The Institute for Sanitary Engineering, Water Quality and Solid Waste Management (ISWA) at the University of Stuttgart, which was established early in the 1950s, is an interdisciplinary institution where engineers and scientists work hand in hand in the area of environmental protection technologies. At the Department of Hydrochemistry and Hydrobiology in Sanitary Environmental Engineering at the ISWA, a research group led by Prof. Dr. Jörg Metzger is focused on biodegradable polymers (BAP) and their use in the treatment of water. Their work  concentrates on the denitrification of water.

The increasing presence of nitrate and pesticides beyond valid standards is a worldwide problem. Huge technical and financial efforts are required to achieve a drinking water quality that conforms to health requirements. “This cannot always be guaranteed in the case of small water treatment facilities in rural areas. This was one reason why we developed a simple technology that not only enables the elimination of nitrate, but also the elimination of organic contaminants,” explains Dr. Wolf-Rüdiger Müller, head of the research group.

Standard methods used for denitrification involve the growth of bacteria on an inert carrier material, such as sand or expanded clay. These bacteria then reduce the nitrate. The bacterial enzymes convert bound nitrogen (nitrate, nitrite) into molecular nitrogen. In this process, which is also known as denitrification, nitrate is used as electron acceptor instead of oxygen. Ethanol is mainly used as carbon source (substrate). The thus prepurified water will subsequently be treated under aerobic conditions and filtered through an activated carbon filter to remove organic constituents.

Instead of using dissolved substrate, the ISWA researchers use a polymer such as poly-ε-caprolacton (PCL). This substance can be biologically degraded, does not dissolve in water and is used by bacteria as solid organic substrate for denitrification. At the same time, it also serves as a carrier matrix for bacteria. "PCL has another advantage," said Müller. "It can absorb pesticides and thus also has the potential to replace an activated carbon filter. However, subsequent aerobic purification is also necessary in this case." The method is being tested at the ISWA and, on a technical scale, in a waterworks using a photobioreactor from the company Formtechnik and the Dynasand reactor of Nordic-Water.

Using biopolymers as sorption substance for pesticides entails the problem of having to remove the pollutant-loaded polymers. "We initially assumed that the further degradation of the polymer is prevented when the polymer has absorbed a specific quantity of pollutants," explained Müller. The endosulfan loading capacity is huge, and inhibition of bacterial growth does not occur even with a pesticide concentration that is 100 times higher than the limit defined in the German Drinking Water Standard (TVO 2001). PCL was used up by the bacteria and endosulfan was once again released. Therefore, the replacement of the polymer with new material is indicated when a certain pollutant load has been reached.

At present, plastics are to a large degree recycled by way of composting. However, the ISWA researchers also envisage an alternative for disposing biodegradable polymers (BAP): using them in sewage plants. In sewage plants, the denitrification of water occurs after primary clarification. The BAPs can be used as substrate for the denitrifying activated sludge. The sludge arising from the purification of wastewater (biomass and residuals) is degraded by bacteria to biogas (methane and carbon dioxide) during what is known as anaerobic sludge decomposition. Biodegradable polymers can also be used as substrate in this process.

But how can one know whether a certain material can be biologically degraded? The ISWA is developing new methods to test the degradability of polymers in aqueous systems. Not all biopolymers can be degraded in aqueous environments in areas with temperatures below 20°C. “Previously, the main focus tended to be on the composting of biodegradable polymers,” said the chemist Dr. Angela Boley. The ISWA has already been involved in committees dealing with these issues and has helped prepare draft standards for test methods. The scientists are testing the degradability of polymers in aqueous systems under aerobic, anaerobic and anoxic conditions using devices developed by the ISWA or devices that have been bought and expanded. It is therefore possible to assess degradation under different environmental conditions and through their use in a digestion tower, for example for the production of biogas.