Sustainable material reuse solutions for dredged sediments

Abstract

The Yorkshire Business Unit of British Waterways (BW) was due to start dredging canals in the South Yorkshire (SY) region. Dredging is vital to maintain navigable waters and help ensure the long-term passage of freight and pleasure craft. The canals in SY had not been dredged for 10 years, which was beginning to impact the effectiveness of the waterways in this area. The most economic means of managing the dredged sediments was to place the material under a Paragraph 19 waste management licence exemption into a dredging deposit located at Long Sandall, Doncaster. However, the Long Sandall dredging site was nearing capacity containing 100,000 m³ of sediment, so it was proposed that this material could be excavated and reused as part of canal stabilisation works proposed several miles up canal at Whitley Lock. In order to reuse the existing sediment material in the lagoon under a Paragraph 19 waste management licence exemption, it had to be established that the sediments could be classified as non-hazardous. Following this, the sediment required an assessment in terms of its risk to human health and controlled waters to demonstrate its suitability for use in the canal bank stabilisation works. Ramboll carried out a detailed assessment and modelling exercise to help BW demonstrate the suitability of the sediments for reuse as construction fill. The work creates the infrastructure for BW to manage dredged sediments in a sustainable manner. This is vital in ensuring the future viability of the waterway and has saved BW's Yorkshire Business Unit potential disposal and material construction costs in the region £1,500,000.

Keywords:

• sediments
• sustainable
• assessment
• dredging
• classification
• hazardous waste

1. Introduction

British Waterways (BW) are responsible for the maintenance and enhancement of over 2200 miles (3540 km) of Britain's inland waterway network. Part of their maintenance works (typically on a 10-year cycle) involves dredging the canals. Therefore, they are required to manage large amounts of dredged sediment from canals and waterways which is usually deposited into waste management sites owned and operated by BW.

In the northeast of England at Long Sandall, 100,000 m³ of sediment previously dredged from the South Yorkshire (SY) region was stored in six lagoons adjacent to the canal. These lagoons were nearing capacity and could only accept a further limited volume of dredged material. The implications of the lack of space at the Long Sandall site were severe since this would have seriously undermined all of BW's dredging work in the SY. Dredging satisfies the boater's requirements and that of freight and in turn helps provide income and support for BW's other beneficial work. By limiting the amount of dredging possible along the SY Navigation, the ability of BW to upkeep the waterway and habitats in this region would have been severely threatened.

Increasing the working life of the Long Sandall deposit site and the ease of disposal of sediments to this site is part of a medium- to long-term project by BW to develop a functional network of in-house disposal facilities. This is important to BW given the cost, uncertainty, practical constraints and environmental issues linked to the use of commercial landfill. BW therefore proposed to use the dredged lagoon material as infill in the canal bank stabilisation works on the Aire and Calder Navigation along an approximately 1 km-length strip between Whitley Lock and Heck Bridge.

Ramboll was instructed to provide a waste classification of the material in terms of the hazardous waste guidance (EA 2005) and the new oily waste guidance (EA 2007) to prove whether the sediments were eligible for reuse under a Paragraph 19 waste management licence exemption.

The waste classification and the potential reuse capabilities of this material were key issues to the future management options of BW. Therefore, the outcome of these investigations had significant implications for both the financial and legal liability of existing BW dredging sites, the ability to use the Long Sandall site in the future and the ability to deposit sediments under waste management exemptions that permit the disposal of non-hazardous material only.

2. Current waste guidance

Currently, there are two key pieces of guidance used to classify waste as either hazardous or non-hazardous: (a) WM2 Hazardous Waste: Interpretation of the definition and classification of hazardous waste (EA 2005) and (b) ‘How to find out if waste oil and wastes that contain oil are hazardous’ (EA 2007). The first document provides guidance on the classification of the inorganic and organic components of the waste. The second document provides guidance on the classification of the oily waste component.

