Breaking new ground

7th April 2014


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Elisha Lickiss

Celtic Technologies has won awards for its work remediating the site for Asda's new Avonmouth distribution depot. the environmentalist reports

Portside in Avonmouth is now home to a 492,000 sq ft chilled distribution centre for the retailer Asda. It opened in December 2013, but the site previously had a long industrial history: from a munitions and chemical weapons plant during the First World War; zinc and lead smelting with sulphuric acid production until 1972; and the manufacture of pharmaceuticals, agrochemicals and refrigerants until the facility finally closed in 2008. This history meant the site was regarded as one of the most contaminated in south-west England and its remediation was a complex and challenging project.

Environmental remediation contractor Celtic Technologies has been recognised by the specialist publication Brownfield Briefing for its innovative work on the site, recently sharing an award with its project partners Environmental Resources Management (ERM) and McLaren. It is fitting that the team won the category of “best use of a combination of remediation techniques” because the project required the use of a diverse range of sophisticated remediating technologies. One of these, the innovative surfactant enhanced aquifer restoration (SEAR) process, had yet to be tried and tested by Celtic on a project of this scale – and with the works bound by tight contractual timescales, there was no room for any slippage if it was not successful first time round.

Sustainable remediation

Celtic Technologies is a subsidiary of EnGlobe, a global provider of environmental services that prides itself on using sustainable solutions to treat contaminated soils and groundwater. EnGlobe is currently working on projects in Israel, North America and France. Established 20 years ago, Celtic was one of the first companies to provide remediation services in the UK and since its inception the company has developed a strong track record. It was also involved, for example, in the clean up of the Buncefield oil storage depot following the explosion in 2005 that destroyed the Hertfordshire site (see below).

Sustainability is central to Celtic’s operations, and the company was one of the first in its sector to gain ISO 14001 accreditation. Historically, remediation projects have been predominantly landfill-driven, although this is now changing, as Martin Holmes, regional operations director, explains: “There are sustainable solutions to carrying out remediation work and Celtic makes every effort to avoid sending waste to landfill. Our aim is to achieve 100% material recovery and reuse onsite.”

Celtic is atypical for a remediation contractor in that it carries out every stage of its work in-house, rather than subcontracting out some elements. This approach enables the company to retain strong ownership of every detail of every project, says Holmes. To achieve this holistic delivery on contracts, the firm employs 60 people, many of whom are highly specialised in their field, including geologists and environment professionals.

Scoping the project

Celtic and its partners, sustainability consultants ERM and construction company McLaren, undertook a detailed assessment of the 62-acre Avonmouth site to develop an appropriate remediation strategy and support their planning application to the local authority and the Environment Agency.

ERM carried out a programme of “high-resolution site characterisation” (HRSC) assessments, a sophisticated technology that uses scale-appropriate measurement and sample density to map the distribution of contaminants. This involved using cone penetrometer testing and a membrane interface probe as well as, for the first time in the UK, a BAT groundwater sampler that enabled the team to collect discrete groundwater samples to provide an accurate assessment of chlorinated solvent distribution. The HRSC works identified that the contaminant mass was present to a depth of 14.5 metres.

The HRSC programme and earlier investigations of the site indicated the scale of the technical challenges facing Celtic. The scoping exercises had identified multiple contaminant source zones with impacts from metals, petroleum hydrocarbons and chlorinated solvents that were mainly chloroform and trichlorofluoromethane at concentrations likely to represent dense non-aqueous phase liquids (DNAPL).

DNAPLs are liquids that are both denser than water and are immiscible in or do not dissolve in water. They tend to sink below the water table when leaked in significant quantities and their penetration into an aquifer, or underground layers of rock, makes them difficult to locate and remediate.

The potential risk of mustard gas and materials left behind as a result of the site’s previous life as a munitions and chemical weapons factory also posed unusual health and safety management challenges. An equally serious hazard during the project was posed by the volatile nature of the chlorofluorocarbons (CFCs) detected resulting from the site’s more recent manufacturing activities.

