Almost a tenth of the world's marine waste consists of ghost nets. In the Baltic Sea alone, up to 10,000 abandoned parts of nets, torn from inshore fisheries or lost by fishing boats enter the water each year. Even after decades, they still present a danger to the marine environment, becoming “a silent trap for marine mammals, sea birds and fish” in the words of Jochen Lamp, manager of the WWF Baltic Sea office.
Alarmed, WWF Germany launched its ghost net dragging operations in 2016 to address the problem, collaborating with local fishermen, the fisheries and cultural heritage authorities and government agencies to identify target areas. Specially trained fishing cutter crews working with local diving teams were deployed to retrieve the lost nets - an approach that has resulted in the successful collection of 5 tonnes of derelict fishing gear (wet weight), entangled with old anchor chains, aluminium and copper cables and other seabed waste, from the sea.
"The material recovered from the seabed has partly been in the water for many years - it is very heterogeneous and extremely polluted," said Andrea Stolte, project manager at the WWF Baltic Sea Office.
The retrieval operations are sponsored by the German recycling company Tönsmeier. While an important aim is to retain as much of the material as possible in the production cycle as secondary raw materials, it was first necessary to obtain information on the composition, the size and the degree of contamination of the recovered nets, explained Dr Michael Krüger, Head of the Research and Development of the Tönsmeier Group, who is overseeing the project for the Group.
To that end tests were carried out this year starting in March at the technology centre of Vecoplan, a recycling equipment and systems producer in Bad Marienberg, Germany.
Large-scale impurities - stones, metals or pieces of wood - had to be removed manually, after which the retrieved waste was sorted into different fractions - nets, control nets and ropes. These fractions were subsequently chopped, ground and passed through a metal separator to eliminate any remaining metals. It was then possible to start the actual separation tests, a number of which were carried out using the float / sink method.
"This method works very reliably for the separation of substances with different densities, and it yielded the desired results in the series of experiments performed at this time, " said Krüger.
The experiments verified that the extensive preparation and cleansing procedure carried out sufficed to allow further processing of the material.
After an addition shredding round, the material could undergo the usual separation processes, rendering it suitable for various recycling processes.
"Which type of utilization is most suitable ecologically and economically remains to be determined in the second half of the year," Krüger said.