Dual-bubble method removes more microplastics from wastewater

Researchers at RMIT University found more effective way to capture microplastics from wastewater, using a combination of microbubbles and nanobubbles to achieve removal rates of more than 90%. The researchers say the dual-bubble approach can be adopted by wastewater treatment plants without major infrastructure changes by optimising existing operating conditions, including air pressure, saturation time and bubble size.

Associate Professor Biplob Pramanik, director of RMIT’s Water Effective Technology and Tools Research Centre, said the approach suggests a practical way to reduce microplastic pollution — a growing global issue.

“Wastewater treatment plants are a major pathway for microplastics as they slip through filtration processes, posing risks to ecosystems and human health,” he said. “Our approach is simple to implement and significantly increases the removal of microplastics during the primary stage of treatment.”

The study investigated an enhanced version of dissolved air flotation, a widely used water treatment process that removes contaminants by attaching them to air bubbles and lifting them to the surface.

The researchers found plastic removal rates increased when microbubbles and nanobubbles were used together, outperforming systems that relied on either bubble type alone. Microbubbles provide the lifting force needed to carry particles to the surface, while nanobubbles improve particle attachment and aggregation by increasing interactions between particles, making the process more effective.

Dr Sirajum Monira, who completed the research during her RMIT PhD studies, said the approach remained highly effective in realistic wastewater conditions.

“Organic matter and fats, oils and grease, which are typically considered barriers to treatment, did not reduce performance,” she said. “In some cases, they improved it by helping microplastics clump into larger, more easily removed particles when combined with standard coagulants.

“By capturing the microplastics before they become concentrated in sewage sludge, we can reduce the amount entering biosolids and ultimately minimise their release back into the environment.”

With the dual-bubble approach successfully demonstrated at a laboratory scale, the team are now keen to collaborate with industry partners to validate this approach under real operating conditions and for a variety of wastewater streams.

The paper ‘Micro-Nanobubble Integrated Dissolved Air Flotation: A High-Efficiency Strategy for Microplastic Mitigation in Wastewater’, by Sirajum Monira and Biplob Pramanik, is published in ACS ES&T Water.

Image: Lead author Associate Professor Biplob Pramanik.