In addition to facing a complex remediation project, Celtic also faced logistical challenges. The works had to be carried out within a tight commercial timescale in parallel with the overall redevelopment programme undertaken by McLaren. This meant that there were a lot of people and activity onsite at the same time. Once underway, the progress of the remediation project was not helped by the wet weather conditions, which made the job that much harder for the workers on the ground. “Despite these obstacles, we met every performance criterion for the project and it was completed on schedule and on time, and in a sustainable manner,” confirms Holmes.

Layer by layer

The project design identified that a combination of techniques were needed to deal with the different types of contamination at the Avonmouth site. The choice of approach also depended on the depth of the contaminated area that Celtic had to remediate. The techniques were applied in stages, with many of the treatments overlapping.

The first step was to deal with the contaminated shallow soils, the topmost layer of material. Celtic’s key aims for this stage of the work were to reduce the volume of soil for offsite disposal to a minimum and apply a method that would allow the material to be treated and reused onsite. The team also wanted to maintain the existing geotechnical properties of the soil and minimise contaminant release to the atmosphere. To achieve these criteria, Celtic installed an ex-situ soil vapour extraction (SVE) system, combined with carbon filtration, which was able to treat all of the material.

This stage of the remediation works involved excavating 22,000 tonnes of contaminated material from five locations across the site. “By applying a stringent excavation, segregation, assessment, classification and stockpile management system, we were able to retain 100% of the excavated material onsite,” says Holmes. “Around 52% of the material was immediately reused as part of the wider redevelopment scheme without being treated. The remainder required treatment.”

To carry out the treatment, Celtic installed more than 4km of aeration pipework underneath the contaminated soils, connected to a high-volume vacuum system. It took 11 weeks for the treatment system to reduce the contaminant concentrations in the soils to a level where they could be safely reused.

The next phase of the remediation works involved dealing with the contaminated perched water. First, Celtic needed to prevent the perched water in the shallow contaminated soils migrating towards the watercourses surrounding the site. Initially, the remediation strategy specified that a protection barrier should be built for this purpose, but once onsite ERM and Celtic developed a more pragmatic and lower-cost approach, installing instead a trench to capture water along the site boundary followed by treatment of the water.

“This demonstrates how it is still possible to develop the remediation strategy once the works are underway if there is a more effective solution,” says Holmes.

Celtic developed a treatment method that not only allowed the perched water in the shallow soils to be removed in a controlled manner, but which could also be carried out at the same time as the soil excavation work. One of the challenges encountered with this process was to remove a large proportion of the suspended solid content in the water to ensure that Celtic’s treatment system could maintain high efficiency.

Celtic designed a series of interconnecting trenches, filtration sumps and settlement lagoons, which were very successful in removing almost all suspended solids prior to the water treatment system. The water treatment system included a dissolved air flotation unit to remove ultra-fine particles, reducing the unnecessary blinding of the granular activated carbon and thereby minimising the volume of offsite disposal. A total of 1,515,000 litres of perched water was collected and treated, with no loss of water as the treated water was pumped back into the ground.

Dig deep

The most technically challenging stage of the remediation works at the Avonmouth site was dealing with the deeper-level contamination, which reached a depth of 14 metres in some areas.

The team considered several potential techniques and eventually settled on conventional hydraulic recovery followed by SEAR. The hydraulic recovery removed some contaminant mass and further defined the location of the contaminated areas. This helped to increase the efficiency and sustainability of applying the SEAR by targeting the wells with the greatest contaminant mass. Gradually, the number of wells needed shrank, reducing the level of pumping activity.

Celtic undertook rigorous tests in its UK and EnGlobe’s Canadian laboratories to determine the most suitable surfactant mix – substances added to a liquid to reduce its “surface tension” – as well as the correct amount and concentration of the mix that should be pumped into the ground to loosen up the contaminated mass. Although not an experimental technique, Celtic had not previously used SEAR on a project of this scale, which presented a significant challenge for the firm. In addition, this heavy stage of the remediation work involved working with large volumes of highly volatile contaminants.

During the phases of groundwater treatment, Celtic extracted 11,690,000 litres of groundwater containing a total of 2,500kg of contaminant mass, which was removed using the company’s treatment system.

The SEAR phase of the works was a success, yielding an increase in contaminant recovery of three to four times what would have been possible to achieve if only conventional hydraulic pumping had been used.

