The amount of litter floating in the Rhine is many times larger than previously believed. Researchers from the University of Bonn, the University of Tübingen and the Federal Institute of Hydrology (BfG) partnered with the Cologne-based non-profit pollution-fighting organization K.R.A.K.E. to collect and classify macro litter in a floating litter trap—the only one of its kind in Germany—over a period of 16 months. Extrapolation models based on the observed volume indicate that roughly 53,000 items of macro waste debris float past Cologne on the Rhine river every day. Disposable plastic products make up a large proportion of the litter found in the Rhine. The findings have now been published in the scientific journal "Communications Sustainability."
It is impossible to quantify exactly how many tonnes of anthropogenic litter are in our oceans, but estimates suggest it is several millions. And more litter is added every year. A large part of this volume flows into the oceans via rivers. "The estimate of these large scale assessments are very difficult to do and we need reliable field data," says Dr. Leandra Hamann from the Institute for Organismic Biology at the University of Bonn, who has since moved to the University of Alberta in Edmonton, Canada. "In previous studies, visual macro litter observation was a common method to get a reliable ballpark estimate of the actual litter volume, but this has only been done occasionally in the Rhine. In the past, this has mainly involved watching the litter floating past from a bridge. In this case you can easily imagine that some debris will go unnoticed and some things are floating deeper down. Now, we are using a more reliable, continuous and long-term monitoring process."
Some 20,000 bits of waste collected in 16 months
Partnering with Katharina Höreth of the University of Bonn Department of Geography, Nina Gnann of the Department of Geosciences at the University of Tübingen and the Cologne-based non-profit organization K.R.A.K.E. e. V., Dr. Hamann supervised the citizen science part of the litter trap project in which the researchers and volunteers from the organization systematically collected and classified macro litter floating in the Rhine over a 16-month period. This was done using the "RheinKrake"—a floating litter trap installed in 2022 near the Zoobrücke bridge in Cologne. Spanning three meters of the river, the litter trap captures individual items of debris and garbage which are larger than one centimeter, down to a depth of 80 centimeters. RheinKrake initiator Nico Schweigert discussed the project background: "The idea behind the RheinKrake was to reduce the amount of litter that ends up in the Wadden Sea nature reserve and other places, while raising awareness among the responsible authorities. So we contacted the University of Bonn right at the start about gathering scientific data on the swimming litter being carried along." A long measurement period was a highly important project feature, as well as measurements being taken day and night.
Relying on a host of volunteers, between September 2022 and January 2024 20,339 macro waste items were collected and classified in accordance with international standards, falling into 183 litter categories within nine material types. Extrapolated to the total volume, this amounts to approximately 53,000 pieces of litter passing through Cologne per day in the linear scenario. This corresponds to a total weight of 2,169 tons per year. Weighted scenarios reach values of up to 3,391.8 tons of waste per year in the Rhine in Cologne. "Extrapolated to the entire Rhine, this is 22 to 286 times higher than previous estimates from other studies," says Hamann.
"Plastic accounts for 70% of macro litter items, but less then 15% by weight—the weight difference being attributable to textiles, glass, ceramics and other man-made materials polluting our waters." Analysis reveals that consumer items are the main source of macro litter, comprising over 50 percent of the total. Such items include wooden sticks used in fireworks, glass bottles and plastic caps from beverage bottles. The team often finds fragmented items, for example made of foamed or unfoamed plastic, where it is no longer possible to tell what it originally was without closer study.
Study leads to action recommendations
The researchers have derived a number of action recommendations based on the obtained data. "Disposable products account for 40% of the collected litter—more than half of which is plastic," relates Katharina Höreth, "but reusable products made up less than 8%; the rest was not clearly identifiable." Making bottles and packaging part of the deposit scheme could reduce the amount of litter in rivers on a sustained basis.
Another thing revealed by the RheinKrake project is that macro litter volume varies greatly at different times in the year, ranging from around 70 to over 2,700 litter items per trap emptying. "On New Year's Day, for example, the Rhine carries away remains of fireworks," says Nina Gnann, "and we have also observed garbage left behind on the banks of the Rhine being washed into the river when water levels rise." This could be avoided to a significant extent, the researchers have pointed out, by targeted cleanup campaigns and making sure that trash bins are emptied before the water level rise.
Institutions involved and funding secured:
The University of Bonn, the University of Tübingen, K.R.A.K.E. ("Kölner Rhein-Aufräum-Kommando-Einheit," or "Cologne Rhine Cleanup Command Unit") and the Federal Institute of Hydrology were involved in the project. The volunteering organization K.R.A.K.E. provided resources. The boat used by K.R.A.K.E. to transport people and waste from the shore to the waste collector was provided by Rheinau Marina in Cologne. Aside from this assistance, K.R.A.K.E. operates on a self-funded basis through donations.