A flow diagram is presented as Figure 3.1 (p. 9) in the Hazardous Waste Guidance WM2 (EA 2005), which describes the key steps in the hazard classification process. Steps 1–3 require the assessor to determine whether the waste is a Directive Waste, is covered by domestic legislation and has an EWC 2002 Code. Step 4 asks whether the composition of the waste is known. Therefore, Ramboll carried out chemical analysis of the sediments (30 no. samples) from the Long Sandall deposit and established that the sediments contained several inorganic compounds (typically heavy metals, including arsenic, cadmium, chromium, lead, mercury, nickel and selenium), dioxin compounds and hydrocarbon compounds (typically diesel range organics (DROs) and other heavy-end organic compounds).

Step 5 requires the assessor to assess whether the waste contains dangerous substances. As the waste contains heavy metals (e.g. arsenic, cadmium and chromium) and hydrocarbons, some of these elements/compounds have the potential to be dangerous (as defined in WM2, EA 2005). Step 6 requires the assessor to determine whether the waste possesses any hazardous properties H1–H14. The 14 hazardous properties are defined in Annex III of the Hazardous Waste Directive. For the inorganic components, the current waste regulations require the waste producer to assume that the elemental composition of the waste soil comprises the worst-case speciation of the elements contained within the soil (unless there is data to suggest otherwise). For example, for wastes containing chromium, the current approach is to assume that all chromium present in the soil is in the form of toxic chromium trioxide (CrO₃) rather than the significantly less toxic chromium oxide (Cr₂O₃). This approach can result in inappropriate classification of the soil.

3. Hazard assessment

3.1 Inorganics

Applying the current approach to waste classification (e.g. worst-case speciation), the sediments typically failed the hazardous property H7, carcinogenic, mainly due to the presence of heavy metals and hydrocarbons (i.e. concentration ranges, As 2.0–143, Cd 1.0–687, Ni 6.0–4720 mg/kg, etc.). An intensive literature review of the speciation of the compounds assuming neutral pH and positive redox conditions was carried out drawing heavily on data published in Heavy Metals in Soils edited by Alloway (1995). The results of the literature review suggested that the compounds would most likely be in the sulphide form. Also, the worst-case speciation of the metal compounds is unlikely to exist within the sediments concerned as these compounds tend to be soluble and hence would have most likely dissolved in the aqueous environment where they are found. Ramboll carried out X-ray diffraction and X-ray fluorescence testing and analysis using a Siroquant and SedNorm software. The results of this analysis supported the conclusion that within this environment sulphide compounds would be the most likely speciated form.

A hazardous property assessment was then carried out (using the most likely speciated form) to determine whether the sediment triggered any of the hazardous properties given in H1–H14. None of these properties were triggered during the assessment suggesting that in terms of the metal constituents the sediment could be classified as non-hazardous waste.

3.2 Dioxins

Material from the Long Sandall lagoons contained a potentially very toxic group of compounds called dioxins. Dioxins possess a number of toxicological properties, including carcinogenic, immunological, reproductive and human development effects (WHO 1998, DEFRA 2007).

Dioxins are classified as carcinogens, with the associated hazard property H7 (EPA 1987, RoC 2005), and therefore may be classified as dangerous substances according to the updated version of Council Directive (67/548/EEC, 2001/59/EC). According to the WM2 Hazardous Waste Guidance (EA 2005), the H7 (carcinogenic) hazardous threshold value is set at 1000 mg/kg, therefore as dioxins trigger H7, the hazardous threshold value for dioxins would be 1000 mg/kg (Environment Agency, personal communication, 2007). Other hazardous properties triggered by dioxins include H5 and H6 (harmful and toxic, respectively), and again the threshold levels for H5 and H6 hazardous properties are set at 1000 mg/kg.

Using such a high threshold value for dioxins concerned Ramboll, as the 1000 mg/kg threshold concentration is six orders of magnitude greater than the US EPA human health guidance action level which is set at 1000 ng toxic equivalent (TEQ) kg.

Given the inconsistency of the waste threshold value provided in WM2 (1000 mg/kg) with the human health safe exposure limits (1000 ng TEQ/kg), a detailed literature review of the available guidance for dioxins was therefore undertaken in order to present a more reasonable maximum threshold value for waste contaminated with elevated concentrations of dioxins.