An award-winning project

There are several reasons why Celtic feels that the Avonmouth project deserves its accolade from Brownfield Briefing. “Throughout the project we made every effort to follow best practice, from the conceptual approach to the remediation, design and implementation of the works,” says Holmes. “This approach allowed the project to achieve the best possible outcomes, on time and within realistic commercial boundaries.”

Celtic’s award entry cites many instances of best practice, ranging from the project’s strong focus on carbon reduction throughout the works – for example, the use of HRSC and the methods to increase soil reuse rather than offsite disposal – to the use of innovative techniques, such as SEAR. The application of leading-edge technologies typically went hand in hand with significant programme acceleration and further opportunities to reduce energy consumption, delivering even greater sustainability benefits.

Given the nature of the project, health and safety considerations were paramount. The remediation area was completely segregated from all other subcontractors, and Celtic and McLaren delivered regular “toolbox” talks to staff on health and safety. There were no incidents or accidents recorded throughout the entire project and, after intensive daily environment monitoring of the site and along the site boundary, neither were any environmental issues raised throughout its duration.

Celtic believes that the way it designed and delivered the project also helped to achieve a significant reduction in the potential pollution burden. “The remediation approach developed by Celtic and ERM for the contaminated soils was very successful as the process did not modify or change any of the geotechnical properties of the soil, and it provided a treatment system that could treat all of the material in one batch over a relatively short time,” says Holmes. “On completion of the treatment all of the remediated material has been reused on the site.”

The strong ethos of collaboration that characterised the working relationship between Celtic and its partners was also a key factor to the project’s success, believes Holmes. “A more traditional contractual model would not have achieved the high level of communication and collaboration that we established at Avonmouth,” he argues. “This relationship enabled aspects of the project, such as water treatment rates, to be discussed and assessed throughout the works and modified to meet the required timescales.”

The project team also made every effort to develop a solid working relationship with regulators, such as the Environment Agency. Holmes’ advice to other practitioners leading on remediation projects is to engage with regulators as early in the process as possible to ensure that they are fully aware of what to expect. The regulators were kept regularly informed throughout the Avonmouth works and were also invited to the site on occasion so that ERM and Celtic could demonstrate the remediation techniques, as well as to share information about the works and to address any questions that the officers had. “The early buy-in of the regulators was key to closing the site out efficiently,” adds Holmes.

Cleaning up after Buncefield

In March 2013, Celtic Technologies reported that it had completed the remediation work at the site of the Buncefield oil storage depot in Hemel Hempstead, which in December 2005 was destroyed by a massive explosion and fire, injuring more than 40 people and contaminating the area with oil and chemicals.

Remediation of the site involved major excavation and segregation, and the disposal of contaminated soils. The site contained a number of contaminants of concern, such as fuel oils and firefighting foam additives. These additives included perfluorooctane sulfonic acid and its derivatives (PFOS) which were banned under the Persistent Organic Pollutant Regulation (EC) 850/2004 in 2010 but had previously been used in some foams to tackle fires involving flammable liquids such as fuel. It was essential that these contaminants were disposed of appropriately by selecting the correct disposal route of the contaminated soils. Significant volumes of soil were removed from site, not only to dispose of the contaminated materials but also to accommodate a lowering of ground level as required for the future development.

Christine Mardle, project manager at Buncefield for Celtic, says that the company worked closely with British Pipeline Agency (BPA), which is responsible for redeveloping the site. “Working with BPA we were able to amend our soil disposal strategy as the project developed and being able to use Biogenie’s soil treatment facility in Redhill meant that we were able to dispose of highly contaminated soils in a compliant and cost-effective way.”

After the successful remediation, in March 2013 BPA announced details of the Buncefield aviation storage terminal rebuild project. This will involve the construction of eight tanks totalling around 65,000m3 of fuel storage and all associated pumping terminal and control facilities.

The 2005 explosion was the largest peacetime fire in Europe and lasted five days. Five companies – Total UK, BPA, Hertfordshire Oil Storage, Motherwell Control Systems and TAV Engineering – were fined £9.5 million for their part in the incident, including £1.3 million in fines for pollution offences, which is a record for a single incident in the UK.

Header image source: Celtic Technologies

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