New process from Freiberg: removing pollutants from mining water

The pilot plant for a new type of water treatment process has been in operation at the Hammerberg slag heap on the eastern outskirts of Freiberg since July 2024. It was developed by a team from the Institute of Biosciences at Freiberg University of Mining and Technology (TU Freiberg) together with G.E.O.S Ingenieurgesellschaft mbH and the Förderverein Montanregion Erzgebirge e.V. It purifies the contaminated leachate in several stages using a combination of (micro)biological and chemical processes. It only needs electricity for the pumps, which is provided on site with the help of a small photovoltaic system with battery storage. The treatment itself is passive and requires hardly any maintenance.

"The leachate we work with here is still heavily contaminated with aluminum, cadmium, zinc, manganese and other metals more than 50 years after the closure of the slag heap," explains project manager Prof. Sabrina Hedrich, Professor of Microbiology and Biohydrometallurgy at TU Freiberg. "They are also contaminated by low pH values and high sulphate concentrations. Even if there is no danger to humans, we urgently need to find solutions to minimize the impact on nature."

• Three steps to clean water

The plant itself looks unspectacular: The water flows from storage tanks through the cascading treatment plant, where it is purified step by step. In the first treatment stage, the pH value of the acidic water is raised from around 4.9 to neutral pH values between 6.5 and 7.0 with the help of a limestone filter. The aluminum dissolved in the water also precipitates as aluminum hydroxide and can be collected.

The second treatment stage is a so-called bioreactor, which contains a mixture of limestone, wood chips, straw and compost - the so-called biomix. It provides the best living conditions for bacteria that produce hydrogen sulphide from the sulphate in the leachate. This in turn ensures that the metals contained are precipitated as metal sulphides and retained in the biomix. The third and final stage is a plant basin, which acts as a natural sewage treatment plant, filtering residual pollutants and suspended matter from the water.

• Promising results and potential applications worldwide

The MindMontan project comes to an end at the end of this year. The results that the research team has achieved over the last few months are very promising. "We were able to show that our process is able to efficiently remove pollutants from water," says Prof. Sabrina Hedrich. "It is also cost-effective and can later be naturally integrated into remediation landscapes as a large-scale plant. This makes it interesting not only for our region, but also for mining landscapes worldwide."

According to the EU Water Framework Directive, all European water bodies should have achieved good ecological and chemical status by 2027. The process developed in the MindMontan project can contribute to this.