Learning together in the fight against microplastics

Install a bubble screen on the bed of a river or another waterway and you have a way to remove plastic waste from the water. The bubbles send the waste to the surface, 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, who devised the technique with the other three founders: “Our success led to contacts with PWN, the joint organiser of the Makathon. During discussions with the drinking water company, the idea came up of looking at the effect our bubble screen would have on microplastics in the treated waste water from a wastewater treatment plant before it goes to the surface water. That matched our company’s mission wonderfully. We want to use our invention to remove as much plastic as possible from the rivers before it enters the sea.”

Natural alliance

That was how the Preventing the discharge of microplastics via effluent to surface water project came about, and the trial location was soon found: the WWTP in Wervershoof in North Holland. “We are already working on other research here with PWN,” says Bob de Boer of the Hoogheemraadschap Hollands Noorderkwartier (HHNK) water authority. “For example, we are applying techniques from drinking water production to make the effluent even cleaner. We use ozone to test, for example, the removal of residual medicines.” De Boer thinks it’s a natural step for a water authority to work with a water company and he sees the benefits. “For example, you can work together on optimising operational processes such as pipeline management. And we can help drinking water companies look for new sources of water intake, such as treated effluent. That can’t be used as drinking water but it may be suitable for other applications such as cooling systems. And we both benefit from as few microplastics in the water as possible.”

Microplastics in effluent

Microplastics from homes enter waste water when, for example, fabrics like fleece are washed. The wearing of car tyres on the road is also a source of microplastics. WWTPs can remove some 90 to 99 percent of microplastics from effluent. De Boer would like to see a solution for the remainder. “As a water authority, we are constantly looking at how we can make our waste water even cleaner. During the trial, we looked at how good the bubble screen is at capturing plastic particles measuring 0.02 to 0.5 millimetres. No demonstrable difference was found between the amount of microplastics on either side of the bubble screen.” Nevertheless, in their report, the research consortium states that they do not believe this necessarily means the technique will not work.

“As a water authority, we are constantly looking at how we can make our waste water even cleaner.”

External factors

External factors that affect the result, such as the sampling method or the height in the water column where the measurements were taken, have to be considered. Weather conditions such as rain and wind also have to be taken into account. In addition to the technology, there are many more variables that have to be studied in order to come to a conclusion about the effectiveness of the bubble screen for microplastics. “In earlier pilot projects with the bubble screen, plastic particles of 1 millimetre and larger were captured, which shows that it may also capture microplastics,” says Eveleens. Microplastics are plastic particles smaller than 5 millimetres and they should therefore, in principle, be in this range. “So one of our conclusions is that we could repeat the test in the laboratory in standardised conditions.”