Ultrafiltration creates clear conditions

Ultrafiltration creates clear conditions
Seas and oceans are the biggest water reservoirs on Earth. Since decades sea water is turned to drinking water by desalination in large scale. In Ghana, West Africa, a modern desalination plant is currently being established to produce up to 60,000 cubic meters of drinking water per day - enough to supply half a million people. In this plant ultrafiltration membranes made by the BASF subsidiary inge play an important role: They are used to pretreat the salt water in order to optimize the actual desalination and to protect the downstream salt filters from contamination. These salt filters operate according to the reverse osmosis principle - hereby the water diffuses as individual molecules through the sensitive membrane. As high pressure of up to 80 bar is required for this process, the pre-purification by means of ultrafiltration additionally contributes to the limitation of the energy input.

The water taken out of the sea is forced under pressure through the very fine-pore Multibore membranes and can pass through them, while undesired substances such as sand, clay, algae and even pathogenic germs are intercepted. At first glance, the ultrafiltration membranes look like thin white tubules, but the cross-section reveals their complex inner life: The fiber contains seven capillaries into which the raw water runs. The walls of the capillaries have tiny pores with a diameter of about 20 nanometers - this is 500 times thinner than a filament of a spider's web. All the particles larger are retained here by the membrane. Only the purified water passes through the pores into the plastic fiber and emerges again on the outside of the fiber.

Production of the membranes requires extensive know-how and experience. "The challenge is to create pores during the production process that are small enough and evenly distributed over the membrane surface," explains Dr. Nicole Janssen, Laboratory Team Leader at Performance Materials Research. Together with her team, she is optimizing the conditions and the starting material from which the membrane fibers are manufactured: the BASF plastic Ultrason E, a polyethersulfone. "We can now adjust the Ultrason solution and the process so accurately that the membranes offer dependable filter performance."

For the filters to work reliably in practice, however, not only the size and distribution of the pores have to be correct, the fibers also have to be resistant. This is ensured by the honeycomb structure inside the fibers designed by the experts of the BASF subsidiary inge. "The arrangement of the seven capillaries in the supporting structure makes the fiber mechanically stable and thereby resilient," explains Martin Heijnen, Head of Membrane Development at inge, who adds: "This protects the membranes against fine cracks through which otherwise bacteria or viruses could pass."

In a filter plant through which, for example, the sea water in Ghana will be pumped, the membrane fibers are bundled together in white plastic cylinders. The ends are stuck to the housing with epoxy resin. During operation, the lower surface is sealed so that the capillaries are only open at the top. Here the raw water is pumped in at a pressure of about 0.5 bar. The only path it can take from here is through the pores in the internal capillary walls of the fibers - and out again as clean water on the outside.

Over time, the residues collect in the capillaries. To ensure that the water can penetrate this so-called filter cake, the water pressure has to be increased. This requires large amounts of energy and causes stress to the membranes. The filter system is therefore cleaned regularly every one to two hours by reversing the water flow: Clean water is briefly forced from outside into the fibers and rinses the filter cake out of the capillaries.

Nevertheless, blockages in the pores or sticky substances like sugar or proteins may still remain behind. These are removed chemically at longer intervals, for example using sodium hydroxide, acid or hypochlorite. In time, however, oxidizing agents can attack the plastic Ultrason E. The material expert Janssen and her colleagues want to improve this situation. For example, they are working on making the filter surface of the capillaries more hydrophilic, in other words more water-loving. In this way, it would be more difficult for the contaminants to be deposited. This would make cleaning easier and chemical cleaning steps would also be reduced. “Membrane service time and lifetime are thereby prolonged,” adds Janssen. These improvements would not only be useful for the pre-cleaning of sea water but also for the processing of drinking water or the treatment of waste water.

Water filter

Read more: Polymers 97

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