molecular Weight PAHs 1700 9600

Organics-non-PAHs (µµµµg kg-1 dry wt.)

Total PCBs 23 180

Organometallics (µµµµg TBT l-1 in interstitial water)

Tributyltin* 0.15 0.15

Note: *The contaminant level is considered to have exceeded the UCEL if it is greater than the value shown.

Following the chemical assessment of the CDMS the sediment is classified into distinct

categories based on the contaminant concentrations present and associated risks to marine life

during dredging and disposal. The following categories are used in Hong Kong to classify

sediments:

• Category L Sediment with all contaminant levels lower than or equal to the LCEL.

The material must be dredged, transported and disposed of in a manner that

minimises the loss of contaminants either into solution or re-suspension.

• Category M Sediment with any one or more contaminant levels exceeding the

LCEL and none exceeding the UCEL. The material must be dredged and

transported with care, and must be effectively isolated from the environment upon

final disposal unless appropriate bioassay (biological) tests demonstrate that the

material will not adversely affect the marine environment.

• Category H Sediment with any one or more contaminant levels exceeding the

UCEL. The material must be dredged and transported with great care, and must be

effectively isolated from the environment upon final disposal.

Biological Screening

The biological testing component of the framework is only required for those CDMS’ that have

contaminants above the SQG chemical screening criteria and such an approach is cost effective

and negates the need to test all CDMS using expensive biological assays. The bioassays use a

suite of ecologically relevant test species that interact with bedded sediments in different ways

and are, therefore, exposed to contaminants through different routes of exposure. The test

species used in the whole-sediment bioassays are non-native to Hong Kong but local test species

are not widely available (some trials using native species have been undertaken) and use test

species for which testing protocols are already well developed. The species typically used in the

bioassays comprise an epibenthic amphipod (e.g., Ampelisca abdita), a burrowing polychaete

(Neanthes arenaceodentata) and either bivalve (e.g., Mytilus sp., Crassostrea gigas) or

echinoderm embryos. In practice the bivalve embryo assay has been used in preference to the

Appendix B4


echinoderm test. In order to maintain test validity, the Management Framework requires the

implementation of stringent QA/QC so as to reduce the probability of extraneous factors

unrelated to sediment toxicity confounding the test results111. The test species, endpoint and

protocol used in the bioassays are summarised below in Table B2.

Table B2 Whole-Sediment Bioassay Test Species and Endpoints

Test Endpoint Failure Criteria Typical Protocol

Amphipod 10-day Survival Mean survival in the test sediment is significantly different (p≤0.05) from mean survival in the reference sediment and mean survival in test sediment <80% of mean survival in reference sediment.

USEPA, 1994

Polychaete 20-day Growth Mean dry weight in test sediment is significantly different (p≤0.05) from mean dry weight in reference sediment and mean dry weight in test sediment <90% of mean dry weight in reference sediment

PSEP, 1995

Bivalve or echinoderm embryo

48 to 96-hour Survival and Development

Mean normality survival in test sediment is significantly different (p≤0.05) from mean normality survival in reference sediment and mean normality in test sediment <80% of mean normality survival in reference sediment

PSEP, 1995

ETWB (2002) provides full details of the test protocols and statistical analysis used to determine whether sediments pass or fail the biological screening.

Disposal of Type 3 (Special Treatment/ Disposal) CDMS

Under the Management framework, CDMS that is highly contaminated and fails the various

chemical and biological tests is required to under go special treatment prior to disposal. Any

Type 3 treatment and/or disposal of exceptionally contaminated CDMS would need to be agreed

with the regulator (Environmental Protection Department) and is likely to be project-specific, as

guidelines cannot be prescribed. Trials have been conducted at the CAD with material placed

into sealed geosynthetic liners that were subsequently disposed by barge into the pit. Such a

technique may also be applicable for disposal of Type 3 CDMS.

To date, no dredged sediment has been categorised as requiring special treatment and/or

disposal although pressure to develop certain coastal areas of Hong Kong cannot preclude the

possibility of such material being present and require dredging in the future.

