Dual-bubble approach boosts microplastic removal from wastewater past 90%

Researchers at RMIT University have engineered a highly effective method for microplastic removal from wastewater by combining microbubbles and nanobubbles

The breakthrough technique achieves a microplastic removal rate of over 90% during the primary stage of water treatment.

The peer-reviewed study, published in the journal ACS ES&T Water, was led by Associate Professor Biplob Pramanik and Dr Sirajum Monira at RMIT’s Water Effective Technology and Tools (WETT) Research Centre.

Upgrading dissolved air flotation: Microplastic removal

Wastewater treatment plants are one of the primary pathways through which microplastics slip into natural ecosystems. Because these microscopic plastic fragments easily bypass standard filtration setups, they pose a severe and growing risk to marine life and human health.

To address this, the RMIT team focused on optimising Dissolved Air Flotation (DAF), a water purification method widely utilised by treatment facilities. Standard DAF operates by injecting air bubbles into wastewater; these bubbles latch onto suspended contaminants and float them to the surface, where they can be skimmed off.

The new approach enhances this baseline process by simultaneously introducing two distinct bubble scales, which outperform systems relying on just one size:

• Microbubbles:

  • These larger bubbles provide the necessary upward buoyant force to rapidly carry trapped plastic particles up to the water’s surface.

• Nanobubbles:

  • Due to their sub-microscopic size, these bubbles significantly increase the physical interactions between floating particles. They enhance particle attachment and promote aggregation, forcing individual microplastics to clump together.

Resilience against complex organic waste

A major hurdle for new water treatment technologies is that they often fail outside controlled laboratory environments when exposed to raw, messy sewage. However, the dual-bubble system proved remarkably resilient under realistic wastewater conditions.

Testing revealed that common treatment barriers—such as dissolved organic matter, fats, oils, and grease—did not degrade the system’s performance. In fact, when combined with standard chemical coagulants, these organic compounds actually assisted the process. The grease and organic matter helped glue the microplastics into larger, heavier clusters, making them even easier for the micro-nanobubble matrix to sweep upward.

By capturing these pollutants during the primary phase of treatment, the method prevents microplastics from becoming deeply embedded in the downstream sewage sludge. This effectively blocks them from contaminating biosolids, which are frequently recycled as agricultural fertilisers, minimising their release back into the environment.

Low-cost, infrastructure-free integration

Because the technology relies on a process already standard in modern sanitation plants, it can be adopted globally without requiring major infrastructure overhauls or expensive new machinery. Facilities can deploy the system simply by optimising their existing operating conditions, such as adjusting air pressure ratios, tuning saturation timelines, and modifying bubble generation sizes.

Having successfully validated the dual-bubble framework at a laboratory scale, the RMIT research team is actively seeking industry partners and municipal water authorities to scale the technology for real-world operating conditions across diverse wastewater streams.