A clean thing

Without a fourth treatment stage, micro-pollutants such as drug residues pass through wastewater treatment plants undisturbed to a large extent. Take diclofenac, for example: only a fraction of the active ingredient frequently used in painkilling ointments and tablets is actually utilized by the body. Up to 70 % finds its way into wastewater via the toilet. Conventional wastewater treatment plants with the three classic purification stages mechanical-biological-chemical cannot remove the tiny trace substances from circulation; like many other drug residues, hormones and chemical compounds, they end up in nature. They accumulate in streams, rivers and lakes, causing disease and infertility in many aquatic organisms. Environmental chemicals can have hormone-like effects and feminize male fish. Martin Spruck, Head of the Energy & Process Technologies research focus, sums up the problem: Austria's flowing waters contain a veritable cocktail of pharmaceuticals. This is also confirmed by a corresponding water study by the Federal Ministry for Sustainability and Tourism.

How far-reaching the problem is only became clear through more precise analytical methods. The low concentrations are difficult to detect, but can already have their harmful effects. In addition, the consumption of pharmaceuticals is constantly increasing. According to Marc Koch, a research associate at the MCI's department of Environmental, Process and Energy Engineering, it is also not always clear at what concentration the substances are harmful and how thoroughly they need to be removed. For various reasons, it is difficult to apply the currently functioning methods of the fourth purification stage across the board. Systems modeled on seawater desalination plants require a lot of space and energy, and the fourth purification stage used in model regions in Switzerland is expensive. Treatment with ozone works from a technical point of view, but means higher safety requirements and unknown degradation products, some of which are also harmful. Martin Spruck, Marc Koch and their team are researching an optimized hybrid approach: membrane filters with embedded activated carbon.

Based on the results of an earlier project, the team developed membranes made of a plastic that is harmless to health. Wastewater is forced under pressure through pores, of which those with the smallest dimension are around 500 times smaller than a human hair. Particles and dirt are retained, with virtually only the tiny micropollutants and water passing through. Embedded deeper in the membrane structure is adsorption-friendly activated carbon, which thanks to its particularly porous nature has an enormous surface area - around 1000 m² per gram. Thanks to upstream membrane filtration, the activated carbon only comes into contact with the critical molecules and remains usable for longer. In the laboratory, the MCI research team is testing and optimizing the membranes with synthetic wastewater to determine their long-term behavior.

The next step is a larger-scale field test in a suitable wastewater treatment plant. By the middle of the year, the hybrid system, for which a patent application has already been filed, is to be used to treat a partial stream of wastewater and remove the trace substances it contains in the best possible way.