Water is essential for life on earth. Clean drinking water is a precious commodity that is not available everywhere in the world. For a sufficient supply of clean water, unconventional sources (seawater, waste water, etc.) must therefore also be used. With the help of membrane technology, for example, drinking water is produced from salty seawater (desalination by reverse osmosis). However, membrane technology is also used to remove pollutants, viruses or bacteria from raw water. In most cases, porous membranes are used, the pore size of which ranges from a few nanometers (nm) to micrometers (µm), depending on the separation task (for comparison: the average thickness of human hair is 50 - 80 µm). Separation is by size exclusion (sieving effect). This means that substances in the water that are larger than the pores are retained. The membrane filtration process and the influence of the pore size on the separation behavior is demonstrated using an ultrafiltration membrane (pore size 100 nm) and a microfiltration membrane (pore size >> 100 nm).
Porous polymer membranes are commonly made by a solution precipitation process. That is, a polymer solution is drawn into a film (flat membrane) or spun into a hollow fiber (hollow fiber membrane) through a nozzle. The film or hollow fiber is then transferred to a non-solvent, typically water, thereby precipitating the polymer. Adjusting the process parameters (concentration, temperature, additives, etc.) controls the pore size and other membrane properties. This manufacturing process can be reproduced by visitors in their own experiments and the membranes obtained can be tested in a simple filtration experiment.