Successful treatment of PFAS-contaminated soils on large scale: practical experience with soil washing.

The method of soil washing is currently the only economically feasible cleaning method for PFAS-contaminated soils on large scale.

A total of about 430,000 t of soil contaminated with PFAS and HCs was washed from August 2018 to October 2021 at the site of a former refinery in Bavaria. The cleaned soil could then be refilled at site-specific costs of about EUR 50 per ton. Since September 2022, Züblin Umwelttechnik GmbH has been operating another soil washing plant in Northern Germany, which was specially designed to treat soils contaminated with PFAS. Several hundred thousand tons of sandy soil with around 10% fines will be washed and refilled on site in the next years.

Data obtained from these two large-scale remediation projects are presented. The technical concept and challenges to treat 1,000 to 3,000 t of material per day are discussed, along with regulatory obstacles and the boundary conditions to process these quantities while limiting the emissions. A special focus lies on the heterogeneous nature of input material, with varying contaminant load, soil quality and soil structure as well as affiliated contaminants. In both attempts, the washing water is circulated in a closed water cycle, therefore elaborate treatment of sludge and polluted water is required.

One of the critical issues in soil washing is to optimise the washing process so that the washing liquid serves as the predominant contaminant sink. In this way, steady washing results are achieved, and all output fractions can be successfully processed while water consumption is kept to a minimum.

The complex nature of the contaminant itself, the multitude of singular PFAS substances and precursors, further complicate operation and controlling. Progress shows that soil washing can even be a viable tool to treat the fine fraction, and various strategies in different treatment steps have been tested on multiple scales, including several washing agents.

Since after treatment, the washed soils can be safely reused on site, large emissions from transportation and properly disposing the PFAS-materials is avoided while increasingly limited landfill space is maintained.

Future research is required for expanding the applicability of the method to highly challenging materials such as high fines and/or high organic content. Further projects for washing of PFAS contaminated soils in Europe are already in the planning stage.