Innovative remediation

Dr Russell Thomas and Dr Steve Wallace explain how innovation has been applied to the remediation of contaminated land

You will be aware of the raft of guidance and legislation affecting contaminated land that has been implemented in recent years.

This has been brought into force to identify and mitigate the risks posed by contaminated land, improve site investigations and remediation, and at the same time reduce our dependency on landfill as a remediation option.

The guidance in the form of documents such as CLR 11 has helped improve the processes involved in site investigation and remediation, and although Contaminated Land Exposure Assessment (CLEA) may have its problems, it is still a more pragmatic risk-based system than that in force in some other European countries (see EB December 2005, page 39).

Difficult and expensive

Recent legislation, such as the landfill regulations, has made disposal of some hazardous wastes much more difficult and expensive. This has opened up the market to more innovative remediation, such as thermal desorption and soil washing. Like other sectors, there has always been a willingness by organisations such as National Grid (formerly SecondSite) to improve environmental standards, but market forces have accelerated the uptake of new technologies as landfill disposal costs have increased. The cost differentials that have recently emerged between landfill disposal and other innovative technologies is making these a matter of preference rather than a risky alternative. And as such they are becoming more commonplace in the UK.

Many of these technologies have been sourced from Western Europe and the US, where these technologies have been tried and tested. Despite market forces being historically in the favour of landfill, the UK has lead the way in a number of areas of innovation, one example being the permeabel reactive barrier project, discussed later in this article.

It is important to recognise that landfill will need to remain a viable option for some time to come. And in some cases it may represent the most sustainable option. For example, waste deposited in a correctly engineered landfill can allow biodegradation to occur, leachate to be collected and treated, and gas collected which can be burnt for energy production, all within a contained system.

Is an energy-intensive thermal desorption system more sustainable? On this basis, and the fact that technologies are not available for some waste types, particularly on small sites, landfill can still play a useful role in the toolbox of remediation technologies.

There is still an opportunity for further innovation in the contaminated land market because much of the UK has undergone industrialisation, with a variety of different industries and a great diversity in regional geology requiring different solutions. Even though greater innovation is often required for complex sites, innovation can still play a key role on simple sites because some of the best solutions can be both simple and innovative. Below are two examples of how Parsons Brinckerhoff (PB) has worked with National Grid to deliver innovative approaches to contaminated land remediation.

Risk to groundwater

A recent project that required a complex remediation system was an operational gas distribution depot and former gasworks in southern England belonging to National Grid. Following a site investigation, it was realised that remedial action was required to mitigate the risk to groundwater from contaminants.

As an active site, the suitable remediation technologies available were limited. A permeable reactive barrier (PRB) was chosen as it provided a cost-effective means of managing risk at the site boundary while on-site activities continued.

An industrial project team was assembled to oversee the project, which included ourselves, as well as Bob Kalin, Queens University Belfast (QUB), Paddy Daly, National Grid and Jamie Robinson, Parsons Brinckerhoff (PB).

A major investigation of the site was undertaken using adaptive site investigation methods to develop models of the site’s soil, hydrogeological and biogeochemical parameters. Groundwater and contaminant flux modelling was used to predict both the mass of water and contaminants to be treated and the size of cut-off wall required to capture the contaminant plume. Laboratory treatability studies were undertaken to determine rates of contaminant degradation and risk assessment models were used to agree contaminant discharge parameters with the Environment Agency (EA).

High concentrations of contaminants such as cyanides and coal tar were present in the groundwater at the site, and this required a new type of PRB called a Sequential Reactive Barrier (SEREBAR) to be developed. This required a high degree of innovation, both from the applied science required to develop the biotreatment system and also from an engineering perspective to design and install it.

The SEREBAR contract was let as design-and-build and awarded to Keller Ground Engineering with their subconsultant RD Geotech.

A cement bentonite wall was installed through the gravel aquifer and keyed into the Breccia beneath the site, including sealing around two gas mains, to channel groundwater into the treatment system. This was composed of six canisters installed underground and arranged by height to treat the water in series via gravity flow.

Two of the canisters contain gravels populated by indigenous anaerobic bacteria, two contain gravels populated by indigenous aerobic bacteria, and two canisters contain granular activated carbon (GAC) as a failsafe protection measure. An interceptor also prevents free phase hydrocarbons from entering the treatment system, which can be fully monitored online using telemetry.

The project had significant financial support from National Grid and the Government through the Bioremediation LINK Programme supported by the Department of Trade and Industry, the Biotechnology and Biological Sciences Research Council, the Engineering and Physical Sciences Research Council and the Natural Environment Research Council. The LINK funding supported the large research aspect of the project, which was undertaken by the research groups of: Bob Kalin (PRB technology) and Mike Larkin (microbiology) at the Queen’s University of Belfast; Ian Thompson (microbiology) at NERC CEH Oxford; and Stephen Jefferis (civil engineering) at the University of Surrey.

This research has discovered important information, such as novel micro-organisms capable of degrading these difficult contaminants, and improved models for the degradation kinetics.

The SEREBAR project has been unique in its innovative approach to developing a new remediation system from scratch and within a short timescale. It has shown that, as long as the right teams are assembled, innovative technology can be applied within rapid timescales to solve complex environmental problems.

A totally different type of innovation was required at a similar site in the east of England. A very different problem was encountered at a former gasworks, again owned by National Grid. Constraints included: a small working area, all of which required remediation; an elderly tenant, who would not be rehoused during the work, occupied the house; residential properties in close proximity of the site; and access to the site was also restricted and required a wall to be demolished, which restricted the size to plant.

PB undertook the project on a design-and-build basis, which included all stages from initial desktop survey, through to regulatory sign-off.

The principal risks of contamination were to site tenants and groundwater within the chalk aquifer. The only solution was excavation and disposal to landfill, but this was not straightforward. The remediation works needed to be undertaken within restricted space on a split-level and within close proximity of residential properties.

Uncontaminated material was also recycled. The remediation was completed within the programme and budget without any HSE incidents, to a standard suitable for the existing residential use to continue. Even on relatively small projects like this, there is still room for innovative methods.

Contaminated land still offers great opportunities for innovation – whether it is in site investigation techniques, analytical methods or remediation technology, and it should be remembered that the ability to innovate is open to all involved in the field – from contaminated land site engineers to academics.

Indeed, a partnership approach that brings together all of these disciplines, often yields the best results.

  • Dr Russell Thomas is from Parsons Brinckerhoff. Dr Steve Wallace is from National Grid

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