For years, the Voltastraat site functioned as a large-scale storage facility for firefighting foam, which contains hazardous PFAS compounds. Over time, this foam leaked into the ground, significantly contaminating both the soil and groundwater. A remediation operation consisting of two phases was begun to address this contamination. The municipality asked ACD to be involved in the preparation and execution of the first phase.

Specifically, we were tasked with producing technical designs – in particular for the on-site water treatment installation – drafting specifications, supporting the tender process, and supervising works execution. In the first phase, a source-based approach is being employed in which as many kilograms of PFAS as possible are removed from the soil (known as ‘bulk removal’) through wet excavation. The second phase tackles the remaining PFAS in the groundwater.

The soil excavation is being conducted ‘in the wet’ – in other words, below the groundwater level and without applying dewatering techniques. This results in a pit filled with water up to the natural groundwater level. The excavated, highly PFAS-contaminated soil (mainly clay) gets transported to a recognised processing firm.

Wet excavation was chosen over dry excavation because the latter requires the extraction and treatment of large volumes of groundwater which is then discharged into surface water. This approach was considered too risky in view of the very high PFAS levels in the on-site groundwater and the low regulatory limits for discharging PFAS into surface water.

To remove PFAS from the groundwater in the first phase, it is rinsed in a water treatment installation during excavation. The water is then returned for further rinsing. For this process step, Witteveen+Bos created the technical design for the water treatment installation. The installation, which is approximately half the size of a tennis court, consists of separate process steps. Each step has either previously been tested in the field or in a lab, or is already being deployed on a large scale. The first process step involves pretreatment using coagulant dosing and pulverised coal.

This is followed by a flotation step, which removes particles of clay and a great deal of the PFAS. During this process, foam is produced which also contains PFAS. The formation of this foam can be controlled and the foam ‘skimmed’ off. Pretreatment is necessary to protect the carbon filters in the following step. If the water contains too many clay particles, the carbon filters will silt up too quickly, reducing the effectiveness of the treatment. The treated water then goes back to the excavation site, with the residual streams being disposed of together with the contaminated soil. The groundwater at the excavation site is rinsed multiple times until its PFAS levels meet the limits in the specifications.