Dioxins are classified as a persistent organic pollutant (POP). European Regulations (EC 1195/2006) indicate that wastes containing dioxins exceeding the low POP content must be ‘disposed of or recovered… to ensure that the POP content is destroyed or irreversibly transformed’ (EC 850/2004). The low POP content adopted by the EU and the United Nations Environment Programme (UNEP 2006a, 2006b) is a threshold of 15,000 ng TEQ/kg. Wastes with concentrations of dioxins above this level must be disposed of or transformed in an environmentally sound manner, such as permanent storage in a hazardous landfill, provided the waste is solidified and the concentration of dioxins does not exceed 5 mg/kg (EC 172/2007). Ramboll considered that a more reasonable dioxin concentration should be used as a trigger value for hazardous waste classification. Using European Regulations, it is considered appropriate that the low POP concentration limit for dioxins of 15,000 ng TEQ/kg be used as a trigger value for hazardous waste classification.

The concentrations of dioxins within the dredged material were well below the trigger value for hazardous waste with the maximum concentration recorded at 3100 ng TEQ/kg. Therefore, with respect to the dioxin concentration, the waste would be classified as non-hazardous.

3.3 Hydrocarbons: oily waste

The final group of contaminants in the dredging material to be assessed were the hydrocarbons. In 2007, the Environment Agency released a hazardous waste document for wastes containing oils; ‘How to find out if waste oil and wastes that contain oil are hazardous’ (EA 2007).

Oils are associated with three hazard phrases: H5 (harmful), H7 (carcinogenic) and H14 (ecotoxic). The guidance recommends that if the oil in the waste is unknown, it should be analysed to determine whether the oil content causes the waste to have any hazardous properties (as detailed above). Therefore, the wastes were analysed for fuel and lubricating/other oils and compared to the hazardous waste thresholds for each hazard phase.

The sediments were analysed for speciated hydrocarbons and the results were assessed using the current EA guidance (HRW08, EA 2007). The concentration of total petroleum hydrocarbons (TPHs) in the waste did not exceed the threshold concentrations to trigger either H14 or H5 hazardous properties.

To assess whether the TPH concentrations in the sediments trigger the hazardous property H7, the EA guidance (HRW08 2007) recommends that the various hydrocarbon bands, e.g. petrol range organics (PROs) and DROs, and the lubricating/other oil content of the waste are compared to the specific threshold values:

• PRO (C₆–C₁₀) – 0.1% w/w (1000 mg/kg)

• DRO (C₁₀–C₂₄) – 1% w/w (10,000 mg/kg)

• lubricating/other oils (C₂₄–C₄₄) – 0.1% w/w (1000 mg/kg)

The sediments did not exceed the threshold values for PRO or DRO fractions; however, some samples contained lubricating/other oils in excess of 1000 mg/kg. Typical concentration ranges recorded were as follows:

• PRO – always < 50 mg/kg

• DRO – 170–1500 mg/kg

• lube oil – 620–2400 mg/kg

The HRW08 guidance states that the PROs are assumed to be all categories 1 and 2 carcinogens and that the DROs are assumed to be category 3 carcinogens (hence the higher threshold value of 10,000 mg/kg). The lubricating/other oil in the waste is ‘assumed to be a category 2 carcinogen unless it can be demonstrated that it does not possess carcinogenic properties’.

As some samples recorded the lubricating/other oil fraction in concentrations exceeding the waste threshold, Ramboll assessed this fraction in more detail as follows: (i) by carrying out speciated TPH analysis, speciating the TPH into 13 bands giving data for both the aliphatic and aromatic fractions and (ii) by carrying out speciated polycyclic aromatic hydrocarbon (PAH) analysis, and found that the lube oil fraction contained categories 1–3 carcinogens. The categories 1 and 2 carcinogens (most carcinogenic) are due to the presence of aromatic hydrocarbons and specifically PAH compounds, e.g. benzo(a)pyrene. The category 3 carcinogens (less carcinogenic) are due to aliphatic compounds within the lubricating/other oil fraction. Ramboll considered it was extremely conservative to assume that the entire lubricating/other oil fraction contained categories 1 and 2 carcinogens when the results of the speciated TPH and PAH analyses of the waste provided a greater understanding of the oil composition in the waste.

Therefore, Ramboll assessed the TPH compounds within the waste using the following method:

• Categories 1 and 2 carcinogens. PRO (C₆–C₁₀) and aromatic fraction of lubricating/other oils (C₂₄–C₄₄) summed and compared to the waste threshold of 0.1% w/w (1000 mg/kg).

