A bubble curtain for a plastic-free Wadden Sea

Innovative bubble curtains for cleaner surface water

Amsterdam-based company The Great Bubble Barrier installs inventive bubble curtains that filter plastic litter and microplastics from the water system. Behind the Tsjerk Hiddesluizen in the Harlinger Van Harinxmakanaal*, the latest Bubble Barrier has been installed, combined with a DAAS flow system that helps map water behaviour. Beneficial for a cleaner Wadden Sea.

Dutch rivers, canals and waterways transport about 27 kg of plastic bags, tyre rubber, paint particles, pellets and artificial grass granules to the sea every day, a process in which plastic residues fragment into microplastics (< 5 mm). RIVM estimates microplastics emissions in the Dutch river basin at more than 10,000 kilotonnes annually, a figure equivalent to the weight of about 990 Eiffel Towers. These synthetic polymers have the property of barely degrading, dissolving very poorly in water and entering the marine environment. The plastic residues settle in the soil, accumulate in sediment and threaten the marine food chain, including humans.

A Bubble Barrier as a water retention facility

Cleaner surface water is a target task in the Netherlands, especially where waterways and industrial agglomerations are concentrated. How do you safeguard the quality of a freshwater system in a way that continues to allow shipping passage, that fits well with the practices of water authorities and drinking water companies, that can be managed cost-effectively and that does not disrupt the natural system hydrology? The Great Bubble Barrier has the answer: with a Bubble Barrier.

“You can think of the Bubble Barrier as an underwater curtain”, explains Louis Le Duc, who, as a Research and Development engineer at The Great Bubble Barrier, is involved in implementing and improving the innovative water treatment technology. “A Bubble Barrier consists of a diagonal tube at the bottom with small holes, which creates air bubbles when compressed air is blown through it. These bubbles float to the surface by upward pressure, taking the plastic with them.

The diagonal position of the tube ensures that the plastic is pushed to the side of the waterway, where it can be easily collected. This depends on many factors, such as location, depth, wind and current. The unique added value of our technology lies in fine-tuning the interplay between hoses, the angle of the tube, how much air is needed to create effective bubbles and how you are going to generate that air in the right place.”

Dynamic bubble production in Harlingen

Since 23 May 2024, the Municipality of Harlingen has installed a bubble curtain in front of the Van Harinxmakanaal’s discharge sluices. Right where the Wadden Sea begins and the canal ends: after 37.5 km of Frisian waterway through agricultural areas, past industry and right through towns and villages. The ambition? Safeguarding the Wadden Sea World Heritage Site from microplastic pollution: a vulnerable intertidal area that serves as a nursery of fish species, is a foraging habitat for 10 to 12 million birds and is home to unique saltwater plant species.

To optimise the insights from the bubble curtain, the system in the Van Harinxmakanaal is combined with River Insight’s Discharge Data As A Service technology, DAAS for short. The Acoustic Doppler Current Profiler (ADCP) in the station provides The Great Bubble Barrier with real-time current measurements and data on the water level in the channel. Essential water data is readily available thanks to a secure Low Power Wide Area Network (LPWAN) and allows the team to remotely monitor how the water system is functioning. This mode of operation improves monitoring, minimises maintenance and ensures that the Bubble Barrier can be operated energy-efficiently. The station generates its own solar energy and is self-sufficient.

“It fits with our vision. In Harlingen, we eventually want to integrate a dynamic control system, where we can adjust the air supply in the bubble curtain based on actual data on water movements”, says Le Duc. “We want to map that water behaviour, model it and integrate it into software that controls the system. The bubble curtain then becomes a sustainable, learning system and can respond to changes itself.”

Amsterdam expands Bubble Barriers

The success of The Great Bubble Barrier in Harlingen does not stand alone. In 2019, the first bubble curtain was installed in Amsterdam’s Westerdokkanaal, where more than a tonne of plastic pollution in water is collected every year. “In addition, The Great Bubble Barrier also has ongoing projects in Katwijk, Portugal’s Vila do Conde, in collaboration with the EU project MAELSTROM, and discussions with water managers in the Netherlands, as well as in Asia and the United States”, Le Duc says.

The City of Amsterdam is currently actively looking with partners Waterschap Amstel, Gooi en Vecht and the company Waternet for new city locations for additional bubble curtains. It illustrates the impact, but also the good scalability of the technology: over 86% of floating material is intercepted by the barrier.

“At moderate flow rates at the intended location, our solution works with unprecedented effectiveness. It means our technology can add value in sustainable water management in all kinds of environments, from urban canals to rural rivers.”

Scientific research

Collaborations with local partners and scientific institutes, such as Deltares, are crucial for developing innovative water treatment technology. Extensive testing and data modelling should continuously improve the effectiveness of the technology, but also ensure that the systems are fully adapted to specific water conditions.

“We conducted several tests in Deltares’ labs to measure and ensure the effectiveness of hoses and air pressure. This collaboration also helps us better understand the additional benefits of the technology, such as promoting biodiversity through more added oxygen, or combating saltwater intrusion.”

Even after installing a barrier, the research continues. It is common practice for local partners to conduct a monitoring project for at least a year to measure plastic capture and influences of bubble curtains in the water column. “This is also happening in Harlingen, in a study on glass eel migration. The final report will be released soon and promises good news.”

Research and data modelling also help refine the technical functionality of components. For example, River Insight’s DAAS system is being used to lay a foundation for more knowledge about rough current velocities in the Van Harinxmakanaal.

The largest bubble curtain in Portugal is 235 metres long, but the limits of this technology have not yet been reached. The mesh size of the collection system is basically designed for plastic particles as small as one millimetre. But The Great Bubble Barrier does not stop there.

“We are working hard to expand those boundaries further, but also look for integration with entirely new functionalities. Integrating sonar, sensors or cameras to find and detect microplastics could potentially detect waste faster and better. This results in a technology that provides an effective and environmentally positive solution to plastic pollution in water, captures microplastics and helps monitor water quality, which in itself is of great value.”

Plastic-free Dutch waters

There is growing awareness in the Netherlands that additional effort is needed to filter plastics out of the ecological chain and eliminate existing plastic pollution.

Polyethylene bags at the supermarket have cost money since 1 January 2016, films and packaging are increasingly biodegradable, polypropylene caps on soft drink bottles are required to be fixed, and there has been a deposit on PET bottles since 1 January 2021. These are efforts that are part of pan-European policies against plastic litter and microplastics. Rijkswaterstaat is investigating the feasibility of new programmes to clean up plastic litter on banks and in the water column. The OSPAR Convention** (Oslo-Paris Convention, Convention for the Protection of the Marine Environment of the North-East Atlantic) looks – among other things – at improving sustainable discharge management, awareness and waste disposal.

The ‘Plastic-free Rivers’ programme supports municipalities is acting against plastic litter and runs until the end of 2025. Because the plastic that does not end up in the water does not need to be cleaned up. In the Netherlands, tackling all litter represents an annual cost for municipalities of around €280 million.

For more information, please visit the website of The Great Bubble Barrier.

*Bubble Barrier Harlingen is an initiative of the Municipality of Harlingen, the Province of Fryslân, Rijkswaterstaat, Port of Harlingen, the Municipality of Leeuwarden, the Municipality of Waadhoeke and is supported by the Waddenfonds.

**The OSPAR Convention protects the marine environment in the North-East Atlantic and is important for North Sea policy coordination. The Great Bubble Barrier only uses the OSPAR method to monitor plastic capture with Bubble Barriers. This is a protocol developed by OSPAR to classify plastics into more than 100 categories.