Debunking the Myths around Thermal Desorption

‘No call for it in the UK’, ‘too costly’, and ‘it can’t treat the sort of soils we have in the UK’ are just some of the myths that have attached themselves to thermal desorption. Fortunately, none of them are true.

What is Thermal Desorption?

Thermal desorption is a two-stage process primarily suitable for soils contaminated with organic compounds.

Firstly, soil is heated in a rotating drum to a temperature at which the contaminants vapourise (desorb), and secondly, these ‘off-gases’ are treated in an abatement process, principally using thermal oxidation. The treated soil is essentially sterilised and free of contaminants.

Myth 1 – ‘No call for it in the UK’

Thermal Desorption is a significant player in the remediation sector in mainland Europe and the USA – but not the UK. The primary reason for this has been the dominant role that cheap landfill has traditionally played in the UK market. The ability to dispose of contaminated soils for under £10/tonne essentially prevented other technologies from gaining a foothold. However, the Environmental Protection Act 1990, the introduction of Landfill Tax in 1999, the recent flurry of environmental legislation that has seen a sharp decline in the number of hazardous waste sites, the implementation of IPC controls to landfill, new Hazardous Waste Regulations and the need for Waste Acceptance Criteria tests (WAC) have made the life of landfill operators and waste producers much more challenging.

These developments are in line with the strategic policy of the European Union: This is to reduce the impact of contaminants on the environment, and to recover value from waste. Since the UK is a long way from this position at present, it can be expected that more actions will be introduced to accelerate the conversion. The structure of the UK waste market differs from Europe because of our reliance on landfill as can be seen in the following chart that considers all controlled wastes (see figure 1.0)

Landfill capacity and capability is declining whilst overall waste production levels continue to rise. The Greater London Authority has projected these two trends in a report published in 2003 from which the following chart is extracted (see figure 2.0)

The chart shows the growth in London’s waste production while at the same time landfill plays a smaller and smaller role. It represents the situation in southern England, but other parts of the UK have similar issues to resolve.

Considering the two charts together, we have a waste management system that is highly dependant on landfill whilst at the same time landfill availability is declining rapidly.

The Planning regime for landfill is protracted, costly and fraught with risk. Consequently, the UK is consuming landfill void space at ten times the rate it is being replaced. There has recently been a number of operating landfill sites refused IPC permits, which would lead one to believe they will have to cease operations, creating a step-change reduction in the UK landfill capacity.

It is generally accepted that incineration will not play a more significant role in the UK in the medium term, primarily due to distrust of the technology and the NIMBY effect.

This leaves the ‘Recycling and Composting’, or more properly ‘Treatment’ sector to pick up the slack. The South East of England Development Agency says it requires over 420 new waste treatment facilities to replace regional landfill which it says will be fully consumed by 2010. The Mayor of London says he needs 280 plants alone. Elsewhere in the UK the issue is slightly less pressured but the same issues remain to be resolved.

As the supply of landfill declines, landfill and transport costs will rise until demand and supply are balanced. This will be achieved by pricing a significant volume of material out of the landfill market. For contaminated soil, either developers accept lower margins, alternative treatment methods are used as they become cost-effective, or projects are shelved until market forces make them financially attractive again.

Therefore, there is a growing demand for treatment technologies encouraged by declining landfill capacity – there is clearly growing demand for thermal desorption in the UK.

Myth 2 – It is ‘too costly’

As a technology, thermal desorption could not penetrate the UK because it was more expensive than landfill. As we have seen above, that situation has changed and will continue to move in favour of all treatment technologies.

The cost of thermal desorption is largely based on three factors: the size of the project, the range and concentration of contaminants, and the moisture content of the soil.

Size – Where mobile plant is used to treat material on site, a mobilization and demobilization cost applies, which includes issues such as site assessment and agreeing site specific working plans with the EA. This cost needs to be amortized across the volume of material treated and this, in turn, suggests a minimum project size. At present market rates, this is in the order of 5,000 tonnes but this is by no means a fixed volume and will decline over time as landfill becomes more expensive. Fixed location thermal desorption plant have no minimum volume limits.

Range – The range and concentration of contaminants impact the rate at which the soil can be treated, the fuel demand, and the complexity of the off-gas abatement system.

· Different contaminants have varying desorption temperatures. This may mean the plant has to be operated at higher temperatures thereby increasing fuel costs.

· High concentrations may require an increase in the soil residence time in the plant, thus reducing the rate of treatment and therefore increasing the overall cost per tonne.

· Very high concentrations may require pre-treatment in a Thermopile© to reduce concentrations to a level suitable for thermal desorption.

· Some contaminants, once desorbed, are not affected by thermal oxidation and require additional treatment as part of the off-gas abatement process. These include, for example halide compounds.

Moisture – Moisture content directly influences fuel usage since the moisture has to be driven off before the soil can be raised to the desorption temperature required to remove the contaminants. The higher the moisture content the more fuel is required.

Thermal desorption can treat wastes at costs ranging from £40/tonne to £100/tonne. At these prices it is already price competitive with hazardous waste landfill, and of course, on-site treatment entails no transport charges.

Myth 3 – ‘It can’t treat the sort of soils we have in the UK’

Some have a view that UK soils cannot be treated using thermal desorption. Certainly the UK has more clayey soils – which is why landfill has achieved a dominant market position – but even clays are processed by thermal desorption plant.

In thermal desorption, particle size is the key mechanical soil characteristic. For the process to be successful, the soil needs to be raised to the required temperature throughout. If clods of soil are allowed to develop, the soil in the centre can theoretically be left untreated. Mechanical pre-processing and the addition of large grain size material to improve the friability of soil are techniques used to overcome this issue.

Conclusions

Thermal desorption is not a panacea for all contaminated soils – no treatment technology can economically treat everything. But it is a well-understood technology that is used widely in other countries and fulfils a need in the UK market.

As landfill becomes less available and more costly, developers will seek out economic solutions to their needs. Thermal desorption is already cost competitive and will only become even more attractive as a solution over time.

Andy Wheatley

Managing Director

Deep Green (UK) Ltd.

Tel: 0117 311 1250

Fax: 0117 930 4222

Mobile: 07921-694144

www.DeepGreenUK.com