• Category 3 carcinogens. DRO (C₁₀–C₂₄) and aliphatic fraction of lubricating/other oils (C₂₄–C₄₄) summed and compared to the waste threshold of 1% w/w (10,000 mg/kg).

When assessed using more detailed methods mentioned above, the sediments would be classified as ‘non-hazardous’. The above process for classifying the lube oil contaminants in sediments has been applied in SY under approval of the Environment Agency, and BW intend to take this forward nationally.

4. Risk assessment

Once the dredged sediments were classified as non-hazardous, the material then had to be assessed in terms of its risk to human health and controlled waters. The proposed reuse of the materials was as backfill material at the embankment renewal works of the Aire and Calder Navigation Networks. The infilled material will be used on both sides of the canal between Whitley Lock and Heck Bridge (Figure 1).

Figure 1 Aire and Calder Navigation, Whitley Lock to Heck Bridge (Crown copyright Ordnance Survey. All rights reserved.)

4.1 Controlled waters risk assessment

The leachate analysis identified minor isolated incidents of contaminant concentrations exceeding the recommended generic assessment criteria. However, based on the low concentrations and isolated occurrence of these incidents, the sediments are considered unlikely to present a significant risk to controlled waters if these sediments are placed at the Heck Bridge site.

Following this assessment, it was considered that no further assessment of the risks presented to controlled waters was required.

4.2 Human health risk assessment

As part of a full quantitative risk assessment (QRA), the potential risks to human health from dioxins and other elevated contaminants were assessed, so recommendations could be made regarding the suitability of the material for use as a structural fill material for canal bank stabilisation works.

These works consisted of a review of previous datasets collected by other consultants, an updated generic QRA (GQRA), a conceptual site model of the site identifying the source of contamination, principal receptors at the location proposed for the placement of the sediments and likely contaminant pathways and quantitative modelling to assess the significance and degree of risk associated with the placement of sediments at the proposed Heck Bridge site.

The updated GQRA identified site-wide impacts in the sediments of:

• Arsenic

• Nickel

• Naphthalene

• Benzo(a)pyrene

• Aromatic TPH (C₁₆–C₃₅)

• Aliphatic TPH (C₁₂–C₁₆)

• Dioxins

These contaminants were assessed as having the potential to present a risk to human health and therefore required further assessment as part of a detailed QRA (DQRA).

The conceptual model was then refined based specifically on the Heck Bridge site where the contaminants are to be placed (Figure 2). From Figure 2, it can be seen that steel sheet piles were to be inserted into the canal and the void created between the existing canal embankment and the steel sheet pile was to be filled by sediments placed on both sides of the canal. Note, there was no public access to the northeast bank. A 200 mm-thick layer of clay was to line the bottom of the sediment infill and a 200 mm-thick layer of topsoil was to cap the top of the sediment infill. The infill material on average would be approximately 2 m away from the tow path on the southwestern bank. The potential source of contamination was the sediments used as infilled material identified in the GQRA. The sensitive receptors likely to be at risk were site users.

Figure 2 Conceptual site model.

This assessment would take into account the nature and duration of visits and the age of the receptors. The three main receptor groups identified as potentially at risk from placement of the dredged material are as follows:

• 0–6-year-old. Most susceptible/sensitive but not the most frequent site user or the most likely to come in contact with the dredged sediments.

• Angler. Potentially a frequent user of the site and likely to be exposed to the sediments for a significant duration per visit and most likely to come in contact with dredged sediments.

• Adult commuter. Most frequent user but exposure for a limited duration and less likely to come into contact with dredged sediments.

Dermal contact with soils and ingestion of soils and/or dust were identified to be the active exposure pathways on the site. These sources, pathways and receptors were then subjected to further site-specific human health risk assessment modelling to provide a more specific assessment of the significance of any pollutant linkages.

Using an RISC Workbench software (Spence and BP Oil 2003), a DQRA was carried out to generate site specific assessment criteria (SSAC) for the contaminants of concern. SSAC were calculated for each contaminant using a worst-case base model for each of the three critical receptor groups as detailed above. The base model combined worst-case assumptions of frequency/duration of the visits, dermal exposure and ingestion based on all receptor likely activity patterns. These models then formed the basis for a detailed sensitivity analysis to assess the significance of each exposure input in order to provide a more realistic exposure assessment for the site situation and use.