Disposal of CDMS at the CAD Facility

There are a series of thirteen purpose built seabed pits that receive CDMS (Figure B2). The pits

started to receive CDMS in December 1992 and when the original pits were exhausted, further

CADs (termed ‘Contaminated Mud Pits’ or CMPs) were excavated, the more recent of which

111 Nicholson, S., Clarke, S.C., Word, J.Q., Kennish, R., Barlow, K.L., and Reid, C.A. 2000. Quality assurance in the

toxicological assessment of Hong Kong dredged sediments: the potential influence of confounding factors on

bioassay results. In: Proceedings of the ISWA International Symposium & Exhibition on Waste Management in Asian

Cities (Eds. C.S. Poon and P.C.K. Lei), pp. 196-203. The Hong Kong Polytechnic University Press.

Appendix B5


comprise three adjacent pits designated CMP IVa-IVc. The CMP IVa-IVc pits are

comparatively large subsea structures (each pit is ~1km x 1.2km) and deep (CMP IVa, IVb and

IVc were dredged to approximately –30, -31 and –44mCD, respectively) and were originally

excavated to provide fill material for construction of the international airport at Chek Lap Kok.

The CMP IV series of pits have an overall capacity of over 30mM3 and have provided Hong

Kong with a discrete location for contained CDMS disposal and have an operational life span of

about twelve years.

The CAD facility has proven effective to meet Hong Kong’s needs for disposal of CDMS in an

environmentally responsible manner although the pits are nearing their operational life span and

there are proposals for another CAD facility. The design and operation of the pits and CDMS

disposal operations are highly regulated and independent environmental consultants are

responsible for an extensive EM&A programme that aims to detect any impacts to aquatic

receptors in the receiving environment81.

The Environmental Monitoring & Audit Programme

The disposal of CDMS is in waters that have numerous ecological and human receptors and

disposal operations are highly regulated through an extensive environmental monitoring and

audit (EM&A) programme that collects environmental samples and is used to identify any

adverse impacts disposal activities may have on the receiving environment and to determine

appropriate mitigation measures to prevent and ameliorate any adverse impacts detected81. The

focus of the EM&A programme is on water and sediment quality (both sediment chemistry and

whole-sediment bioassays) and contaminant concentrations in fisheries resources (together with

ecological and human health risk assessment). On completion of capping, grab samples are

taken and used in an assessment of the macrofauna that colonise the inert material used to cap

the CDMS within the pits.

A 24-hour on-site management barge is in operation year-round at the CAD facility and the on-

site management team register all incoming barges and allocate a disposal area in cells within

the pits depending on the tidal current at the time of disposal. For example, during the ebb tide

the disposal cells in the northern area of the pit are most commonly used as any sediment in

suspension will be carried to the south and likely settle out of suspension within the pit. Further

precautions also include ensuring that the barges used to dispose the CDMS are allowed to drift

over the target cells to minimise the movement of vessels under power over the pit as this may

cause some propeller wash of the placed sediments. Disposal of CDMS material within the

CAD facility is also evenly distributed and regular bathymetric surveys are conducted to

determine the levels of contaminated mud within the pit.

The results of the water and sediment quality assessments over a ten year period of intensive

monitoring revealed that the CDMS disposal did not appear to result in elevated CDMS losses

from the CAD as contaminants in the ambient environment were not significantly above

background. It has, therefore, generally been concluded that the CADs are highly successful in

isolating CDMS and will retain sediment even during storm events81.

Appendix B6


Discussion

The management of CDMS in Hong Kong is highly regulated and based on stringent

environmental regulations. The tiered sediment management framework used in Hong Kong is

based on a ‘weight of evidence’ risk-based approach, involving chemical analysis of both

inorganic and organic contaminants together with biological screening to determine the

likelihood of risk to receptors in receiving waters and hence disposal option. The CDMS

management framework incorporating synoptic chemical and biological screening is a practical

tool in dredged sediment management and can serve as a model for other countries including the

UK.

department for environment, food and rural affairs

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