Afværge teknikker for MTBE-forurenet grundvand

Summary

MTBE in Danish groundwater

MTBE (Methyl Tert. Butyl Ether) is an ether-compound which is used as an additive in certain gasoline products in order to increase the octane number. In the past few years, MTBE has been found in groundwater at several locations in Denmark at very different concentrations (µg/L to mg/L), most often in a mixture with other hydrocarbons. The administrative guideline value for MTBE in drinking water is 30 µg/L.

Need for review-project

Only a few remediation projects have been implemented for MTBE-contaminated groundwater. This review-project on potential methods to clean MTBE-contaminated groundwater and soil was initiated within the Remediation Programme for Soil- and Groundwater Contamination organized by the Ministry of Environment in Denmark.

Evaluation of remediation technologies

The purpose of this project is to evaluate the feasibility of different remediation technologies for MTBE-contaminated groundwater. The evaluations are limited to techniques for which data are available for MTBE. The information is obtained from literature databases, from the Internet, and through contacts to persons at universities and companies in particular in the US, and through contacts to Danish counties. Generally, there is little experience on the remediation of MTBE-polluted groundwater, but internationally, a strong development has been observed within the area. It can be expected that in the near future, the literature on the subject will increase significantly.

7 remediation methods described

Within this report, 6 remediation technologies for groundwater and one for soil are described. The technologies have been tested for MTBE, or the methods are considered particularly relevant for MTBE. The technologies are reviewed in relation to their way of working, case stories, their advantages and disadvantages, remediation efficiency, and economy to the extent relevant data are available. The details are reported in appendix 2. In addition, since on-site treatment is an integrated part of groundwater and soil remediation, this report contains a short description of on-site technologies for treating waste air and wastewater from waste sites.

The remediation technologies for MTBE-polluted groundwater can be categorized according to the mass of MTBE that can be removed per unit of time, the concentration in the groundwater that can be achieved after a certain time, control of the MTBE-plume, economy, etc.. The suitability of a given technology in a given situation is thereby closely linked to the local requirements of clean-up efficiency, time frame, and economy. A comparative discussion of the methods is given in chapter 6.

Stripping methods work

Generally, it can be concluded that the stripping techniques are well suited for clean-up of gasoline-contaminated groundwater containing MTBE. Due to the relatively low volatility of MTBE from water compared with the volatility of the hydrocarbons (BTEX), removal of MTBE from groundwater is less efficient than for the BTEX. Moreover, the removal of MTBE from the waste air by using activated carbon is less efficient for MTBE than for BTEX. Therefore, removal of MTBE is more costly than for BTEX, however, from an overall point of view, it is expected that the costs of remediation are not increased significantly. The reason is that the capital costs and many of the operating costs are the same weather MTBE is present or not. In addition, the benzene concentration in groundwater has to be reduced to a level of 1 µg/L, whereas the MTBE-concentration only has to reduced to 30 µg/L. There is a potential of reducing the costs of remediation through the development of techniques for biological degradation of MTBE and hydrocarbons either in on-site biological filters or in biological filters implemented in the unsaturated part of the soil.

Vacuumventilation at recent spills efficient

At recent MTBE-spills where there is a considerable amount of MTBE in the unsaturated zone, an efficient and rapid removal of MTBE can be achieved by vacuum-ventilation due to the high volatility of MTBE from the gasoline phase.

MTBE can be oxidized chemically

Remediation of MTBE-contaminated groundwater through stimulated degradation by addition of oxidizing agents (oxygen releasing compounds, ozone, etc.) through wells within the plume of MTBE is also possible, but before these methods are used on a larger scale, the technical-economical documentation should be improved including the applicability of the methods in reduced groundwater aquifers.

Natural degradation has a considerable potential

Removal of MTBE from groundwater by natural degradation is at a stage of research, but there is a potential for an important future remediation technique. If this method turns out to work, one could imagine a combination of stripping and natural degradation, whereby a significant mass removal is obtained over a few weeks or months leading to a moderate residual concentration in the aquifer, followed by natural degradation to a very low final concentration after some years. Considering the economic potential, there is a significant need for research on natural degradation of MTBE in groundwater.