Even if research cannot directly demonstrate that technology works in practice, the Topconsortia voor Kennis en Innovatie (TKI) approach pays off. This is proven by a project that looked at whether a special screen of air bubbles could be used to remove microplastics from waste water treatment plant (WWTP) effluent. Much has been learned from solving social issues together, say the organisations involved.
Create a bubble screen from the bottom of a river or other waterway and you have a way to remove plastic waste from the water. The bubbles push the waste to the surface of the water where it can be collected. This concept won the Plastic Free Rivers Makathon in 2016. Anne Marieke Eveleens, co-founder of The Great Bubble Barrier, and the three other founders came up with the technique. "This success brought us into contact with PWN, the co-organiser of the Makathon. During a conversation with the drinking water company, the idea arose to see what effect our bubble barrier would have on microplastics in the treated wastewater from a sewage treatment plant before it flows towards surface water. This fits beautifully with our company's mission. We want our invention to remove as much plastic as possible from rivers before it flows out to sea."
Collaboration between water company and water board
Thus, the ‘Preventing the outflow of microplastics via effluent to surface water’ project was born, and the test location was soon found. It was the Wervershoof sewage treatment plant in the province of North Holland. "We already do research here with PWN," says Bob de Boer of North Holland’s Hoogheemraadschap Hollands Noorderkwartier (HHNK) water board. "For instance, we apply techniques from drinking water preparation to make the effluent even cleaner. We use ozone, for example, to test the removal of medicine residues." De Boer finds it only logical for a water board to cooperate with a water company and sees the advantages of doing so. "You can work together to optimise operational processes, such as in pipeline management. And we can help water companies find new sources of water intake, such as treated effluent. This is not directly usable as drinking water but could be used for other applications, such as cooling systems. And both benefit from as few microplastics in the water as possible."
Microplastics in effluent
In households, microplastics, from washing fleece or polyester clothing, for example, end up in wastewater. The wearing down of car tires on roads is also a source of microplastics. Sewage treatment plants can remove about 90 to 99 percent of microplastics from effluent. De Boer would like to see a solution for the rest. "As a water board, we are constantly looking at how we can make our wastewater even cleaner. The trial examined how well the bubble screen can capture plastic particles measuring from 0.02 to 0.5 millimetres in size. No demonstrable difference was found between the amount of microplastics on one side or the other of the bubble screen. Nevertheless, in their report, the research consortium states that this does not necessarily mean that the technique would not work.
As a water board, we are constantly looking at how we can get our wastewater even cleaner.
Bob de Boer
Working for North Holland’s Hoogheemraadschap Hollands Noorderkwartier (HHNK)
External factors that affect the result should be considered, such as the sampling method or the height which the measurement is taken in the water column. Weather conditions should also be considered, such as rain and wind. Apart from the technique, many variables need more research before a statement can be made about the effectiveness of the bubble barrier for microplastics. "In previous pilots with the bubble barrier, plastic particles of one millimetre and larger were captured, which therefore shows that it could also capture microplastics," says Eveleens. Microplastics are plastic particles smaller than five millimetres and should thus fall within this range. "So one of our conclusions is that we could repeat the test in the laboratory under standardised conditions."
Frank Oesterholt, a researcher at KWR, agrees that a laboratory test would be very useful. The research institute was also involved in the project to further improve the measuring method for microplastics in water, given that a standard method for detecting microplastics does not yet exist. "We used the LD-IR method, a laser technique that is quite complicated and requires special pre-treatment of the samples. You must be sure you are measuring microplastics in the effluent and not something else,” Oesterholt explains. “We have been able to visualise this process better, which means we are a little closer to a good detection method of microplastics in different water streams. A scientific paper on this will be published soon."
We have moved a little closer towards a good detection method of microplastics in different water streams.
Researcher at KWR
Unique knowledge from different perspectives
After the success of the bubble barrier with macroplastics in rivers, was it perhaps opportunistic to immediately do a field trial with microplastics in effluent? Oesterholt doesn't think so. "The effluent channel in question was a well-defined environment to do the test and the choice to do so was an obvious one. It is very good that we were able to hone our methodologies, where we also found that the effluent contained 40 to 50 particles of microplastics per litre. This is in line with what you can find in the literature about this." De Boer is not disappointed with the project either. As far as he is concerned, the door to the bubble barrier remains open. "This trial was a first step. You then need to take a step back and fine-tune the methodology. Although the water board is also looking at other ways to tackle microplastics in effluent, we are certainly interested in what any follow-up research will yield." Should there be follow-up, the partners know how to reach each other quickly, says Eveleens, who, like the others, is enthusiastic about collaboration. "The lines of communication are short. We keep each other on our toes with constructive feedback. Everyone contributes unique knowledge from different perspectives." This is the power of TKI in practice.
The research consortium consists of drinking water company PWN, PWN Technologies (PWNT), Hoogheemraadschap Hollands Noorderkwartier (HHNK), The Great Bubble Barrier, and KWR. In this joint venture, all the partners take their social responsibility in looking for solutions that give greater insight into the problem of microplastics in surface water and help counteract them.