Wastewater from washing machines is widely seen as a major source of microplastics -- tiny plastic particles suspected of harming both people and animals. To help reduce that pollution, researchers at the University of Bonn have created a new filter based on a natural model: the gill arch system in fish. In early tests, the patent-pending device removed more than 99 percent of plastic fibers from washing machine wastewater. The findings were published in the journal npj Emerging Contaminants.

A washing machine used by a four-person household can generate up to 500 grams of microplastics each year, largely due to fibers wearing off textiles during washing. That makes everyday laundry one of the most important sources of these particles. Today, much of that material ends up in the sewage sludge produced at wastewater treatment plants. Because this sludge is often applied as fertilizer, the fibers can ultimately be spread onto fields.

Why existing washing machine filters fall short

Because of the scale of the problem, many manufacturers have been looking for ways to remove microplastics from wash water before it reaches the environment. "The filter systems available so far, however, have various disadvantages," explains Dr. Leandra Hamann from the Institute for Organismic Biology at the University of Bonn. "Some of them quickly become clogged, others do not offer adequate filtration."

Hamann, working with her doctoral supervisor Dr. Alexander Blanke and other colleagues, turned to biology for ideas. The team focused on fish that act as expert natural filters -- the result of hundreds of millions of years of evolution.

Some fish feed by filtering water, including mackerel, sardines, and anchovies. They swim with open mouths and capture plankton using their gill arch system. "We took a closer look at the construction of this system and used it as the model for developing a filter that can be used in washing machines," says Blanke, who is a member of the transdisciplinary research areas "Life & Health" and "Sustainable Futures" at the University of Bonn.

How fish gills inspired a self-cleaning filter design

Over time, these fish evolved a strategy similar to cross-flow filtration. Their gill arch system forms a funnel that is widest at the mouth and narrows toward the gullet. The funnel's walls are formed by branchial arches, which include comb-like structures. Those structures are covered in tiny teeth, creating a mesh stretched across the arches.

"During food intake, the water flows through the permeable funnel wall, is filtered, and the particle-free water is then released back into the environment via the gills," explains Blanke. "However, the plankton is too big for this; it is held back by the natural sieve structure. Thanks to the funnel shape, it then rolls towards the gullet, where it is collected until the fish swallows, which empties and cleans the system."