Inside the mouth - of this anchovy, plankton particles are captured by the gill arch system. © Photo: Jens Hamann

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Wastewater from a washing machine in a four-person household produces up to 500 grams of microplastics each year, mainly caused by textile abrasion. The household appliances are thus one of the most important sources of the tiny particles. Microplastics currently make their way directly into the sewage sludge of wastewater treatment plants. As this sludge is often used as fertilizer, the fibers ultimately end up on the fields.

Many manufacturers have thus been searching for ways to remove microplastics from washing water to prevent them from entering 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.”

Looking inside the mouths of fish

The scientist, alongside her doctoral supervisor Dr. Alexander Blanke and colleagues, has thus turned to the animal kingdom in her search for possible solutions. The team focused on fish that can be considered true masters of filter technology – and have evolved this filtration over hundreds of millions of years.

Some fish feed by means of filtration; these include, for example, mackerel, sardines, and anchovies. They swim through the water with their mouths open and sift out the plankton with 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.

During their evolution these fish have developed a technique similar to cross-flow filtration. Their gill arch system is shaped like a funnel that is widest at the fish’s mouth and tapers towards their gullet. The walls of the funnel are shaped by the branchial arches. These feature comb-like structures, the arches, which are themselves covered in small teeth. This creates a kind of mesh that is stretched by the branchial arches.

Self-cleaning: plankton rolls towards the gullet

“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.”

This principle prevents the filter from being blocked – instead of hitting the filter head-on, the fibers roll along it towards the gullet. The process is also highly effective, as it removes almost all of the plankton from the water. Both are aspects that a microplastic filter must also be able to deliver. The researchers thus replicated the gill arch system. In doing so, they varied both the mesh size of the sieve structure and the opening angle of the funnel.

Filter achieves high efficiency

“We have thus found a combination of parameters that enable our filter to separate more than 99 percent of the microplastics out of the water but not become blocked,” says Hamann. To achieve this, the team used not only experiments but also computer simulations. The filter modelled on nature does not contain any elaborate mechanics and should thus be very inexpensive to manufacture.

The microplastics that it filters out of the washing water collect in the filter outlet and are then suctioned away several times a minute. According to the researcher, who has now moved to the University of Alberta in Edmonton, Canada, they could then, for example, be pressed in the machine to remove the remaining water. The plastic pellet created in this manner could then be removed every few dozen washes and disposed of with general waste.

The team from the University of Bonn and the Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT has already applied for a patent for its development in Germany; EU-wide patenting is currently underway. The researchers now hope that manufacturers will further develop the filter and integrate it into future generations of washing machines. This would stem the spread of microplastics from textiles, at least to some extent. And that is also necessary: analyses indicate that the particles may cause serious damage to health. They have already been found in breast milk and in the placenta – and even in the brain.

Microscopic image: - the gill rakers are covered with denticles forming a mesh structure that catches the particles. © Photo: Leandra Hamann

The filter element in the center - imitates the gill arch system of the fish. The filter housing enables periodic cleaning and installation in washing machines. © Illustration: Christian Reuß/Leandra Hamann

The research team at Uni Bonn: - front Dr. Leandra Hamann, right Dr. Alexander Blanke, center material researcher Christian Reuß, left biologist Dr. Hendrik Herzog. © Photo: Peter Rühr/Uni Bonn