The critical exposure parameters were found to be the number of days/year and the soil ingestion rate. The critical receptor for all scenarios was the child. A reasonable worst-case model was developed based on the child receptor with a frequent exposure to a small quantity of soil. This scenario was used to calculate the final SSAC.

The 95th percentile concentrations of the dredged sediments were then calculated and compared to the SSAC calculated. In conclusion, none of the representative site concentrations from the Long Sandall sediments exceed the calculated SSAC. This suggests that, based on the assumptions made in the development of the conceptual site model and the existing site data reported, the materials at Long Sandall would be suitable for reuse as embankment fill along the Whitley Lock to Heck Bridge stretch of the Aire and Calder Network.

A qualitative risk assessment for the construction phase of the project (excavation, shipping and compaction on the new site) was also carried out. Currently, there is no occupational exposure limit set by the Health and Safety Executive (HSE) for dioxins and there is no validated method for personal sampling for dioxins. As the upper 95th percentile dioxin concentration (600 ng TEQ/kg) was below the US EPA human health safe exposure limit (1000 ng TEQ/kg), it was considered that sediments would not present a significant risk to site personnel through dust generation as long as certain control measures were adhered to, namely:

• during excavation and compaction operations, material was damped down using a water mist,

• excavation and compaction operations were not carried out during very sunny or windy days,

• site personnel wore dust masks.

5. Conclusion

When ‘special waste’ became ‘hazardous waste’, in the Hazardous Waste (England and Wales) Regulations 2005 (EA 2005) and the Special Waste Amendment (Scotland) Regulations (2004), it was not anticipated that there would be significant changes. The reality was different, fairly innocuous soil-based material (previously defined as inert or controlled waste) was now classified as ‘hazardous’ by the regulators. This reclassification had dramatic cost implications for the waste producer increasing the cost of disposal of the reclassified soil by four- to fivefold. It also meant that a scarce resource (i.e. space in a hazardous waste landfill) was being used inappropriately by materials that do not need such disposal.

The Long Sandall to Heck initiative brings a great number of environmental and cost benefits. For example, the transport of sediments in 250 tonne loads using the waterway network negates the need for up to 10,000 heavy goods vehicle movements on a 24 mile journey on largely congested roads. The 17 mile waterway journey across three navigations, six swing bridges, six lift bridges plus one aqueduct utilises the waterway network to its full potential and harks back to the days of the industrial revolution and the heyday of the waterway system. In addition, if this material had been classified as hazardous, it would have resulted in the loss of 100,000 m³ of space within a hazardous landfill and would have required a maximum of 10,000 heavy goods vehicle movements on at least an approximately 95 mile journey to the nearest hazardous landfill accepting waste. This would have resulted in carbon dioxide emissions of approximately 1350 tonnes,2 which have subsequently been saved as a result of this work.

Reusing the sediments taken out of Long Sandall avoided a requirement for virgin materials and limited the uptake of natural resources. BW estimated a minimum saving of around £500,000 on the cost of buying in a comparable tonnage of virgin material. In addition, it was estimated that potentially £1,000,000 was saved when BW considered the costs they would have incurred by disposal to a commercial facility.

On behalf of BW, Ramboll reassessed certain elements of the waste guidance by constructively engaging with the waste regulators who were able to use high-quality information and assessment of waste materials to enable more appropriate ways of dealing with some wastes. Where regulators and operators work together constructively, it is possible to find sensible and sustainable solutions with regard to classification of a waste stream conserving valuable resources.3

Acknowledgements

The authors would like to thank the Yorkshire Business Unit of British Waterways for the opportunities provided to Ramboll.

Additional information

Notes on contributors

Zoë M. Miller

1. 1. [email protected]

Notes

^1. [email protected]

^2. Guidelines for Company Reporting on Greenhouse Gas Emissions – Annexes. DEFRA (2005).

^3. This project has recently been recognised in the Ground Engineering Awards Sustainability Category for demonstrating how innovative and sustainable engineering solutions can be developed by challenging current thinking.