1.1.11 Heat as an accelerator – heat lances turning soil pollutants into vapours

High temperatures are a good way to extract pollution from contaminated soil. This has been proven by researchers at the Versuchseinrichtung zur Grundwasser- und Altlastensanierung (research facility for the remediation of groundwater and abandoned waste, VEGAS) within the University of Stuttgart’s water engineering institute. Their thermal in-situ remediation technology (THERIS) – just one of the procedures used for thermal remediation – uses a strong infeed of heat to clean out contamination from underground, which is faster than the conventional means of “cold” soil gas suction. Experts at the VEGAS research facility (see project 1.1.09) have compared both processes with each other as part of a research project – with very clear results.

Medium to low solubility liquid hydrocarbons (LNAPL/DNAPLCombination of ->activated sludge procedure and membrane filtration. The aim of this process is the extensive cleaning of biologically contaminated wastewater such that it can be used as service or process water.) contaminate soil and pose a risk to groundwater. Landowners frequently turn to a process called soil gas suction (SGS) for remediation. This involves the pollutants, which convert into gas at a sufficiently high natural temperature, being sucked out of the contaminated ground together with the air within through pipes (soil gas level). A discharged air treatment facility filters out and removes the pollution from the contaminated soil air on site and the system then releases the cleaned air into the atmosphere. However, this method quickly reaches its limits: the organic pollutants only convert from liquid to gas in very small amounts at the usual soil temperature of about 10°C, which makes the remediation process much slower and much more expensive. Furthermore, fine-grain soils – in which pollutants most often accumulate – are not permeable enough for this process. The soil gas is then very difficult to suck out. After several years, it is still uncertain whether many of these facilities are achieving their remediation objective.

Heat infeed boosts contaminant discharge

These problems can be overcome by applying heat, e.g. via fixed sources of heat such as the electrical heat lances on which the THERIS procedure is based. The soil heats up, pollutants convert more quickly to gas and are also discharged from smaller areas of permeable soil. SGS sucks out gaseous contaminants directly from under the ground along with the soil gas. The suctioned, contaminated soil gas is then cleaned on site. THERIS can be used with various soil types, particularly those with loose stones (sand, silt, clay) in the soil zone above the groundwater, i.e. the non-saturated soil zone. Unlike thermal in-situ remediation procedures – vapour or vapour-air injection – no heat medium is injected during the THERIS process. This makes THERIS also suitable for heating and cleaning large, low-permeabilityMeasure of the ability of a porous material, such as rock or soil, to allow gases or fluids to pass through it. soil layers. Comparative experiments in the large tank at the VEGAS research facility and at a field site should now clarify the differences between SGS and THERIS in terms of remediation time, degree of effectiveness and energy consumption.

Experiment in the large VEGAS tank

The researchers began testing the THERIS procedure in a 150 cubic metre tank at the VEGAS research facility, which was filled with a layered soil structure and equipped with measuring instruments. They added 30 kilograms of trimethylbenzene (TMB, flash point 169°C) to a localised area of a one-metre thick layer of fine low-permeabilityMeasure of the ability of a porous material, such as rock or soil, to allow gases or fluids to pass through it. sand.

To begin with, they applied cold soil gas suction for two months. They then put four quadratically arranged, fixed heat sources (THERIS) into operation in this fine-structured layer, which heated up to 500°C. The SGS continued to deliver a constant rate of suction. As the TMB was fully removed after just 20 days, the soil temperature between the heating elements had not even hit 100°C.

Remediation in the field test

The researchers also began a field test: an approximately 3.5 metre thick clay-marl layer over an area of around 80 square metres still contained high concentrations of CHC after several years of cold SGS, primarily perchloroethyleneSee: Tetrachloroethylene.◄(PCE, flash point 121°C). To begin with, the SGS was left to continue without any change. Then 22 heat lances from the THERIS system were implemented, equipped with the latest measuring, control, data recording and transfer technology. They heated the soil evenly and in a way relatively uninfluenced by differences in geological structure. After just one month the temperature was around 80°C, and largely over 90°C after two. The researchers sucked out the gaseous pollutants mobilised by THERIS using SGS.

THERIS has the edge

The evaluation of the tests showed that THERIS cut the remediation time by about 90% compared with cold SGS and reduced the amount of energy required by two-thirds. Differences in the absolute values between the tank and the field test were traced to location factors such as geology, the type of contamination and its distribution plus system-specific details (e.g. the well arrangement).

THERIS takes just a few weeks to clean even low-permeabilityMeasure of the ability of a porous material, such as rock or soil, to allow gases or fluids to pass through it. soil quickly, reliably and with sustainable results. The significant increase in performance is predominantly attributable to:

• the increased gas permeabilityMeasure of the ability of a porous material, such as rock or soil, to allow gases or fluids to pass through it. and more effective diffusion as a result of the dried soil
• the accelerated conversion of liquid hydrocarbons to gas as a result of the higher temperatures.

The THERIS facilities are robust, enabling them to be installed quickly and operated safely whatever the weather. Due to the cost of putting the heating elements and monitoring systems in place, the installation costs are higher than for cold soil gas suction. However, these additional costs are compensated for by the shorter remediation time, lower operating costs (energy, equipment rental etc.) and lower personnel costs.

Project website www.vegasinfo.de

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1.2 River basins

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1.1.10 Remediation using alcohol