One of the last two remaining single-track sections between Malmö and Gothenburg is the track through Varberg. To accommodate double tracks in the city, a new three-kilometer-long railway tunnel is being excavated beneath the community. Or more precisely two tunnels - a railway tunnel, and a parallel service tunnel. Both tunnels are drilled and blasted simultaneously, from both ends. Thus, there are a total of four excavation points. When the contractor Implenia entered the project, they set up their own water treatment equipment. While it primarily handles suspended material and adjusts pH, it struggles with water-soluble metals.

Seeking a Solution for Chromium

After the project had been underway for six months, issues arose with chromium leaching from the cement used in the tunnel as a sealant to prevent groundwater lowering. Even though cement producers work to minimize chromium content, it's impossible to eliminate it entirely. Meanwhile, the maximum allowed chromium levels have been significantly reduced. To treat the drainage water and handle the chromium, the project attempted to solve the problem – easier said than done.

– It was challenging to find a solution for chromium. We explored various options, such as ion exchangers, but it's difficult to say if they would have worked, says Elina Myrestaf, responsible environmental coordinator at Implenia.

However, Implenia in Varberg received good news from their colleagues in Stockholm working on the Johannelund tunnel project, which has a similar setup to the Varberg Tunnel: There is a company that has developed a solution for chromium issues.

Chromium a Thing of the Past

This is how Elina Myrestaf got in touch with Swedish Hydro Solutions. The company visited the Varberg project and conducted bench tests on water samples. The result was the installation of a treatment plant comprising RedOx3 technology, chitosan flocculation, and pH adjustment using carbon dioxide. The treatment works by reducing highly water-soluble hexavalent chromium to less water-soluble trivalent chromium with RedOx3.

The trivalent chromium is then precipitated, and the metal is subsequently flocculated by chitosan before being sedimented. After the initial adjustment of the plant, high chromium levels became a thing of the past.

– I am very pleased with the treatment. The reduction in chromium is very clear, allowing us to meet the requirements. For instance, we can have incoming total chromium at 24 micrograms per liter and outgoing at 1 microgram. And we have very few issues, despite water treatment being a bit tricky depending on the flow.

Unexpected Bonus – Zinc Removal

In the Varberg Tunnel, there were occasional issues with zinc levels exceeding limits. However, despite attempts, the source could not be identified. Zinc is not used in the project, and it is not present in the groundwater either. But with Swedish Hydro Solutions' plant, an unexpected bonus was achieved.

–We also see a clear reduction in zinc, from incoming 420 micrograms per liter to outgoing 59. It is well below the limit, says Elina Myrestaf.

Moreover, Elina Myrestaf finds Swedish Hydro Solutions' chromium treatment plant very compact.

– It takes an unreasonably small space with the extra step for chromium management, which is very convenient since construction projects like these often have limited space.

To ensure treatment effectiveness, Implenia and Swedish Hydro Solutions maintained continuous contact throughout the project until the end of 2023 when the tunnel halves are connected during breakthrough. According to the plan, trains will start running in the tunnel in December 2024.