Pilot remediation with MOSAM in Zeitz within the framework of TansIT und SAFIRA II

At a former hydrogenation plant in Zeitz, Germany, on a site which is part of the "Ökologisches Großprojekt ÖGP" of Zeitz (ecological large-scale project of Zeitz) a pilot-scale trial of thermal in-situ remediation using steam-air injection was carried out.

The source area is located underneath a former benzene distillation unit of the hydrogenation plant. The pilot trial had a volume of approx. 1.500 m³ and extended to a depth of 12 m b.g.s. Three injection wells were installed surrounded by six extractions wells. The main objective was the removal of a part of the contaminant source zone (partial source removal) of benzene and in consequence the reduction of the downstream contaminant flux in the groundwater (plume). This was monitored by an accompanying project of the UFZ.

The pilot trial started in May 2007. After a short period of a "cold soil-vapor extraction" and following air-sparging the steam-air injection started. During several phases steam was injected into both the unsaturated and the saturated zone. Steam-air injection at different wells and depths ensured a safe extraction of benzene from the different layers particularly from a silt layer located above the capillary fringe. Steam/heat propagation was continuously monitored by direct temperature measurements (Pt100 array) and by innovative surface geophysical methods (carried out by UFZ). The steam injection was terminated in November 2007 after exceeding an average temperature of 90°C in the subsurface. The cooling phase lasted through December 2007.

After 6 days of injection steam breakthrough at the extraction wells was observed on a depth of 8-10 m b.g.s. The "thermal radius" (horizontal, radial steam propagation) of more than 5 m for an injection rate of approx. 220 kg/h of steam-air was larger than predicted (2 m) based upon gravity number related type curves (Ochs S.O., 2006). This indicates an anisotropic permeability distribution.

In total 6.780 kg of benzene were extracted, the final benzene concentration in the soil vapour was below 0.2 g/m³, indicating a removal of more than 99%. The remediation efficiency was proofed by soil sampling from the unsaturated zone (max. 0.1 mg benzene per kg soil) and from the saturated zone (max. 2.2 mg/kg) meeting regulatory limits. Benzene concentrations in the groundwater were reduced by a factor of 4 to 10.

In total more than 97% of the removed mass of benzene was extracted from the unsaturated zone. For efficiency reasons it is advised to focus on this zone and the capillary fringe. With respect to the high mass removal during cold soil vapour extraction phase a remediation scenario should focus on this standard technology. In order to reduce the contaminant mass from the silt layers, to reduce the remediation time and to meet legal limits the application of thermal methods can be advised.

Based on the pilot performance data and costs the design for a thermal in-situ remediation of 14,000 m³ (8,000 m³) volume of highly contaminated soil resulted in site-specific costs ranging between 80 (95) EUR/ton of soil.

In close co-operation between the UFZ and the authorities (LAF, Landesanstalt für Altlastenfreistellung Sachsen-Anhalt) responsible for the ÖGP, the results will be integrated in the further site development concept for the ÖGP.

For further information please have a look at the following publication.
Paper in proceedings of Consoil 2008, Milano

Koschitzky, Hans-Peter

Trötschler, Oliver
Limburg, Berit

VEGAS

From: 2006-08-01

To: 2007-12-31

Helmholtz-Zentrum für Umweltforschung (UFZ) Leipzig-Halle